&EPA
United States
Environmental Protection
Agency
Office of Research and
Development
Washington, DC 20460
EPA/625/R-94/003
September 1994
Manual
Alternative Methods for Fluid
Delivery and Recovery
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EPA/625/R-94/003
September 1994
Manual
Alternative Methods for
Fluid Delivery and Recovery
Center for Environmental Research Information
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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Notice
The information in this manual has been funded wholly or in part by the U.S. Environmental
Protection Agency (EPA). It has been subjected to the Agency's peer and administrative review and
approved for publication as an EPA document. Mention of trade names or commercial products
does not constitute endorsement or recommendation for use.
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Contents
Page
Chapter 1 Introduction 1
1.1 Role of Alternative Methods of Delivery and Recovery 1
1.1.1 Containment 1
1.1.2 Restoration 2
1.1.3 Technical Impracticability 2
1.2 Overview of Contents 2
1.3 Summary of Important Issues 2
1.3.1 Access 4
1.3.2 Depth 4
1.3.3 Recovered Phase 4
1.3.4 Geology 4
1.3.5 Availability 5
1.3.6 Current Experience 5
1.3.7 Operating and Maintenance Costs 5
1.3.8 Monitoring 5
1.4 References 6
Chapter 2 Horizontal and Inclined Wells 7
2.1 Well Construction 8
2.1.1 Directional Drilling Components (or "Equipment") 8
2.1.2 Drilling Fluids 11
2.1.3 Inclined Wellbores Created With Sonic Drilling 12
2.1.4 Well Installation and Completion 14
2.1.5 Well Development 17
2.1.6 Pumps 17
2.2 Design Considerations 18
2.2.1 Pattern of Flow 18
2.2.2 Wellbore Specifications 18
2.2.3 Geologic Site Conditions 21
2.2.4 Distribution of Contaminants 24
2.2.5 Wellbore Hydraulics 24
2.2.6 Site Conditions 25
2.3 Applications 25
2.3.1 Pump and Treat 25
2.3.2 Vapor Extraction 26
2.3.3 Free-Product Recovery 26
2.3.4 Sparging and Vapor Extraction 27
2.3.5 Bioremediation 27
2.3.6 Flushing 27
2.3.7 Soil Monitoring and Sampling 28
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Contents (continued)
Page
2.4 Case Histories 28
2.4.1 Overview 28
2.4.2 Typical Costs of Completed Projects 29
2.4.3 Selected Examples 29
2.5 References 36
Chapter 3 Induced Fractures 39
3.1 Methods of Inducing Fractures 39
3.1.1 General Considerations 40
3.1.2 Monitoring Fracture Location 41
3.1.3 Equipment 42
3.1.4 Well Completion 43
3.2 Design Considerations 43
3.2.1 Flow to a Gently Dipping Fracture 45
3.2.2 Forms of Fractures 46
3.2.3 Geologic Conditions 47
3.2.4 Fracture Permeability 48
3.2.5 Fracture Size 49
3.2.6 Well Completion 50
3.2.7 Site Conditions 50
3.3 Applications 50
3.3.1 Vapor Extraction 50
3.3.2 LNAPL Recovery 51
3.3.3 Dual-Phase Recovery 52
3.3.4 NAPL Recovery 52
3.3.5 DNAPL Recovery 52
3.3.6 Bioremediation Applications 52
3.3.7 Air Injection 53
3.3.8 Steam Injection 53
3.3.9 Electrokinetics 53
3.3.10 In Situ Treatment Zones 54
3.3.11 Barriers 54
3.3.12 Monitoring 54
3.4 Case Histories 54
3.4.1 Overview 54
3.4.2 Selected Examples 56
3.5 References 59
Chapter 4 Interceptor Trenches 63
4.1 Trench Construction 63
4.1.1 Conventional Methods 63
4.1.2 Specialized Methods 64
IV
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Contents (continued)
Page
4.2 Design Considerations 66
4.2.1 Pattern of Flow to a Trench 66
4.2.2 Site Geology 67
4.2.3 Hydrologic Conditions 68
4.2.4 Trench Hydraulics 71
4.2.5 Contaminants 72
4.2.6 Site Conditions 72
4.2.7 Trenches and Horizontal Wells 72
4.3 Applications 73
4.3.1 Hydrodynamic Control 73
4.3.2 Plume Interception 73
4.3.3 NAPL Recovery 74
4.3.4 Vapor Extraction 74
4.3.5 Other Applications 75
4.4 Case Histories 76
4.4.1 Overview 76
4.4.2 Selected Examples 76
4.5 References 85
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Figures
Figure Page
2-1 Horizontal well used to intercept a plume 7
2-2 Some applications of horizontal wells 8
2-3 Directional drilling rig 8
2-4 Directional drilling with a downhole motor 9
2-5 Potential damaged zones around a horizontal wellbore 11
2-6 Resonant stress wave providing energy to the drill bit 12
2-7 Sonic drill head 13
2-8 Creating a horizontal well with the pull-back method 15
2-9 Completing a blind well using the washover pipe method 15
2-10 Types of force normal to axes during installation of casing or well materials 16
2-11 Flow paths to a horizontal well in a thin aquifer with no regional flow 18
2-12 Radial flow in a vertical plane in the vicinity of a horizontal well 18
2-13 Nomenclature describing a horizontal well 19
2-14 Orientations of horizontal well relative to principal directions of permeability 22
2-15 Drawdown resulting from a horizontal well as a function of time and dimensionless length 23
2-16 Discharge from horizontal wells of different lengths operated at constant drawdown 24
2-17 Schematic of distribution of influx into a horizontal well 24
2-18 Two approaches using horizontal wells to intercept contaminant plumes 26
2-19 Schematic of flow patterns with horizontal wells 26
2-20 Recovery of LNAPL with one horizontal well, and water control with another well below 27
2-21 Inclined wellbore created with resonant sonic drilling, Sandia National Laboratories 28
2-22 Distribution of total lengths of horizontal wells drilled by summer 1993 29
3-1 Injection pressure during creation of a fracture by injecting a sand-laden slurry 41
3-2 Injected fluids leaking out of fracture during propagation 41
3-3 Typical pattern of uplift over a shallow, gently dipping hydraulic or pneumatic fracture 42
3-4 Aboveground equipment used for pneumatic fracturing 42
3-5 Fracture created by isolating a zone with a straddle packer 43
3-6 Fractures created at the bottom of driven casing 44
3-7 Methods of completing wells with induced fractures 44
3-8 Air discharge from a well intersecting a flat-lying circular fracture as a function of the permeability
of the fracture 45
3-9 Pressure, flux and travel time to a conventional well and a well intersecting a sand-filled fracture ... 46
3-10 Plan and section of a typical hydraulic fracture created in overconsolidated silty clay 47
3-11 Locations of major areas of soils related to glaciers and vertisols 48
3-12 The effective aperture of an open slot that is equivalent to that of a fracture filled with proppant. ... 49
3-13 Factors that affect how fractures should be propped 49
3-14 Flux of air to three flat-lying fractures shown in cross section 51
3-15 Well configuration to recover liquid and vapor phases using suction at the ground surface 52
vi
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Figures (continued)
Figure Page
3-16 Schematic application of steam injection into induced fractures 53
3-17 Temperature as a function of depth and time 54
3-18 Area of LNAPL recovery using fractured and conventional wells 57
3-19 Methods of completing a well containing hydraulic fractures to recover LNAPL 58
3-20 Discharges from C-10, C-12, and PW-1 as functions of time 58
3-21 Cross-section along line of piezometers showing change in head after five days of pumping 59
4-1 Cross sections of a basic interceptor trench configuration showing permeable backfill and
perforated casing 63
4-2 Schematic of continuous excavation and completion technique 65
4-3 Schematic of deep trench excavation equipment 65
4-4 The pattern of flow to a trench with no regional gradient 66
4-5 Drawdown at the midpoint of a trench where discharge is constant 68
4-6 Discharge from a trench held at constant drawdown 69
4-7 Cross section of a trench partially penetrating an aquifer in a regional flow, showing the
vertical extent of aquifer captured by the trench 69
4-8 Dimensionless arrival times as a function of location for various strengths and directions of
regional flow 70
4-9 Map of the distribution of influx and flow along an idealized trench 71
4-10 Controlling the flow through a contaminated area by pumping from an upgradient trench and
injecting into a downgradient trench 73
4-11 Interceptor trench coupled with a hydraulic barrier to minimize recharge from an adjacent
surface waterbody 73
4-12 Trench used to capture plume and prevent migration to an offsite well 73
4-13 Trench systems for DNAPL recovery 74
4-14 Enhanced recovery of DNAPL by reintroducing water to increase the hydraulic gradient 74
4-15 Excessive pumping rates from the ground-water and DNAPL drains causing "pinch off' of the
DNAPL plume 75
4-16 Plan of the Westminster gasoline recovery site 80
4-17 Plan of the Laramie DNAPL recovery site 80
4-18 Map of the northeastern Illinois vapor extraction site and detail of trench completion 81
4-19 Typical subsurface profile 82
4-20 Site map showing locations of monitoring wells, storage silos, and probable contaminant
source area 82
4-21 Extent of TCE plume in 1984 and 1986 82
4-22 Model domain showing location of the interceptor trench, the 1986 TCE plume limit, and extent
of the capture zone 83
4-23 TCE concentration distribution through time 83
4-24 Site plan showing locations of initial interceptor trench segments 84
4-25 Schematic cross section of trench construction 84
4-26 Monthly extraction from interceptor trench sumps 85
4-27 Observed and predicted discharges from trench system 85
VII
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Tables
Table Page
1-1 Issues Affecting Application of Alternative Methods for Delivery or Recovery 3
2-1 Specifications of Directional Drilling Rigs 9
2-2 Wellbore Curve Measurements 19
2-3 Strengths of Prepacked Screens 21
2-4 Tensile Strengths of Pipe 21
2-5 Summary of Variables 23
2-6 Summary of Applications of Horizontal Wells 28
2-7 Sites Where Horizontal Wells Have Been Used for Ground-Water Recovery 30
2-8 Sites Where Horizontal Wells Have Been Used for Vapor Extraction 31
2-9 Sites Where Horizontal Wells Have Been Used for Assorted Environmental Applications 32
2-10 Sites Where Horizontal Wells Have Been Used for Several Purposes 32
2-11 Specifications of Horizontal Well, Geismar, Louisiana 34
3-1 Design Considerations for Induced Fractures 44
3-2 Summary of the Effects of Induced Fractures Used To Improve the Recovery of Wells 55
3-3 Specific Capacities Before and After Hydraulic Fracturing 56
3-4 Specifications of Fractures Used During the Pilot Test 57
3-5 Average Discharges and Ratios of Discharge 58
4-1 Variables Used in Trench Analysis 68
4-2 Summary of Applications of Trenches for Delivery or Recovery: Site Characteristics 77
4-3 Summary of Applications of Trenches for Delivery or Recovery: Trench Parameters 78
4-4 Summary of Applications of Trenches for Delivery or Recovery: Product Recovery 79
VIM
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A cknowledgments
This manual was prepared under Contract No. 68-CO-0068 by Eastern Research Group, Inc. (ERG),
forthe U.S. Environmental Protection Agency's (EPA's) Office of Research and Development (ORD),
Center for Environmental Research Information (CERI), Cincinnati, Ohio. Edwin Earth of CERI
served as the Project Director and provided technical direction and review. Heidi Schultz of ERG
directed the editing and production of this manual.
Principal authors were Larry Murdoch, Center for GeoEnvironmental Science and Technology,
University of Cincinnati, Engineering Research Division, Cincinnati, Ohio, and David Wilson, Inde-
pendent Environmental Consultants, Arvada, Colorado. Kevin Savage, Bill Slack, and Jim Uber of
the University of Cincinnati assisted in preparing sections of the manual. The following individuals
deserve special acknowledgment for their review: Steven Acree and Randall Ross, EPA ORD,
Robert S. Kerr Environmental Research Laboratory, Ada, Oklahoma; Randy Breeden, EPA Office
of Emergency and Remedial Response, Washington, DC; Subijoy Dutta, EPA Office of Solid Waste,
Washington, DC; Rene Fuentes, EPA Environmental Services Division, Seattle, Washington;
George Losonsky, International Technology Corporation, Lake Charles, Louisiana; and Eugene
Donovan, Leslie Sparrow, and Bruce McClellan, HydroQual, Inc., Mahwah, New Jersey.
IX
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Chapter 1
Introduction
Controlling subsurface fluids is among the highest pri-
orities in managing sites with in situ contamination.
Some applications direct fluid movement continually in-
ward towards the site, whereas others attempt to re-
cover contaminants and, ultimately, close the site. Both
these types of applications use physical methods of
delivery and recovery, perhaps in conjunction with other
methods, to meet their respective goals. Probably the
most common physical system used to recover fluids at
contaminated sites is a vertical well with a submersible
water pump. However, this system is by no means the
most effective in every situation. Therefore, this manual
focuses on alternatives to conventional applications of
vertical wellsalternatives that improve performance or
reduce cost.
Developments in drilling technology over the past dec-
ade have resulted in considerable improvements. For
example, technological advancements now allow wells
to curve to a horizontal orientation, potentially placing
great lengths of screen in a contaminated zone and
offering several additional advantages over their vertical
counterparts. Other developments involve methods of
fracturing rock and soil to improve the performance of
vertical and, in some cases, horizontal wells. This tech-
nique is particularly suited to increasing recovery from
low permeability formations, but it also has some spe-
cialized applications. Trenches filled with gravel have
long been used for dewatering, and they can still be a
valuable option at a contaminated site.
This manual presents these three alternative methods
of enhancing delivery and recovery. The scope of this
document is confined to physical enhancement tech-
niques. For this reason, detailed discussion of chemical
methods (e.g., surfactant flushing) and thermal methods
(e.g., steam stripping) has been omitted. These chemi-
cal methods are mentioned, however, because the al-
ternative methods can be used in conjunction with a
variety of other processes, including flushing and steam
stripping. In fact, nearly any in situ method of remedia-
tion that involves subsurface fluid flow either has been
or could be used with the methods described herein.
Additional information on related technologies is avail-
able in the following U.S. Environmental Protection
Agency (EPA) documents: Handbook on In Situ Treat-
ment of Hazardous Waste Contaminated Soils, Subsur-
face Contamination Reference Guide, Leachate Plume
Management, and Evaluation of Ground-Water Extrac-
tion Remedies, Volumes 1-3 (1-4). Additional informa-
tion related to the existence, movement, and transport
of ground water is described in Volumes 1 and 2 of the
EPA document Ground Water (5, 6).
1.1 Role of Alternative Methods of
Delivery and Recovery
Alternative methods of delivery and recovery are in-
tended to improve the performance of remedies ranging
from hydrodynamic containment, which arrests offsite
migration, to restoration, which reduces contaminant
concentrations. The methods are applicable during in-
terim measures, in particular by improving the effective-
ness of containment, and they also can augment the
performance of a variety of remedial actions selected as
possible long-term remedies.
1.1.1 Containment
Recent studies (4) have evaluated the performance of
ground-water extraction systems. These studies have
identified factors related to hydrogeology, contaminant
properties, and system design that may impede the
ability of those systems to reduce concentrations to
targeted values over the entire area of contamination.
Heterogeneities, such as natural fractures, karstic fea-
tures, or variations in stratigraphy, result in preferential
flow paths between wells, thus limiting recovery capa-
bilities. Moreover, nonaqueous-phase liquid (NAPL) is
present in the subsurface at many contaminated sites
(7, 8) and, with components of the NAPL slowly dissolv-
ing in ground water, may act as a persistent source of
contamination.
Those issues have led to a higher appreciation of the
difficulty of remediation at some sites, and a recognition
of the current technical impracticability of complete re-
mediation at other sites (9). Nevertheless, the ability to
halt migration and prevent the spread of contamination
is well within the scope of current technology. Thus,
emphasis grows on expeditiously implementing sys-
tems to halt dissolved phase migration by controlling
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hydraulic gradients early in the investigation of a site.
Aggressively pursuing free-phase NAPL recovery also
is recommended at the earliest possible time to reduce
the potential for further contamination. Interim measures
must be coordinated with final remedies so that they
constitute the first phase of the overall remedial strategy.
Accordingly, containment and source recovery during
interim measures are a viable method of risk reduction
at many contaminated sites.
The effectiveness of interim measures and the extent to
which they reduce risks of exposure are based on their
performance. In many cases, systems of alternative
methods improve the ability to control hydraulic gradi-
ents beyond the capabilities of vertical wells. Central to
improved performance is the geometry of flow fields that
the systems produce. Whereas vertical wells are limited
to producing radial flow (in the absence of regional flow),
horizontal wells and trenches produce linear flows in a
horizontal plane, and horizontal fractures produce verti-
cal flows. This expands the versatility of containment
systems, allowing them to be tailored more closely to
the configuration of the site. An additional benefit of the
alternative methods is the improvement in the amount
of fluid recovered per unit of power. Increased values of
specific capacities occur when using enhanced systems
in productive aquifers. Moreover, in tight formations
where recovery typically occurs at fixed drawdown, the
enhanced systems are capable of increasing the dis-
charges compared with conventional vertical wells.
Where impenetrable surface structures cover NAPLs or
aqueous plumes, horizontal wells offer the capability to
recover contaminants close to their source.
1.1.2 Restoration
Fluid recovery by pumping vapor, aqueous, or
nonaqueous phases may be sufficient to restore some
sites, but heterogeneities, adsorption, low vapor pres-
sure, and other factors conspire to limit the effectiveness
of fluid recovery alone as a remedial tool. A wide range
of other methods has been and are currently being
developed to accelerate in situ remediation. Some
methods involve injecting liquids or vapors that carry
nutrients and electron acceptors to stimulate in situ
organisms, thereby promoting contaminant degrada-
tion. Other methods involve degrading contaminants by
injecting an oxidant or mobilizing the contaminant with
a surfactant to enhance recovery. Another approach is
to heat the subsurface by injecting steam or hot air to
vaporize organic compounds.
This diverse range of technologies shares the common
need to control the flow of subsurface fluids. Where the
magnitude or uniformity of fluid flow is impaired, the
performance of a conventional remedial method can be
significantly reduced. Formations of low permeability or
those marked by heterogeneities or other hydrologic
complexities present particular difficulties. Stagnation
zones in the flow field resulting from well placement and
design further limit fluid flow. In view of these issues, the
alternative methods described here can be a valuable
asset when used in conjunction with other remedial
methods involving fluid flow. In this application, the pri-
mary purpose of the delivery and recovery techniques is
to improve the effectiveness of remedial methods involv-
ing fluid flow. Improved effectiveness reduces costs or
may allow a technique to be successfully applied under
conditions where it would otherwise be unsuccessful.
1.1.3 Technical Impracticability
Currently, it is technically impracticable to restore some
sites to accepted standards due to the presence of
immobile NAPL, strongly sorbed contaminants, forma-
tion heterogeneities, or other factors (9). As a result, the
methods described herein may be no better than vertical
wells in addressing limitations to restoration at some
sites. Alternative methods of delivery or recovery should
be implemented only when they provide a substantive
benefit to technically achievable goals.
1.2 Overview of Contents
The handbook contains chapters describing:
Horizontal wells
Induced fractures
Interceptor trenches
Each chapter contains a description of construction
methods, with particular emphasis on factors to consider
when selecting the method. The factors affecting design
are summarized, with particular emphasis on the pattern
of flow induced by the method, the geologic and hydro-
logic site conditions that affect performance, and access
requirements for site implementation. In addition, an
overview of applications suggests various possibilities,
although by no means have all the potential applications
been described. Each chapter closes with several case
histories describing site conditions, system design, and
results. Cost information is provided based on published
or informally reported descriptions. The cost of a particu-
lar technique can vary greatly, however, and the reader
should use the data given here as a preliminary guide
to be supplemented by the cited references and site-
specific vendor estimates.
1.3 Summary of Important Issues
Horizontal wells, induced fractures, and interceptor
trenches present slightly different options for controlling
subsurface fluids, and each has slightly different advan-
tages and disadvantages among the issues facing ap-
plications at contaminated sites. Table 1-1 summarizes
essential issues.
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Table 1-1. Issues Affecting Application of Alternative Methods for Delivery or Recovery
Issue Horizontal Well Induced Fracture
Trench
Access
Fragile structures
over target
Poor access over
target
Depth
<6 m
6-20 m
>20 m
Recovered Phase
Aqueous
LNAPL
DNAPL
Vapor
Geology
Normally consolidated
clay
Swelling clay
Silty clay till
Stratified sediment or
rock
Vertically fractured
sediment or rock
Coarse gravel
Thick sand
Rock
Availability
Current Experience
(Approximate)
Minimal surface disturbance D Evaluate effects of surface
displacement
Standoff required
1 m minimum depth
Cost of guidance system
increases at >6 m
No depth limit within
environmental applications
O Requires accurate drilling;
best if water table
fluctuations are minor
© Requires accurate drilling
and site characterization
Consider omitting gravel
pack to save costs
© Smearing of bore wall may
reduce performance
© Smearing of bore wall may
reduce performance
© Smearing of bore wall may
reduce performance
O Anisotropy may limit vertical
influence of well
Orient well normal to
fractures when possible
O Possible problems with hole
stability; penetrating cobbles
© May be difficult to access
top and bottom of
formation; hole stability
problems
O Feasible, but drilling costs
more in rock than in
sediment
10 to 20 companies with
capabilities; nationwide
coverage but may require
equipment mobilization
150 to 250 wells at 50 to
100 sites
O Possible with horizontal well
1-2 m minimum depth
No depth limit within
environmental applications
© Best with access to individual
fractures
O Caution; steeply dipping
fractures may cause downward
movement
Best with access to individual
fractures
O Induced fractures may be
vertical and limited in size
Relatively large, gently dipping
fractures expected
Relatively large, gently dipping
fractures expected
© Stratification may limit upward
propagation and increase
fracture size
© Good where induced fractures
cross-cut natural fractures
(overconsolidated sediment and
rock)
D Permeability enhancement may
be unnecessary
D Permeability enhancement may
be unnecessary
Widely used in oil, gas, and
water wells drilled in rock
Several companies offer service;
nationwide coverage with equipment
mobilization
200 to 400 fractures at
20 to 40 sites
D Excavation expected to be
infeasible
D Excavation expected to be
infeasible
Installation with common
equipment
© Excavation costs increase with
depth
O Specialized excavation
methods required
Widely used to ensure capture;
accommodates water table
fluctuations
Assuming mobile phase
present and accurately located
© Requires tight seal on top of
trench
Large discharge expected
relative to alternatives
Large discharge expected
relative to alternatives
Large discharge expected
relative to alternatives
Good way to access many thin
beds or horizontal partings
© Orient trench perpendicular to
natural fractures when possible
Stability a concern during
excavation
Stability a concern during
excavation
D Excavation difficult but blasting
possible to make trench-like
feature
Shallow trench (<6 m) installation
widely available from local
contractors; deep trench will require
mobilization
1,000+ trenches at many hundreds
of sites
Key
Good application
© Moderately good
O Fair, with possible technical difficulties
D Poor; not recommended using available methods
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1.3.1 Access
Access to the ground overlying the contaminated region
is commonly restricted, or access may be obstructed by
fragile structures such as product lines, tanks, and build-
ings. Horizontal wells are ideally suited to address ac-
cess problems because the drill rig can be located at
some distance from the obstructed area. Alternatively,
induced fractures created from vertical wells are limited
to areas where access allows creation of the well. Frac-
tures have been induced in the vicinity of horizontal
wells, although experience with this application for envi-
ronmental purposes is currently limited. Because in-
duced fractures displace the ground surface, this
technique may be inappropriate where such displace-
ment could damage overlying structures. Finally, the
excavation required to create trenches precludes their
application in many areas of limited access.
1.3.2 Depth
Depth is a minor factor for horizontal wells and induced
fractures; both can be created at depths far greater than
those that environmental applications require. Minimal
depths of 1 to 2 meters (3.3 to 6.6 feet) are commonly
required to contain induced fractures within the subsur-
face. Drilling horizontal wells requires similar minimum
depths if high pressure jets are used to cut the bore. The
current maximal limit of radio-beacon guidance sys-
tems, which are the most economical method of guiding
horizontal wells, is 6 to 8 meters (19.7 to 26.2 feet).
Therefore, creating a horizontal bore deeper than 6 to 8
meters (19.7 to 26.2 feet) requires a more sophisticated
and expensive guidance system. Trench depth greatly
influences cost, because deeper trenches increase both
the volume of contaminated soil that must be handled
(and possibly disposed of) and the cost of excavation.
The maximal limit of conventional excavation equipment
is approximately 4 to 6 meters (13.1 to 19.7 feet), so
creating deeper trenches requires specialized equip-
ment. Currently, trenchlike structures 100 meters (328
feet) or more in depth can be created using specialized
excavators (Table 1-1).
1.3.3 Recovered Phase
All the alternative methods can recover aqueous phase
contaminants, but they may differ in their ability to re-
cover NAPL. It is feasible to recover free-phase light
(LNAPL) and dense (DNAPL) nonaqueous-phase liquid
using a horizontal well, but this application requires
particularly accurate depth control during drilling, as well
as a detailed understanding of each site's subsurface
variability. Moreover, even with accurate placement of
the well, rises and falls in the water tableand thus the
LNAPL layercan significantly hamper recovery of
LNAPL. Induced fractures can improve the recovery of
LNAPL from tight formations. In addition, creating sev-
eral fractures over a range of depth can account for
water table fluctuations. Trenches are widely used to
recover shallow LNAPL because they can intercept lat-
eral migration and accommodate changes in the water
table.
Each of the three methods can be used to increase
recovery of DNAPL. However, the benefits of recovering
some DNAPL must be balanced against the risk of
aggravating the recovery of liquid that remains in the
ground. Addressing this issue entails locating the
DNAPL, which is a formidable task unto itself, and using
methods that limit the creation of vertical channels in the
area containing DNAPL. Horizontal wells offer the pos-
sibility of accessing a DNAPL layer without creating a
vertical conduit. Conversely, the location of induced frac-
tures cannot be determined precisely, so it is possible
that they will create pathways for downward migration.
Moreover, methods of sealing induced fractures remain
untested. Therefore, the feasibility of abandoning the
fractures is currently unknown. Trenches also create
vertical channels, but the trench location can be deter-
mined precisely, and methods of sealing and abandon-
ing trenches are available. Feasibility tests, modeling,
and pilot studies are recommended before conducting
any DNAPL recovery project.
1.3.4 Geology
It is possible to use horizontal wells in fine-grained
sediments, although smearing along the wall of the bore
may require aggressive development. Anisotropy pro-
duced by stratigraphic layering limits the vertical influ-
ence of a horizontal well. In contrast, anisotropy
produced by vertical fractures tends to enhance the
performance of a well oriented perpendicular to the
fractures. Although drilling through coarse gravel and
rock is technically possible, these formations are rela-
tively difficult to penetrate and thus increase the cost of
the well. In thick sand, a horizontal well may present
limited access to the upper or lower regions. Moreover,
steering a directional bore is difficult in soft sand, so
drilling accuracy may be compromised.
Induced fractures can increase the recovery of subsur-
face fluids from fine-grained sediments and particularly
from overconsolidated deposits, such as glacial drift or
swelling clay. However, induced fractures may tend to
dip steeply in normally consolidated deposits, which can
limit their size and effectiveness. Stratigraphic layering
can inhibit upward propagation of an induced fracture
and improve performance, even in normally consoli-
dated formations. However, a horizontal fracture may
offer little benefit in finely interbedded sands and clay.
Where vertical fractures are the dominant flowpaths,
induced fractures can be a benefit if they are flat-lying
and cut across multiple natural fractures. In contrast,
where induced fractures follow one vertical fracture,
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they may be of limited benefit. Both types of behavior
have been noted. Highly permeable formations, such as
sands and gravels, will benefit little from the permeability
enhancement that induced fractures offer, although
there may applications where treatment materials are
injected into induced fractures in sand. Induced frac-
tures are widely used to increase the discharge from
wells in rock formations.
Trenches are suited to applications in a wide range of
geologic formations. In fine-grained sediments, excava-
tion is relatively easy, and trenches offer significantly
greater discharge than vertical wells. In stratified sedi-
ments, they can cut across and access multiple thin
permeable beds. Of course, excavating rock is generally
infeasible, although trenchlike features can be created
using explosives placed in a line of vertical boreholes.
1.3.5 Availability
Horizontal wells, induced fractures, and trenches are all
methods available within the United States and Canada.
As of the summer of 1993, approximately a dozen com-
panies had installed a horizontal well, with more enter-
ing the market since that time. Most companies that
create horizontal wells for environmental applications
are specialists in that market, although some are capa-
ble of vertical drilling or directional drilling for utility
installation. Depending on the location of the site, drilling
equipment may need to travel several hundred miles or
more.
At least two companies specialize in designing and
creating induced fractures for environmental applica-
tions, and several others offer fracturing capabilities
along with other environmental services; these compa-
nies are capable of mobilizing within the United States
and Canada. Hydraulic fracturing of water wells is avail-
able from well drilling companies in many locations,
although their experience with environmental applica-
tions typically is limited.
Widely available excavation equipment can create
relatively shallow trenches and should require minimal
mobilization. Deeper trenches require the more sophis-
ticated equipment and engineering capabilities that spe-
cialist construction companies can offer.
1.3.6 Current Experience
Approximately 150 to 250 horizontal wells have been
installed during the past 7 years for environmental ap-
plications at 50 to 100 sites. The drilling capabilities
used to install horizontal wells draw upon the experience
from directional boring of hundreds of kilometers to in-
stall utility conduits and oil wells. Specialized drilling and
completion methods designed to meet the challenges of
environmental applications continue to be developed.
From 200 to 400 fractures have been created at 20 to
40 sites. Fracturing capabilities draw upon the experi-
ence of thousands of applications in the petroleum and
water well industries and on the methods and effects of
injection grouting. Because creating induced fractures
requires particular sensitivity to site conditions, the
methods of creating fractures and anticipating their per-
formance for environmental applications continues to
develop. This is the newest of the three alternative
techniques discussed in this manual.
Trenches are widely used to recover contaminated fluids
from sites. Experience includes more than 1,000 appli-
cations at hundreds of sites, making this the most ma-
ture of the techniques discussed here.
1.3.7 Operating and Maintenance Costs
Methods other than a field of vertical wells may reduce
operating and maintenance (O&M) costs in many ways,
such as by decreasing the number of pumps that must
be used, reducing the time for routine sampling and
measurement of water levels, and decreasing power
consumption. Some maintenance aspects, such as re-
conditioning, may be more expensive for a horizontal
well or trench than for a vertical well, although the cost
on a per-well basis may be offset if the horizontal well
or trench is equivalent to several vertical wells. One
comparative study (10) found that the capital cost of one
horizontal well was similar to five vertical wells, but the
O&M cost for the horizontal well was less than one third
that of the five vertical wells. In general, however, costs
are sensitive to site conditions and design quality, and
the relative economic advantages of the alternative
methods are difficult to generalize.
1.3.8 Monitoring
Induced fractures and horizontal wells in some circum-
stances can provide better monitoring capabilities than
vertical wells. Small induced fractures can be used to
increase the area sampled and the fluid recovered by a
monitoring well. This application is particularly attractive
in fractured clay or rock, where it can be difficult to obtain
a water sample of adequate volume from the formation.
Horizontal wells present the possibility of monitoring
beneath potential sources, such as tanks or lagoons,
that cannot be penetrated by a vertical well.
In situ monitoring of the distribution of head or pressure
when using alternative methods of delivery or recovery
can be accomplished using vertical piezometers in
much the same manner as monitoring recovery by ver-
tical wells. One exception is during the resolution of
vertical head gradients; these gradients can be impor-
tant in the vicinity of an induced fracture, horizontal well,
or trench that partially penetrates an aquifer but are
often ignored when vertical wells are used. Head distri-
butions caused by vertical wells are commonly moni-
-------
tored using piezometers with long screens, whereas
vertical gradients can only be measured with clusters of
multiple piezometers with short screens at different
depths or piezometers with multiple ports separated by
packers (11).
Monitoring the rates and concentrations of recovered
fluids or the concentrations of compounds in the subsur-
face is independent of the method of recovery; similar
sampling and analytical techniques are used for alterna-
tive and conventional recovery methods.
1.4 References
1. U.S. EPA. 1990. Handbook of in situ treatment of
hazardous waste-contaminated soils. EPA/540/2-
90/002.
2. U.S. EPA. 1990. Subsurface contamination refer-
ence guide. EPA/540/2-90/011.
3. U.S. EPA. 1985. Leachate plume management.
EPA/540/2-85/004.
4. U.S. EPA. 1989. Evaluation of ground-water extrac-
tion remedies, Vols. 1-3. EPA/540/2-89/054b,c.
5. U.S. EPA. 1990. Handbook: Ground water, Vol. 1.
Ground water and contamination. EPA/625/6-
90/016a (September).
6. U.S. EPA. 1991. Handbook: Ground water, Vol. 2.
Methodology. EPA/625/6-90/016b (July).
7. U.S. EPA. 1993. Evaluation of the likelihood of
DNAPL presence at NPL sites. EPA/540/R-93/073.
8. Cohen, R.M., J.W. Mercer, and J. Matthews. 1993.
DNAPL site evaluation. U.S. Environmental Protec-
tion Agency. Ada, OK.
9. U.S. EPA. 1993. Guidance for evaluating the tech-
nical impracticability of ground-water restoration.
EPA/540/R-93/080.
10. Losonsky, G., and M.S. Beljin. 1992. Horizontal
wells in subsurface remediation. Hazardous Mate-
rials Control Conference, New Orleans, LA (Febru-
ary).
11. Rehtlane, E.A., and F.D. Patton. 1982. Multiple port
piezometers vs. standpipe piezometers: An eco-
nomic comparison. Second Aquifer Restoration and
Ground Water Conference, Columbus, OH (May
26-28), pp. 287-295
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Chapter 2
Horizontal and Inclined Wells
Recent advances in directional drilling have forever
changed the well's image. No longer must a well be a
vertical cylinder; directional drilling methods can create
wellbores with almost any trajectory. Wells that curve to
a horizontal orientation are particularly suited for envi-
ronmental applications.
Horizontal wells are not technically an innovation; the
water supply industry has used horizontal wells for many
years to collect water from beneath rivers and other
bodies of water (1). However, these wells are drilled
radially from large caissons, making installation expen-
sive and rarely practical for environmental applications.
Directional drilling methods use specialized bits to curve
bores in a controlled arc; trajectory is monitored with
electronic sensors. This enables bores to be initiated at
a relatively shallow angle from the ground surface and
gradually to curve to horizontal (Figure 2-1). Blind well-
bores terminate in the subsurface. In some cases, how-
ever, the well is turned upward and returns to the ground
surface, which makes it accessible from both ends. This
is called a continuous wellbore. Most horizontal well-
bores are drilled in roughly a straight line, but lateral
curves are certainly possible and may be important in
certain circumstances.
Figure 2-1. Horizontal well used to intercept a plume. The solid
line represents a blind well; the solid line plus the
dashed line represents a continuous well.
Directional drilling can create two basic types of bores:
Wellbores preserve the permeability of the host for-
mation.
Boreholes penetrate the subsurface without particular
regard to the permeability of the host.
Creating wellbores requires special methods to avoid or
restore permeability damage, whereas methods used to
create boreholes are less demanding.
Inclined, or slant, wells are installed in a straight well-
bore typically created with conventional drilling equip-
ment that is tilted. Inclined wells drilled with conventional
drilling equipment cannot be horizontal if they originate
from flat-lying ground.
In some cases, such as when drilling into a slope, an
inclined well can be horizontal or even slant upward.
Directional drilling equipment can create wells that du-
plicate the geometry of inclined wells.
The primary reason for including inclined wells in this
chapter is that they have recently received renewed
interest as a product of sonic drilling. This technique has
the remarkable ability to create wellbores without drilling
fluid and extraneous drill cuttings; all of the material in
the path of the wellbore is collected as continuous core
sample. These attributes make sonic drilling ideal for
certain sampling and environmental drilling applications.
Thus, the treatment of inclined wells in this chapter is
limited to applications involving sonic drilling.
Because of its orientation, a horizontal well is particu-
larly suited to recovering contaminants distributed as
broad, flat layers (Figure 2-2a). Such a distribution may
occur when LNAPL floats on a water table or when
DNAPL accumulates on a low-permeability bed. Direc-
tional drilling can place 100 meters (328 feet) or more
of well screen in the layer, whereas the screen of a
vertical well may intersect less than a meter of an
LNAPL layer. Often, when contaminants have moved
with the regional flow, the compounds distribute as a
long, narrow, roughly horizontal plume. Asingle horizon-
tal well placed along the long axis of the plume (Figure
2-2b) offers an ideal geometry for recovering those con-
taminants.
In tight formations such as bedrock or till, where vertical
fractures provide the primary flow paths, a horizontal
well can intersect many vertical fractures (Figure 2-2c).
This application facilitates access to the preferred flow
paths, increases well discharge, and controls fluid flow
in the formation (2, 3).
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2.1 Well Construction
a.
b.
c.
d.
Figure 2-2. Some applications of horizontal wells: a) intersect-
ing flat-lying layers, b) intercepting plume elon-
gated by regional gradient, c) intersecting vertical
fractures, and d) access beneath structures.
Some advantages of a horizontal well are unrelated to
hydrologic performance. In many locations, such as be-
neath landfills, tanks, buildings, roads, lagoons, or bod-
ies of water, access limitations prohibit entry by a drill
rig, prevent penetration by a vertical hole, or restrict the
aboveground facilities necessary for recovery opera-
tions. As a result, recovery, sampling, or monitoring with
conventional drilling technology is difficult beneath many
structures that may be sources of contaminants. Hori-
zontal and inclined wells, however, overcome those dif-
ficulties by allowing the rig to be adjacent to the
obstructing structure, and the wellbore to be created
beneath it (Figure 2-2d).
This chapter contains four sections. The first section
describes current methods of constructing wellbores
and completing horizontal wells. The second section
summarizes technical and economic factors that need
consideration when planning to use horizontal wells at
a contaminated site. Following this is a section that
presents some possible environmental applications.
The chapter closes with an extensive summary of envi-
ronmental projects that have used horizontal wells and
a detailed description of four case histories.
Constructing a horizontal well entails directionally drill-
ing a wellbore and placing perforated tubing in the well-
bore to hold it open and provide access to subsurface
fluids. The well is developed to increase permeability in
the vicinity of the bore, and a pump is installed to recover
fluids. Details of this process follow.
2. 1. 1 Directional Drilling Components
(or "Equipment")
Directional drilling uses three specialized components:
A drilling rig to power the system
A bit to create a curved hole
A guidance system to locate and steer the bore
2.1.1.1 Directional Drilling
Directional drill rigs that currently create horizontal wells
for environmental applications typically consist of a car-
riage that slides on a frame and holds the drill rods at
an angle of 0 to 45 degrees (Figure 2-3). In most cases,
hydraulic power energizes a motor on the carriage and
rotates the drill rods. A chain drive, rack and pinion drive,
or hydraulic cylinder may push or pull the carriage to
advance or retract the drill string. A pump on the rig,
capable of handling slurries at 1 to 30 megapascals
(MPa) (145 to 4,351 pounds per square inch [psi]), is
typically used to inject drilling fluid.
Chain
Drive
Drill Rod
Bit
Figure 2-3. Directional drilling rig.
Drill rigs are available in a range of sizes and are distin-
guished chiefly by the torque and push/pull force they
provide (Table 2-1). Despite the range in sizes and
various details that manufacturers offer, the rigs share
some common features. The drill rig provides thrust to
the drilling tool and pull-back to the drill string. When
drilling a vertical well, the weight of the drill motor and
the drill string provide the downward force on the drill bit.
In contrast, when drilling a directional wellbore, the drill
-------
Table 2-1. Specifications of Directional Drilling Rigs (May
1994)
Mini Rigs
Midi Rigs
Maxi Rigs
Thrust/Pullback
Maximum
Torque
Drilling Speed
Carriage Speed
Carriage Drive
Drill Pipe
Length
Drilling
Distance3
Power Source
< 66.7 kN
(15,000 Ibs)
2.7 kN-m
(2,000 ft-lb)
>130 RPM
>30 m/min
Cable or
chain
1 .5-3 m
<200 m
<150 HP
66.7-444 kN
(15,000-
100,000 Ibs)
2.7 kN-m - 27
kN-m (2,000-
20,000 ft-lb)
130-100 RPM
28-30 m/min
Chain or rack
and pinion
3-9 m
200-600 m
150-250 HP
>444 kN
(>1 00,000 Ibs)
27 kN-m
(20,000 ft-lb)
<100 RPM
<28 m/min
Rack and
pinion
9-1 2m
>600 m
>250 HP
a Assumes nominal 12-inch wellbore, which is the effective maximum
diameter wellbore for a mini-drill rig.
rigtypically a cable, chain, or rack and pinion sys-
temmust provide the forward force on the drill string.
The rig must be anchored to provide a reaction against
which the mechanical system on the rig can operate.
Anchoring is typically accomplished by driving stakes
through openings at the front of the rig (Figure 2-3) and
attaching the drill rig to a buried weight ("dead man") or
simply attaching it to a large, heavy piece of equipment
on the surface. The drill rig must provide sufficient thrust
to advance the drill string the full length of the proposed
wellbore, and sufficient pulling force to retract casing
into the completed wellbore. The relationship between
thrust and drilling distance depends on the formation
type and use of drilling fluids.
The drill rig must also provide torque to the drill string.
Most drilling methods require that the drill string rotates
while it advances into the wellbore in order to reduce
friction on the drill string. The drill rig must have sufficient
capacity to overcome wellbore friction and supply the
necessary torque to the drill string throughout the pro-
posed length of the wellbore.
In addition, some rigs use pneumatic or hydraulic ham-
mers to help advance the drill string, a method rarely
used for environmental applications. In most cases, the
hammers are mounted on the drill rig, but several manu-
facturers also offer downhole hammers.
Some contractors have directional drill rigs of varying
sizes, whereas others may only have one size of drill rig.
In general, the midi-size rigs (Table 2-1) are the most
versatile and can complete most projects.
The oil industry has used other methods of directional
drilling, with perhaps the most notable technique using
a high energy water jet to create the bore (4). This
method employs a whipstock to bend continuous steel
tubing in a tight arc (less than a 0.5-meter [1.6-foot]
radius of curvature) at the bottom of a vertical hole. The
result is a roughly straight horizontal hole extending
radially from a vertical access bore. This geometry pre-
sents some advantages over the inclined entry rigs for
applications where access is extremely limited. Although
some have proposed environmental applications of this
method (5), apparently no demonstrations have been
conducted.
2.1.1.2 Creating and Steering the Wellbore
Directional drilling uses a downhole assembly that cre-
ates the wellbore and induces a curve in the trajectory.
Wellbores are created by cutting the formation with a
rotating bit, water jet, hammer, or combination of these.
The trajectory is curved using a tool that is eccentric with
respect to the axis of the drill rod. In some cases, there
is a slight bend in the rod behind the bit, whereas in other
cases the bit itself has a beveled surface. Most com-
monly, when creating directional wellbores, bits are
driven by downhole motors, jetting tools, and compac-
tion tools.
Downhole Motors. A downhole motor uses pressurized
drilling fluid to rotate a cutting bit. The advantage of
downhole mud motors is that they eliminate drill string
rotation and make it possible to drill a wellbore with a
short radius of curvature. Downhole motors are gener-
ally preferred when drilling rock or resistant sediments,
where wellbore control is critical.
To curve the wellbore, a bent rod or "sub" is fixed behind
the motor. Rotating the entire drill string slightly offsets
the bent sub, causing the path of the wellbore to be
approximately straight. Pushing the drill string without
rotation, however, curves the wellbore by an amount
dictated by the angle of the bent sub (Figure 2-4).
Motor Bit
Path With Push Only
Bent Sub
Figure 2-4. Directional drilling with a downhole motor.
Jetting Tools. Jetting tools use hydraulic pressure either
to cut the geologic formation or to assist rotary drilling
with bits. Water, mud, polymer, or other drilling fluid can
be used to form the jet. The hydraulic jet is directed from
either a bent housing or from a drilling fluid port on a drill
bit that is attached to a bent subassembly. A pump on the
drill rig controls the pressure of the hydraulic jet. To drill
a curved section, the drill string follows the advancing
-------
bent subassembly along a curved path. To drill a straight
segment of the wellbore, the drill string is rotated (in one
direction or by alternating directions/every 180-degree
turn); rotation prevents the hydraulic jet from having a
preferred orientation, and the drill string will not deviate
from the wellbore path.
Compaction Drilling Tools. A compaction drilling tool
creates a wellbore by compacting sediments as it ad-
vances into the formation. The typical compaction bit
resembles a wood chisel, with the beveled part of the
chisel providing the eccentricity to curve the wellbore.
Just as with the bent subassembly, rotation of a beveled
bit results in a straight wellbore, whereas pushing the bit
causes it to turn. Accordingly, compaction drilling re-
quires significant torque and thrust by the rig at the
ground surface. Compaction drilling is usually assisted
by cutting the formation with a water jet or by advancing
the drilling tool with a hammer. Water is used to help
lubricate and cool the drilling tool, but neither drill cut-
tings nor drilling fluid return to the surface.
Steering during compaction drilling requires the resis-
tance of the formation to be sufficient to bend the drill
rods. In loosely consolidated sand, it may be difficult to
steer using conventional beveled compaction bits, al-
though some bits with enlarged cutting surfaces are
available to make steering in soft formations easier.
2.1.1.3 Location and Guidance Systems
Typically, an electronics package placed behind the cut-
ting head:
Locates the end of the drill rod.
Provides the azimuth and inclination of the bottom-
hole assembly.
Provides the orientation of the drill face (bent sub-
assembly).
This information, combined with the measured length of
drill pipe, can be used to calculate the position of the
bottomhole assembly. The most common guidance sys-
tems are magnetometer-accelerometer systems, gyro-
scopes systems, and electronic beacons.
Magnetometer-Accelerometer System. This system
uses three magnetometers to measure the position (azi-
muth) of the tool in the earth's magnetic field and three
accelerometers to measure the position (inclination) of
the tool in the earth's gravitational field. The system
sends information continuously to a surface computer
that calculates and displays the tool azimuth, inclination,
and drill-face orientation. An advantage of this system is
that the location of the bottomhole assembly is available
in real time.
If subsurface magnetic interference, for example from
metal pipes or tanks, is suspected of influencing the
azimuth readings, an electromagnetic secondary survey
system may be used in conjunction with a magnetic
guidance tool. The secondary system consists of a cable
laid out on the ground in a rectangular configuration,
with the well path running along the center of the long
dimension of the rectangle. A direct current is applied to
the cable, inducing an electromagnetic field of known
intensity and size in the subsurface around the tool. The
induced magnetic field generally is strong enough to
overcome any indigenous magnetic interference, allow-
ing the tool to display its location in the induced mag-
netic field. The cable is unobtrusive and can be used at
most sites without fear that it will inhibit surface activi-
ties. The combination of a magnetometer-accelerometer
tool and the secondary electromagnetic system pro-
vides the most accurate (plus or minus 2 percent of the
vertical depth [VD]) guidance system for shallow direc-
tional drilling. The system loses accuracy at depths
exceeding 35 meters (114.8 feet).
Gyroscope System. The gyroscope system is based on
the same navigational principles used in guided missiles
and airplanes. The system uses three gyroscopes to
measure azimuth and three accelerometers to measure
the inclination. Before the survey is made, the gyro-
scopes are aligned to true north at the ground surface.
The gyroscopes detect any deviation from true north
during the survey and relay the information to the sur-
face, where a computer calculates the azimuth, inclina-
tion, and drilling tool orientation. This system is
unaffected by magnetic interference and therefore may
be used in areas that do not allow the use of a magnetic
guidance system.
Electronic Beacon. The electronic beacon is a battery-
operated sonde that sends a radio signal from the bot-
tomhole assembly. A hand-held surface unit then locates
the position of the beacon, calculates the depth to
the beacon, and displays the drill-face orientation.
This method is often called the "walkover" method be-
cause a technician carries the surface unit over the
beacon. The system cannot be used in areas where
surface obstructions prohibit access above the bottom
assembly.
Electronic beacons were developed to use with direc-
tional drilling for utility installation. They are widely avail-
able and relatively easy to use, with satisfactory results.
Widely available equipment can be used no deeperthan
approximately 8 meters (26.2 feet); however, recent
advances in the technology provide walkover systems
that can be used to approximately 17 meters (55.8 feet)
in depth. Electronic beacons currently provide the least
expensive method of locating and guiding directional
wellbores. Vertical accuracy of current systems is at
least within 5 percent of the VD.
10
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2.1.2 Drilling Fluids
Drilling fluids have a variety of applications during direc-
tional drilling:
Clean cuttings from the bit and the end of the well-
bore, and transport the cuttings to the surface.
Provide wellbore stability.
Control subsurface pressures.
Cool the drill bit and lubricate the drill string.
Drive a downhole drill motor, if one is used.
Ensure that formation information is obtained from
cuttings, cores, or geophysical tools.
Act as conduit for pressure pulses that communicate
information from the guidance tool to the ground sur-
face, if such a communication system is used.
Drilling fluids must provide these applications while mini-
mizing both fluid loss to the formation and reduction in
permeability around the wellbore. No single fluid readily
meets these demands, requiring a tradeoff between
drilling needs and treatment objectives.
Drilling needs may constitute a higher priority than well
efficiency for many horizontal wells used for environ-
mental applications. However, the drilling fluid should be
compatible with well materials and formation soil and
water. Drilling fluid that penetrates the formation can be
difficult to remove and, thus, can have lasting negative
consequences (6). For instance, the fluid may change
the pH, redox potential, or ionic strength of the formation
waters. These changes can cause minerals in the host
formation to dissolve or precipitate. Clay-sized particles
in some drilling fluids also increase the surface area
available for precipitation of minerals. Depending on
formation and drilling fluid chemistry, the minerals most
likely to dissolve or precipitate are magnesium, ara-
gonite, oxides, hydroxides of iron, and possibly dolomite
(7). Drilling fluids that cause precipitation of minerals
reduce pore space and permeability of the host forma-
tion (Figure 2-5). The worst place for reduced perme-
ability is the vicinity of the wellbore because all the fluid
that the well recovers must flow through this zone. Ac-
cordingly, it is critical to select drilling fluids and additives
that minimize damage to the formation adjacent to the
wellbore.
In addition to damaging the formation, it is possible for
drilling fluids to contain contaminants that increase
disposal costs and possibly contribute to migration or
sorption of contaminants during drilling. Minimizing
or eliminating liquid drilling fluids is the ideal. As men-
tioned above, compaction drilling techniques require a
minimum amount of drilling fluid. Compaction drilling,
however, is incompatible with some hydrogeologic con-
ditions. Therefore, many horizontal environmental wells
Filter or Mud Cake
0-1 inch
Flushed Zone
1-5 inches
Transition Zone
5-8 inches
Axial View of Wellbore
Figure 2-5. Potential damaged zones around a horizontal
wellbore.
require fluids. Some experiments with air-based drilling
fluids have been conducted in environmental directional
drilling, and use of air to contain cuttings transported
from the wellbore has recently been demonstrated (8).
Investigation of cryogenic drilling fluids also is presently
underway (9). Cryogenic drilling fluids have the advan-
tage of freezing the wellbore wall to provide stability and
improve sample recovery. In addition, they vaporize and
minimize cross contamination after use.
Bentonite slurry and guar gum gel are the least expen-
sive and most widely used drilling fluids. Many drillers
are familiar with bentonite-based drilling fluid, which
they can easily control to maintain a stable wellbore and
efficiently remove cuttings. The major disadvantage is
that this type of drilling fluid creates a mud cake that
must be removed during well development to restore the
original permeability of the formation. A selected drilling
fluid should form an easily removed mud cake (less than
a few millimeters thick).
Guar gum is a cellulose-like polymer derived from the
guar bean and commonly used as a food additive. As a
drilling fluid, guar gum is mixed with water to form a gel
with the following strengths:
It reduces friction on the drilling tool, thereby increas-
ing drilling tool penetration rates.
It lacks fine-grained clays that can clog formation
pores.
It either biodegrades naturally or it can be degraded
with fluid additives.
It is nontoxic.
Guar gum has several disadvantages, however:
Biodegradation of guar gum can increase biomass in
the vicinity of the wellbore; this can reduce perme-
ability, which must be addressed during completion.
Low gel strength can limit the ability to transport
cuttings.
11
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Crosslinking to create a stiff fluid improves ability to
transport cuttings but increases problems with circu-
lation.
Elevated temperatures accelerate bioactivity, which
could cause premature degradation in the mud pit.
Guar gum is also used with the other alternative meth-
ods discussed in this manual. Those applications are
described in detail in following chapters.
2.1.3 Inclined Wei I bores Created With Sonic
Drilling
This section describes two sonic drilling methods with
respect to inclined wellbores: rotosonic and resonant
sonic. Both methods use the same type of sonic drill
head and dual drill pipe or casing advance method, but
they differ in the capabilities to rotate the casing during
drilling. These methods entail advancing a casing into
the formation and removing drill cuttings from the inside
of the casing via an inner drill pipe/core pipe. In some
cases, the inner drill pipe advances first into the forma-
tion, and the outer casing follows. The major difference
between the two sonic drilling methods is the rotosonic
method relies on rotary power and water as a drilling
fluid to aid in drilling, whereas the resonant sonic
method uses rotary power and drilling fluid only when
drilling rock. Following is a discussion of the principles
of sonic drilling and sonic drilling equipment, and a
description of sampling methods used in sonic drilling.
2.1.3.1 Sonic Drilling Principles
The axial oscillations that the sonic drill head produces
induce a sinusoidal energy wave in the drill pipe (10).
Maximum energy transfers from the drill head, through
the drill pipe, and to the formation when the sinusoidal
wave is in resonance with the drill string and a standing
wave is created in the pipe (Figure 2-6). More than one
frequency of axial oscillations can produce resonance
and a standing wave. The fundamental resonance fre-
quency, Rf, relates to the length of the pipe by the
equation
Rf = v/(2L),
(2-1)
where vis the speed of sound through steel and L is the
length of the drill pipe. For instance, for a steel drill pipe
30 meters (97.8 feet) long,
= (5,000 m/sec)/(2 x 30 m) = 83 Hz.
(2-2)
Additional resonance frequencies are whole number
multiples (overtones) of the fundamental resonance fre-
quency. Higher resonance frequencies are used when
greater energy is required to advance the drill string
through more resistant formations or at greater depths.
The maximum sonic drilling depths depend on the host
Resonant Sonic
Drill Head
Antinode Location:
Point of Maximum Strain
(Compression or Expansion)
in Molecular Structure
Superimposed
Induced Pressure Wave
and Reflected Pressure Wave
Expanding and Compressing Pipe
Steel Drill Pipe
Node Location:
Point of Minimum Strain
in Molecular Structure
Standing Wave:
Fundamental
One-Half Wavelength
Condition
Amplitude of
Wave Cycle
Drill Bit
Wave Variation With Time
Figure 2-6. Resonant stress wave providing energy to the drill
bit (10).
formation. For example, for unconsolidated sand (sedi-
ments most compatible with sonic drilling), the maxi-
mum drilling depth is approximately 200 meters (656
feet).
The drill pipe's penetration rate can be optimized by
controlling its frequency of oscillations between reso-
nance overtones. As a result, the soil particles adjacent
to the drill pipe cannot vibrate in unison with the drill
pipe, and they begin to vibrate in random directions. This
random movement fluidizes the soil within approximately
0.6 centimeters (1/4 inch) of the drill pipe, thereby re-
ducing the friction on the drill pipe.
2.1.3.2 Sonic Drilling Equipment
The sonic drilling method uses a combination of tech-
niques to penetrate the subsurface:
Thrust or pull-down
Mechanically induced vibrations
Rotary power
A hydraulic chain driven winch on the drill tower provides
the pull-down (and hoist). The sonic drill head provides
the vibrations (axial oscillations) and rotary power.
12
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2.1.3.3 Sonic Drill Head
2.1.3.4 Drill Pipe
The sonic drill head contains three main components
within its housing (Figure 2-7): an oscillator that consists
of two out-of-balance counter rotating rollers, an air spring
isolator, and a rotational drive (10).
The out-of-balance counter rotating rollers are hydrauli-
cally driven. The rotation of the rollers is timed and
synchronized to provide a sinusoidal oscillation in the
drill string (Figure 2-7). The energy from the rollers
transfers through the drill string to the cutting head and,
thus, to the formation. The oscillation can have a fre-
quency as high as 150 Hz.
The air spring isolator is a large-diameter piston set
within a closed cylinder. The piston connects to the
portion of the drill head that contains the counter-rotating
rollers, and the cylinder is attached to the drill tower. An
air compressor supplies the volume of air on each side
of the piston; this air acts as a soft spring that isolates
the compression waves from the drill rig.
A hydraulically driven motor superimposes rotary move-
ment on the axial vibration of the drill string. The pipe
rotation is used to attach and remove pipe from the drill
string and to aid in drilling. The rotation of the pipe also
helps remove cuttings from the drill bit and keeps the
cutting edge of the bit against the formation.
The sonic method causes the drill pipe to, in part, be-
have like a spring (10). The axial oscillations that the drill
head creates cause the pipe to expand and contract (to
the extent allowed by its natural elasticity and inertial
properties). The alternating expansion and contraction
causes the pipe to dilate over its length, and the outside
diameter of the pipe to decrease cyclically and increase
with each expansion and contraction. This movement
decreases the friction on the outside of the pipe but also
induces stresses that common drill pipe does not nor-
mally experience. The stresses, which are concentrated
on surfaces found at the drill pipe joints, can cause
occasional failure of the drill pipe. The rate of failure of
the drill pipe, however, can be minimized by:
Using threaded drill pipe with hardened steel joints
Avoiding use of welded drill pipe connections
Periodically replacing drill pipe
2.1.3.5 Drilling Tool
Sonic drilling penetrates a formation by displacement,
shearing, or fracture by impact (11). Displacement drill-
ing tools are used to drill through sands and light grav-
els. Displacement occurs by fluidizing the soil particles
and causing them to move either into the formation or
into the center of the drill pipe. The displacement drill
tool has no special shape or hardening requirements,
Upper and Lower
Air Chambers
Center Column
Air Spring
Piston
Oscillator
Housing
Force
Generating
Roller
Air Spring
Housing
Thrust
Bearing
Oscillator
Orbital Race
Drill Head /
Outer Case
Column to
Drill Steel
Adapter Flange
Figure 2-7. Sonic drill head (10).
13
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but a hard welded facing generally is used to prevent
surface erosion.
The shearing drilling tool is used to drill clayey soils. The
clayey soils can only be sheared if the axial oscillations
of the drill pipe overcome the elastic nature of the ma-
terial. Because clayey soils have a high dampening
effect on the drill pipe, the tool is designed with an
increased wall thickness at the cutting face. This pro-
vides clearance for the external and internal pipe sur-
faces that follow.
The fragmentation tool is used to penetrate lithified sedi-
ments and rock. The axial oscillation of the drill pipe
causes the drilling tool to impact and fracture the rock.
The continuous rotation of the drill pipe clears away rock
fragments from the cutting surface. A flushing medium
often is used to remove rock fragments from the well-
bore. A fragmentation tool generally has a hemispherical
array of tungsten carbide buttons on a reinforced tool
surface. The tool is built with passages to allow the
flushing medium to remove cuttings from the drill face.
2.1.3.6 Sampling Equipment and Methods
Traditionally, sonic drilling has used three types of sam-
pling methods: a core tray or plastic sleeve, a split tube,
and a core barrel liner (12). These methods can collect
core samples at any angle. The core tray or plastic
sleeve is a commonly used core retrieval method. The
core sample is taken with the inner drill pipe either inside
the outer casing or below the outer casing. The inner
drill pipe is removed from the wellbore. Then, the final
length of drill pipe that contains the core sample (still
attached to the drill head) is swiveled to about 55 de-
grees from vertical, and the drill head vibrates the drill
pipe. This causes the core sample to slide out of the pipe
and into either a plastic sleeve or onto a core tray placed
at the open end of the pipe. The core tray or plastic
sleeve method works well for lithologic characterization
and/or soil sampling. Because the samples are dis-
turbed when they slide out of the drill pipe, however, they
cannot be used to identify fine sedimentary structures or
to characterize hydraulic properties.
The split tube sampler is constructed of split carbon
steel tubing, which has tapered threads and open caps
on each end. The tube attaches to the end of the inner
drill pipe and is driven into the formation. Then, the outer
casing is drilled over the split tube, and the inner drill
pipe and tube are removed. After removing both the split
tube from the drill pipe and the open end caps, the tube
is split open. This sample method provides an undis-
turbed core sample that can be used for lithologic char-
acterization (including fine sediment structure analysis),
hydraulic properties characterization, and soil sampling.
A core barrel liner method was investigated at the De-
partment of Energy's (DOE's) Hanford, Washington,
site. The method uses a polycarbonate liner installed
inside the lowest piece of the inner drill pipe. The drill
pipe and liner are drilled into the formation and then
removed. The liner and core sample are removed by
vibrating the drill pipe. The core sample this technique
provides is equal in quality to a split tube core sample.
The temperatures of the core samples collected using
all three methods described are generally higher than
the ambient subsurface temperatures. The sample tem-
peratures can be kept near ambient temperature (less
than 27°C [80.6°F]) by decreasing the drilling rate and
cooling the core sampler before it is inserted in the
formation (12).
2.1.4 Well Installation and Completion
Horizontal well construction can create two types of
wellbores: continuous wellbores and blind wellbores.
2.1.4.1 Continuous Wellbores
Many drilling contractors are familiar with methods of
completing continuous wellbores. These methods are
based on techniques developed in the trenchless tech-
nology industry for installing cable and pipe below roads
and rivers. The continuous wellbore is an advantage for
treatment systems that require access at both ends of
the horizontal screen.
Some disadvantages of continuous wellbores are that
they require twice the surface access (exit and entrance
access) as a blind wellbore. In addition, the continuous
wellbore path is longer and possibly more expensive to
drill. Also, well installation in a continuous wellbore is
more stressful on the well materials because of two
wellbore curves. Therefore, the strength requirements of
the well materials are greater than those in a blind
wellbore.
The continuous wellbore generally has a 7- to 25-degree
approach angle and a medium to long radius of curva-
ture (greater than 45 meters [147.6 feet]). To construct
a continuous wellbore, a pilot hole first is drilled from the
surface, through the curved section to the target depth.
The pilot hole then is drilled along the horizontal section,
through another curved section, and back to the ground
surface. The well materials, such as casing, screen, and
filter packs, are assembled at the exit hole and attached
to the drill string in the wellbore. Generally a hole opener
or reamer is placed at the end of the drill string, just
before the well materials, to facilitate their installation.
The drill string, with the hole opener and well materials
in tow, is pulled back through the wellbore. The hole
opener removes any excess cuttings or formation mate-
rials that may have sloughed into the wellbore and
enlarges the pilot hole to the desired diameter. When all
the drill string has been removed from the wellbore, the
well materials are in place. Figure 2-8 illustrates this
14
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a.
Casing and Screen
Figure 2-8. Creating a horizontal well with the pull-back
method: a) drilling a continuous wellbore, b) ream-
ing and pulling in casing, c) the finished well.
technique of installing casing, which is often referred to
as the pull-back method; the name is sometimes used
to refer to the entire task of installing a continuous well.
2.1.4.2 Blind Wellbores
Blind horizontal wellbores have an entrance hole and
terminate at the vertical depth of the target zone (Figure
2-8). This type of horizontal wellbore was first developed
in the petroleum industry. The radius of curvature for a
blind wellbore can be short, medium, or long. An advan-
tage of a blind wellbore is that only one wellbore curve
needs to be negotiated during well installation. In addi-
tion, there is no need for site-access at an exit hole, and,
because a blind wellbore is shorter than a continuous
wellbore, it may be less expensive to create. However,
these cost savings generally are offset because the
drilling and well installation methods are more complex
than for continuous wellbores. Drilling methods usually
require multiple trips in and out of the hole, as opposed
to a single round trip for continuous wells.
Blind wellbores can be drilled using three techniques:
washover pipe, open wellbore, and pulling casing while
drilling. Washover pipe drilling entails initially drilling a
prior hole (Figure 2-9). Then, the washover pipe is drilled
over the pilot drill rods to enlarge the wellbore and
provide a conduit for well installation. The washover pipe
installation is difficult because it requires drilling two
pipes into the ground, thereby doubling the chances of
experiencing drilling problems. However, well installa-
tion is much easier in a washover pipe than in an open
wellbore.
Open wellbore installations have the advantage of being
the simplest and easiest to complete but are limited to
sites where wellbore stability is not a problem. In this
method, the wellbore is drilled and cased to the end of
the curve. Then, a pilot hole is drilled in the horizontal
section, and a hole opener is used to make the horizon-
tal section the desired diameter. Finally, the well casing
is installed in the open wellbore. The obvious risk in this
method is that the wellbore may collapse before the well
casing is completely installed.
The most complicated technique for completing a blind
wellbore involves pulling a casing as drilling proceeds.
A problem associated with this technique is that casing
materials stick because the hole cannot be properly
cleaned as it is drilled (13). A modified method involves
drilling a pilot hole and installing the casing as the pilot
hole is reamed to a larger diameter.
Either a washover casing or an open hole can be used to
install well materials into blind wellbores. The well materi-
als are assembled at the surface and pushed into the
wellbore. When well materials are in place, the washover
casing, if used, is withdrawn the desired distance.
2.1.4.2 Stresses on the Casing During
Installation
Friction against the wellbore is the primary cause of
stress on the casing during installation. Pushing or pull-
ing casing or well materials into a well requires force
greater than the friction generated along the surface of
contact between the hole and the casing or screen.
Figure 2-9. Completing a blind well using the washover pipe
method.
15
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Friction force is characterized as the product of a friction
coefficient and force normal to the surface between
moving objects. Although neither of these two parame-
ters can be precisely characterized, identifying and con-
sidering several influential factors should assist in
minimizing the force required to complete a horizontal
well.
Friction coefficients between solid objects at rest range
from 0.1 to 4. The exact value depends significantly on
the smoothness of the surface and nature of the mate-
rials, and especially on surface properties such as inter-
facial energy. Thus, steel and plastic casings will have
different coefficients of friction. Likewise, coarse sand
will differ from clay. If solid surfaces are separated by
small amounts of a third material, such as a liquid,
effective friction coefficients can be markedly less than
dry surface friction coefficients. Small particles can also
lubricate surfaces by acting as ball bearings, thus yield-
ing lower effective friction coefficients. Consequently,
friction coefficients during drilling operations can vary,
although Ta Inglis (14) recommends values of 0.2 to 0.4.
Several types of forces perpendicular to casing or well
screen can exist simultaneously. Figure 2-10 illustrates
the origin of some of these forces, all of which can occur
in blind and continuous wells. The long arc of the curve
section between the entrance angle and the horizontal
section bends casing and well screen. The stiffness of
these materials requires that a force normal to the axes
be realized to affect the shape. This force should be
inversely proportional to the radius of curvature of the
hole. In the horizontal section, the normal force can be
as small as the weight of the casing or screen. This
lower limit requires that the hole be open and straight,
so that it bears no other force. If the surrounding forma-
tion contacts the well materials, then the in situ stresses
transmit as a normal force. Formation materials also can
exert normal forces through capillary action; for in-
stance, the stickiness of clay is, in large part, a manifes-
tation of the strong capillary action exhibited by clays.
Perfectly straight horizontal holes are difficult to drill. The
highs and lows of the trajectory deflect the pipe from a
straight path. Even though the deflection may not be
Figure 2-10. Types of force normal to axes during installation
of casing or well materials: A) flexural stress in
long arcs, B) casing weight in straight open
sections, C) lateral stress created by tension
through serpentines, D) pressure by surrounding
formation.
sufficient to cause flexural reaction force in the pipe, an
additional normal force is generated when the force
along the trajectory is resolved into components parallel
and perpendicular to the general direction of the well. In
a mechanistic sense, the normal force components try
to straighten the serpentine wellbore. This force varies
inversely with radius of curvature. The force is propor-
tional to the tension force that is placed on the casing or
well materials. This has the consequence of compound-
ing the friction force along the length of the well.
Friction is generated at all the points along the well, and
the required force for placement can be represented by
the integral of friction force per unit length along the well.
Considering the types of normal forces described, an
expression of tractive force is
Fp = JV| F_L dx = JV| (Fflex + W + 2ji r N + a Fp) dx, (2-3)
where Fp is the force required for placement, r| is an
effective coefficient of friction, and F± is the sum of
normal forces per unit length of casing or well materials.
The normal force components are the reaction force Fflex
of flexing the pipe normalized by length, the weight W
of pipe per unit length, stresses and pressure N exerted
by the formation, and the perpendicular component of
the overall tractive force. In this equation, ris the radius
of casing or well materials and a is a constant that
accounts for the geometry of serpentine sections.
For the pull-back method of installing well materials in a
continuous well, the pulling force is transmitted as a
tensile stress in the casing material. It follows that the
tensile stress, o, in the casing is
PFp Ffiex + W+2KPN ,. ^
o =
ji (2 r w - w2) a n (2 r w - w2)
(2-4)
where x is the length of casing in the ground, w is the
wall thickness, and p is a factor that accounts for the
loss of material due to the slots. (A simple approximation
of p is the ratio of the total to the intact cross-sectional
area of the casing, while more complex analysis might
include the stress intensity imposed by the sharp com-
ers of the slots.) The origin of coordinates is where the
casing enters the ground. The casing breaks in tension
when o exceeds the tensile strength of the material.
According to Equation 2-3, stress increases as more
casing is pulled into the ground, and the increase in
stress is exponential if the well is serpentine. The maxi-
mum stress occurs where the combination of x and p is
the greatest. (The otherterms are constant.) This means
that the casing will break either near the end that is
being pulled or at the first piece of screen; this prediction
is consistent with observations of broken casing in the
field.
16
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Simple calculations using Equation 2-3 show that some
screens break if they are pulled into the ground unsup-
ported. As a result, a cable or rod is sometimes threaded
from the reamer through the casing and used as an
interior support. The interior support fastens to the end
of the casing so that much of the load during pull-back
is supported by the pipe or cable rather than the casing.
This allows materials that are relatively weak in tension,
such as plastic casing with threaded or barbed joints, to
be pulled into a continuous wellbore. The elasticity of a
cable, however, must be sufficiently small for the cable
not to stretch enough to allow the well materials to part.
Equation 2-3 can be similarly extended to assess the
compressive forces that occur while pushing casing and
well materials into a washover pipe or open hole. Wu
and Juvkan-Wold (15) present an analysis of compres-
sion stress that includes buckling of the well material.
2.1.4.4 Well Completion
Two methods of installing a gravel pack are available for
horizontal wells. One method places a tremie pipe along
the casing and injects the filter material into the annulus
between the casing and the wellbore. Placement of the
tremie pipe to the end of the well screen must be ac-
complished either during well material placement or as
part of a washover casing system. If the tremie pipe
clogs, however, it can be difficult to replace. Placing
continuous sandpack around the well screen can be
particularly difficult because the rate required to flush the
sand through a tremie pipe over a long horizontal dis-
tance (greater than 90 meters [295.3 feet]) tends to
wash out the wellbore during backfilling.
An alternative method is to install a prepacked screen,
which consists of an inner and outer screen with sand
in the annulus between them. Thus, the filter pack is
contained within the prepacked screen. Installation of a
prepacked screen requires a larger radius of curvature
for installation than standard wire wrapped screens.
Another alternative is to wrap the well screen with a
geotextile filter membrane.
Another option is to use tremie pipe to install a bentonite
seal and grout in the annulus around the riser casing. A
grout basket placed on the riser casing before installa-
tion keeps bentonite and grout from flowing into the well
screen. It is advisable to first install sand in the grout
basket to ensure that the bentonite and grout do not
seep around the edge of the basket.
A gravel pack may be unnecessary for some applica-
tions. Where horizontal wells are placed in sands, re-
moving the fine-grained sediments from the vicinity of
the well may not be possible during development. This
results in a natural sand pack that resembles similar
applications used for vertical wells (16). Moreover, a
gravel pack often is unnecessary for applications involv-
ing vapor extraction or bioventing. Eliminating a gravel
pack can significantly reduce the costs of a horizontal
well.
2.1.5 Well Development
Well development concerns and techniques for horizon-
tal wells are similar to those for conventional vertical
wells; they depend on the hydrogeologic setting and
remediation objective of the well. The drilling techniques
for installing a horizontal well in the vadose zone for the
purpose of vapor extraction or injection ideally will not
use a drilling mud because of concerns of reducing the
gas permeability of the formation. However, if a drilling
mud is required, the well development technique would
be similarto that used for a well installed in the saturated
zone.
When drilling a well installed in the saturated zone and
using a bentonite- or polymer-based drilling fluid, well
development should begin as soon as possible after well
installation and/or well completion. The drilling fluid and
mud cake need to be removed to restore formation
permeability around the well screen and/or filter pack.
Well development may include volume flushing the an-
nular space with water, jetting water into the well screen
to remove drilling fluid, treatment with an acid or base
chemicals to dissolve the drill fluid or mud cake, or
treatment with a disinfectant to prohibit bacterial growth
in the case of a biodegradable drilling fluid. The devel-
opment fluid should be chemically compatible with the
formation water to avoid detrimental effects on the for-
mation.
2.1.6 Pumps
The pump used in an inclined or a horizontal ground-
water extraction well must be able to function continu-
ously in an inclined or a horizontal position. Currently
available well pumps are designed to operate vertically,
but no studies have been published to date on the
long-term performance of well pumps in nonvertical en-
vironmental wells.
Electrical centrifugal pumps can function in the horizon-
tal position, but the life of the motor and impeller bear-
ings, which were designed to operate vertically, may
limit their endurance. Of the pumps that are currently
available, electrical centrifugal pumps are the most com-
monly used in horizontal wells because of their versatil-
ity and ability to be used at any depth.
Pneumatic pumps can be either diaphragm or bladder
actuated and typically make use of two check valves.
Usually the valves must be vertical to seat properly.
Apparently some diaphragm pumps function horizon-
tally, because Wilson and Kaback (17) used diaphragm
pumps to recover NAPLs in shallow, low-volume hori-
zontal wells.
17
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2.2 Design Considerations
In horizontal well applications, it is important to consider
many factors that can affect the performance of a well
and the ability to drill and complete the wellbore. The
pattern of flow in the vicinity of a well is a major factor
affecting performance. In addition, geologic and hydro-
logic conditions and contaminant distribution affect de-
cisions regarding:
Drilling methods
Horizontal wellbore specifications
Length and elevation of the well
Composition and design of well materials
2.2.7 Pattern of Flow
The pattern of flow created by a horizontal well is a
fundamental aspect of performance (18). Under ideal
conditionsa thin, confined aquifer of infinite extent with
no regional flowradial flow adjacent to the well is
confined to a small area and streamlines in the vicinity
of the well run roughly parallel and perpendicular to the
long axis of the well (Figure 2-11). At great distances
from the well, streamlines run approximately radially, so
that in a thin aquifer the pattern of flow to a horizontal
well resembles flow to a trench (19). At steady state in
a thin aquifer, the specific discharge from a horizontal
well with no head loss along the casing approaches that
of a vertical well with a radius equivalent to one-fourth
the total length of a horizontal well. Clearly, this can
represent a very large vertical well.
The pattern of flow changes as the formation becomes
thicker and the zone of radial flow around the well
becomes larger (18). In this case, the pattern is similar
to Figure 2-11 in plan but develops components of flow
in the third dimension (depth). Head losses associated
with the vertical component of radial flow adjacent to the
well (Figure 2-12) limit the advantages that a horizontal
well may offer over a fully penetrating vertical well, which
causes no vertical flow. The effect is exacerbated by
anisotropic conditions, in which the ratio of vertical to
horizontal hydraulic conductivity is less than one (3),
such as in interbedded fine- and coarse-grained sedi-
ments. Nevertheless, for many commonly encountered
geologic conditions, effects of formation thickness and
anisotropy are small relative to improvements in specific
capacity resulting from a horizontal well's longer screen
length (2).
Figure 2-12.
Radial flow in a vertical plane in the vicinity of a
horizontal well.
Figure 2-11. Flow paths to a horizontal well in a thin aquifer
with no regional flow. Lines are nearly straight and
parallel within the patterned area.
A secondary benefit of a long screen is reduced velocity
of fluid in the vicinity of the well. This effect reduces the
possibility of inducing turbulence and associated non-
linear head losses (2, 20). Decreasing fluid velocities in
horizontal wells also should reduce damage to well
screens and filter packs due to migration and entrap-
ment of fine-grained sediment (2).
2.2.2 Wellbore Specifications
The well trajectory and materials of construction are the
principal wellbore specifications. To some extent, these
specifications are interdependent. Installation and resi-
dence in the subsurface put stress on the wellbore
tubing. The magnitude of the stresses depends on the
method of installation, curvature and depth of the bore,
and other factors. To ensure subsurface access, the
wellbore tubing selected must withstand these stresses.
2.2.2.1 Specification of Trajectory
Horizontal wells currently used for environmental appli-
cations typically enter the ground at a shallow angle,
18
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gradually curve until they are horizontal, proceed
through the contaminated interval, and then either ter-
minate or curve upward to the ground surface. Accord-
ingly, this trajectory is specified by the following
parameters (Figure 2-13):
Well head location
Entry angle
Vertical depth to target
Measured depth to target
Wellbore curve
Step-off distance
Plan path
KOP
EOC
VDkop
VDeoc
VDtarget
MDeoc
MDkop
MDtotal
Heoc
Kickoff Point
End-of-Curve
Vertical Depth to KOP: Equation 2-5
Vertical Depth to EOC: Equation 2-6
Vertical Depth to Target: Given
Measured Depth to EOC: Equation 2-7
Measured Depth to KOP: Equation 2-8
Total Measured Depth: MDeoc + Horizontal Length
Horizontal Distance to EOC: Equation 2-9
Horizontal Distance to KOP: Equation 2-10
Total Horizontal Length: Heoc + Length of Horizontal
Section
Figure 2-13. Nomenclature describing a horizontal well (14).
Well Head Location. Generally the well head is in line
with the horizontal well at a practical distance from the
target zone. It is possible, however, to curve laterally
before reaching the target zone to offset the well head
from the major line of the well. Lateral curves stress the
drill rods, reduce the distance capabilities of the rig, and
increase stresses on the well materials. The following
points should be considered when selecting the well
head location:
Surface obstructions may inhibit the selection of a
well head location. There must be sufficient room at
the well head location for the safe operation of the
drilling equipment.
The length of surface piping to a treatment facility
should be minimized.
Multiple horizontal wells may radiate from the same
well head location. This enables well heads to be
located in a single well vault and one treatment sys-
tem to be used for an extensive area.
Entry Angle. The entry angle (l|), sometimes referred to
as the approach angle, is the angle between the drill
stem and the ground surface at the entry hole. The entry
angle may be between 7 to 90 degrees from horizontal
(10 to 35 degrees is most common), depending on the
type of drill rig being used. Shallow horizontal well in-
stallations (less than 8 meters [26.2 feet] VD) generally
have a 10- to 15-degree entry angle, whereas deeper
horizontal well installations (greater than 8 meters [26.2
feet] VD) have a 15- to 35-degree entry angle.
Vertical Depth to Target. The VD to target is the differ-
ence in elevation between the wellbore entry location
and the target zone.
Measured Depth to Target. The measured depth (MD)
to the target is the distance along the wellbore path from
the wellbore entry to the target zone. The MD is equal
to the length of drill rod in the ground.
Wellbore Curve. A radius of curvature defines the curved
portion. By industry convention, the radius of curvature
is measured in feet, and a well is drilled with a buildup
rate (BUR) measured in degrees of angle per 30.5
meters (100 feet) of wellbore drilled. The curve begins
at the kickoff point (KOP) and ends at the end-of-curve
(EOC). The curve BUR that provides the desired radius
of curvature is given in Table 2-2.
Selection of a radius of curvature depends on the target
zone location, well materials, and drilling equipment.
Table 2-2. Wellbore Curve Measurements
Radius of Curvature (RoC)
(feet)
Buildup Rate (BUR)
(degrees/100 feet)
RoC x BUR = 5,730
50
100
150
200
250
300
400
500
750
1,000
114.6
57.3
38.2
28.7
22.9
19.1
14.3
11.5
7.6
5.7
19
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Wellbore curves have been classified based on their
radii of curvature as short-, medium-, or long-radius
wells: a short radius of curvature is less than 45 meters
(147.6 feet); a medium radius of curvature is 45 to 250
meters (141.6 to 820 feet); and a long radius of curva-
ture is greater than 250 meters (820 feet).
As the radius of curvature tightens, both the bending
stresses on the drill rods and the frictional resistance
between the rods and the formation increase. These
stresses conspire to effectively reduce the total length
that can be achieved by a particular combination of drill
rods and boring machine. Horizontal sections are limited
to approximately 100 meters (328 feet) in short radius
of curvature boreholes (13, 14). The diameter and ma-
terial of the casing also affect the wellbore radius. A rule
of thumb in the river crossing industry is that the radius
of curvature, measured in feet, should be 100 times the
diameter of the installed pipe measured in inches. For
example, if diameter of the well casing is 4 inches, then
the radius of curvature should be approximately 120
meters (394 feet).
The radius of curvature of a borehole should be as long
as practical within the constraints of the other borehole
criteria. A shallow entry angle and large radius of curva-
ture increase the borehole length and reduce the possi-
ble locations for the well head. The increase in drilling
cost associated with drilling a longer borehole must be
weighed against the benefit of reduced stress on well
materials.
Step-Off Distance. The step-off distance (Heoc) is the
horizontal distance between the entry hole and the be-
ginning of the horizontal section or the EOC of the
wellbore. The step-off distance may be determined by
site-specific conditions, such as the available surface
area for the drilling equipment, well head(s), and asso-
ciated treatment systems.
Plan Path. Normally a horizontal well is planned to be
straight from entry point to wellbore termination or exit.
The size and shape of the plume and subsurface ob-
structions, however, may require the well path design to
include lateral curves. A horizontal wellbore can be
drilled with a lateral curve, but the radius of curvature
should be large to reduce stress on well materials.
2.2.2.2 Estimating Wellbore Specifications
A detailed determination of the wellbore trajectory is
required during drilling. This allows the expected loca-
tion to be compared with information from the electronic
systems that locate the bore in the subsurface. Typically,
designs of wellbore trajectories are developed and re-
vised graphically in conjunction with site conditions.
After establishing the trajectory, either graphical or geo-
metric methods should quantify it. A few equations that
may be helpful follow.
For the VD of the KOP (VDkop),
VDkop = VD, - [k-,(cos If - cos 1^1 BUR,
(2-5)
where VD, is the VD to the target zone, k1 is a constant
that equals 5,730 when BUR is in degrees per 30.5
meters (100 feet), lf is the final wellbore angle (0 degrees
for a horizontal well), and /, is the entry angle. The VD
to the EOC is equal to the VD to the target zone if the
surface elevation at the entry point is equal to the ele-
vation of the point above the target zone. If those eleva-
tions are unequal, then the VD to the EOC relative to
the entry point is given by
(2-6)
= MDkop + [(nkJ/180] [(I, - IJ/BUR], (2-7)
where MDkop is the MD to the KOP and is given by
MDkop = VDkop/sin(lj. (2-8)
If the step-off distance is not fixed, then it can be deter-
mined by the entry angle (/,) and the radius of curvature
of the borehole (or BUR) as follows:
VDeoc = VDkop - Ik, (cos lf - cos
The MD to the EOC (MDeoc) is given by
= Hkop +
(2-9)
where the horizontal distance to the kickoff point (Hkop)
is given by
Hkop =
2.2.2.3 Casing and Screen
(2-10)
Casing and screen in a horizontal well must be stronger
than in vertical applications. Horizontal wells require the
extra strength to withstand tensile or compressive
stresses along the axis of the casing that result from
pulling or pushing the casing into the bore. Furthermore,
the weight of the overburden produces stresses that
may collapse a horizontal casing.
Strength of casing depends on the casing material,
diameter, and wall thickness. In general, stainless and
carbon steel casings are stronger than plastic casing.
However, plastic casing, particularly high-density poly-
ethylene (HOPE), can bend in a tighter radius than steel
casing. The collapse strength, which is the maximum
allowable force applied perpendicular to the casing, de-
creases with increasing diameter (Table 2-3). Con-
versely, the tensile strength, which is the maximum
allowable axial force, increases with increasing diame-
ter. A typical formation exerts approximately 0.02 MPa
per meter depth (0.8 psi/foot), so the collapse strength
is related to the allowable depth at which the casing can
20
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Table 2-3. Strengths of Prepacked Screens (According to
Johnson Filtration Systems, Minneapolis,
Minnesota)
Nominal Size
Collapse
Strength (MPa)
Tensile
Strength (kN)
Prepacked 304 Stainless Steel
2 inch 10.3
4 inch 4.14
6 inch 1.38
Prepack Schedule 40 PVC
2 inch 1.72
4 inch 1.03
24
58
170
4.9
7.1
be used. Tensile strength is important when pulling cas-
ing into a horizontal bore, so the allowable stress has
been converted to allowable pulling force in Table 2-3.
To estimate allowable pulling force for casings of other
sizes and materials, multiply the effective pulling area
times the strength of the tubing (Table 2-4). The effective
pulling area is the continuous cross-sectional area,
which for solid casing is rcdw/2 (with d the outer casing
diameter and wthe wall thickness). Most casings that
fail in tension crack at either the slots in the screen or
the joints between individual pieces of casing. To esti-
mate the upper limit of the strength of a screen made
from plastic pipe with slots cut in it, use the approach
outlined above, but reduce the effective pulling area to
account only for the solid material in between the slots.
The strength of joints between casing sections can
range widely, from essentially the same as the casing
itself for welded joints to small values for threaded joints
designed for vertical wells.
Table 2-4. Tensile Strengths of Pipe
Pipe
Tensile Strength (MPa)
PVC
HOPE
Fiberglass
9.4
6.5
1.3-6.9
Casing and screen in a horizontal well used for environ-
mental applications must also be resistant to whatever
chemicals exist at a particular site. In this respect, the
requirements for horizontal wells are the same as those
for vertical wells.
New materials and joining capabilities are being devel-
oped for casing and screen for environmental wells.
Distributors of well supplies should be able to supply
information about the capabilities of these new products.
2.2.2.4 Filter Pack
Filter packing around a well screen is recommended for
horizontal wells that produce liquids from formations
containing fine-grained sediments, just as it is for verti-
cal wells in similar sediments. To select a size gradation
for a granular filter pack, use the same approach as for
a vertical well (16), although keep in mind that entrance
velocities into horizontal wells can be considerably less
than those into vertical wells.
Several methods exist to place filter packing into the
annulus around a casing or screen, most of which in-
volve injecting the particles of the filter pack in a liquid
or air stream (21). Moving fluid more readily transports
HOPE particles, but these methods are more expensive
than sand. Quality assurance is problematic in the injec-
tion of filter pack materials; it is difficult to ensure that
the packing is continuous around the entire casing.
An alternative to injecting filter pack materials is to use
a prepacked screen, which consists of an inner screen,
an annulus filled with sand, and an outer screen. The
prepacked screen currently is the most popular method
of installing a gravel pack in horizontal wells because it
ensures that the pack is continuous and uniform. Pre-
packed screens are available in most sizes and casing
materials.
Unlike most vertical wells, filter packing in a horizontal
well can be a significant component of the cost of the
well. Therefore, careful examination of each application
should establish the need for a filter pack and determine
whether the cost of the pack will result in a valuable
increase in the performance of the well. Many wells used
for vapor extraction, for example, may require only a well
screen. Likewise, liquid-phase recovery wells com-
pleted in clean sands formations may have naturally
developed filter packs (16), thereby avoiding the cost of
the filter pack.
2.2.3 Geologic Site Conditions
Geologic conditions, ranging from composition of the
formation to the heterogeneities caused by depositional
environment, affect details related to drilling and com-
pletion and the use of horizontal wells.
2.2.3.1 Formation Composition
The composition of the formation affects the methods
used to drill a directional wellbore. Highly resistant for-
mations, such as siliceous sandstones and metamor-
phic or igneous rocks, are difficult to cut. Loosely
consolidated formations that are cobble-rich can be dif-
ficult to penetrate because the cobbles rotate with the
drill bit. Generally, loosely consolidated granular sedi-
ments present steering problems because the bit cannot
exert enough lateral force on the formation to deflect the
drill rods. Specialized technology can address all these
issues, although it also may increase the cost of creating
the wellbore.
21
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2.2.3.2 Stratification
Drilling in a stratified formation may present problems in
some cases, particularly when intersecting a resistant
unit at a shallow angle. In this case, the wellbore tends
to ride along the top of the resistant formation. If recog-
nized, however, this problem can be addressed.
Formation stratification largely affects the application of
horizontal wells, particularly when the distribution of
contaminants follows a sequence of alternating fine- and
coarse-grained sediments (impermeable and perme-
able layers). In this case, recovery would be primarily
from thick, high permeability strata in the vicinity of the
well, with recovery from strata overlying or underlying
the well less than anticipated. The degree to which this
occurs depends on the thickness and permeability of the
stratigraphic layers.
In general, contaminants flow in a horizontal plain un-
less hydraulic, chemical, or gravitational forces provide
an impetus for flow in the vertical direction. A horizontal
well should be placed as near the contaminant as pos-
sible to decrease the likelihood of an impermeable layer
impeding the flow between the well and the contaminant.
2.2.3.3 Heterogeneities in the Horizontal Plane
Horizontal wells are suited to accessing formations that
are heterogeneous in a horizontal plane. Formations
whose major component of permeability comes from
vertical fractures are excellent candidates. In addition,
horizontal wells can access braided stream deposits
with laterally discontinuous sand bodies. Many glacial
sediments contain both vertical fractures and laterally
discontinuous sand bodies, and many near-surface rock
formations contain vertical fractures.
2.2.3.4 Aquifer Thickness
Horizontal wells can be effective in aquifers of any thick-
ness. However, the efficiency of a horizontal well relative
to a fully penetrating vertical well increases as the aqui-
fer becomes thinner (2). This is because flow must
converge from increasing distances both above and
below the well as the aquifer thickens (Figure 2-12). A
vertical well that fully penetrates an aquifer, however,
induces primarily horizontal flow and does not suffer
from the effects of vertical flow (assuming that draw-
down is modest). The specific capacity of both horizontal
and vertical wells increases as the aquifer thickens.
Therefore, the performance (measured by specific ca-
pacity) of a horizontal well in a thick aquifer is better than
the same well in a thin aquifer. This occurs because less
interaction with the upper and lower no-flow boundaries
occurs in a thick aquifer than in a thin layer.
2.2.3.5 Anisotropy
The permeability of most formations is anisotropic, al-
though the principal directions depend on the geologic
conditions. Interbedded sediments, for example, typi-
cally are more permeable in a horizontal than in a verti-
cal plane, and even in the horizontal plane permeability
is greater along one direction than other directions. Ac-
cordingly, the directions of greatest, intermediate, and
least permeability are mutually perpendicular in an or-
thogonal set. In other geologic formations, the principal
directions of permeability may differ significantly. In any
case, these directions should be considered when locat-
ing horizontal wells.
For situations with distinction only between vertical and
lateral permeability, the effects of anisotropy resemble
the effects of aquifer thickness; an anisotropic aquifer of
a certain thickness behaves the same as an isotropic
aquifer of different thickness (20, 22). The effective
thickness he of an anisotropic aquifer of actual thickness
h is given by he = h /ch//cv, where /ch and kv are the
horizontal and vertical permeabilities, respectively. Ac-
cordingly, an anisotropic aquifer, for which /ch//cv = 100,
that is 5 meters (16.4 feet) thick behaves like a homo-
geneous aquifer 50 meters (164 feet) thick with k = kh.
Similarly, if kv is greater than /ch, as is possible where
vertical fractures are common, then the effective thick-
ness of the aquifer is less than the actual thickness.
In general, a horizontal well drains the largest volume if
placed normal to the plane that contains the axes of
maximum and intermediate permeability (Figure 2-14a).
This only can be possible if the direction of greatest
permeability is vertical. Where the greatest permeability
is horizontal, a horizontal well placed perpendicular to
the direction of intermediate permeability drains the larg-
est volume (Figure 2-14b).
a. Ideal Orientation b. Fair Orientation c. Poor Orientation
I/.
' ^max
^v min
K
intermediate
<*""
->*"
Figure 2-14. Orientations of horizontal well (dashed line) rela-
tive to principal directions of permeability.
When locating the well, other factors to consider along with
the direction of anisotropy include surface obstructions,
direction of regional flow, and location of contaminants.
Well tests are available to estimate formation anisot-
ropy (23).
2.2.3.6 Drawdown and Discharge
The drawdown and discharge of a horizontal well depends
on aquifer and well characteristics. Aquifer characteristics
22
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include permeability, storage coefficient, and thickness
and boundary conditions. Relevant well characteristics
are length, height in the aquifer, and details of the hy-
draulics resulting from flow through the slots in the
screen and along the axis of the wellbore. Most of these
factors also are common to analyses of vertical wells,
with the exception of wellbore hydraulics, which are
commonly ignored for vertical wells that have fairly short
screens. The next section includes a discussion on well-
bore hydraulics; therefore, this section examines the
effects of the other factors.
Horizontal wells placed in productive aquifers commonly
use a pump operating at constant discharge. The pump
produces a drawdown that increases with time, just as
with a vertical well, and the magnitude of the drawdown
depends on the factors listed above. As an example,
consider a horizontal well of length L at midheight in a
confined aquifer of thickness h and infinite lateral extent.
The dimensionless drawdown Pd as a function of time
depends on the ratio of well length to aquifer thickness,
and on the ratio of vertical to horizontal permeability
(Figure 2-15). This is consistent with the conclusions
presented in the qualitative discussion above. At any
given time, the drawdown increases as the well
becomes shorter, the aquifer becomes thicker, or the
ratio of vertical to horizontal hydraulic conductivity be-
comes smallerall of which contribute to decreasing Ld
(Figure 2-15). It is noteworthy, however, that these vari-
ables cause the most severe differences early on, but
the differences diminish and approach the results for
Ld = oo. The interpretation of Ld = oo is that the well cuts
the full thickness of the aquifer, which is impractical for
most horizontal wells but is fairly typical of many instal-
lations of trenches. Thus, based on Figure 2-15, the
drawdown in a horizontal well apparently approaches
that of a trench with increasing time. The abscissa of
Figure 2-15 is dimensionless time, which must be trans-
lated to real time using the aquifer properties and well
length (18).
The results for a well operating at constant discharge in
a confined aquifer are by no means applicable to all
horizontal wells. For example, results from cases in
which horizontal wells extract vapor from areas where
the ground surface is open to the atmosphere differ
significantly from results when the ground surface is
sealed. Moreover, it would be inappropriate to use Fig-
ure 2-15 to analyze cases in which horizontal wells are
used in confined aquifers of relatively low permeability;
the difficulty in this case results from the requirement of
constant discharge. In many applications in low perme-
ability formations, horizontal wells operate at constant
drawdown using a water level sensor to control the
pump. This results in a well with a roughly constant
drawdown but with a discharge that decreases with
time.
10
10-1
10-2
10-3
I I I I I I I I
Trench Solution
I I I
10-6
10-3
103
Figure 2-15. Drawdown resulting from a horizontal well as a
function of time and dimensionless length. Draw-
down and time are dimensionless.
For example, consider a well at the top of a confined
aquifer, where the aquifer thickness is h = 20 (variables
are defined in Table 2-5). The dimensionless discharge
decreases as a function of time and also as a function
of well length (Figure 2-16). The effect of well length on
dimensionless discharge occurs because the discharge
per unit length of the well (Q/L), not the total discharge,
decreases with increasing well length. For example, at
fd = 10,000, the dimensionless discharge is 0.078 when
L = 10, whereas it is 0.055 for L = 20. As a result, the
actual discharge, obtained using the expression in Fig-
ure 2-16, is proportional to 10 x 0.078 = 0.78 for L = 10,
whereas it is proportional to 20 x 0.055 = 1.1 forL = 20.
Therefore, as the well length increases from 10 to 20,
the actual discharge increases 40 percent. By present-
ing the results in dimensionless form, estimates can be
obtained of the actual discharge for any combination of
aquifer conditions. Details of this analysis are presented
in Murdoch and Franco (24).
2.2.3.7 Vertical Fractures
The presence of naturally occurring vertical fractures
enhances the performance of a horizontal well accord-
ing to the principals outlined above, but such fractures
Table 2-5. Summary of Variables
Ky Vertical Hydraulic Conductivity
Kh Horizontal Hydraulic Conductivity
K Average Hydraulic Conductivity
h Aquifer Thickness
S Storage Coefficient
L Well Length
t Time
rw Well Radius
AP,,, Drawdown at the Well
23
-------
0.12
0.10
0.08 -
0.06 -
0.04 -
0 2,000 4,000 6,000 8,000 10,000
td
Figure 2-16. Discharge from horizontal wells of different lengths
operated at constant drawdown (24).
are absent in many formations. Vertical fractures can be
induced from horizontal wells using methods developed
for vertical wells, described in Chapter 3. Such induced
fractures can significantly reduce limitations related to
formation thickness, anisotropy, and layering. The petro-
leum industry routinely induces hydraulic fracturing of
horizontal wells. In addition, techniques developed for
soil (25) have been used to create fractures from hori-
zontal wells used for environmental applications.
2.2.3.8 Regional Gradient
Horizontal wells can provide hydrodynamic control in
aquifers with a regional gradient. Special considerations
are necessary, however, if the wells are intended to
intercept contaminated plumes. In some cases, horizon-
tal wells can be placed in the direction of the regional
gradient, particularly to intercept an elongated plume
that is moving relatively slowly.
In other cases, horizontal wells can be placed perpen-
dicular to a regional gradient to intercept ground water.
This application is best suited to relatively thin aquifers
and relatively minor regional flow rates. This is because
the certainty of all the water being intercepted decreases
as the aquifer becomes thicker or the regional flow rate
increases. Other aquifers above or below the well also
may affect interception. Monitoring piezometric heads
with multilevel vertical piezometers and estimating gra-
dients at various depths in the vicinity of the horizontal
well are recommended to ensure that all water is being
intercepted.
2.2.4 Distribution of Contaminants
Once the distribution of contaminants is known, or as it
becomes resolved in more detail, locating horizontal
wells ensures hydrodynamic containment before recov-
ery begins. Details of the plume geometry also may
affect the design of a horizontal well.
2.2.4.1 Elongated Plume
Elongated plumes can resemble the shape of the cap-
ture zone of a horizontal well, which also is elongated
(2). Therefore, elongated plumes are ideally suited to
this application. A regional gradient, however, affects
the capture zone of a horizontal well with travel time
contours forming teardrop shapes. The narrow end of
these teardrops is on the downgradient side of the well.
Inasmuch as many plumes are elongated in the direction
of regional flow, regional effects must be considered
when designing the horizontal well. Using a few vertical
wells downgradient from the horizontal well may be
advisable to ensure complete capture.
An elongated plume may be wide enough to warrant
placing a horizontal well perpendicular to its long axis to
intercept migration with the regional flow.
Some elongated plumes result from elongated sources
and can be independent of regional flows (e.g., contami-
nants beneath a leaking pipe). Horizontal wells are ide-
ally suited to this application.
2.2.4.2 Equidimensional Plume
Horizontal wells that access equidimensional plumes
provide the best performance when located perpendicu-
lar to the direction of maximum permeability.
2.2.5 Wei I bore Hydraulics
A notion carried over from experience with vertical wells
is that a horizontal well recovers water uniformly along
its length. This is true with wells that have a large screen
diameter (greater than 15 centimeters [6 inches]) and
are installed in low-permeability formations. Head loss
in large diameter screens is negligible when compared
with the head loss in the formation next to the screen.
However, when small diameter wells are installed in
highly permeable formations, the head loss along the
screen can cause the rate of inflow into the well to vary
with distance from the pump (Figure 2-17).
Although a detailed analysis of wellbore hydraulics is
beyond the scope of this manual, the effects of various
parameters can be anticipated. Consider a very long,
small-diameter well with a small pump operating at one
Pump
Influx
Figure 2-17. Schematic of distribution of influx into a horizontal
well.
24
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end. Fluid flows into the well adjacent to the pump at
some rate per unit length of well screen. That rate of
influx must decrease with increasing distance along the
wellbore due to head losses associated with flow
through the casing. It follows that the inflow rate, and
thus the effect of the well, may be negligible beyond
some critical distance where the sum of all the influx is
equal to the discharge of the pump (26). This effect
would be most severe where long, narrow wellbores are
installed in highly permeable formations (as mentioned
above). In some applications, however, the influx would
decrease along the wellbore but then increase as it
approaches the far end of the well (27). This occurs
because water converges from beyond the end of the
well (as in Figure 2-17). This effect from the flow pattern
in the aquifer offsets the effects of wellbore hydraulics.
Hydraulics cause variations in inflow along the length of
the wellbore, although the magnitude of this effect dur-
ing field applications has yet to be quantified. Formation
heterogeneities (e.g., vertical fracture) that are inter-
sected by the wellbore also cause variations in the rate
of inflow and may dominate the effects of wellbore hy-
draulics. Logging techniques that use a borehole flow-
meter (16) to determine the inflow into a vertical well are
available, and similar methods could be applied where
the distribution of inflow into a horizontal well is critical.
2.2.6 Site Conditions
The major site conditions that affect well design are
related to access and obstructions of the well trajectory.
A horizontal or inclined well typically is selected when
surface structures restrict subsurface access. In some
cases, such as at refineries or other large industrial
complexes, surface structures exist a considerable dis-
tance from the contaminated zone and restrict access
to even a directional wellbore. In other cases, sites are
simply too small to permit sufficient step-off distance.
The entry location for a horizontal well must provide
sufficient space to set up the drill rig and related equip-
ment. The required area for this activity currently is at
least 5 by 10 meters (16.4 by 32.8 feet), and can be
considerably more depending on the type of drill rig.
The following are additional considerations related to
site conditions:
Elevation differences in excess of 16 meters (52.5
feet) between the entrance hole and the exit hole
should be avoided because they may cause prob-
lems with the drilling fluid system.
The wellbore path should avoid existing piles, moni-
toring wells, metal footings and pipelines. Subsurface
metallic objects decrease the accuracy of a primary
magnetometer-type steering tool. Drilling projects
that use a magnetometer-type steering tool and re-
quire wellbore accuracy in an area that has subsur-
face magnetic interference need a secondary subsur-
face survey system.
Drilling should not occur in areas where overhead
structures or wires may limit the use of construction
equipment.
A continuous wellbore design should provide at least
a 12-meter (39.4-foot) wide area for the exit hole and
an available area for laying out the well string during
pull-back installation.
2.3 Applications
Horizontal or inclined wells have been used for most of
the same purposes as vertical wells (28): air sparging,
soil vapor extraction, ground-water extraction, and injec-
tion. The following sections discuss only the effects of
horizontal and inclined wells on the efficacy of these
methods.
2.3.1 Pump and Treat
Recovery and treatment of ground water, "pump and
treat," is perhaps the most common method of address-
ing ground-water contamination, accounting for approxi-
mately one-quarter of horizontal well applications (28).
Horizontal wells can recover aqueous-phase com-
pounds at reduced energy costs by using fewer pumps.
It is important to recognize that processes such as
sorption, diffusion, and dissolution from NAPL sources
are unaffected by well geometry. Accordingly, limitations
of pump and treat systems now widely recognized for
vertical wells (29) also apply to horizontal wells.
Horizontal well design approaches depend on specific
site conditions, but the following general approaches are
applicable to many field cases:
Use horizontal wells to increase the rate of recovery
from low-permeability formations. This is particularly
applicable to relatively massive formations with ver-
tical fractures. The well should be placed in the area
of greatest contamination, and perpendicular to ver-
tical fractures. To pump, maintain constant drawdown
to maximize discharge.
Place a horizontal well along the axis of an elongated
plume to recover contaminated water and arrest mi-
gration (Figure 2-18a). Because the shape of many
plumes resembles the elongated capture zone of a
horizontal well, an excellent opportunity exists to re-
cover a majority of the plume with one well. Keep in
mind, however, that an elongated plume probably is
moving, and that regional gradients can skew the
capture zone of the well. Well placement should an-
ticipate the effect of the regional gradient. For in-
stance, it may be appropriate to extend the well
further downgradient than the current extent of con-
tamination. Pumping should begin immediately after
25
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Horizontal Well
Capture Zone
Source
Figure 2-18. Two approaches using horizontal wells to inter-
cept contaminant plumes.
creating such a well because, if neglected, the well
offers a path along which contaminants can migrate
under the natural gradient. Horizontal wells also may
combine with vertical wells to contain downgradient
migration.
Place a horizontal well perpendicular to the direction
of migration of a plume (Figure 2-18b). This ap-
proach, which resembles that of an interceptor
trench, is most applicable when a relatively thin stra-
tigraphic unit contains a broad plume. Because some
of the plume may travel above or below the horizontal
well, this approach may be appropriate when trench-
ing is not an option because of access, depth, or
other factors.
All of the applications described above would benefit
from modeling studies to improve the chances of plume
capture. This effort requires that site hydrogeology and
distribution of contaminants are well known. Moreover,
the pattern of flow that a long horizontal well induces
may take some time to become steady, so transient
analyses are recommended during the design process
to avoid using the overly optimistic conditions seen at
steady state.
2.3.2 Vapor Extraction
Vapor extraction projects account for approximately
one-quarter of the applications of horizontal wells. This
is because of the relatively large area covered by the
well and the ability to access beneath structures and
place recovery wells close to the source of volatile or-
ganic compounds (VOC). The source of air is a key
factor during the design of vapor extraction systems that
use horizontal wells. One horizontal well beneath an
open surface causes flow that is vertically downward
and converges toward the screen (Figure 2-19a). This
focuses the extraction on the ground overlying the well,
although the lateral extent may be limited. It is possible
to increase the lateral extent affected by the well by
placing a layer, or cap, with low pneumatic conductivity
over the well (Figure 2-19b). In this case, air flows into
the subsurface beyond the covering layer and expands
the size of the area affected by the well (30). This
increased area, however, comes at the expense of re-
ducing the magnitude of air flux through the ground
directly above the well. In some cases, the region above
the well may become stagnant and receive little reme-
dial effect. Installation of air inlets in the cap may be
advisable so that the benefits of both covered and un-
covered sites may be realized. Other horizontal wells or
vertical wells may also be used to provide air inlets in
the subsurface.
Figure 2-19. Schematic of flow patterns with horizontal wells:
a) without cap and b) with cap.
2.3.3 Free-Product Recovery
A horizontal well can present an ideal geometry to re-
cover layers of free-phase LNAPL or DNAPL, although
complicating factors can make this application difficult to
execute effectively. Free-phase LNAPL, and in some
cases DNAPL, commonly occurs as broad, flat-lying
layers and can be recovered by a horizontal well placed
within or slightly below the layers. This, of course, re-
quires that the NAPL location is known and well place-
ment is accurate. However, accurate determination of
LNAPL location can be difficult, and even approximate
26
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determination of DNAPL location can be difficult (31).
Assuming that the NAPL can be identified, to optimize
recovery, well placement must have a relatively narrow
window of depth. For example, consider a LNAPL layer
0.3 meters (1 foot) thick. The best placement of the
recovery well would be at or slightly below the
NAPL/water interface. If the well is a few decimeters too
shallow, it would overlay the NAPL, and recovery would
be minimal. (Suction would have to be used to recover
any liquid.) If it is a few decimeters too deep, water
recovery must precede NAPL recovery. Lowering the
water table significantly may cause the newly created
vadose zone to trap some of the NAPL (32). For depths
within 10 meters (32.8 feet) of the ground surface, cur-
rent methods can drill accurately to within a few vertical
decimeters (less than a foot) (accuracy is better than 5
percent of the depth).
Natural fluctuations in the water table may change the
depth of the interface between NAPL and water. This
can be particularly problematic if fluctuations cause the
NAPL to move significantly above or below the horizon-
tal well. The severity of this problem can be assessed
by monitoring water levels for at least 1 year and, pref-
erably, by accessing historical records. Using an inter-
ceptor trench may be preferable at NAPL sites with large
seasonal fluctuations in water level.
NAPL recovery deflects the interface with water so that
it may be advantageous to simultaneously recoverwater
using another pump either below (for LNAPL) or above
(for DNAPL) the interval of contaminant recovery. Using
two pumps is a common method of recovering LNAPL
with vertical wells (32), and this method has been used
to improve the recovery of DNAPL (33). A similar geome-
try is feasible with a horizontal well, except an additional
well must be created either above or below the contami-
nant recovery well (Figure 2-20). Alternatively, it may be
cost effective to control water levels with vertical wells
while a horizontal well recovers free product.
At least eight horizontal wells have been installed to
recover free product. One of those wells was designed
to recover machining oil beneath a building and, al-
though specific data are unavailable, it is reportedly
producing more oil than expected (17). At least seven
wells were installed for free-product recovery and hydro-
dynamic control to halt the migration of petroleum-
contaminated ground water into a residential area (34).
2.3.4 Sparging and Vapor Extraction
A technique that couples sparging with vapor extraction
has been used with horizontal wells at a minimum of five
locations since the DOE Savannah River site in Aiken,
South Carolina, first demonstrated this approach (35).
During an initial 139-day demonstration, air was injected
into one well below the water table, and air and VOCs
were recovered from another well above the water table.
This approach recovered 7,200 kilograms (15,873
pounds) of trichloroethylene (TCE), which was five times
more than was estimated for recovery using vertical
wells (35).
This application consists of two horizontal wells, one
located over the other as shown in Figure 2-20. The
primary advantage of using horizontal wells in this ap-
plication is that they encourage the injection of air along
the length of the well, thus lengthening the area covered
compared with sparging using vertical wells. This can
represent a significant improvement because in the ab-
sence of confining layers, air channels from a vertical
well may rise rapidly, remain close to the well, and cover
a small portion of the aquifer. A horizontal well used for
the upper well in this application is less critical and can
be replaced with several vertical wells to recover in-
jected air and stripped VOCs. As of 1993, at least 19
horizontal wells have been installed in integrated
sparging and vapor recovery systems at five sites.
Figure 2-20. Recovery of LNAPL with one horizontal well, and
water control with another well below.
2.3.5 Bioremediation
Horizontal wells can be used to enhance bioremedia-
tion, either by injecting liquids, such as nutrients and
hydrogen peroxide, or by injecting gases, such as air or
oxygen. The design considerations and approaches are
similar to applications for pump and treat or vapor ex-
traction.
2.3.6 Flushing
Uniform infiltration from a horizontal well can be tapped
to enhance soil washing or flushing. Twenty-six horizon-
tal wells have been installed as surfactant infiltration
galleries, and two wells as recovery wells. Information
on their performance is unavailable, but Wilson and
Kaback (27) provide some information resources.
27
-------
2.3.7 Soil Monitoring and Sampling
Another potentially beneficial application of horizontal
wells involves collecting samples and/or monitoring be-
neath structures. Two known cases of horizontal wells
used for monitoring or sampling exist. A slotted pipe was
installed beneath a mixed-waste landfill at Sandia, New
Mexico, to obtain soil vapor samples (36). At another
site, soil samples were taken while drilling a horizontal
well. The sampler was a hydraulically powered coring
tool with an inner barrel actuated by the drilling fluid
pump pressure (37). The inner barrel was hydraulically
pushed into the formation, forcing the sample into an
enclosed plastic cylinder. After withdrawing the cylinder
into an outer housing, a valve closed to protect it from
contamination while the drill string and core sample
were withdrawn from the wellbore. This method allowed
access to high-quality samples during drilling, but it
required removal of the drill bit, insertion and removal of
the core barrel, and reinsertion of the drill bit. Each drill
rod required handling four times during this operation,
which illustrates the excessive labor involved in recov-
ering one sample from a long wellbore. Moreover, this
sampling method requires that the wellbore remain open
while the drill rods are removed, which either requires
specialized drilling techniques or a stable formation.
Methods of obtaining soil samples without removing drill
rods are currently unavailable. Such a method would
markedly increase the value of horizontal drilling and
thus, is currently being investigated. Lower drilling costs
and improved methods should increase horizontal well
applications for sampling and monitoring.
Inclined wells created by sonic drilling greatly comple-
ment horizontal wells because they promise to markedly
improve sampling capabilities. Sonic drilling uses high-
energy vibrations to resonate a drill string of heavy
casing. The vibrations enhance penetration and force
formation material into the casing. A sample retrieval
system clears the casing and brings the sample to the
surface. Thus, sample recovery is integrated with the
drilling process. The quality of the samples that this
technique obtains can be excellent because no fluids
are required during drilling.
Sonic drilling has created approximately 12 inclined
wellbores and slant wells. The Sandia National Labora-
tory in 1993 obtained soil samples beneath a mixed-
waste landfill using two wellbores inclined at 15 degrees
from horizontal and 43 meters (141.1 feet) long (Figure
2-21) (38). Soil samples were taken ahead of the drill bit
to reduce sample temperature and analyzed in the field
for metals. (Chromium was the primary contaminant.) A
pipe 15 centimeters (6 inches) in diameter was driven to
create the wellbore. Then, a 10-centimeter (4-inch) poly-
vinyl chloride (PVC) casing and screen was inserted as
the pipe was jacked out. An inverted plastic sleeve was
inflated in the screen to obtain vapor samples.
Chemical Waste Landfill /
Figure 2-21. Inclined wellbore created with resonant sonic drill-
ing, Sandia National Laboratories (38).
Similar studies have been conducted at the DOE Han-
ford site, where approximately five wellbores were cre-
ated for sampling and one inclined wellbore was
completed for soil vapor extraction by 1993 (12). The
"Case Histories" section that follows describes the soil
vapor extraction well. Cobble-rich sediments at Hanford
present extremely challenging conditions for drilling and
sampling, and the success of sonic drilling at this loca-
tion is a powerful testament to the capabilities of this
application.
Using sonic techniques to drill three inclined wellbores,
continuous cores were recovered at Newark Air Force
Base (AFB), Newark, Ohio (39). The wellbores were
drilled under a building at 45 degrees to a VD of approxi-
mately 5.5 meters (18 feet). The wellbores readily pene-
trated a reinforced concrete beam.
2.4 Case Histories
This section overviews known applications, discusses
average costs of previous projects, and describes four
selected applications of horizontal or inclined wells.
2.4.1 Overview
As of the summer of 1993, there were more than 100
applications of horizontal wells at 30 contaminated sites
(17). Approximately one-quarter of the wells strictly ex-
tracted ground water, one-quarter extracted vapor, and
the remaining half was used for a variety of techniques
including free-product recovery, sparging, sampling, or
combinations of two recovery techniques (Table 2-6).
The majority of the wells were created at depths of less
than 8 meters (26.2 feet). This partially reflects the
tendency to find more contaminants at shallow depths
than at great depths. The depths of the majority of these
Table 2-6. Summary of Applications of Horizontal Wells (28)
Vertical
Depths
<8 m
8-30 m
>30 m
Totals
Ground-
Water
Extraction
Wells
13
9
4
26
Soil
Vapor
Extraction
Wells
18
4
3
25
Other
Wells
52
1
2
55
Totals
83
14
9
106
28
-------
wells also reflect the lower limit of electronic beacon
locating systems. The electronic beacon, or "walkover,"
method currently is significantly less expensive than any
alternative. Therefore, the cost of creating wells in-
creases abruptly if they are deeper than 8 meters (26.2
feet).
The measured depth, or total length, of horizontal wells
created for environmental purposes generally ranges
from 10 to 670 meters (32.8 to 2,197 feet), but the
majority of the wells are in the range of 30 to 150 meters
(98.4 to 492 feet) (Figure 2-22). Like the range of
depths, the range of well lengths probably also results
from both technical and economic factors. Wellbores
150 meters (492 feet) long can be drilled with relatively
small, compaction-type rigs and can be completed with
pull-back techniques. Accordingly, this type of well is
typically less expensive than those created with more
sophisticated rigs and completion methods.
20
15
10
-------
Table 2-7. Sites Where Horizontal Wells Have Been Used for Ground-Water Recovery
Reported
Contaminant
Type/Phase
TCE/APL and
DNAPL
TCE/APL and
DNAPL
Halogenated
hydrocarbons/
APL and DNAPL
TCE/APL and
DNAPL
TCE/APL and
DNAPL
TCE/APL and
DNAPL
Petroleum
hydrocarbons/
APL and LNAPL
Petroleum
hydrocarbons/
APL and LNAPL
Petroleum
hydrocarbons/NR
Petroleum
hydrocarbons/NR
JP-4jetfuel/APL
and LNAPL
Leaking
underground
storage tank/APL
and LNAPL
Petroleum
hydrocarbons/NR
Petroleum
hydrocarbons/NR
Lime Lake
leachate/APL
TCE/NR
TCE/NR
Petroleum
hydrocarbons/NR
Petroleum
hydrocarbons/NR
Site
Taylor, Ml
Savannah River
Site, Aiken, SC
Geismar, LA
Union City, CA
San Francisco
Bay area, CA
Tinker AFB, OK
Sacramento, CA
Carson City, NV
Fullerton, CA
Beaumont, TX
Williams AFB,
AZ
Houston, TX
Geismar, LA
Geismar, LA
Barberton, OH
Savannah River
Site, Aiken, SC
Savannah River
Site, Aiken, SC
Los Angeles,
CA
Norwalk, CA
Date
1987
June 1991
August
1992
July 1992
1992
November
1992 to
February
1993
December
1991
April 1992
April 1992
July 1992
July 1992
September
1992
November
1992
February
1993
February
1993
May 1991
November
1992
February
to March
1992
August to
September
1992
Number
and
Type of
Wells
2
1
2
1
1
5
1
1
1
2
2
1
1
2
9
1
2
2
2
GWE
GWE
GWE
GWE
GWE
GWE
GWE
GWE
GWE
GWE
GWE
GWE
GWE
GWE
GWE
SVE
SVE
SVE
SVE
Site Geology
Unconsolidated
sediments
Fine silty sand,
silt, and clay
Sand and clay
Fine silty sand
and clay
Sand, sand
and gravel,
intermittent clay
Fine sand, silt,
and clay with
shale and
sandstone units
Sandy clay
Unconsolidated
channel sands
Silt, clay, and
intermittent fine
sand
Silty sand with
clay stringers
Fine sand and
silt clay with
gravel and
cobbles
Clayey soil
Sand and clay
Clay and sand
Clay and sand-
size tailings
Fine silty sand,
silt, and clay
Fine sand, silt,
and clay
Unconsolidated
silty sand
Silty sand and
sandy silt
Type of
Wellbore
Blind
Blind
Blind
Blind
Blind
Blind
Blind
Blind
Blind
Blind
Continuous
Blind
Blind
Blind
Blind
Blind
Blind
Blind
Blind
Measured
Depth/
Horizontal
Length
(Feet)
550/
488/258
435/363
469/400
1 00/70
200/80
920/200
362/100
250/1 00
460/325
/300
2,200/500
320/
185/
295/
320/
750/
300/1 52
725/420
720/420
600/400
360/
320/
Vertical
Depth
(Feet) Well Materials
<25
152
14
16
15
25
110
150
3x35
50
25
110
25
235
35
18
36
20
40
105
110
45
85
25
7 in. ID
6 in. ID
6 in. ID,
HOPE;
4 in. ID,
SS filter
Sin. ID,
PVC
Sin. ID,
SS
6 in. ID,
HOPE
4 in. ID,
SS filter
3 in. ID,
PVC
Sin. ID,
PVC
6 in. ID,
HOPE
SS
HOPE
20-slot
12-slot
pack
1 0-slot
1 0-slot
20-slot
12-slot
pack
20-slot
20-slot
20-slot
6 in. ID 20-slot
HOPE;
4 in. ID HOPE
filter pack
6 in. ID
prepack
4 in. ID,
PVC
6 in. ID,
HOPE
SS
1 0-slot
20-slot
6 in. ID
Enviroscreen
4 in. ID,
HOPE
6 in. ID
Sin. ID,
PVC
Sin. ID,
PVC
4 in. ID,
HOPE
20-slot
HOPE
1 0-slot
1 0-slot
20-slot
Drilling
Contractor
Drilex
Eastman
Cherrington
Eastman
Cherrington
Eastman
Cherrington
Eastman
Cherrington
Eastman
Cherrington
Eastman
Cherrington
Eastman
Cherrington
Eastman
Cherrington
Eastman
Cherrington
Michels
Environmental
Eastman
Cherrington
Eastman
Cherrington
Eastman
Cherrington
Eastman
Cherrington
Eastman
Cherrington
Eastman
Cherrington
Eastman
Cherrington
Eastman
Cherrington
30
-------
Table 2-7. Sites Where Horizontal Wells Have Been Used for Ground-Water Recovery (continued)
Reported
Contaminant
Type/Phase Site
Date
Number
and
Type of Type of
Wells Site Geology Wellbore
Measured
Depth/
Horizontal Vertical
Length Depth Drilling
(Feet) (Feet) Well Materials Contractor
Petroleum
hydrocarbons/NR
Petroleum
hydrocarbons/NR
Hydrocarbons/NR
Hydrocarbons/NR
Hydrocarbons/NR
Orcutt, CA
Charlotte, NC
Las Vegas,
Las Vegas,
Cherry Hill,
NV
NV
NJ
October
1992
June 1993
1993
1993
1993
4SVE
2SVE
1 SVE
1 SVE
2 SVE
Sandy
Sandy
sandy
NR
NR
NR
soil
silt and
clay
Continuous
Continuous
Continuous
Continuous
Continuous
1 x 1207
3 x 707
150/100
120/110
4267
8407
/140
1 x 4
3x5
8
7
<5
<5
<5
2 in. ID, 10-slot
PVC
2 in. ID, 10-slot
PVC
4 in. ID HOPE
4 in. ID HOPE
2 in. ID HOPE
UTILX
UTILX
UTILX
UTILX
UTILX
APL = aqueous phase liquid; DNAPL = dense nonaqueous phase liquid; GWE = ground-water extraction;
LNAPL = light nonaqueous phase liquid; NR = not reported; SS = stainless steel; SVE = soil vapor extraction; TCE = trichloroethylene
Table 2-8. Sites Where Horizontal Wells Have Been Used for Vapor Extraction
Reported
Contaminant
Type
TCE
TCE
Petroleum
hydrocarbons
Petroleum
hydrocarbons
Petroleum
hydrocarbons
Petroleum
hydrocarbons
Hydrocarbons
Hydrocarbons
Hydrocarbons
Site
Savannah
River Site,
Aiken, SC
Savannah
River Site,
Aiken, SC
Los Angeles,
CA
Norwalk, CA
Orcutt, CA
Charlotte, NC
Las Vegas,
NV
Las Vegas,
NV
Cherry Hill,
NJ
Date
May 1991
November
1992
February
to March
1992
August to
September
1992
October
1992
June 1993
1993
1993
1993
Number
and
Type of
Wells
1 SVE
2 SVE
2 SVE
2 SVE
4 SVE
2 SVE
1 SVE
1 SVE
2 SVE
Site Geology
Fine silty sand,
silt, and clay
Fine sand, silt,
and clay
Unconsolidated
silty sand
Silty sand, and
sandy silt
Sandy soil
Sandy silt and
sandy clay
NR
NR
NR
Measured
Depth/
Horizontal
Type of Length
Wellbore (Feet)
Blind 300/152
Blind 725/420
720/420
Blind 600/400
Blind 360/
320/
Continuous 1 x 120/
3 x 707
Continuous 150/100
120/110
Continuous 426/
Continuous 840/
Continuous /140
Vertical
Depth
(Feet) Well Materials
105
110
45
85
25
1 x 4
3x5
8
7
<5
<5
<5
6 in. ID HOPE
Sin. ID,
10-slot PVC
3 in. ID,
10-slot PVC
4 in. ID,
20-slot HOPE
2 in. ID,
10-slot PVC
2 in. ID,
1 0-slot PVC
4 in. ID HOPE
4 in. ID HOPE
2 in. ID HOPE
Drilling
Contractor
Eastman
Cherrington
Eastman
Cherrington
Eastman
Cherrington
Eastman
Cherrington
UTILX
UTILX
UTILX
UTILX
UTILX
APL = aqueous phase liquid; DNAPL = dense
LNAPL = light nonaqueous phase liquid; NR =
nonaqueous phase liquid; GWE = ground-water extraction;
not reported; SVE = soil vapor extraction; TCE = trichloroethylene
31
-------
Table 2-9. Sites Where Horizontal Wells Have Been Used for Assorted Environmental Applications
Measured
Depth/
Horizontal Vertical
Purpose Site
Characterization, Kirtland
monitoring at AFB, NM
landfill
Bioventing-diesel Cincinnati,
OH
Ground water Seattle, WA
and petroleum
recovery-LNAPL
Product recovery- Ponca City,
petro-hydro OK
Date
May 1993
July 1993
June 1993
1988
Number
of Wells Site Geology
1 Sand, gravel,
and cobbles
2 Fluvial
sediments
1 Medium to
coarse sand
7 Fine sand, silt,
and clay
Type of
Wellbore
Continuous
Continuous
Continuous
Continuous
Length
(Feet)
400/
1 90/1 40
320/1 80
980/
Depth
(Feet)
40
10
13
17
Well Materials
None
2 in. ID,
20-slot PVC
4 in. ID,
20-slot
prepack PVC
4 in. ID
slotted PVC
Drilling
Contractor
Charles
Machine
Works Ditch
Witch
University of
Cincinnati
UTILX
UTILX
APL = aqueous phase liquid; DNAPL = dense nonaqueous phase liquid; GWE = ground-water extraction;
LNAPL = light nonaqueous phase liquid; SVE = soil vapor extraction; TCE = trichloroethylene
Table 2-10. Sites Where Horizontal Wells Have Been Used for Several Purposes
Measured
Purposes
In situ stripping of
TCE -contam-
inated soil and
ground water
Air injection, SVE,
bioactive barrier
hydrocarbon
Dual well system
for SVE and GWE
with TCE in soil
and ground water
Radio frequency
heating/SVE of
TCE in soil
Dual well system
for SVE and GWE
of TCE-contam-
inated soil and
ground water
Washing soils
contaminated with
gasoline
Air injection, SVE,
and gasoline
remediation
system for
underground
storage tank
Site
Savannah
River Site,
Aiken, SC
Ponca
City, OK
Tinker
AFB, OK
Savannah
River Site,
Aiken, SC
South San
Francisco
Bay Area,
CA
Minden, NV
Del City,
OK
Date
October
1988
September
1988
November
1991
September
1992
October
1992
October
1992
October
1992
Number
and Type
of Wells
1 SVE
1 injection
3 SVE/
bioinjection
1 SVE
1 GWE
and control
1 SVE
1 SVE
1 GWE
22
infiltration
galleries
2 recovery
4 recovery
3 GWE/
SVE/
injection
Depth/
Horizontal Vertical
Site Geology
Fine silty sand,
silt, and clay
Unsaturated
fine silty sand,
silt, and clay
Fine silty sand,
silt, and clay
Fine silty sand,
silt, and clay
Sand, gravel,
and
intermittent clay
Sandy clay
with gravel
Fine silty sand,
silt, and clay
Type of
Wellbore
Blind
Blind
Continuous
Blind
Blind
Continuous
Blind
Blind
Blind
Blind
Blind
Continuous
Length
(Feet)
290/200
485/300
MOO
270/70
270/70
570/230
210/110
210/110
20/
35 and
65/
135/
300/
Depth
(Feet)
75
155
20
15
25
40
18
20
3
3
3
12
Well Materials
4 in. ID,
10-slot SS
screen;
2 in. ID steel
perforated pipe
2-4 in. ID,
20-slot PVC
6 in. ID
prepack SS
6 in. ID
prepack SS
4 in. ID,
10-slot
fiberglass
3 in. ID,
20-slot HOPE;
Sin. ID,
20-slot SS
2 in. ID PVC
4 in. ID PVC
2 in. ID PVC
2 in. ID,
20-slot PVC
Drilling
Contractor
Eastman
Cherrington
S&S Harris
Michels
Environmental
Charles
Machine
Works
Eastman
Cherrington
UTILX
S&S Harris
32
-------
Table 2-10. Sites Where Horizontal Wells Have Been Used for Several Purposes (continued)
Purposes Site
Dual well system Cincinnati,
for diesel recovery OH
and GWE and
ground-water
control
Number
and Type
Date of Wells Site Geology
December 1 product Fluvial
1992 recovery sediments
1 GWE
and control
Measured
Depth/
Horizontal
Type of Length
Wellbore (Feet)
Continuous 297/
Continuous 325/
Vertical
Depth
(Feet)
19
21
Drilling
Well Materials Contractor
4 in. ID PVC Underground
Research, Inc.
4 in. ID PVC
In situ stripping of Fallen, NV December 3 SVE
soil and ground 1992
water contam- 4 air
inated by injection
underground
storage tank
Silty sand Continuous 170/ 4 2 in. ID,
1707 5 10-slot PVC
2807 4
Continuous 1507 14 2 in. ID,
1807 10-slot PVC
2407
2807
UTILX
Air injection, SVE, Moore, OK March
and gasoline 1993
remediation
system for
underground
storage tank
Dual wall system Portland, April 1993
for SVE and GWE OR
of contaminated
soil and ground
water by
underground
storage tank
4 GWE/ Fine silty sand, Continuous 5007 12
SVE/ silt, and clay
injection
1 SVE Silt Continuous 475/300 6.5
1 GWE Continuous 475/300 23
and control
4 in. ID,
20-slot PVC
4 in. ID,
20-slot PVC
4 in. ID,
20-slot PVC
prepack
screen
S&S Harris
UTILX
APL = aqueous phase liquid; DNAPL = dense nonaqueous phase liquid; GWE = ground-water extraction;
LNAPL = light nonaqueous phase liquid; SS = stainless steel; SVE = soil vapor extraction; TCE = trichloroethylene
significantly less costly than the ones detailed here,
were unavailable at the time of writing. Each discussion
includes the following information:
Location of site, date of study, contact for the con-
tracting agency, and contact for the drilling contractor.
Remediation objective.
Geology of site as related to drilling operations.
Design, description of well trajectory, completion
technique, intended purpose, and well performance
(if available).
Results, discharge, contaminant concentration with
time, area where drawdown is affected, comparison
to model, and cost of project (if available).
2.4.3.1 Ground-Water Recovery, Geismar,
Louisiana
Location. Two ground-water extraction wells were in-
stalled in Geismar during August 1992 at an abandoned
herbicide manufacturing plant (37).
Objectives. The wells were installed as components of
a pump and treat ground-water extraction system. The
system was designed to remediate ground water con-
taminated with ethylene dichloride (EDC) and mono-
chlorobenzene (MCB) beneath the plant. There were
two ground-water contaminant plumes: the south plume
had an area of approximately 1,486 square meters
(16,000 square feet), with MCB concentrations of 100 to
200 ppm and EDC concentrations of 10 to 100 ppm; the
north plume had an area of approximately 557 square
meters (6,000 square feet), with MCB concentrations of
100 to 370 ppm and EDC concentrations of 10 to 70
ppm. The conceptual plan was to install two horizontal
wells with screens 122 meters (400 feet) long. The goal
was to install each horizontal well on top of a clay layer
along the base of the affected zone, parallel and through
the longitudinal axis of each plume.
Site Conditions. The two wells, H-50 and H-51, were
installed beneath an abandoned wastewater pipeline
(south plume) and a product loading area (north plume),
respectively. The contaminated portion of the aquifer
consisted of Holocene silty clay.
The wellbore path for H-50 was designed to run west to
east through the principal area of contamination. The
wellbore path had to run through a narrow corridor of
vertical subsurface drill shafts and pilings that supported
an overhead steel superstructure. The corridor was
33
-------
6.1 meters (20 feet) wide, allowing a maximum of 2.4
meters (8 feet) clearance to the south and 3.7 meters
(12 feet) clearance to the north. The abandoned waste-
water pipe ran about 1.5 meters (5 feet) above the
proposed well path.
The wellbore path for H-51 was adjacent to an active
railcar loading area and beneath a concrete road. The
H-51 wellbore was allowed a 0.6-meter (2-foot) vertical
and a 2-degree horizontal deviation.
Well Geometry. The well geometry for the two horizontal
wells is presented in Table 2-11.
Table 2-11. Specifications of Horizontal Wells, Geismar,
Louisiana
H-50
H-51
Entry Angle
Kickoff Point
End-of-Curve
Horizontal Length
15° to Horizontal
19ftMD/5ft VD
59ftMD/12ft VD
459 ft
15° to Horizontal
19ftMD/5ft VD
67ftMD/14ftVD
467 ft
Drilling Method. The drill rig used for this job was rated
to 610 meters (2,000 feet) for vertical drilling, with a
hydraulic drill stem hoist that had a 311-kilonewton (kN)
(70,000-pound) hoist and a 133-kN (30,000-pound) pull-
back capacity. The drill rig mast could incline from verti-
cal to 15 degrees to horizontal.
This application used two different downhole assem-
blies. In the curved section of the wellbore, the downhole
assembly consisted of a 30.5-meter (100-foot) radius,
and a 17.1-centimeter (6.75-inch) diameter RDM mud
motor with a 31.1-centimeter (12.25-inch) expandable
bit. Wellbore surveying was done with a tool face indi-
cator, which provided inclination and tool face orienta-
tion but did not provide wellbore azimuth. Data from the
surveying system were communicated to the ground
surface by creating pressure pulses in the drilling mud.
When drilling neared the end of the curve, a film-based
multishot magnetic survey tool confirmed the tool face
indicator readings and the wellbore location. (The tool
face indicator and the multishot magnetic survey tools
are not commonly used in environmental directional
drilling.) In the horizontal section of the wellbore, the
downhole assembly consisted of a 91.4-meter (300-
foot) radius and a 12.1-centimeter (4.75-inch) diameter
RDM mud motor with a 22.2-centimeter (8.75-inch)
expandable bit. Here, wellbore surveying was accom-
plished with a wireline magnetometer-accelerometer lo-
cator and an electromagnetic secondary survey system.
The wellbores were drilled and the curved section of the
wellbore was cased with a 10-inch HOPE casing. The
horizontal section of the wellbore was left open for the
well installation. The open hole method was possible
because the formation had a high clay content. In addi-
tion, a mixed metal hydroxide (MMH) additive was used
with bentonite in the water-based drilling fluid. MMH is
an insoluble crystalline inorganic compound containing
two or more metals in a unique hydroxide lattice. The
bentonite/MMH mixture extended the gel strength of the
drilling fluid and allowed suspension of soil cuttings
indefinitely when the drilling fluid circulation was halted.
Well Materials. The wells were constructed of 6-inch
internal diameter (ID), 0.5-millimeter (0.02-inch) slotted
HOPE with a 4-inch ID stainless steel prepack inside.
Well Installation. The well materials were pushed into
the open wellbore. The horizontal section of the wellbore
was relatively clear of cuttings, so the well materials did
not experience extra friction during well installation.
Each well was wash developed before the drill rig was
removed from the drill site. Drilling fluids were displaced
with potable water and pumped into tanks for treatment
and disposal. The drill crew washed the HOPE liner with
a wash tool to remove the filter cake and the fine-grained
sediments. Water discharging from the wells became
relatively free of sediments after 6 to 8 hours of flushing
with water.
Field Effort. Well H-50 required 17 days of construction,
11 days longer than anticipated. A mechanical problem
with the steering mechanism on the downhole mud
motor resulted in two aborted curved section attempts
before the problem was recognized and corrected.
There also was one aborted attempt to drill the horizon-
tal section due to overcompensating while controlling
the elevation of the wellbore. The well was completed
with a 110-meter (363-foot) horizontal well screen (as
opposed to the 122-meter (400-foot) planned length).
Most of the wellbore was steered along the specified
trajectory, except a 12.5 meter (41-foot) section that fell
0.3 meters (1 foot) below the specified trajectory. The
wellbore was successfully steered through the narrow
corridor and away from the vertical pilings and monitor
well. Soil cores were also retrieved successfully.
H-51 was constructed in 6 days. The completed well had
122 meters (400 feet) of well screen, as planned. Most
of the wellbore was placed as specified, although
a 21.6-meter (71-foot) section lay 0.3 meters (1 foot)
below the maximum specified depth of 4.9 meters
(16 feet), according to the surveys performed after
completion.
Results of Tests Performed on Wells. Three steps were
required to determine the pumping performance of each
well:
1. The well was pumped for 8 hours to evaluate pump
size and flow rates.
34
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2. The well pump control was manually operated to
obtain the optimum settings for the continuous auto-
matic pumping operation.
3. The well was placed in continuous automatic opera-
tion.
Each well elicited different pumping results because of
the heterogeneity of the sediments' hydraulic properties.
The results for H-50 were:
A long-term pumping rate of 4.9 liters (1.3 gallons)
per minute.
An area of influence that spanned 48.8 meters (160
feet) on either side of the well.
An estimated one pore volume of the well capture
zone that pumped once every 149 days.
The results for H-51 were:
1. A long-term pumping rate of 22.3 liters (5.9 gallons)
per minute.
2. An area of influence that spanned 21.3 meters (70
feet) on either side of the well.
3. An estimated one pore volume of the well capture
zone that was pumped once every 15 days.
Current Status. The wells are pumping as part of a
full-scale remediation system.
Cosf.The wells cost approximately $400,000 each.
2.4.3.2 Ground-Water Recovery, Barberton, Ohio
The horizontal well project in Barberton exemplifies a
multiple-well installation. A pilot well was installed to
characterize drilling conditions, determine sediment hy-
draulic properties, and test the well screen and filter
pack design. The pilot well installation provided impor-
tant information that was used to optimize the design of
the remaining eight horizontal wells (40).
Location. One pilot and eight production ground-water
extraction wells were installed in Barberton during 1993.
Objectives. The wells were installed to collect leachate
seeping from the base of two Solvay Process waste
impoundments (lime lakes) into adjacent streams. The
leachate had a pH of 12 to 13 and contained various
hazardous constituents, including chlorinated hydrocar-
bons, asbestos, and lead (40).
Site Conditions. The lime lakes, Lime Lake 1 and 2, are
located within a large chemical manufacturing facility
that includes several portions of major waterways. There
are two streams that flow along the north, east, and
south boundaries of Lime Lake 1 and the north and
northeast boundaries of Lime Lake 2. The lakes are
about 12.2 meters (40 feet) above local grade and con-
tain clay- to sand-sized lime spoils that are 12.2 to 15.2
meters (40 to 50 feet) thick and rest upon a hard,
chemically altered layer of native soils. The hard layer
acts as an aquitard and causes the leachate to form a
mound within the lime lakes. A thick slaker sand and
cinder cover was spread across the surface of the lakes
to prevent surface erosion (40).
Well Geometry. The pilot well was constructed in a blind
wellbore with a 17-degree to horizontal entry angle,
drilled with a 183-meter (600-foot) radius of curvature,
231-meter (757-foot) MD, 152-meter (500-foot) horizon-
tal length, and 11.6-meter (38-foot) VD. The geometry
of the eight horizontal wells resembled that of the pilot
well. Two wells had the horizontal section shortened to
91.4 meters (300 feet) due to a rapid change in the
elevation of the hard layer (40).
Drilling Method. The pilot well was drilled with a medium-
sized utility-type drilling rig equipped with a hydraulic
spud jet drilling tool, a mud motor to drill through hard
layers, and a magnetometer-accelerometer steering
tool. A washover pipe enlarged the wellbore. A guar
gum, water-based drilling fluid was used during drilling.
The eight production wells were drilled in a similar man-
ner, except for modifications based on the experience
gained from drilling the pilot well. The modifications
included additives mixed to the guar gum drilling fluid to
improve its ability to clean the wellbore and maintain
wellbore integrity and a reduced rate of penetration
during drilling.
Well Materials. The pilot well was constructed of 10-
centimeter (4-inch) ID, 0.5-millimeter (0.02-inch) slotted
HOPE casing, and a 20/40 sand filter pack. The other
eight wells were constructed of 15-centimeter (6-inch)
ID HOPE, 0.5-millimeter (0.02-inch) slotted HOPE cas-
ing, and no filter pack. The decision not to include a filter
pack in the eight production wells was based on the
assumption that it would become encrusted and clogged
with carbonate minerals. The fine materials that did
enter the pilot well were attributed to over-pumping and
not the well design. The decision to omit the filter pack
in the production wells proved correct because fine-
grained materials did not enter the wells during well
development and testing. An electrical centrifugal pump
was used in each well.
Well Installation Method. The well materials for the pilot
well were pushed into the washover pipe. The filter pack
was installed around the well screen with a "sand shoe"
as the washover pipe was withdrawn from the wellbore.
Installation of the remaining wells was similar except
that they did not have a filter pack.
Field Effort. The pilot well installation required two at-
tempts. Some problems occurred while constructing the
pilot well:
Drilling fluid and lime spoils fractured to the surface.
35
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The hydraulic spud jet met refusal at several thin,
hard layers.
The washover pipe became stuck twice and was re-
trieved by pulling on it with bulldozers.
These problems were overcome by changing the drilling
fluid rheology, slowing the rate of penetration of the drill
stem, and using a downhole mud motor to drill through
the hard layers.
Results of Tests Performed on Well. The pilot well pro-
duced 6.1 liters (1.6 gallons) per minute after 45 days of
pumping. The eight production wells produced 15.1 li-
ters (4 gallons) per minute on an intermittent pumping
schedule that was determined for each well. The rate of
15.1 liters (4 gallons) per minute was selected to provide
a flow velocity that would keep the pump discharge line
clear of solids and minimize the infiltration of fine mate-
rials into the wells.
Current Status. The wells currently are being used to
collect the lime lake leachate.
Cost. The total cost of the nine-well system has not been
released.
2.4.3.3 High-Angle Inclined Boreholes, Newark
AFB, Ohio
Location. Three high-angle boreholes were drilled at
Newark AFB in Newark during 1993.
Objectives. The purpose of the boreholes was to obtain
continuous core samples beneath Building 4. The soil
beneath the building was believed to be contaminated
with chlorofluorocarbons.
Site Conditions. The soil was silty clay with some sand
and gravel.
Well Geometry. Three boreholes were drilled, each at a
45-degree angle. The boreholes reached vertical depths
of 5.2 to 5.5 meters (17 to 18 feet).
Drilling Method. The wellbores were drilled using the
rotosonic technique described earlier with the drill rig
mast at a 45-degree angle.
Field Effort. The wellbores successfully penetrated a
reinforced concrete beam.
Cost. The three boreholes were drilled at a total cost of
$7,530.
2.4.3.4 Inclined Well for Soil Vapor Extraction
Well, Hanford Site, Washington
Location. The DOE (through the Westinghouse Hanford
Company and Pacific Northwest Laboratory) and Water
Development Corporation formed a Cooperative Re-
search and Development Agreement (CRADA) for the
purpose of developing a resonant sonic drilling method
that can meet the rigorous drilling conditions found at
the DOE's Hanford site in southeast Washington. Phase
I of the research program included installing a 45-degree
angle soil vapor extraction well at the 200 West Area
Carbon Tetrachloride Expedited Response Reaction site
(200 West area) at the Hanford site (12).
Objectives. The purpose of the inclined well was to
demonstrate the ability of the resonant sonic drill rig to
create an angled wellbore, and to install a soil vapor
extraction well at the 200 West area.
Site Conditions. The upper geologic unit of the Hanford
formation beneath the 200 West area consists of two
fades: coarse-grained sand and granule-to-boulder
gravel from which matrix is commonly lacking; and fine-
to coarse-grained sand and silt that commonly display
normally graded rhythmites from an inch to several
inches. In general, the coarse fades is 5-percent boul-
der and ranges in thickness from 6.1 meters (20 feet) to
greater than 61 meters (200 feet). The underlying fine
fades consists of 1.5 to 18.3 meters (5 to 60 feet) of silts
and fine sands, which in turn overlay sediments of Plio-
Pleistocene Ringold formation (12).
Well Geometry. The wellbore was drilled at a 45-degree
angle to a 36.3-meter (119-foot) VD and a 51.2-meter
(168-foot) MD.
Drilling Method. The wellbore was drilled using the reso-
nant sonic technique with a 300-horsepower drill head
and the drill rig mast at a 45-degree angle.
Well Installation Method. The well materials were
pushed into the wellbore.
Well Materials. The well materials consisted of a 7.6-
centimeter (3-inch) ID stainless steel screen and riser.
Field Effort. The wellbore was drilled and the well was
installed in 9 days.
Results of Tests Performed on Well. The inclined well
yielded satisfactory flow rates compared to an adjacent
soil vapor extraction well drilled with a cable tool drill rig.
Current Status. The well will be used to remediate the
vadose zone at the 200 West area.
Cost. No cost analysis was performed on this well.
Phase II of the CRADA will include cost analysis of the
resonant sonic drilling method.
2.5 References
When an NTIS number is cited in a reference, that
document is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
703-487-4650
36
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1. Hantush, M.S., and I.S. Papadapulous. 1962. Flow
of ground water to collector wells. Proceedings
of the American Society of Civil Engineers.
88(HY5):221-244.
2. Losonsky, G., and M.S. Beljin. 1992. Horizontal
wells in subsurface remediation. Hazardous
Materials Control Conference, New Orleans, LA
(February).
3. Mukherjee, H., and M.J. Economides. 1991. A para-
metric comparison of horizontal and vertical well
performance. Society of Petroleum Engineers
(June), pp. 209-216.
4. Dickinson, W.W., R.W. Dickinson, J. Nees, and E.
Dickinson. 1991. Field production results with the
ultrashort radius radial system in unconsolidated
sandstone formations. Presented at the 5th UNI-
TARJUNDP International Conference on Heavy
Crude and Tar Sands, Caracas, Venezuela
(August).
5. Dickinson, W.W., R.W. Dickinson, PA. Mote, and
J.S. Nelson. 1987. Horizontal radials for geophysics
and hazardous waste remediation. In: Superfund
'87. Silver Spring, MD. Hazardous Waste Control
Research Institute, pp. 371-375.
6. Howsam, P., and R. Hollamby. 1990. Drilling fluid
invasion and permeability impairment in granular
formations. Quarterly J. Engin. Geol. (London) 23:
161-168.
7. Hayatdavoudi, A.Z. 1994. Personal communication
from drilling fluid specialist, Eastman Cherrington
Environmental Corporation, to D. Wilson, Inde-
pendent Environmental Consultants, LLC (Febru-
ary).
8. Westmoreland, J. 1994. Evaluation of an air drilling
cutting containment system. Sandia Report
SAND94-0214 UC-721 (April). Albuquerque, NM:
Sandia National Laboratories.
9. Simon, R., and G. Cooper. 1994. Use of cryogenic
fluids for environmental drilling in unconsolidated
formations. In: Vozniak, J.P, ed. Drilling technology.
New York, NY: American Society of Mechanical En-
gineers. PD-Vol. 56: pp. 199-207.
10. Barrow, J.C. 1994. The resonant sonic drilling
method: An innovative technology for environmental
restoration programs. Ground Water Monitor. Rev.
14(2):153-161.
11. Dance, D.R., and N.W Beattie. 1981. Sonic drilling.
Northwest Mining Association Fall Short Course (no
address available).
12. Westinghouse Hanford Co. 1994. Phase I: Reso-
nant sonic CRADA report. WHC-SD-EN-TRP-007.
Richland, WA: Westinghouse Hanford Company.
13. Westinghouse Savannah River Co. 1993. Sum-
mary report of the drilling technologies tested at the
integrated demonstration site. WSRC-TR-93-565.
Aiken, SC: Westinghouse Savannah River Co.
14. Ta Inglis, T.A. 1987. Directional drilling, petroleum
engineering, and development studies, Vol. 2. Bos-
ton, MA: Graham and Trotman.
15. Wu, J., and H. Juvkan-Wold. 1994. The effect of
wellbore curvature on tubular buckling and lockup,
drilling technology. In: Vozniak, J.P, ed. New York,
NY: American Society of Mechanical Engineers.
PD-Vol. 56. pp. 17-25.
16. Driscoll, F.G. 1986. Ground water and wells, 2nd
ed. St. Paul, MN: Johnson Division Publisher.
17. Wilson, D.D., and D.S. Kaback. 1993. Industry sur-
vey for horizontal wells. Final report. WSRC-TR-93-
511. Aiken, SC: Westinghouse Savannah River
Company.
18. Joshi, S.A. 1991. Horizontal well technology. Tulsa,
OK: PennWell Books.
19. Murdoch, L.C. 1994. Transient analysis of an ide-
alized interceptor trench. Water Resour. Res. (in
press).
20. Bear, J. 1979. Hydraulics of ground water. New
York, NY: McGraw-Hill.
21. Karlsson, H., G. Losonsky, and G.E. Jacques.
1992. Horizontal wellbore completions for aquifer
restoration and their economics. Proceedings of the
10th Annual Hazardous Materials and Environ-
mental Management Conference, Atlantic City, NJ
(June), pp. 154-181.
22. Muskat, M. 1937. The flow of homogenous fluids
through porous media. Ann Arbor, Ml: J.W Ed-
wards, Inc.
23. Streltsova, T.D. 1988. Well testing in heterogene-
ous formations. New York, NY: John Wiley and
Sons.
24. Murdoch, L.C., and J. Franco. 1994. The analysis
of constant drawdown wells using instantaneous
source functions. Water Resour. Res. 30(1):117-
124.
25. U.S. EPA. 1991. The feasibility of hydraulic fractur-
ing of soil to improve remedial actions. Final report.
EPA/600/2-91/012 (NTIS PB91-181818).
26. Dikken, B.J. 1990. Pressure drop in horizontal wells
and its effect on production performance. JPT
Nov:1,426-1,433.
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27. Tarshish, M. 1992. Combined mathematical model
of flow in an aquifer-horizontal well system. Ground
Water 30(6):931-935.
28. Wilson, D.D., and D.S. Kaback. 1993. Results of a
survey of horizontal environmental well installa-
tions. In: Vozniak, J.P., ed. Drilling technology 1994.
New York, NY: American Society of Mechanical En-
gineers. PD-Vol. 56. pp. 193-199.
29. U.S. EPA. 1993. Guidance for evaluating the tech-
nical impracticability of ground-water restoration.
EPA/540/R-93/080.
30. U.S. EPA. 1991. Soil vapor extraction technology
handbook. EPA/540/2-92/003.
31. Cohen, R.M., J.W Mercer, and J. Matthews. 1993.
DNAPL site evaluation. Ada, OK: U.S. Environ-
mental Protection Agency
32. American Petroleum Institute. 1989. A guide to the
assessment and remediation of underground petro-
leum releases. API Publication 1628. Washington,
DC: American Petroleum Institute.
33. Sale, T.C., and D. Applegate. 1994. Oil recovery at
a former wood treating facility. Proceedings of the
Innovative Solutions for Contaminated Site Man-
agement, Water Environment Federation, Miami,
FL.
34. Downs, C. 1993. Personal communication from
Conoco Corporation to D. Wilson, Independent En-
vironmental Consultants, LLC (June).
35. Looney, B.B., T. Hazen, D. Kaback, and C. Eddy.
1991. Full-scale field test of the in situ air stripping
process at the Savannah River Integrated Demon-
stration Test Site. WSRC-RD-91-22. Aiken, SC:
Westinghouse Savannah River Co.
36. Wemple, R. 1994. Personal communication from
drilling specialist, Sandia National Laboratories, to
D. Wilson, Independent Environmental Consult-
ants, LLC (April).
37. Conger, R.M., and K. Trichel. 1993. Aground-water
pumping application for remediation of a chlorinated
hydrocarbon plume with horizontal well technology.
In: Proceedings of the Seventh National Outdoor
Action Conference and Exposition, Las Vegas, NV.
Dublin, OH: National Ground Water Association, pp.
47-61.
38. Sandia National Laboratories. 1993. Sonic drilling
rig hits heavy metal at closed landfill. Lab News
45(10). Albuquerque, NM: Sandia National Labora-
tories.
39. Clough, R. 1994. Personal communication from Al-
liance Environmental, Inc., to D. Wilson, Inde-
pendent Environmental Consultants, LLC (March).
40. Gaillot, G. 1994. Design and installation of an in situ
leachate collection system utilizing horizontal wells
within two former waste impoundments. Proceed-
ings of the Eighth National Outdoor Action Confer-
ence and Exposition, Minneapolis, MN (May 23-25).
Dublin, OH: National Ground Water Association (in
press)
38
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Chapter 3
Induced Fractures
More than 50 years ago, petroleum engineers recog-
nized that induced fractures could increase the produc-
tivity of oil wells. Within the past 5 years, EPA research
has shown that similar techniques can improve the per-
formance of environmental wells (1, 2). This chapter
focuses on the methods currently used for environ-
mental applications that primarily involve injection of
fluids to drive fracture.
Several fluid injection methods induce fractures using
fluid pressure to dilate a wellbore and open nearby
cracks. Once fluid pressure exceeds a critical value, a
fracture begins to propagate and continues to grow until
injection ceases, the fracture intersects a barrier (or the
ground surface), or the injected fluid leaks out through
the fracture walls. After injection is complete, fractures
are held open either by asperities on the fracture walls
(naturally propped fractures) or by permeable material,
or proppant, injected during propagation. The result is a
layer designed to be more permeable than the adjacent
formation.
The maximum dimension of fractures created by inject-
ing fluids is limited by either a tendency for the fracture
to climb and intersect the ground surface or by the loss
of fluid through the fracture walls. Therefore, all else
being equal, the maximum dimension increases with
increasing depth and decreasing permeability of forma-
tion. At a depth range of 1.5 to 5 meters (4.9 to 16.4 feet)
in overconsolidated silty clay, the typical maximum di-
mension of a fracture is approximately three times its
depth.
The primary application of induced fractures is to in-
crease the discharge of and the area affected by a well.
Field demonstrations conducted in rock or silty clay
have shown that, compared with a conventional well,
induced fractures can increase well discharge 10 to 50
times and increase the distance for detecting pressure
effects 10 times. According to theoretical modeling stud-
ies, these results are consistent with the expected
change in performance when adding a permeable layer
to the vicinity of a well.
Even though the improvement in performance is strik-
ing, induced fractures do not solve all the problems of
remediation in tight formations. The relative increase in
performance is greatest in the tightest formations, where
the performance of conventional wells is poorest. Ac-
cordingly, it may be possible to improve the rate of
recovery of a contaminant by an order of magnitude or
more and still have a rate that is less than desired for
timely closure. In tight formations, induced fractures
may improve the performance of hydraulic control and
containment at the site, or they can be combined with
other enhancement technologies, such as hot air injec-
tion (3), to accelerate recovery.
A secondary application of fracturing techniques is to
deliver solid compounds to the subsurface. This appli-
cation fills fractures with granular compounds that im-
prove the remedial process in various ways. For
example, injecting solid nutrients or slowly dissolving
oxygen sources can improve bioremediation, or inject-
ing electrically conductive compounds (e.g., graphite)
can improve electrokinetics.
Hydraulic (injecting air) and pneumatic (injecting liquid)
fracturing both are recognized methods of inducing frac-
tures for environmental applications. Both the mecha-
nisms and the results of the methods share some
striking similarities (3, 4), yet they each have distinc-
tions. Capabilities are evolving so rapidly, however, that
a direct comparison of hydraulic and pneumatic fractur-
ing methods is unnecessary. For this reason, this chap-
ter focuses on the characteristics and factors affecting
performance of induced fractures, and avoids direct ref-
erence to hydraulic or pneumatic fracturing except in the
description of techniques.
Most induced fractures that have been used for environ-
mental applications to date are shaped like gently dip-
ping disks, or in some cases they are slightly bowl
shaped. Vertical fractures have been used infrequently.
Therefore, this chapter focuses on gently dipping frac-
tures. Applications of induced vertical fractures hold
promise, however, particularly to improve the perform-
ance of horizontal wells.
3.1 Methods of Inducing Fractures
Methods used to induce fractures and improve the per-
formance of wells range from injecting fluid to detonat-
ing explosives. Many of the environmental applications
39
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involve fluid injection, although a few cases have involved
use of explosives to enhance permeability of bedrock and
improve contaminant recovery (5,6). The following section
focuses on methods that use fluid injection.
3.1.1 General Considerations
The fundamental process of inducing fractures by inject-
ing a fluid is straightforward: the fluid is injected into a
borehole until the pressure exceeds some critical value
and a fracture nucleates. This method can create frac-
tures in most naturally occurring materials, from rock to
unlithified sediment or soil. Once the fracture nucleates,
fluid continues to be injected, driving propagation away
from the borehole.
3.1.1.1 Injection Fluids
Three fluids have been used to create fractures for most
environmental applications: gas, water, and gelled
water. The major issues affecting choice of injection fluid
are:
The equipment required for injection.
Safety concerns.
The potential to mobilize contaminants or other injec-
tion restrictions.
The ability to transport solid grains into the fracture.
Air is the primary gas used to create fractures, although
other gases may have applications in specialized con-
ditions, such as when anaerobic conditions are required.
Air injection requires relatively simple equipment. Large
injection rates, however, demand special safety precau-
tions. There is relatively little possibility of mobilizing
liquid phases, although there is a strong possibility of
mobilizing vapor phases. Local regulations may govern
injection of air into the subsurface.
Fine-grained particles or powders can readily be trans-
ported into fractures by injecting air (7). The ability to
transport particles decreases with increasing grain size
and density, however, limiting capabilities of injecting
significant volumes of coarse-grained sand. This is a
topic of current research, and the development of spe-
cialized equipment and proppants promises to markedly
improve the transport of proppant during air injection.
Injecting water can create fractures in low-permeability
rock formations. Relatively simple pumps and packers
are used, although pressures in excess of 5 MPa may
be required to initiate the fracture. Safety precautions
relate to potentially high pressures. Injection of water is
restricted by regulations in some locations. Injecting
water will have limited effect on mobilization of vapors,
although it may mobilize liquids. In most cases, the
injected water and any fluids mobilized as a result of
injection should be readily recovered through the result-
ing fracture. Water can transport solid grains into a
fracture, although the best results are achieved using
plastic particles that have a density similar to water (8).
Guargum gel is a viscous fluid commonly used to create
fractures. Guar gum is a food additive derived from the
guar bean. Mixed with water, guar gum forms a short-
chain polymer with the consistency of mineral oil. Adding
a crosslinker causes the polymer chains to link and form
a thick gel capable of suspending high concentrations
of coarse-grain sand. This property makes guar gum gel
ideal for filling fractures with solid material. An enzyme
added to the gel breaks the polymer chains, allowing
recovery of the thinned fluid from the fracture.
Fracturing with guar gum gel requires several special-
ized pieces of equipment. A mixer is required to blend
the gel, crosslinker, and enzyme, as well as sand or
other solids. This method also requires a pump capable
of handling a slurry containing a high concentration of
sand grains. The safety precautions are similar to those
for pressurized water. Injecting guar gum gel negligibly
effects mobilization of vapors, although it may slightly
mobilize liquids after the gel breaks down. The fracture
confines the gel during injection, however, and prompt
recovery of the gel should eliminate interaction with pore
fluids. Areas that regulate subsurface injection may re-
strict injection of guar gum gel. Because in situ organ-
isms metabolize the organic components of guar gum
gel, its use is commonly avoided when fractures are
created to enhance discharge from a drinking water well.
The major benefit to using guar gum gel is the ability to
suspend a high concentration of coarse-grained sand
(1.2 to 1.8 kilograms of sand per liter of gel [10 to 15
pounds of sand per gallon of gel]) as a slurry in the gel.
3.1.1.2 Injection Pressure and Rate
The pressure required to initiate a fracture in a borehole
depends on several factors, including confining stress,
toughness of the enveloping formation, initial rate of
injection, size of incipient fractures, and pores or defects
in the borehole wall. In general, the injection pressure
increases with increasing depth, injection rate, and fluid
viscosity. For instance, propagating a fracture by inject-
ing liquid into soil at 75 liters (19.8 gallons) per minute
and at a 2-meter (6.6-foot) depth requires 60 to 85 kPa
(8.7 to 12.3 psi) of pressure, which increases approxi-
mately 20 kPa per meter (0.9 psi per foot) of depth. In
contrast, the pressure required to create a fracture by
injecting air, with injection rates of 20,000 to 30,000 liters
(706 to 1,059 cubic feet) per minute, is in the range of
500 to 1,000 kPa (73 to 145 psi).
The pressure during propagation decreases in most
operations, but the details of the pressure history de-
pend on a variety of factors. For example, slight in-
creases in pressure occur when sand concentration in
slurry increases (Figure 3-1).
40
-------
400
,300
200
100
Propagation Starts
Sand Arrives
Sand Content Increases
Pump Off
01 234567
Time (min)
Figure 3-1. Injection pressure during creation of a fracture by
injecting a sand-laden slurry.
3.1.1.3 Leakoff
Propagation could continue indefinitely if the fracture is
created in infinitely impermeable material, but in real
materials several factors limit the size of the fracture.
Some of the injected fluid flows out through the walls of
the fracture and into the pores of enveloping soil or rock
(Figure 3-2). Workers in the oil industry dubbed this
process "leakoff." The rate of leakoff increases as the
fracture grows and offers more surface area through
which the injected fluid can flow. Other factors that affect
the leakoff rate include the relative permeability of the
fractured formation and the viscosity and pressure of the
fluid. Accordingly, the rate of fracture propagation de-
creases as the rate of leakoff increases, and propaga-
tion ceases entirely when the leakoff rate equals the rate
of injection.
In most environmental applications, leakoff generally
controls the size of the gas-driven fractures. For exam-
ple, injecting gas at 25 to 50 cubic meters (883 to 1,766
cubic feet) per minute into sandstone for approximately
20 seconds typically results in a fracture roughly 10 to
Casing
Fracture
Figure 3-2. Injected fluids leaking out of fracture during
propagation.
20 meters (32.8 to 65.6 feet) in maximum dimension. A
longer injection period negligibly affects fracture dimen-
sion (3). Thus, the rate of injection is a critical design
variable affecting fracture size during air injection (9).
The fast rates of leakoff during air injection also may
yield the secondary benefit of dilating fractures or pores
adjacent to the main fracture.
Leakoff tends to be more significant when creating frac-
tures by injecting slurries into sand or gravel. This is
because in other situations (e.g., when creating frac-
tures at shallow depths in silty clay by injecting water or
viscous gel), other effects (e.g., intersecting the ground
surface) become important before the fractures become
large enough to be affected by leakoff.
3.1.1.4 Other Fracturing Methods
Although effective, injection of fluid is by no means the
only method of inducing useful fractures in the vicinity
of wells. Propagating at high rates can create multiple
fractures in the vicinity of a bore. This process involves
detonating explosives (10) or igniting rapidly burning
propellants to drive fractures at high rates (11,12). Early
practitioners of this technique lowered glass bottles filled
with nitroglycerin into wells and detonated the explosive
with a sudden shock. This procedure probably helped to
clear the zone adjacent to the well that was plugged
during drilling. A variety of modern propellants, which
rapidly produce gas without an explosive shock, have
been used to fracture wells. In some cases propellants
lowered into a well perforate the wall of a bore, whereas
in other cases the propellant rapidly drives a gas-filled
fracture into the adjacent formation (11).
Another method involves applying an electric field to the
vicinity of a bore to induce fractures. This method has
been used to a limited extent to affect the productivity
of wells (13). Myriad microfractures in low permeabil-
ity formations accompany the application of radio-
frequency heating, presumably resulting from the rapid
boiling of pore water.
3.1.2 Monitoring Fracture Location
The most widely used method of monitoring fracture
location is measuring the displacement of the ground
surface (3, 4, 9). The displacement over a gently dipping
fracture at shallow depths appears as an asymmetric
dome (Figure 3-3). Net displacements can be deter-
mined by surveying a field of staffs with finely graduated
scales before and after fracturing. Alternatively, tilt-
meters can measure extremely gentle slopes of the
ground surface in real time while the fracture is being
created. The former method is inexpensive and provides
reliable data on the final displacements, whereas the
latter method supplies information on the growth of the
fracture, which may be necessary when creating frac-
tures in the vicinity of sensitive structures.
41
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Uplift Measured
1 mm
5 meters
Figure 3-3. Typical pattern of uplift over a shallow, gently dip-
ping hydraulic or pneumatic fracture.
The pattern of uplift indicates characteristics of the frac-
ture at depth. A broad, symmetric dome indicates that
the fracture gently dips and is roughly symmetric. Many
fractures propagate in a preferred direction, which is
reflected by displacements that are asymmetric with
respect to the borehole. At shallow depths (where the
ratio of fracture length to depth is roughly 3), the mag-
nitude of uplift appears roughly equivalent to the aper-
ture of the fracture. Thus, in these cases, fracture
aperture and extent can be estimated directly from the
uplift. Lesser ratios of length to depth (deeper fractures)
generally require mathematical inversion of appropriate
analyses (14-16) to estimate geometry of the fracture.
Borehole extensiometers fitted to open bores also can
detect the location of induced fractures. This application
directly detects the opening of the fracture tip as an
increase in the length of the bore. This provides a direct
determination of the aperture of the fracture that is more
reliable than measurements of uplift. In one application,
borehole extensiometers placed in the vicinity of a re-
taining wall ensured that induced fractures were termi-
nated before they reached the wall (17).
Monitoring induced fractures through their displacement
field is not without its limitations. Accurately determining
the dip and aperture of deep fractures is difficult, even
when using numerical inversion methods (14,18). More-
over, details such as fracture bifurcation or intersection
of adjacent fractures generally cannot be detected.
Heterogeneities in the subsurface may produce results
that are difficult to interpret.
3.1.3 Equipment
The equipment used to induce fractures consists of an
aboveground system, which must be capable of inject-
ing the desired fluid at the required pressures and rates,
and a belowground system, which must be capable of
isolating the zone where injection will take place.
3.1.3.1 Aboveground Equipment
The type of fluid to be injected largely determines the
aboveground equipment. Pneumatic fracturing, which
entails injecting air to create fractures, requires equip-
ment that rapidly delivers air to the subsurface. The
most versatile equipment developed for pneumatic frac-
turing employs a series of high-pressure gas cylinders
with a pressure regulator to control injection (Figure 3-4)
(9). This equipment injects air at rates of 25 to 50 cubic
meters (883 to 1,766 cubic feet) per minute and at
pressures of 0.5 to 2 MPa (72.5 to 290 psi). The process
can be tailored to site conditions and is particularly
suited to delivering air at high rates. Moreover, this method
can create fractures with compressed gases other than
air, which may have applications during bioremediation,
in situ oxidization, or other remedial processes.
Pressure Regulator
Figure 3-4.
To the Subsurface
Aboveground equipment used for pneumatic
fracturing (9).
Injecting air directly from a compressor may induce
fractures under some circumstances. Filters or special-
ized compressors are required to eliminate traces of oil
in the air stream. Typically, compressors are unable to
supply the pressure or the rate available from pressur-
ized cylinders; therefore, this approach may be limited
in relatively permeable formations where leakoff limits
the size of fractures that are created by injecting at
modest rates.
Injecting liquid to induce fractures, hydraulic fracturing,
requires equipment to prepare and inject the liquid. Hy-
draulic fractures created by injecting water alone require
equipment that consists primarily of a high-pressure
positive displacement pump with associated pressure
relief devices (19). Hydraulic fractures that are filled with
sand or other granular proppant require a mixer to cre-
ate the slurry. A batch mixer consisting of one or two
open tanks fitted with agitators can be used to create
42
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the slurry, a labor-intensive approach that can limit field
productivity when desired fracture volumes are larger
than the tanks. An alternative is to use a continuous
mixer, which blends metered streams of gel, crosslinker,
breaker, and sand to form slurry. This type of device
represents a larger capital investment than a batch
mixer, although it reduces the time, labor, and thus the
cost required to create fractures. In most cases, positive
displacement pumps inject slurry to create hydraulic
fractures. Duplex and triplex piston pumps, as well as
progressive cavity pumps, are widely used for this
purpose.
3.1.3.2 Belowground Equipment
Belowground equipment for inducing fractures consists
of a device for isolating the zone of injection. Both
pneumatic and hydraulic fracturing use straddle packers
in open holes (Figure 3-5) (19). A specialized nozzle
assembly (3, 20) fits between straddle packers and
improves gas delivery during pneumatic fracturing. This
technique allows fractures to be spaced approximately
every 0.5 meters (1.6 feet) along an open borehole. In
some cases, the seal that straddle packers provide in
open holes in saturated silts and clays may be insufficient.
When fractures are created in unlithified sediments,
driven casing (4) offers an alternative to straddle pack-
ers. One example of this approach is to drive a casing
Figure 3-5. Fracture created by isolating a zone with a straddle
packer.
with an inner-pointed rod to depth (Figure 3-6). After
removing the rod, a high-pressure pump injects a water
jet to cut a notch in the sediments at the bottom of the
borehole. The notch reduces the pressure required to
start propagation, much as the notch in plastic packag-
ing reduces the effort required to open the package. The
notch also ensures that the fracture starts in a horizontal
plane at the bottom of the casing. A fracture can be
created at the bottom of the casing by injecting either
gas, liquid, or slurry. After creating the fracture, the rod
can be reinserted and driven to greater depth to create
another fracture, or the casing can be left in place to
access the fracture during recovery.
Using this approach in unlithified sediments allows cas-
ing to be advanced by either hammering (using a drop-
weight, pneumatic, or hydraulic hammer) or direct push
(using the weight of a drill rig or cone penetrometer).
Packers or related methods are appropriate in rock and
in some unlithified sediments.
3.1.4 Well Completion
The method used to complete a well that has been
hydraulically fractured affects the versatility and cost of
the well. Completions that allow access to individual
fractures provide the most versatility, but they can con-
sume more time and money than completions that offer
access to all fractures by one casing. Access to individ-
ual fractures can improve performance during vapor
extraction. For example, this method can allow alternat-
ing between air inlet and suction on adjacent fractures,
or can provide dewatering capabilities from lower frac-
tures and vapor recovery from upper fractures. It also
can improve recovery of NAPL by directing aqueous and
nonaqueous phases to separate pumps. Other applica-
tions, such as water recovery from a confined aquifer,
may benefit little from access to each fracture.
Completion techniques that access all fractures simul-
taneously (Figure 3-7a) resemble standard well comple-
tion methods (10). To provide individual access, a
grouted zone along the bore can isolate each fracture
(Figure 3-7b). Alternatively, casing driven to create frac-
tures in unlithified sediments can be left in place to
access the fracture during recovery (Figure 3-7c).
3.2 Design Considerations
Several factors affect the form and permeability of in-
duced fractures. These factors should be evaluated
when considering using induced fractures to increase
the performance of wells at a site. Geologic conditions
play a major role in affecting fracture form and dictate
the need for using a sand proppant. Site conditions,
particularly structures that may interfere with propaga-
tion, also are important considerations. Table 3-1 sum-
marizes the important factors and their favorable or
43
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Rod
Pressure From
Soil Seals
Casing
Figure. 3-6. Fractures created at the bottom of driven casing.
Table 3-1. Design Considerations for Induced Fractures
Factor Favorable Unfavorable
Fracture form Gently dipping for
vertical wells
Moderate to low
(k < 10'8 cm2)
Formation
permeability
Fracture
permeability
Formation type Rock or fine-grained
sediment
Vertical (fracture may
reach ground surface)
Unnecessary in high
permeability formations
(clean sand)
\ ,000 times formation k <100 times formation k
Coarse-grained
sediment
Sand proppant Unlithified, saturated May be unnecessary
in rock formations
Figure 3-7. Methods of completing wells with induced frac-
tures: a) screen across all fractures b) casing to each
fracture, and c) driven casing to each fracture.
sediments
No proppant Rock formations
State of stress Horizontal stress >
vertical stress
(overconsolidated)
Soft formations where
fracture may close
Horizontal stress <
vertical stress
(normally consolidated)
Well completion Access to each fracture Screen to several
most versatile fractures less versatile
but less costly than
individual access
Site conditions Open ground over
fracture
Structures sensitive to
displacement over
fracture
-------
unfavorable results. These factors specifically target
relatively shallow applications typical of many contami-
nated sites. The table is based on current findings
and is subject to change as fracturing techniques are
modified.
3.2.1 Flow to a Gently Dipping Fracture
To better understand the impact of some of the factors
listed in Table 3-1, consider the effects of creating a
fracture in the vicinity of a well. One perspective ignores
the geometry of the fracture and views the improvement
in well performance as an increase in the effective per-
meability of the material enveloping the well (21). From
another view, the fracture is a discrete layer that affects
the pattern of flow in the subsurface. The former pro-
vides a simple method of assessing the improvement
resulting from the fractures, whereas the latter provides
more detailed insight into effects in the subsurface.
For example, consider the effect of a sand-filled fracture
on subsurface air flow during a field test (22). The frac-
ture in this case was 1.5 meters (4.9 feet) deep and
3 meters (9.8 feet) in radius and had an average thick-
ness of 6 millimeters (0.2 inches). The fracture was
accessed via a well 5 centimeters (2 inches) in radius
and screened from 1.4 to 1.7 meters (4.6 to 5.6 feet). An
identical well with no fracture was used for control. Silty
clay, of permeability 10"9 square centimeters (10"12
square feet), underlays the site. The surface of the site
was exposed to the atmosphere (no cap), and an imper-
meable boundary was assumed at 7 meters (23 feet)
(bedrock was at this depth). Air was pulled from the wells
using a suction head of 2.5 meters (8.3 feet) of water
(absolute pressure head of 7.9 meters [25.75 feet] of
water). To evaluate subsurface flow, field observations
allowed calibration of a numerical analysis of these
conditions.
Assuming the conditions cited above, the discharges
from a vapor well were calculated for fractures with
permeabilities ranging from 10"9 to 10"3 square centime-
ters (10"12 to 10"6 square feet). The results (Figure 3-8)
indicate that discharge increases only slightly if the frac-
ture permeability is less than 10"7 square centimeters
(10"1° square feet), or not even 100 times greater than
the formation permeability. The discharge increases sig-
nificantly, however, if the fracture permeability is greater
than 1,000 times that of the formation. Using Figure 3-8,
field measurements of well discharge, and other analy-
ses, the permeability of the fracture used for this test
was estimated at 5 x 10"6 square centimeters (5 x 10"9
square feet), approximately the permeability of coarse-
grained sand injected into the fracture.
The fracture influences each of the essential measures
of subsurface flowpressure, flux, and travel times.
Pressure contours form concentric shells around the
conventional well screen, whereas they elongate around
12
-10-9
10-7
10-s
10-5 1CH
10-3
Fracture Permeability (cm2)
Figure 3-8. Air discharge from a well intersecting a flat-lying
circular fracture as a function of the permeability of
the fracture. (Formation permeability is 10~9 cm .)
the fracture. The pattern of flow paths also elongates by
the fracture (Figure 3-9). The suction felt by the forma-
tion less than 0.5 meters (1.6 feet) from the control well
is equal to the suction in a broad area 3 meters (9.8 feet)
from the well, and more than 1 meter (3.3 feet) above
and 2 meters (6.6 feet) below the fracture. Remember
that equal suction was applied to both wells; thus, most
of the applied suction was lost within a few tenths of a
meter of the conventional well, whereas significant suc-
tion occurred several meters from the well along the
fracture.
The most easily measured formation parameter in the
field is pressure, or suction. Flux (volumetric flow/unit
area) and travel time, however, are more important than
pressure for environmental applications. The flux pat-
tern resembles the pressure pattern in that it forms
concentric shells around the conventional well and elon-
gated shapes around the fracture. Fluxes in the vicinity
of the fracture are at least two orders of magnitude more
than fluxes roughly a half meter from the conventional
well. The flux 2 meters (6.6 feet) from the conventional
well is roughly the same as the flux 6 meters (19.7 feet)
from the fractured well.
Travel times, which were determined by tracking a par-
ticle from its starting location to the well, are shorter in
the vicinity of the fracture. Travel time is particularly
short in the region over the fracture, and the travel time
roughly 5 meters (16 feet) from the well with a fracture
is less than travel time 2 meters (6.5 feet) from the
conventional well. This is significant because some es-
timates of remediation are based on the number of pore
volumes, and the travel time is a measure of the time
required to exchange one pore volume within that con-
tour. Note that travel times in Figure 3-9 are given in
units of time/effective porosity, so that the actual time of
travel is obtained by multiplying the number on the plot
45
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Conventional Well
6 m
Well Intersecting Sand-Filled Fracture
6m 4 2 0
0.002
0.001
111 \
Travel Time: Days/Effective Porosity^
\ ,\ \ \ V .
Figure 3-9. Pressure, flux, and travel time to a conventional well and a well intersecting a sand-filled fracture. Shaded areas are for
comparison (based on the results of a numerical simulation using field data [22]).
by the effective porosity of the site. The results are
presented this way because estimates of effective po-
rosity were unavailable from this site. Estimates of ef-
fective porosity are 0.001 or less (23) at a similar site
underlain by fractured silty clay till, so that air in the
pores that are available for flow, shown in the shaded
areas in Figure 3-9, would be exchanged in 2.4 hours.
The essential conclusions of this study are:
The fracture increased the discharge by a factor of
20 and increased the radial distance where pressure
is affected by a factor of 10 compared with the control
well.
The fracture changed the flow paths and patterns of
pressure, flux, and travel time in the subsurface. Val-
ues of those parameters in the vicinity of the fracture
were typically more than 10 times greater than in the
vicinity of the control well.
A theoretical model that treated the fracture as a thin
layer of coarse-grained sand aided in predicting field
observations.
The fracture permeability was critical to well perform-
ance. Well discharge was nearly unaffected when the
fracture permeability was less than 100 times that of
the formation, whereas it increased abruptly as the
fracture permeability increased from 100 to 10,000
times that of the formation.
3.2.2 Forms of Fractures
The effectiveness of an induced fracture depends pri-
marily on its form, that is, its shape, aperture, orienta-
tion, length, width, and location with respect to the
borehole. In most cases, fractures created by injecting
fluids consist of one to several fracture surfaces. The
general forms of fractures range from a steeply dipping,
elongated feature to a flat-lying circular disk or bowl-
shaped feature. The flat-lying fractures are useful to
many applications because they can grow to significant
sizes without intersecting the ground surface. Con-
versely, steeply dipping fractures tend to climb upward
and intersect the ground surface.
The form of an induced fracture results from both frac-
turing technique and site conditions. Critical factors re-
lated to fracturing technique include type of fluid, rate or
pressure of injection, and configuration of the borehole.
Critical site conditions affecting form include loading at
the ground surface, permeability, formation heterogenei-
ties, and subsurface borings. Fractures created by in-
jecting sand-laden slurry are easy to identify; their form
is better known than the form of naturally propped frac-
tures, which lack a proppant and can be difficult to
46
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identify in split-spoon samples. Most sand-filled frac-
tures used for environmental applications have been
created in glacial sediments or vertisols. Therefore, the
impression of fracture form is biased toward fractures
created under those conditions.
A database containing characteristics of approximately
140 fractures was analyzed to estimate the charac-
teristics of a typical fracture created using methods
described in Murdoch et al. (4). According to the analy-
sis, the "typical" fracture induced by injecting liquid at
shallow depths in overconsolidated silty clay is slightly
elongated in plan and dips gently toward its parent
borehole. The major axis of the fracture is approximately
three times greater than the depth of initiation and 1.2
times greater than the minor axis (Figure 3-10).
$ Average Extent of Uplift , , 5 , Maximum '
Uplift ' "
Plan
Injection Point
Section
Figure 3-10. Plan and section of a typical hydraulic fracture
created in overconsolidated silty clay.
Ground overlying the fracture displaces upward to cre-
ate a gentle dome, and the amplitude of the dome
resembles the aperture of the fracture at depth. The
fracture closes and the dome subsides after injection,
but the injected sand prevents the fracture walls from
closing completely. The amount of closure depends on
the concentration of sand in the slurry. Here, the ratio of
maximum aperture when the fracture is pressurized to
thickness of sand is similar to the ratio of the total slurry
volume to the bulk volume of sand in the slurry. The
major axis of the extent of uplift ranges from 5 meters
(16.4 feet) to more than 12 meters (39.4 feet), with an
average of 8.5 meters (27.9 feet). The maximum uplift
ranges from a few millimeters to more than 30 milli-
meters (1.2 inches), with an average of 19 millimeters
(0.75 inches).
The typical extent of uplift is roughly elliptical, with an
aspect ratio of 1.2:1. The borehole used to create the
fracture and the point of maximum uplift rarely coincide
with the center of the extent of uplift. Interestingly, both
the borehole eccentricity (ratio of distance between the
center of uplift and the borehole to the major axis) and
the displacement eccentricity (ratio of distance between
the center of uplift and the point of maximum uplift to the
major axis) are 0.14. Those two points, however, typically
are on opposite sides of the center of uplift (Figure 3-10).
The average dips of fractures at seven different sites in
the Midwest and Gulf Coast range from 5 to 25 degrees.
At each site, however, the dips were fairly consistent,
with the standard deviations of dips approximately 5
degrees. The dips at some sites were statistically differ-
ent from dips at other sites, so that dip angle appears to
depend strongly on site conditions.
3.2.3 Geologic Conditions
Geologic conditions significantly affect the forms of in-
duced fractures. Details of all the effects are still being
evaluated, but a discussion of some of the major geo-
logic factors follows.
3.2.3.1 Permeability
A fracture must be significantly more permeable than the
enveloping formation to have a major impact on well
discharge (Figure 3-8). Therefore, the relative improve-
ment resulting from induced fractures increases as the
permeability of the formation decreases. In most cases,
rock or formations of silt or clay are best suited to
induced fracturing because they have the lowest initial
permeabilities.
One exception involves using induced fractures to de-
liver solid compounds to the subsurface. This applica-
tion can address processes independent of formation
permeability and may require creating fractures and
filling them with permeable sand, gravel, or rock.
3.2.3.2 State of Stress
The state of stress in the formation affects the orienta-
tion of an induced fracture once it has propagated away
from the borehole. Induced fractures are usually flat-
lying where horizontal formation stresses are greater
than vertical stresses, whereas they tend to be steeply
dipping where vertical stresses are greatest. In rock
formations that erosion has buried or exposed, the lat-
eral stresses are typically greater than the vertical
stresses near the ground surface, effecting flat-lying
induced fractures. Most shallow rock formations, with
the possible exception of recent lava flows, have rela-
tively high lateral stresses.
47
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The state of stress of soils and unlithified sediments
depends on several factors, including consolidation his-
tory (24) and wetting and drying history. Soils that were
consolidated under a load greater than the present load
are overconsolidated, and many such soils contain hori-
zontal stresses that exceed vertical stresses. For exam-
ple, loading by a glacier results in overconsolidation, so
sediments deposited subglacially are good candidates
for high lateral stresses. Soils containing clay minerals
that undergo a large volume reduction upon drying be-
come overconsolidated with repeated cycles of wetting
and drying. For instance, vertisols (soils rich in swelling
clays) are particularly susceptible to relatively large lat-
eral stresses.
Glacial sediments are common in the northern Midwest
and Canada, and vertisols are common along the Gulf
Coast of Texas (Figure 3-11). The states of stress found
in these areas favor the creation of gently dipping
fractures.
Other conditions also result in favorable states of stress.
Local geotechnical engineers can evaluate the state of
stress at a particular site.
3.2.3.3 Bedding
Induced fractures may follow contacts in interbedded
sediments, or they may follow partings between rock
beds. The effect of bedding can be capricious, with
fractures following beds in some cases and crosscutting
beds in others. In some cases, it appears that flat-lying
fractures are created in interbedded sediments with a
state of stress that would favor vertical fractures. Gen-
erally, field tests need to establish the effects of bedding
at a particular site.
3.2.3.4 Formation Strength
The strength of the formation plays an important role in
determining whether fractures can be naturally propped
or if they should be propped with sand. The section on
fracture permeability includes a more detailed discus-
sion of this issue.
3.2.3.5 Water Content
Water content of a formation appears to have negligible
effect on creating fractures by injecting fluid.
3.2.4 Fracture Permeability
Because the permeability of the induced fracture is half
the ratio between fracture and formation permeability, it
critically affects the performance of wells. To compare
the permeabilities of real fractures induced around bore-
holes, it is helpful to express their performance in terms
of a smooth-walled slot with an aperture we that has the
Figure 3-11. Locations of major areas of soils related to glaciers and vertisols (25, 26).
48
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same effective permeability as the real fracture. Several
methods relate effective permeability ke and effective
aperture (27), but one of the most commonly used (28)
is
(3-1)
The aperture of a real fracture varies greatly along its
length. Thus, the effective aperture is an average that
accounts for how some locations with wide gaps and
other locations that may be closed completely affect flow
through the fracture. Accordingly, only hydraulic ortracer
tests in the field can measure the effective permeability
of natural fractures. The effective apertures of induced
fractures adjacent to boreholes are incompletely known
but have been estimated for several natural fractures. In
one example, several flat-lying fractures in dolomite had
an effective aperture of 0.022 to 0.023 centimeters,
(0.0087 to 0.0091 inches), according to constant head
tests (29).
The effectiveness of a fracture propped open with sand
is expressed in terms of the product of proppant perme-
ability kp and aperture w. Using this basis, it follows that
the equivalent aperture of a proppant-filled fracture is
0.06 1
wp =
!>1/3
According to field tests and tabulated values (30), the
permeability of medium- to coarse-grained sand used as
a proppant is approximately 10"5 to 10"6 square centime-
ters (1.6 x 10'6 to 1.6 x 10'7 square inches). The actual
aperture of sand-filled fractures typically ranges 0.5 to
1.0 centimeters (0.2 to 0.4 inches), so that the equiva-
lent aperture is 0.02 to 0.05 centimeters (0.008 to 0.02
inches) (Figure 3-12). This overlaps with the effective
apertures observed for fractures in rock.
The discussion above suggests that the effective aper-
ture of naturally propped fractures may be similar to that
of sand-filled fractures in rock. In soils, however, the
strength of fracture asperities are less than those in
rock, so naturally propped fractures may close. The rate
of closure increases with decreasing strength of the soil
or increasing driving stress on the fracture. In general,
the strength of fine-grained soil decreases with increas-
ing water content or decreasing consolidation. Thus,
fractures may stay open in dry soils but may close when
the soil becomes saturated. The stress driving closure
is the stress the formation applies (for a horizontal frac-
ture, the unit weight of the formation times the depth)
plus the amount of suction applied to the fracture. Ac-
cordingly, fractures probably can be naturally propped
when soil or rock is strong relative to the closure stress
(9). If strength decreases, depth increases, or suction
increases past a critical value, however, fractures
should be propped with granular materials. Figure 3-13
schematically depicts this concept. Data are currently
0.00
Aperture Proppant-Filled Slot (cm)
Figure 3-12. Effective aperture of an open slot that is equiva-
lent to that of a fracture filled with proppant.
unavailable to determine the critical values, so the axes
of Figure 3-13 are qualitative.
(3-2) 3.2.5 Fracture Size
Fracture size is an important design consideration be-
cause performance generally increases with increasing
size. The rate and volume of injected fluids are the
primary variables affecting size. Where significant leak-
off may occur, such as when injecting air to create
fractures, increasing the rate of injection increases the
size of the fracture. In other cases, the volume of in-
jected fluid determines the size of the resulting fracture.
The major exception is when a fracture climbs and
Area Where Naturally
Propped Fractures
Stay Open /
A
Soil/Rock
Strength
Area Where
Fractures Should
Be Filled With
Sand To Maintain
Permeability x
Asperities Collapse
Proppant Needed
Depth + Suction
Figure 3-13. Factors that affect how fractures should be propped.
49
-------
reaches the ground surface, in which case it may be
smaller than anticipated.
The thickness of sand in a fracture can be manipulated
to some extent by controlling the concentration of sand
in the injected slurry or by changing other process
variables. Sand in the fracture should be thick enough
to provide a large contrast with the permeability of the
formation. Once the contrast is sufficient, however, cre-
ating a thicker sand pack to obtain additional contrast
provides only minor improvement. Once sand in the
fracture is several millimeters thick, a decision must be
made as to whether the cost of additional sand is worth
the incremental benefit achieved by a thicker fracture.
Typical applications (in the depth range of 2 to 5 meters
[6.6 to 16.4 feet]) have created fractures that are 6 to 10
meters (19.7 to 32.8 feet) in maximum dimension and 1
to 2 centimeters (0.4 to 0.8 inches) in maximum thick-
ness. The size of the fracture increases with depth, and
ratios of maximum length to depth of 3:1 to 4:1 are
typical.
3.2.6 Well Completion
The type of well completion affects the flexibility of sub-
surface control, versatility in creating additional frac-
tures, and cost. Some methods of completion provide
access to each fracture or group of fractures, whereas
others simultaneously access all the fractures in a well.
Individual completions provide versatility by allowing
each fracture to be used for either injection or recovery
of fluids. This is particularly beneficial during vapor ex-
traction or the simultaneous recovery of two separate
phases, particularly NAPL and water. Individual comple-
tions are also more costly than continuous screening
across all fractures.
In some cases, it may be necessary to return to a well
and create additional fractures. Additional fractures can
be created in wells that consist of open bores, and
fracture size can be increased where completions con-
sist of driven casing. Using currently available methods,
it is difficult to create fractures using wells that have
already been completed with a screen and gravel pack.
3.2.7 Site Conditions
Both surface and subsurface structures (e.g., buildings,
pavement, buried utilities, wells, piezometers, or back-
filled excavations) may affect, or be affected by, the
creation of subsurface fractures. Creating fractures be-
neath structures may displace them. A structural engi-
neer should be involved to estimate displacement
tolerances for particular structures. In cases where sur-
face displacements are critical, it is advisable to use real
time monitoring of the displacements so that the proce-
dure can be terminated before structures reach dis-
placement tolerances.
Surface structures also may affect the propagation of
fractures by loading the ground surface. In one case
where injecting air created a fracture adjacent to a build-
ing, propagation was away from the building apparently
in response to the surface loading by the structure.
Fractures filled with liquid behave in a similar manner.
A fracture also may displace shallow subsurface utilities,
pipe, or related features that lie above the fracture. A
propagating fracture actually may intersect deeper fea-
tures, such as wells, piezometers, or grouted sampling
holes. There is limited evidence regarding the effects of
this type of interaction.
Fracture propagation may terminate or alter markedly if
the fracture intersects a backfilled excavation. The se-
verity of this effect depends on site details and can only
be evaluated case by case.
3.3 Applications
The typical application for induced fractures is to im-
prove the performance of wells. Induced fractures can
improve most in situ remedial actions involving fluid flow.
Other applications include placing solid compounds in
the subsurface and enhancing electrical conductivity.
The sections that follow outline the principal applications
that have been either demonstrated or proposed.
3.3.1 Vapor Extraction
Vapor extraction is one of the most widespread and
effective methods of remediation, and induced fractures
can improve vapor extraction in a variety of low-perme-
ability formations. In most cases, applications include
creation of multiple fractures at various locations along
the length of vertical wellbores.
The primary purpose of inducing fractures for vapor
extraction is to improve the discharge and areas af-
fected by wells. Typically, results increase discharge by
factors ranging from 10 to 100, and increase the dis-
tance affected by a well 10 times or more compared to
control wells (1, 2). In addition, coupling individual com-
pletion methods with induced fractures allows air flux in
the subsurface to concentrate in different areas.
Design variables to considerwhen inducing fractures for
vapor extraction remediation include selection of prop-
pants and details of completion. Some applications use
fractures that are naturally propped, whereas others
prop fractures with sand. Both methods can increase
vapor discharge and contaminant recovery rate by an
order of magnitude or more. In general, the duration of
improvements of vapor discharges by naturally propped
fractures are greatest in competent formations, such as
sandstone and siltstone. Naturally propped fractures
stay open in competent formations, whereas in soft
sediments they may close due to the weight of the
50
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overburden and applied suction. However, naturally
propped fractures in relatively stiff sediments, such as
glacial drift, can stay open for many months. Propping
fractures with sand offers an alternative that may increase
the duration of the discharge in unlithified formations.
Completion methods range from installing screen over
the entire interval of fractures to installing individual
casings and screen to service each fracture. Screening
the entire interval containing fractures reduces comple-
tion costs, but also reduces versatility and may limit the
effectiveness of the application. The limitation results
from how suction is applied to a stack of fractures. For
instance, suction is applied equally when a screen is
placed across all the fractures. As a result, air flux is
particularly great in the region overlying the upper frac-
ture, and it is significant at the ends of the lower frac-
tures (Figure 3-4). Flux can be limited, however, in the
region below the upper fracture, even close to the well.
Air from either a neighboring inlet well or from the
ground surface converges toward the ends of the lower
fractures and avoids the area close to the well (Figure
3-14). As a result, screening a well across multiple
fractures may result in incomplete remediation in the
vicinity of the well.
An approach to avoiding this problem is to use one or
more fractures as air inlet sources. This results in air
10ft
0-0.1 0.1-0.2 0.2-0.4 0.4-0.6 <10,000
Figure 3-14. Flux of air to three flat-lying fractures (white band)
shown in cross section: a) suction of 100 inches
of water on all three fractures, and b) air inlet (at-
mosheric pressure) at the bottom and suction on
the other two fractures.
flowing from one fracture to another and flux concentrat-
ing in the area between the fractures (Figure 3-14b). To
use this process, it must be possible to access fractures
individually. Several methods of well completion can
accomplish individual access. One method includes bor-
ing through the interval containing fractures and placing
sand or gravel at the depth of each fracture and grout in
the intervals between fractures.
Five sites have used fractures filled with coarse-grained
sand for the purpose of vapor extraction, and naturally
propped fractures have been created at one to several
dozen sites.
3.3.2 LNAPL Recovery
Induced fractures can increase the recovery of free-
phase LNAPL by increasing the discharge of recovery
wells. This application can use either naturally propped
fractures in rock or sand-propped fractures in unlithified
sediments. LNAPL recovery from an aquifer closely re-
sembles oil recovery from a reservoir.
This application requires creating fractures in or slightly
below the contaminated zone. Recovering contaminants
using fractures significantly below the LNAPL should be
avoided because of the potential for trapping NAPL as
a residual phase during drawdown (31).
One strategy for recovering a thick layer of LNAPL is to
create one or several fractures in the contaminated
interval and to complete the well to access all the frac-
tures simultaneously. This approach can increase the
recovery of the NAPL phase by an order of magnitude
compared with a conventional well in a low-permeability
formation.
In many cases, however, the contaminated zone is rela-
tively thin, and recovery of LNAPL causes the upward
displacement of the underlying NAPL/water interface.
When the interface reaches the well, the ratio of NAPL
to water diminishes, even though a significant amount
of LNAPL may exist in the vicinity of the well.
In addressing the problem of water recovery from a
LNAPL well, one approach is to create and separately
access a fracture below the LNAPL/water interface.
Water is recovered from the lower fracture, thereby
preventing upward migration of water. This, in turn, pre-
vents the reduction of the ratio of LNAPL to water recov-
ered from the overlying fracture. A test in swelling clays
near Beaumont, Texas, demonstrated this principle. At
this site, several sand-filled fractures were installed to
recover gasoline. At one location, the interface between
gasoline and water was at a depth of approximately 3.3
meters (10.8 feet). Fractures were created at depths of
3.0 and 3.6 meters (9.8 and 11.8 feet), and individual
pumps accessed each fracture. While both pumps were
operating, the ratio of gasoline to water was between 5
and 6 from the upper fracture, whereas it was 0.01 from
51
-------
the lower fracture. The pump in the lower fracture was
turned off while the upper pump continued to operate.
Four hours later, the total rate from the upper fracture
had increased markedly to slightly less than the rate
produced when both pumps were operating. The rate of
recovery of gasoline from the upper fracture, however,
actually diminished, and the gasoline-to-water ratio de-
creased from 5 to 0.3. The case history section provides
more details of this project.
3.3.3 Dual-Phase Recovery
Simultaneous recovery of vapor and liquid is inevitable
when extracting vapor near saturated zones, and it may
occur when accelerating liquid recovery by placing vac-
uum on a well. This process, called dual-phase recov-
ery, uses a well with an inner tube attached to a vacuum
pump (Figure 2-1) (31, 32). The system induces vapor
flow during normal operation but also aspirates and
removes liquid should it enter the well (Figure 3-15).
Wells containing induced fractures in the vadose zone
of low-permeability formations tend to produce more
water than conventional wells (22). Vapor discharge
diminishes during water recovery, so the dual-phase
recovery approach accelerates dewatering and en-
hances vapor recovery. Most applications that have ex-
tracted vapor using sand-filled fractures have used the
dual-phase recovery approach.
3.3.4 NAPL Recovery
Dual-phase recovery from wells with induced fractures
is similar to applications from conventional wells. The
Air Inlet as Needed
Figure 3-15. Well configuration to recover liquid and vapor
phases using suction at the ground surface.
design uses a conventional well with a tube that passes
through a seal at the well head and extends to the
bottom of the screened section (31). Suction is applied
either to the inner tube or to the annulus between the
tube and the well casing. Vapor is recovered during
normal operation. In addition, water that enters the well
is aspirated in the vapor stream and also recovered.
3.3.5 DNAPL Recovery
Using induced fractures can increase the recovery of
DNAPL, particularly from low permeability formations.
However, the benefits of increased recovery must be
balanced against the possible risks. Creating vertical
fractures or intersecting natural vertical fractures may
result in downward migration of DNAPL that cannot be
captured by the well. As a result, induced fractures may
increase the recovery of some DNAPL while making the
remaining liquid more difficult to recover because it has
migrated to greater depth.
In view of this possibility, only sites where gently dipping
fractures can be created should be considered for
DNAPL applications. The orientation of an induced frac-
ture depends on a variety of conditions, including the
state of stress, density of injected fluid, rate of fracture
propagation, depth, and the nature of bedding, fabric,
and preexisting fractures in the soil or rock. These fac-
tors make the orientation of induced fractures difficult to
predict with confidence prior to site activity. In most
cases, and particularly at DNAPL sites, feasibility testing
is recommended to determine fracture orientation.
A pilot test (21), described in the "Case Histories" sec-
tion, demonstrated that induced fractures increase the
recovery of free-phase DNAPL.
3.3.6 Bioremediation Applications
Using induced fractures can enhance bioremediation in
a low-permeability formation by either of two methods:
Increasing the rate of injection of nutrients and
oxygen-bearing fluids (33).
Filling fractures with solid compounds that provide
the essential ingredients for bioremediation (34).
Induced fractures have been used to accelerate biore-
mediation of fine-grained soils by increasing the rate of
injection of nutrients and oxygen, typically in the form
of hydrogen peroxide. In one application where frac-
tures were created in silty clay glacial drift, the rate of
injection increased by nearly two orders of magnitude
(33). In this example, one location contained sand-filled
fractures at depths of 1.2, 1.8, 2.4, and 3 meters (3.9,
5.9, 7.9, and 9.8 feet). The fractures were gently dipping
and reached maximum dimensions of 5 to 7.5 meters
(16.4 to 24.6 feet). The well was screened from 1.2 to 3
meters (3.9 to 9.8 feet) depth, accessing all the fractures
52
-------
simultaneously. Another well, which lacked induced frac-
tures, was created as a control. A solution of nutrients
and hydrogen peroxide was injected into both wells at
constant head. The rate of injection into the well contain-
ing induced fractures ranged from 2.5 to 4.6 liters (0.7
to 1.2 gallons) per minute, whereas the rate into the
control well ranged from 0.02 to 0.08 liters (0.005 to 0.02
gallons) per minute. This example demonstrated one of
the largest differences of injection rates between wells with
and without induced fractures, with a ratio between 50 and
125.
Pilot-scale field tests have been conducted in which
solid compounds designed to slowly release oxygen
were injected into a fracture (33). A bioventing applica-
tion to promote bioremediation of solvents, in which
sand-filled fractures will be used to enhance air injec-
tion, is currently in progress.
3.3.7 A ir Injection
The rate of air injection also can be increased by induc-
ing fractures in the vicinity of an injection bore (35, 36).
Injecting ambient air can stimulate the activity of aerobic
organisms, or heated air can heat the formation and
increase the vaporization of VOCs. During one test, air
heated to between 100°C and 130°C (212°F to 266°F)
was injected for 90 hours at 70 standard cubic feet per
minute (scfm) into a fractured well. Temperatures in-
creased from 14°C (57.2°F) to between 21 °C and 25°C
(69.8°F to 77°F), according to measurements 1.5 and
3.0 meters (4.9 to 9.8 feet) from the point of injection
(36).
3.3.8 Steam Injection
Induced fractures can enable steam injection to heat
low-permeability formations. This application has been
evaluated during a pilot-scale test. In addition, a pro-
gram designed to lead to a field demonstration at a
contaminated site is currently underway.
The well design for steam injection resembles other
applications except that steel, ratherthan plastic casing,
is recommended because of steam temperature. More-
over, wells should be able both to inject steam and to
recover condensate. One possible design involves in-
jecting steam into the middle one of three fractures while
applying suction to the other two (Figure 3-16). This
approach would induce upward and downward migra-
tion of steam and a condensate front accelerated by the
applied suction.
The lower fracture plays another important role in this
approach. Active suction on the lower fracture should
intercept DNAPL mobilized in the condensate front,
thereby enhancing the reliability of the system. Several
fractures could serve this function, depending on the
requirements at the site.
Suction Out
Steam In
Recovery of Possible Mobilized DNAPL
Figure 3-16. Schematic application of steam injection into in-
duced fractures.
A pilot-scale test of a simplified version of this approach
has been conducted. It involved injecting steam into a
well that accessed a sand-filled fracture at a 2.4-meter
(7.9-foot) depth. The test compared the rate of injection
and the temperature field with results from a control well.
An upper and lower fracture were present, but they
received no suction during this preliminary test. The test
site was underlain by silty clay glacial drift with hydraulic
conductivity of 10"6 centimeters per second. Steam was
injected for 17 days at approximately 34 kPa (4.9 psi).
The rate of injection of steam into the fracture was 0.19
to 0.23 kilograms (0.4 to 0.5 pounds) per minute,
whereas the rate of injection into the control well was
less than 0.006 kilograms (0.013 pounds) per minute.
Thus, the rate of injection into the fracture was more
than 30 times greater than that into the control well.
Moreover, the rate of steam injection was enough to
result in significant heating of the silty clay (Figure 3-17).
A monitoring point 1.2 meters (3.9 feet) from the point
of injection of the fractured well showed that tempera-
tures reached 80°C (176°F) in 4 days, but the tempera-
ture was roughly 25°C (77°F) after 6 days of injection
into the control well. Temperature continued to increase
around the fractured well, and the temperature profile
formed a 0.6-meter (2-foot) thick zone of 95°C (203°F)
after 17 days of injection. This was interpreted as a zone
of live steam that propagated outward into the silty clay.
(A 5°C temperature loss is associated with the method
of temperature measurement.)
3.3.9 Electrokinetics
Graphite can be used as a proppant to create fractures
that are electrically conductive. This application, cur-
rently being evaluated, may enhance migration of water
and contaminants via electrokinetics. The approach is
to create two graphite-filled fractures, one over the
other and separated by several meters. To drive vertical
53
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0
-2
-4
±±_
n
Q. -6
Fractured-^ 0
Well at
Start of _12
Test
-14
Control Well
/ After 6 Days
Fractured Well After.
Days
Fractured Well
at 2 Days
Fractured Well
at 4 Days
20
40 60
Temperature (°C)
80
100
Figure 3-17. Temperature as a function of depth and time.
migration by electrokinetics, a voltage difference is
maintained between the two fractures. This process is
analogous to maintaining a pressure difference between
two fractures to drive flow, except that migration by
electrokinetics can be faster than migration by hydraulic
flow in fine-grained soils.
3.3.10 In Situ Treatment Zones
Chemically or biologically active compounds injected
into fractures can act as broad sheets in the subsurface.
Examples of these types of compounds include fine-
grained 0-valence iron, which degrades chlorinated
solvents on contact (37), or encapsulated sodium per-
carbonate, which releases oxygen to stimulate aerobic
bacteria for several months (33). Fractures filled with
these compounds could be designed to create in situ
treatment zones.
One approach of this application creates flat-lying treat-
ment zones in the vadose zone through which contami-
nants migrate as they flow downward by gravity.
Alternatively, electrokinetics can induce contaminants to
migrate through the treatment zones. The development
of in situ treatment zones is in its infancy, but it can
potentially be a low-maintenance system that offers ma-
jor cost reductions compared to current methods.
3.3.11 Barriers
For years, induced fractures have been recognized as
a possible consequence of grouting (38, 39). Tech-
niques that purposefully fill fractures with grout are also
available (40). Most of these grouting techniques call for
creating many cross-cutting fractures to reduce the per-
meability of a zone. In one application, however, grout-
filled fractures from neighboring boreholes linked
together to form a continuous horizontal sheet (41). The
technique, termed the block displacement method (42),
probably was the first horizontal barrier successfully
created beneath a site. However, this test involved spac-
ing the boreholes a few meters apart and creating a slot,
which was nearly equal to the distance between the
boreholes, with a water jet. Accordingly, the fractures
propagated relatively short distances before they inter-
sected adjacent fractures. With wider spacing between
the wells, which many practical applications would prob-
ably require, it would be difficult to ensure that the
fractures form a continuous sheet.
The use of grout-filled fractures as barriers to flow has
received limited attention since the initial demonstration.
Bifurcation of a fracture into several lobes commonly
occurs. Moreover, current methods cannot detect this
phenomenon. Accordingly, ensuring that multiple frac-
tures connect to serve ultimately as a flow barrier would
be difficult.
3.3.12 Monitoring
Conventional wells in low-permeability formations are
associated with slow rates of sample recovery. There-
fore, monitoring the chemical composition of fluids in
these formations can be difficult. Inducing fractures in
the vicinity of a monitoring point would increase the rate
of sample recovery as well as broaden the area repre-
sented by the sample. This application is currently under
investigation, although results were unavailable as of
this writing.
3.4 Case Histories
Induced fractures have been used to improve environ-
mental applications since the late 1980s. Most of the
applications to date have been pilot-scale tests that
evaluate the technology and provide data for progress-
ing to full-scale. The petroleum industry, however, has
used the general technique for more than 50 years, and
it is a common means for improving the discharges of
water wells, particularly in granitic or metamorphic ter-
rains. All the applications strive to improve the perform-
ance of wells, and the petroleum and water applications
provide important insights into possible environmental
applications.
3.4.1 Overview
Records of oil production establish the benefits of hy-
draulic fracturing; ratios of production rates before and
after fracturing suitably measure those benefits. Accord-
ing to data collected from several dozen oil wells (43),
recovery ratios (production rate after fracture : initial
rate) range from 1.4 to 100 or more. In general, the ratio
ranges from 1.5 to roughly 10 for wells that were pro-
ducing before fracturing (Table 3-2). The ratios are large,
however, for wells that showed negligible production
prior to fracturing.
54
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Table 3-2. Summary of the Effects of Induced Fractures Used To Improve the Recovery of Wells
Location
Purpose
Injection
Fluid
Fracs/
Well
Formation Type
Performance
Without Fractures3
Performance With
Fractures
Ratio
Environmental Applications
Dayton, OH (33)
Beaumont, TX
Beaumont, TX
Cincinnati, OH
(22)
Chicago, IL (1)
Cincinnati, OH
Frelinghuysen,
NJ (9)
Newark, NJ (9)
Bristol, TN (21)
Bioremediation
Liquid recovery
LNAPL
recovery
Air recovery
Vapor
extraction
Steam injection
Air recovery
Air recovery
DNAPL+
water
Gel + sand
Gel + sand
Gel + sand
Gel + sand
Gel + sand
Gel + sand
Air
Air
Water
4
2
2
1
3
1
2
2
2
Silty clay glacial
Swelling clay
Swelling clay
Silty clay glacial
Silty clay glacial
Silty clay glacial
Silty clay glacial
Sandstone
Sedimentary rock
0.14-0.57 L/min m
0.0027 L/min m
0.0024 L/min m
3.34 L/min mH20
4.4 L/min mH20
<0.0019 kg/min mH20
6.6-11 L/min mH20
27-113 L/min mH20
0.32-0.98 L/min m
18-33 L/min m
0.13 L/min m
0.046 L/min m
67 L/min mH20
56-136 L/min mH20
0.055-0.066 kg/min mH20
87-204 L/min mH20
397-580 L/min mH20
1 .6-2.7 L/min m
50-100
50
19
20
22
30+
13-19
5-14
2.8-6.2
Petroleum Applications
California (43)
California (43)
Alaska
Texas (43)
Texas (43)
Texas (43)
West Virginia (43)
Oil recovery
Oil recovery
Oil recovery
Oil recovery
Oil recovery
Natural gas
Natural gas
Gel
Gel
Gel
Gel
Gel
Gel
Gel
1
1
1
1
1
1
1
Reservoir rock
Reservoir rock
Reservoir rock
Reservoir rock
Reservoir rock
Reservoir rock
Reservoir rock
20 bopd
10 bopd
1,128 bopd
6 bopd
50 bopd
15 mcfd
3.5 mcfd
120 bopd
70 bopd
1 ,584 bopd
65 bopd
130 bopd
1,100 mcfd
66.1 mcfd
6
7
1.4
10.8
2.6
73
19
Water Supply Applications
New Hampshire
(45)
New Hampshire
(45)
Massachusetts
(47)
Australia (48)
Water
Water
Water
Water
water
water
water
water
1
1
1
1
Rock
Rock
Rock
Rock
18 L/min
15.6 L/min
0.46 L/min
1 .8 L/min
109.2 L/min
68.4 L/min
11.4 L/min
13.2 L/min
6
4.4
22-25
6.9
a Performance measured as rate or rate/drawdown
bopd: barrels of oil per day
mcfd: millions of cubic feet of gas per day
1 mH2o head = 1.42 psi
Hydraulic fracturing also increases discharge of water
wells, and the relative increases resemble results from
the petroleum industry. Thirty years ago, Koenig (44)
examined data from wells used for waterflooding or
waste disposal and reported that 78 percent of those
wells increased discharge following hydraulic fracturing.
The recovery ratios ranged up to 100, with a median of
5.0 (44). More recent tests in the United States (19,
45-47) support these results (Table 3-2).
Recovery rates typically decrease as a function of time
due to depleting target fluid (oil, gas, water, or NAPL) or
reduced fracture permeability from closure or clogging.
Sediment or mineral precipitate are known to close or
clog fractures induced for petroleum recovery. In many
cases, however, creating another fracture can restore oil
recovery. Although environmental applications would be
expected to experience similar problems, several pro-
jects demonstrated that shallow fractures retain their
permeability for more than a year (9, 22, 49).
The recovery ratios for environmental applications resem-
ble some of the more successful applications from the
petroleum and water well industries, with typical values
between 10 and 50. Many of the environmental applica-
tions involve silty clay or rock of quite low permeability.
Therefore, the initial specific discharges (ratio of discharge
to head differential at the well) are exceptionally small,
55
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leaving a large margin for improvement. Similar ratios
are observed for both naturally propped fractures in rock
(created by injecting air or water) and sand-filled frac-
tures in silty clay. Interestingly, the ratios seem to be
relatively independent of the type of fluid recovered and
the remedial method used; applications for vapor extrac-
tion, liquid injection, steam injection, and NAPL recovery
report similar values (Table 3-2).
3.4.2 Selected Examples
Documents published through the Superfund Innovative
Technology Evaluation (SITE) program describe case
histories of hydraulic and pneumatic fracturing (36, 49).
These projects chiefly involve vapor extraction in tight
soils and rock. The following sections present two appli-
cations of NAPL recovery.
3.4.2.1 DNAPL Recovery From Bedrock,
Bristol, Tennessee
At a site near Bristol, naturally propped fractures were
created in July 1991 in rock at depths of 30.5 to 61
meters (100 to 200 feet) to enhance the recovery of
free-phase TCE and other DNAPLs. Injecting water into
intervals of the well isolated by straddle packers created
the fractures. Pumping tests and vapor extraction tests
were conducted to evaluate the effects of the fractures.
Site Conditions. Sandstone, shale, and limestone un-
derlie the vicinity of the site and form a broad fold and
dip approximately 45 degrees beneath the site itself.
The site lies in a local recharge area of a bedrock aquifer
characterized by downward vertical hydraulic gradients
of approximately 0.5 units (21). The hydraulic conduc-
tivity of the water-bearing formation is approximately 10"6
centimeters per second, based on constant rate tests.
Contaminants. The site contains a free-phase plume of
TCE, other solvents, and cutting oil that extends to a
depth of 100 meters (328 feet). Adissolved-phase plume
of primarily TCE extends to greater depthsat least 130
meters (426 feet)and has migrated at least 300 me-
ters (984 feet) from the suspected source. The specific
gravity of the free-phase liquid is 1.3 (21).
Design. Recovery using a pump and treat system
yielded approximately 3.7 liters (1 gallon) per minute of
water per well and fewer than 1.4 kilograms (3.1
pounds) per day of DNAPL. These low rates of recovery
provided impetus for using hydraulic fracturing tech-
niques to stimulate the wells. The intent was to increase
formation permeability that, in turn, would promote liquid
flow and possibly permit sufficient air flow into wells for
recovery though vapor transport.
Three new wells were drilled to 60 meters (197 feet) with
open hole completion, and the performance of these
wells was characterized before and after hydraulic frac-
turing. Each well was fractured by setting open hole
packers 15 meters (49.2 feet) apart and injecting 4,500
to 9,000 liters (1,188 to 2,376 gallons) of clean water;
no proppants were injected. Injection pressures ranged
between 0.5 and 5 MPa (73 and 725 psi), and the
injection rate was approximately 280 liters (73.9 gallons)
per minute. This approach resembles methods used to
increase the discharge of water wells (19). Injection was
terminated when water flowed around the upper packer
and began to spill to the surface. In one case, an obser-
vation well 2.5 meters (8.2 feet) away responded with
discharge of injected water. The initial discharge was
muddy but cleared with continued injection, suggesting
that fine-grained particles had been removed from frac-
tures in the formation (21).
Results. The specific discharge of the three wells in-
creased by factors ranging from 2.8 to 6.2. These effects
are typical of naturally propped fractures created by
hydraulic fracturing of water wells (Table 3-3). Pumping
tests helped determine the effective hydraulic conduc-
tivity after fracturing. In general, the results indicated
that the effective hydraulic conductivity increased by
factors of 20 or more. (Actual values depend on the
method of solution used to analyze the test data.)
Table 3-3. Specific Capacities Before and After Hydraulic
Fracturing
TW-1
TW-2
TW-3
Before
L/min m
0.32
0.45
0.98
After
L/min m
1.6
2.8
2.7
Ratio
5.0
6.2
2.8
Inducing fractures appeared to make vapor extraction a
feasible remedial technique. After fracturing, vapor dis-
charges were on the order of 285 to 700 L/min, and
suction was detectable 10 meters (32.8 feet) from the
recovery well. In contrast, both discharge and suction in
the formation were negligible prior to fracturing.
During a 2-day test of vapor extraction, DNAPL was
recovered at a rate of approximately 82 kg/day (180
Ibs/day). Concentrations diminished during this test,
probably representing an upper limit of the recovery
rate. Nevertheless, the combination of hydraulic fractur-
ing to increase conductivity and suction to induce dewa-
tering and DNAPL recovery appeared to be a viable
method of increasing contaminant recovery at this site.
Cost. Reportedly, the cost to create the fractures used
during this project was $1,500 per well.
3.4.2.2 LNAPL Recovery From Swelling Clay,
Beaumont, Texas
In July 1993, sand-filled hydraulic fractures were created
in swelling clay to enhance the recovery of free-phase
56
-------
LNAPL at a site in Beaumont. A pilot test followed in late
February 1994.
Site Conditions. Silty clay of the Beaumont formation
underlies the site to a depth of 6 to 8 meters (19.7 to
26.2 feet), with fine-grained sand below it. In general,
the Beaumont formation consists of kaolinite, illite, cal-
cium smectite, and fine-grained quartz (50). Wetting and
drying cycles have resulted in overconsolidation in the
upper 8 to 10 meters (26.2 to 32.8 feet). Drying near the
ground surface largely decreased clay volume there,
commonly resulting in dessicated areas. Moreover, in
the interval containing hydraulic fractures, lateral stress
is two to three times greater than vertical stress in the
upper few meters (50).
The upper meter of the site is fill, composed ofsiltyclay,
gravel, and shells. From 1 meter (3.3 feet) to approxi-
mately 3.6 meters (11.8 feet), the formation is a firm to
stiff, dark gray, silty clay with reddish to olive yellow
mottling. Slickensided partings, which indicate preexist-
ing fractures, are common. A light gray, clayey silt occurs
at approximately 3.6 meters (11.8 feet).
Fractures were created between 2 and 3.6 meters (6.6
to 11.8 feet) deep so most of them were initiated in the
dark gray, silty clay. The deepest fractures, however,
were initiated in the light gray, clayey silt. The watertable
was between 1 and 1.5 meters (3.3 to 4.9 feet) deep, so
all the fractures were created in saturated conditions.
Contaminants. The area of the test contained gasoline
and cyclohexane, which infiltrated from surface spillage.
The contaminant appeared as free-phase NAPL from
approximately 1.5 to 3 meters (4.9 to 9.8 feet) in depth
in the vicinity of Wells I, C, and PW-1, and it thinned to
the east toward Well G (Figure 3-18).
control well. One of the fractured wells in the test con-
sisted of two casings that accessed fractures at different
depths, one in the LNAPL and the other in the water
bearing zone below. The other well only contained one
fracture nearthe bottom of the NAPL zone (Figure 3-19).
The goal of the two-fracture design was to recover NAPL
from the upper fracture and water from the lower one.
This approach, if successful, would limit upward coning
of water, which would both increase the rate of recovery
of NAPL and improve the NAPL-to-water ratio (to reduce
costs of phase separation) compared with recovering
from one fracture. Both fractured wells were expected
to produce at greater rates than the conventional well.
Sand-filled fractures were created at six locations at the
site, but only two of them, I and C, were used during the
pilot test. At 1-12 (Figure 3-18), a single sand-filled frac-
ture was created at a depth of 3.6 meters (11.8 feet). At
Well C, four fractures were stacked one above the other,
but the test used only the deepest two, C-10 and C-12.
The fracture at C-10 was initiated at a 3-meter (9.8-foot)
depth, and it curved upward and cut through much of
the zone containing NAPL. Fracture C-12, which was
initiated at 3.6 meters (11.8 feet), extended mostly be-
neath the NAPL zone (Figure 3-19). The fractures were
planned to be approximately circular, with diameters of
7 to 8 meters (23 to 26.2 feet) and average thicknesses
of 5 to 6 millimeters (0.20 to 0.24 inches) (Table 3-4). A
conventional well, PW-1, was screened from 2 to 4
meters (6.6 to 13.1 feet) in depth and used as control.
Clusters of multilevel piezometers (depths of 1.2, 2.4,
and 3.6 meters [3.9, 7.9, 11.8 feet]) with short screens
were installed along a line from Well I through Well C
(Figure 3-18).
Table 3-4. Specifications of Fractures Used During the Pilot
Test
design. The pilot test was designed to compare the Average
erformance of two designs of fractured wells with a _ .. M"m T1?*nd Average Sand
a Depth Uplift Thickness Diameter Volume
m mm mm m m3 (ft3)
r Storage
Tank
C-10 G_85 G-10
C-8.5 I m ^^^
"Pc-12 " " " OP'12
.'
f Storage
I .10 i - i -p i i
1 0 Meters J
N H
1 A Well Accessing Fracture 1|
I PW-1 tH
^ Multilevel Piezometer ||
A Control Well %
C-10 3 16 6 7 0.23(8)
C-12 3.6 24 5.5 8 0.28(10)
1-12 3.6 22 5.5 8 0.28(10)
A constant head was maintained approximately 1 0 cen-
timeters (4 inches) above each fracture, producing a
drawdown of 1 .5 to 2 meters (4.9 to 6.6 feet). Fluid was
pumped from the wells to storage drums and periodically
diverted to a graduated cylinder to determine discharge
rate. The proportion of NAPL and water in the beaker
was measured to estimate the discharge of each phase.
Figure 3-18. Area of LNAPL recovery using fractured and con-
ventional wells.
Results. Both wells containing fractures produced
LNAPL at rates an order of magnitude greater than the
conventional well (Table 3-5). The C location was par-
ticularly noteworthy, producing both the greatest NAPL
57
-------
Two-Fracture Design
(fractures C-10 and C-12)
One-Fracture Design
(fracture 1-12)
Water
LNAPL
Water + LNAPL
Figure 3-19. Methods of completing a well containing hydraulic fractures to recover LNAPL.
Table 3-5. Average Discharges and Ratios of Discharge
C-10
C-12
C (combined)
1-12
PW-1
NAPL
L/hr
4.33
0.07
4.40
1.85
0.23
Water
L/hr
0.34
8.24
8.58
5.61
0.03
Total
L/hr
4.7
8.3
13.0
7.5
0.26
NAPL/
Water
13
0.008
0.5
0.3
7
NAPL/
PW-1
19
0.3
19
8
Total/P
W-1
18
32
50
29
rate and the greatest NAPL-to-water ratio. Fracture
C-10 produced a high concentration of LNAPL at a rate
that was 19 times greater than the control, whereas
C-12 produced almost completely water. The combined
rate of liquid recovery from the C location was 50 times
greater than from the control.
As an additional test, the rates were evaluated as a
function of time, then the pump in the C-12 fracture was
turned off. These results show that Well C initially rapidly
recovered water, presumably as it drained out water
used to created the fractures. Then C-12 recovered
water at a constant rate while C-10 primarily recovered
NAPL (Figure 3-20). After the pump was turned off at
116 hours, the discharge from C-10 changed abruptly;
the total discharge from C-10 increased, but the recov-
ery of NAPL actually decreased. Apparently, turning off
the pump in C-12 caused water to flow upwards, effec-
tively reducing the area at C-10 available to recover NAPL.
The distribution of head was consistent with the rela-
tively large NAPL recovery by wells intersecting sand-
filled fractures. Bowl-shaped zones of relatively large
drawdown occurred in the vicinity of the fractures (Fig-
ure 3-20). In addition, significant drawdown occurred
throughout the area between Wells I and C, and draw-
down of 7 centimeters (2.8 inches) occurred over a band
25 meters (82 feet) long in the vicinity of the two wells.
15
10
5
1 '
1
, 0
6
. i > i i i
i i
1 1 1 ,
Water
LNAPL
ll
' 1 '
?
O
C
1
"^ O CM T O CM V O CM O CM OCM TO CM O CM T
~> 6 6 £ 66o_6666 66o_66 66c
j
)
~0
LO '
, '
20
40 60
Hours
80 100
120
Figure 3-20. Discharges from C-10, C-12, and PW-1 as func-
tions of time.
Drawdown in the vicinity of PW-1 was unavailable, but
similar tests in the area have shown that drawdown is
negligible within 1 to 2 meters (3.3 to 6.6 feet) of con-
ventional wells.
It is noteworthy that the fractures caused large vertical
head gradients (Figure 3-21). Multilevel piezometers
with short screens (25 centimeters [9.8 inches]) were
required to characterize the head distribution. Conven-
tional piezometers screened over a large interval would
have missed the vertical gradients, resulting in a mis-
leading estimate of the effects of the fractures in the
subsurface.
Cost. The fractures cost approximately $800 to $1,000
each to create and complete as wells. This cost includes
mobilization, materials, labor, and equipment. The pilot
test itself cost approximately $40,000.
58
-------
1-12
C-12
10m
C-10
Piezometric Monitor
Sand-filled Fracture
Contour-Head Change (cm)
3m
Figure 3-21. Cross section along line of piezometers showing change in head after 5 days of pumping.
3.5 References
When an NTIS number is cited in a reference, that
document is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
703-487-4650
1. U.S. EPA. 1993. Hydraulic fracturing technology:
Applications analysis and technology evaluation re-
port. EPA/540/R-93/505.
2. U.S. EPA. 1993. Accutech pneumatic fracturing and
hot gas injection, Phase I. Technology demonstra-
tion summary. EPA/540/SR-93/509.
3. Schuring, J.R., V. Jurka, and PC. Chan. 1991.
Pneumatic fracturing to remove VOCs. Remedia-
tion Winter:51-67.
4. Murdoch, L.C., G. Losonsky, P. Cluxton, B. Patter-
son, I. Klich, and B. Braswell. 1991. The feasibility
of hydraulic fracturing of soil to improve remedial
actions. Final report. EPA/600/2-91/012 (NTIS
PB91-181818).
5. Frappa, R.H., and T.H. Forbes. 1994. Innovative
ground water remediation solutions for the Mercury
aircraft site. Presented at NYS WEA Spring Confer-
ence, Corning, NY (June).
6. Begor, K.F., M.A. Miller, and R.W. Sutch. 1989.
Creation of an artificially produced fractured zone
to prevent contaminated ground water migration.
Ground Water 27(1):57-65.
7. Zinck, E. 1984. Method of and device for loosening
agriculturally used soil. U.S. Patent 4,429,647.
8. Waltz, J.P, and T.L Decker. 1981. Hydro-fracturing
offers many benefits. Johnson Drillers J. 53:4-9.
9. Schuring, J.R., and PC. Chan. 1992. Removal of
contaminants from the vadose zone by pneumatic
fracturing. NTIS PB92-161207. Reston, VA: U.S.
Geological Survey, Water Resources Division
(January).
10. Driscoll, F.G. 1986. Ground water and wells, 2nd
ed. St. Paul, MN: Johnson Division Publisher.
11. Passamaneck, R.S. 1993. Controlled propellant
fracturing for vertical wells. SPE Paper 26271.
12. Chu, T.Y., N. Warpinski, R.D. Jacobson. 1988. In
situ experiments of geothermal well stimulation us-
ing gas fracturing technology. Sandia Report
SAND87-2241. Albuquerque, NM: Sandia National
Laboratories.
13. Sarapu, E. 1985. Electrofrac rock-breaking sys-
tems. Skillings Mining Rev. January:2-8.
14. Du, Y, A. Aydinm, and L. Murdoch. 1993. Incre-
mental growth of shallow hydraulic fracture at a
waste remediation site. 34th Symposium on Rock
Mechanics, Madison, Wl (June).
15. Davis, P.M. 1983. Surface deformation associated
with a dipping hydrofracture. J. Geoph. Resour.
88:5,826-5,834.
16. Sun, R.J. 1969. Theoretical size of hydraulically
induced fractures and corresponding surface uplift
in an idealized medium. J. Geophys. Resour.
74:5,995-6,011.
17. Warpinski, N.R. 1985. Measurement of width and
pressure in a propagating hydraulic fracture. Soc.
Petrol. Eng. J. February:46-54.
18. Holzhausen, G.R., C.S. Haase, S.H. Stow, and G.
Gazonas. 1985. Hydraulic-fracture growth in dip-
ping anisotropic strata as viewed through the sur-
face deformation field. Proceedings of the 26th U.S.
Symposium on Rock Mechanics, Rapid City, SD.
59
-------
19. Smith, S.A. 1989. Manual of hydraulic fracturing for
well stimulation and geologic studies. Dublin, OH:
National Water Well Association.
20. Schuring, J.R., P.C. Chan, J.W Liskowitz, P. Pa-
panicolaou, and C.T. Bruening. 1991. Method and
apparatus for eliminating non-naturally occurring
subsurface, liquid toxic contaminants from soil. U.S.
Patent 5,032,042 (July 16).
21. Lundy, D.A., C.J. Carleo, M.M. Westerhiem. 1994.
Hydrofracturing bedrock to enhance DNAPL recov-
ery. Proceedings of 8th Annual NGWA National Out-
door Action Conference (May).
22. Wolf, A., and L. Murdoch. 1993. Afield test of the
effect of sand-filled hydraulic fractures on air flow
in silty clay till. Proceedings of the Seventh National
Outdoor Action Conference, Las Vegas, NV (May).
23. McKay, L.D., J.A. Cherry, and R.W Gilham. 1993.
Field experiments in a fractured clay till, 1. Hydrau-
lic conductivity and fracture aperture. Water Resour.
Res. 29(4):1, 149-1, 162.
24. Brooker, E.W, and H.O. Ireland. 1965. Earth pres-
sures at rest related to stress history. Can. Geotech.
J.
25. U.S. Geological Survey. 1967. Distribution of prin-
cipal kinds of soils: Orders, suborders, and great
groups. Sheet 86.
26. Flint, R.F. 1971. Glacial and quaternary geology.
New York, NY: John Wiley and Sons.
27. Tsang, YW 1992. Usage of "equivalent apertures"
for rock fractures as derived from hydraulic and
tracer tests. Water Resour. Res. 28(5):1, 451 -1,455.
28. Muskat, M. 1949. Physical principles of oil produc-
tion. Boston, MA: McGraw-Hill.
29. Novakowski, K.S. 1988. Comparison of fracture ap-
erture widths determined from hydraulic measure-
ments and tracer experiments. In: Hitchon, B., and
S. Bachu, eds. Proceedings of the Fourth Cana-
dian/American Conference on Hydrogeology. Dub-
lin, OH: National Water Well Association, pp. 68-80.
30. Bear, J. 1979. Hydraulics of ground water. New
York, NY: McGraw-Hill.
31. American Petroleum Institute. 1989. A guide to the
assessment and remediation of underground petro-
leum releases. API Publication 1628. Washington,
DC.
32. Blake., S.B, and M.N. Gates. 1986. Vacuum en-
hanced hydrocarbon recovery. Proceedings of the
Second Hazazardous Materials Management Con-
ference, Long Beach, California (December), pp.
17-25.
33. Vesper, S. J., L.C. Murdoch, S. Hayes, and WJ.
Davis-Hoover. 1994. Solid oxygen source for biore-
mediation in subsurface soils. J. Haz. Mat. 36:265-
274.
34. Davis-Hoover, W.J., L.C. Murdoch, S.J. Vesper,
H.R. Pahren, O.L. Sprockel, C. L. Chang, A. Hus-
sain, and W A. Ritschel. 1991. Hydraulic fracturing
to improve nutrient and oxygen delivery for in situ
bioreclamation. In situ bioreclamation. Boston, MA:
Butterworth-Heinemann. pp. 67-82.
35. Mack, J.P., and H.N. Apsan. 1993. Using pneumatic
fracturing extraction to achieve regulatory compli-
ance and enhance VOC removal from low-perme-
ability formations. Remediation Summer:309-326.
36. U.S. EPA. 1993. Accutech pneumatic fracturing and
hot gas injection, Phase I. Applications analysis.
EPA/540/AR-93/509.
37. Gillham, R.W, and D.R. Burris. 1992. In situ treat-
ment wells: Chemical dehalogenation, denitrifica-
tion, and bioaugmentation. Proceedings of the
Subsurface Restoration Conference, Dallas, TX
(June), pp. 66-68.
38. Morgenstern, N.R., and PR. Vaughan. 1963. Some
observations on allowable grouting pressures. Pro-
ceedings of the Conference on Grouts and Drilling
Muds. London, England: Institute of Civil Engineer-
ing, pp. 36-42.
39. Wong, H.Y, and I.W Farmer. 1973. Hydrofracture
mechanisms in rock during pressure grouting. Rock
Mechanics 5:21-41.
40. Zuomei, Z., and H. Pinshou. 1982. Grouting of the
karstic caves with clay fillings. In: Baker, W.H., ed.
Proceedings of the Conference on Grouting in
Geotechnical Engineering, New Orleans, LA (Feb-
ruary), pp. 92-104.
41. Brunsing, T.P, and R.B. Henderson. 1984. A labo-
ratory technique for assessing the in-situ construc-
tability of a bottom barrier for waste isolation.
Proceedings of the National Conference on Manag-
ing Uncontrolled Hazardous Waste Sites, Washing-
ton, DC. pp. 135-140.
42. Brunsing, T.P. 1987. The block displacement
method field demonstration and specifications. U.S.
Environmental Protection Agency Project Summary
600-S2-87-023. pp. 1-6.
43. Howard, G.C., and C.R. Fast. 1970. Hydraulic frac-
turing. New York, NY: Society of Petroleum Engi-
neers, AIME.
44. Koenig, L. 1960. Survey and analysis of well stimu-
lation performance. J. Am. Water Works Assoc.
52:333-350.
60
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45. Stewart, G.W. 1974. Hydraulic fracturing of crystal-
line rock stimulates yield of two test wells drilled in
New Hampshire. Ground Water 12:46-47.
46. Stewart, G.W. 1978. Hydraulic fracturing of drilled
water wells in crystalline rocks of New Hampshire.
New Hampshire Department of Resources and
Economic Development and Water Resources Re-
search Center.
47. Mony, P.M. 1989. Hydro-frac: One man's opinion.
Water Well J. Feb:48-50.
48. Williamson, W.H., and D.R. Woolley. 1980. Hydrau-
lic fracturing to improve the yield of bores in frac-
tured rock. Australian Water Resources Council
Technical Paper 55:1-76.
49. U.S. EPA. 1993. Hydraulic fracturing technology:
Applications analysis and technology evaluation.
EPA/540/SR-93/505.
50. Mahar, L.J., and M.W O'Neill. 1983. Geotechnical
characterization of desiccated clay. J. Geotech.
Engin. 109(1):56-71.
61
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Chapter 4
Interceptor Trenches
Other than vertical wells, trenches are the most widely
used method of controlling subsurface fluids and recov-
ering contaminants; trenches are remarkably effective,
and widely available construction equipment can be
used to install them. The construction and agricultural
industries have long used trenches or drains for dewa-
tering, and many environmental practices are based on
dewatering methods. However, new methods of install-
ing and completing trenches for environmental purposes
have recently been developed. For instance, techniques
for analyzing a trench's ability to control ground-water
flow directions and travel times differ from the dewater-
ing techniques.
Atrench provides a long zone with which to collect fluids.
Perhaps the most common applications of this linear
feature are to increase the rate of recovery from low-per-
meability formations or to intercept the migration of a
plume. The former uses the large surface area available
for drainage, whereas the latter places the trench normal
to the regional gradient to enhance plume capture.
Trenches also can have a significant vertical compo-
nent, which cuts across and can allow access to the
permeable layers in interbedded sediments. Alterna-
tively, the vertical component can facilitate LNAPL re-
covery from areas with considerable seasonal
fluctuations in the watertable. Although trenches primar-
ily recover liquids, vapor extraction also benefits from
the large area exposed by a trench.
Although trenches have various applications, most of
this chapter discusses the trenches used to recover and
deliver fluids: interceptor trenches. In their simplest
form, interceptor trenches are slots excavated in the
earth and filled with highly permeable material. More
sophisticated designs involve placing perforated pipe or
casing at the bottom of the excavation (Figure 4-1); the
casing commonly slopes downward from the ends of the
trench towards a sump, which collects and removes
water or contaminant through a vertical access casing
(1). Commonly, low-permeability material caps trenches
to prevent surface water or air (for vapor extraction
applications) from diluting the fluids recovered from the
formation.
Recovered Fluids
Access Casing
Pump
Figure 4-1. Cross sections of a basic interceptor trench con-
figuration showing permeable backfill and per-
forated casing.
This chapter presents methods of constructing trenches
and discusses some of the factors that affect their de-
sign and the decision to use trenches at a site. It also
presents an overview of some of the applications of
trenches and several case histories of sites where
trenches have been used.
4.1 Trench Construction
Most interceptor trenches are created with construction
methods adapted to environmental applications. Some
specialized techniques are available for particularly
deep installations, streamlining the installation and com-
pletion process, or for forming trenches in rock.
4.1.1 Conventional Methods
Creating an interceptor trench typically involves:
Excavating material
Supporting the trench walls
Backfilling with permeable material
Installing casing and pumps
Sealing the surface over the trench
Standard construction equipment, either a backhoe or
a trenching tool, commonly is used to perform the
63
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excavation. Backhoes capable of digging a trench up to
5.5 meters (18 feet) deep are readily available in most
locations. Extended-reach hydraulic excavators (2) may
reach depths approaching 15.2 meters (50 feet), with
widths of 0.9 to 1.5 meters (3 to 5 feet). Trenches of
intermediate depth (6.1 to 9.1 meters [20 to 30 feet])
may require an excavation wide enough to accommo-
date the backhoe or trencher if the equipment is to reach
the desired depth.
Trenching tools are available to construct trenches of
various depths and widths. Small vibratory plow/
trencher units create slots a few inches in width and up
to 0.6 meters (2 feet) deep. Many of these small units
were designed for utility installation (gas line, electric,
telephone) and can install the pipe or cable while creat-
ing the slot.
Medium-sized, tractor-mounted trenching units may
reach depths of 2.4 to 3 meters (8 to 10 feet), with widths
of up to 0.6 meters (2 feet). Attachments for these units
permit trench excavation through concrete and asphalt
pavement. Large trenching systems are discussed in
detail in the next section.
Vertical walls on the side of a trench may stay open if
the trench is relatively shallow and cuts through cohe-
sive soil. In many cases, however, the walls of the trench
collapse if they are unsupported. Traditional methods of
support include a sliding trench box, sheet pilings, or
timber beam bracing (shoring). Bentonite slurry also
sometimes supports trenches, although the bentonite
may significantly reduce the permeability of the forma-
tion adjacent to the trench; therefore, this technique
should be avoided for interceptor trenches.
Recently, a slurry formed from guar gum, a cellulose-
based polymer, has been used to support deep trench
excavations in an effort to avoid the problems associ-
ated with bentonite (2, 3). Guar-based slurries have high
gel strength and viscosity (greater than 40 centipoises)
and low water loss (filtrate less than 25 milliliters); these
properties permit efficient transfer of hydrostatic head
from the slurry to the trench walls. Slurry head of 0.9
meters (3 feet) or more over ground-water head should
stabilize most soil types. Guar gum used to support a
trench is treated so that it degrades when construction
is completed; the guar decomposes to water and biode-
gradable sugars and is pumped from the trench. Usually
three trench pore volumes circulate to remove residual
slurry, in a process similar to well development. Small
amounts of degraded slurry may remain as excess fluid
in the trench and must evaporate, solidify, or be dis-
posed of through a wastewater treatment facility (2-4).
Permeable backfill material (coarse sand or gravel) and
injection/extraction structures (perforated casing, sumps,
access pipes, and wells) are placed in the trench
through the slurry. The fill material typically consists of
clean washed gravel (e.g., crushed stone, pea gravel).
When anticipating fine sediment clogging, engineered
gradation or a geotextile filter fabric may be included.
Engineered gradation involves placing the fill material
into the trench by sliding it down the slope of previously
deposited fill or by tremie tube. (Sand and finer material
must be pre-wetted; gravels may be tremied dry.) Tremie
tube emplacement is preferable around wells and per-
forated casing. Using woven geotextile fabrics can in-
hibit siltation. Geotextile filters generally must be
weighted to sink through the slurry. Overlapping sheets
of geotextile is usually sufficient to ensure filter continuity.
The backfill should extend near the ground surface and
always above ground-water level. Excavated soil can
usually fill the remainder of the trench, although regula-
tions may prevent backfilling with contaminated material
at some sites. Backfill or clay usually seals the top of the
trench to prevent infiltration of surface runoff or air.
Vertical access pipes installed in the fill material are the
most common injection/extraction structures. Perforated
pipe, which is commonly laid along the bottom of the
trench, connects to the vertical access pipes. This tech-
nique is particularly prevalent in relatively shallow
trenches created in cohesive soil with free-standing
walls. Slurry in the trench may buoy the perforated pipe
and make installation cumbersome in deep excavations.
Practitioners disagree on the need for perforated pipe in
trenches; some argue that permeable backfill is suffi-
cient. A variety of casing types exist, including PVC,
polyethylene, and galvanized and stainless steel. Sub-
mersible ejector and progressive cavity pumps have
been successfully implemented.
4.1.2 Specialized Methods
Most interceptor trenches are installed using methods
similar to the ones outlined above. Some cases, how-
ever, require specialized methods to increase the effi-
ciency of the installation procedure orto create trenches
that are particularly deep or in rock.
4.1.2.1 Continuous Excavation and Completion
A recently developed technology combines trench exca-
vation, well installation, and barrier installation proc-
esses in a single step (5). The equipment digs a trench
up to 6.1 meters (20 feet) deep, lays in a flexible hori-
zontal well screen, and backfills the trench with the
original soil. Typically, a vertical excavation 35.6 centi-
meters (14 inches) wide is completed to the desired
depth. Then, a vertical casing is installed through the
trenching head. The well is coupled to the flexible well
screen. The trenching machine then moves along the
trench line (Figure 4-2), installing the horizontal well
screen at an average rate of 0.9 to 1.5 meters (3 to 5
feet) per minute (7). Sections of screen can be coupled
to create continuous well screens in excess of 610
64
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Well Screen
1°^^
Figure 4-2. Schematic of continuous excavation and comple-
tion technique (6).
meters (2,000 feet). A permeable polyester and glass
filter system encases the flexible well screen to reduce
or eliminate clogging by fine sediments. Moreover, the
well screen/filter system may be installed within a sand
or gravel filter pack in the trench. (The sand or gravel is
placed at the same time as the well screen.)
Reportedly, continuous excavation and completion sys-
tems are about half the cost of bored (drilled) horizontal
well systems, and as much as 70 percent less expensive
than vertical well systems (5). At a site in North Carolina,
initial remediation plans called for 100 vertical wells to
recover a hydrocarbon plume at an estimated cost of $1
million. Instead, a continuous excavation and comple-
tion system was installed at a cost of under $350,000. A
second benefit under some settings is faster remedia-
tion. A fuel storage facility in Florida opted for a continu-
ous excavation and completion system with nine wells
over a conventional vertical well system requiring 41
wells. Actual time for successful remediation was just 8
weeks, compared with an estimated 30 months for the
vertical well system (5). Finally, this technology reduces
worker exposure to contaminants.
This technique has some limitations:
The trenching head cannot penetrate bedrock.
The equipment cannot access beneath buildings or
landfills.
The equipment is currently limited to a depth of less
than 6 meters (19.7 feet).
4.1.2.2 Deep Applications
Traditional excavation of deep trenches requires the use
of chisels or grabs suspended on cables or guided by
kelly bars. Recent developments have elicited new tech-
nologies for deep excavation for trench and slurry wall
construction. An example is "trench cutting." Large-
toothed cutting wheels loosen the soil material and mix
it with a bentonite slurry (Figure 4-3). The equipment
then pumps the soil-slurry suspension to the surface,
where oil particles are removed and the slurry is re-
turned to the trench for reuse. This technique may be
used to construct trenches 0.5 to 1.5 meters (1.6 to 4.9
feet) wide and up to 120 meters (394 feet) deep. Exca-
vation rates in unconsolidated sediments and soft rock
of 25.2 to 39.8 cubic meters (33 to 52 cubic yards) per
hour have been reported (8); excavation rates in rock
with high compressive strength 145 to 179 MPa (21,000
to 26,000 psi) are reported to be 1.1 to 1.9 cubic meters
(1.5 to 2.5 cubic yards) per hour. This technique is
effective in all types of unconsolidated sediments and in
medium to hard bedrock formations (e.g., limestone,
sandstone). To create cutoff walls, the trench may be
filled with low- permeability materials (e.g., bentonite,
concrete). Alternatively, to create an interceptor trench,
fill may consist of high permeability materials. Additionally,
a crane can be used to install precast/preconstructed pan-
els. Benefits of the trench cutting technique include:
Higher rates of productivity than traditional deep ex-
cavation methods.
Greater control on verticality of the trench because
of the rigid excavation platform.
Reduced shock and vibration (commonly associated
with grab and chisel excavation), resulting in de-
creased risk of damage to adjacent structures and of
trench wall collapse.
4.1.2.3 Trenches in Rock
Economic reasons generally preclude excavating
trenches in rock. Trenchlike structures can be created in
To Setting Pond
Steel Frame
Hydraulic Motor
Centrifugal Pump
Spade With Pump Inlet
Cutting Wheels
Figure 4-3. Schematic of deep trench excavation equipment
(redrawn from material provided by Coastal
Caisson Corp., Clearwater, FL).
65
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rock, however, by detonating explosives in closely
spaced boreholes. One case in New York (9) created an
elongated fracture zone resembling a trench by placing
approximately 900 kilograms (1,984 pounds) of explo-
sive in 60 boreholes on a line 100 meters (328 feet) long
and perpendicular to ground-water flow. The bores were
7 meters (23 feet) deep and penetrated a sandstone
aquifer with a hydraulic conductivity of 2.8 x 10"4 centi-
meters per second. After detonation, fractures extended
between boreholes, from 7 meters (23 feet) deep to the
ground surface, and 3 to 4 meters (9.8 to 13.1 feet) into
adjacent rock. A conventional well placed in the zone
recovered water. Other instances of blasting to create a
permeable wall in rock exist, although published ac-
counts are scarce.
4.2 Design Considerations
The design of an interceptor trench system requires
information related to geology, hydrology, trench hydrau-
lics, and nature of contaminants. These factors influence
the length, depth, and location of a trench, as well as
the method of operation and type of completion.
4.2.1 Pattern of Flow to a Trench
The advantage a trench offers over other physical meth-
ods of recovery mostly results from the pattern of flow
that the trench induces in the contaminated formation.
Unlike a vertical well, which causes flow along radial
paths, the pattern of flow to a trench changes with time.
Under ideal conditions, the flow pattern to a trench
progresses through three basic periods: linear flow, tran-
sition, and radial flow. These periods are best recog-
nized when the trench penetrates a laterally extensive,
confined aquifer with minimal recharge and regional
gradient. In the presence of boundaries, recharge, or a
regional gradient, the flow pattern differs from the ideal-
ized case; however, the pattern still changes with time,
and this change still affects the trench performance.
Figure 4-4. The pattern of flow to a trench with no regional
gradient: a) linear flow, b and c) transition, and
d) radial flow (11).
The period of linear flow occurs early on, shortly after
the onset of pumping, when ground water flow is nearly
perpendicular to the plane of the trench (Figure 4-4a).
During the linear flow period, flow vectors are essentially
parallel, and the region affected is small when compared
with the entire length of the trench.
In contrast, the period of radial flow occurs after the
trench has been operating for a relatively long time.
During this period, flow converges from great distances
toward the trench (Figure 4-4d), much as flow converges
toward a well. Except in the vicinity of the trench, flow
vectors are essentially radial during this period, causing
the trench to behave like a well of large diameter.
Details of the flow pattern during the transition period,
as the flow changes from linear to radial, depend on
details of the trench geometry. For example, if the trench
partially penetrates the aquifer, the linear period is fol-
lowed by a period when the flow is linear and adjacent
to the trench, radial and converging upward beneath the
bottom of the trench, or radial and converging horizon-
tally toward the ends of the trench (Figure 4-4b). With
increasing time, the lower radial zone expands down-
ward and affects the bottom of the aquifer, and the area
influenced by the trench increases (Figure 4-4c). Even-
tually, the area of horizontal radial flow grows until it is
dominant and the period of radial flow begins. The tran-
sition period is somewhat simpler when the trench fully
penetrates the aquifer because it lacks the early period
of upward flow toward the bottom of the trench.
The pattern of flow underlies much of the hydrodynamic
performance of an interceptor trench. For example,
analyses that consider an interceptor trench in section
view only include flow normal to the trench and omit flow
from areas beyond the ends of the trench (published
analyses of trenches are summarized in Beljin and Mur-
doch [10]). The consequences of this assumption are
relatively minor early on, but amplify as time increases.
Neglecting the effects of the ends of the trench may
significantly underestimate the discharge (11).
One approach to estimating the discharge of a trench at
late times represents it as a well of large diameter. It is
possible to represent a trench of infinite hydraulic con-
ductivity (1,000 or more times the conductivity of the
aquifer) by considering a well whose radius is one-quar-
ter the total length of the trench (11).
The actual times when the flow pattern changes depend
on the length of the trench, the hydraulic conductivity of
the aquifer K, aquifer thickness h, aquifer storage S,
trench length 2\, and perhaps other quantities. Rather
than examining all the different possibilities of trench
length and aquifer properties, it is convenient to combine
those quantities with time t to give dimensionless time:
td =
(4-1)
66
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The periods of flow to a fully penetrating trench follow
(11):
Linear flow (early time): fd < 0.25
Transition: 0.25 < td < 25
Radial flow (late time): td > 25
This framework shows that the duration of the different
flow periods in real time t can vary widely, depending in
particular on the hydraulic conductivity of the aquifer and
the length of the trench. Consider, for example, a trench
200 meters (656 feet) long (x, = 100 meters [328 feet]),
in a layer of silty sand that is 5 meters (16.4 feet) thick,
with K= 10"4 centimeters per second and S= 0.1. The
linear flow period would last approximately 0.4 year, and
the radial flow period would not begin before 40 years.
However, if the trench were half that long (x, = 50 meters
[164 feet]) and installed in a clean sand (K= 10"2 centi-
meters per second), the linear flow period would be only
8.7 hours, and the radial flow period would begin after
36 days. Note that when the trench is very long, the
linear flow period lasts an extremely long time and the
cross-sectional analyses are appropriate.
4.2.2 Site Geology
Site geology affects the design process in two areas:
performance efficiency and trench construction. Several
geologic factors affect the performance of the trench
system, including heterogeneity of the aquifer, formation
composition, and degree of saturation (12).
4.2.2.1 Heterogeneities
Formation heterogeneity is one of the most notorious
obstacles to contaminant recovery, and trenches prob-
ably present the best physical method of addressing this
obstacle. This is particularly true when the heterogeneity
involves horizontal stratification because the trench can
cut across the strata and access all the permeable units
simultaneously.
Vertical heterogeneities, such as fractures, may be
drained effectively if the trench is roughly perpendicular
to the fracture strike. Pumping tests or geological map-
ping may help delineate the orientation of vertical frac-
tures when locating the trench.
Vertical fractures or other heterogeneities may cause
preferential flow of fluids in the subsurface. Therefore,
even though a trench can access many fractures and
produce fluids at an impressive rate, vertical fractures
may cause the flow to channel and avoid unfractured
areas. In this case, diffusion from the unfractured areas
can control recovery of contaminants. Accordingly,
trenches are an excellent method of controlling fluid flow
in fractured or heterogeneous formations, although even
the advantage of accessing many of the fractures (com-
pared with a vertical well) may be insufficient to recover
contaminants from unfractured areas.
4.2.2.2 Formation Composition
The composition of the formation primarily affects how
the trench is constructed. Most excavation methods are
capable of removing formations from unlithified sedi-
ments to weathered rock. However, they are unable to
effectively penetrate well-cemented sediments or meta-
morphic or igneous rocks. In the latter case, it is still
possible to use a trenchlike approach for recovery, al-
though the approach requires specialized trench-cutting
methods (8) or blasting to create the permeable zone (9).
To prevent collapse of the trench walls during excava-
tion, many sediments must be supported, typically by
filling the trench with a dense fluid. The need for support
of the trench walls increases as the depth and the
degree of saturation increase. Moreover, granular sedi-
ments, such as sand, are more prone to collapse than
cohesive sediments, such as clay. Should a fluid be
necessary to hold open the trench, the type of fluid
selected should minimize potential reductions in perme-
ability adjacent to the trench. Accordingly, bentonite
slurry, which supports trenches in standard construction
practice, should be avoided. Proper support of the
trench is critical when installing perforated pipe and
access casing for pumps, which require that the trench
remain open for several days.
4.2.2.3 Degree of Saturation
The degree of saturation affects both the construction
and application of trenches. The stability of trench walls
increases as the soils become drier, so it may be possi-
ble to create and complete trenches in relatively dry soils
without holding them open using fluids. When fluids are
used to hold open trenches in unsaturated soil, some of
the fluids may leak out of the trench during construction.
In many cases, the fluids are designed to form filter
cakes, or they have sufficient gel strength to minimize
penetration into even relatively dry soils.
The degree of saturation also affects the method of
remediation, although the extent to which this is appli-
cable is similar to other methods of recovery.
Vibratory plow trenching units are used where the per-
centage of large rocks near the ground surface are low.
Earth saws are commonly used when the trench exca-
vation must cross pavement, such as a sidewalk, road-
way, or parking lot. Earth saws have also allowed
trenching through shallow, "soft" bedrock, such as lime-
stone. Using chain-type trench cutters is possible with
most unconsolidated sediments; the use of tungsten-
carbide tipped bits allows chain-type cutters to be used
in hard, rocky soils, and even to trench through asphalt
paving.
67
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4.2.3 Hydrologic Conditions
Hydrologic conditions affect the location, depth, and
sizing of a trench. Of particular importance are the ef-
fects of hydraulic conductivity, regional flow, and aquifer
extent.
4.2.3.1 Hydraulic Conductivity
A trench is an effective method of enhancing the recov-
ery of subsurface fluids if it is more permeable than the
enveloping formation. Based on field experience, a
trench requires a contrast of at least 100 between the
permeability of trench filling and that of the aquifer,
although extremely long trenches may require either a
greater contrast or several pumps. Accordingly, it may
be difficult to obtain sufficient permeability contrast when
the trench is in a very permeable sand or gravel. Con-
sequently, a trench oriented perpendicular to regional
gradient may fail to capture the regional flow, with water
flowing in one side and out the other. One option in this
situation is to install a low permeability liner on the
downgradient side (or the side containing the least
amount of contamination) of the trench.
The permeability and thickness of the aquifer also play
central roles in determining the operating parameters of
the recovery system. For example, trenches installed in
relatively permeable material typically operate at con-
stant discharge, and the drawdown decreases with time.
The drawdown decreases rapidly in trenches installed
in low-permeability formations, so a float switch is com-
monly used to hold the drawdown constant above the
intake of the pump. In the latter case, the drawdown is
held constant, and the discharge decreases with time.
Similar behavior occurs with wells. When a well oper-
ates at constant discharge, the Theis solution is com-
monly used to estimate the drawdown as a function of
time.
A solution similar to the Theis solution for a well can help
predict the drawdown at a trench operating at constant
discharge (11). Following the assumption of Theis, the
trench is analyzed as a sink over which the inflow is
uniform. This is an idealization, but it results in head
gradients along the trench that approximate field condi-
tions where the permeability of material in the trench is
approximately 100 times greater than the aquifer. A
similar analytical solution can predict drawdowns if ma-
terial of infinite permeability fills the trench. (A perme-
ability contrast between the trench filling and the aquifer
that is greater than 1,000 is essentially an infinite con-
trast.)
As in a well, the drawdown increases with time (Figure
4-5). The major difference, however, is that drawdown
increases at a much slower rate than it does for a well
of reasonable size. Table 4-1 lists the variables used to
analyze trenches, including the ones represented in
Uniform
Flux
Pd = 47tKhAP/Q
10-2
1CH
10°
102
Figure 4-5. Drawdown at the midpoint of a trench where dis-
charge is constant (11).
Table 4-1. Variables Used in Trench Analysis
Q
S
K
R
n
h
a
Pumping Rate
Storage Coefficient
Hydraulic Conductivity
Retardation Factor
Effective Porosity
Aquifer Thickness
Regional Gradient
Figure 4-5. The results in Figure 4-5 allow forecasting of
the drawdown so that the pump can be sized and lo-
cated properly. Forecasting drawdown can also help in
estimating aquifer parameters from a pumping test, just
as aquifer parameters are estimated from pumping tests
using wells. The uniform flux case in Figure 4-5 resem-
bles a trench with a permeability contrast of roughly 100.
Keep in mind, however, that the results assume that the
trench does not interact with lateral boundaries, nor is it
affected by recharge. A more comprehensive analysis
should consider both conditions because they affect
drawdown.
The performance of a trench held at constant drawdown
is markedly different from one operating at constant
discharge. The discharge from a trench held at constant
drawdown is greatest at the onset of pumping. Dis-
charge then decreases with time (Figure 4-6) as head
gradients in the vicinity of the trench flatten. Note that
this effect may decrease the discharge by an order of
magnitude or more. The real time of this decrease
68
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10.0
1.0
0.1
Qd = Q/(APw8Kh)
1C-2 -10-1
10°
101 102 103 1Q4 105
Figure 4-6. Discharge from a trench held at constant draw-
down (11).
depends on the aquifer properties, just as the real time
separating the different flow periods depends on the
properties described earlier. As a result, an appreciable
decrease in discharge may take place over several
minutes or several years, depending on the aquifer
properties. Over long periods, fluctuations in natural
hydrologic conditions may mask this effect.
Nevertheless, the discharge from a trench held at con-
stant drawdown may decrease with time as an inevitable
consequence of the dynamics it creates in the aquifer.
Therefore, this effect should be considered along with
others when trying to diagnose diminishing discharge.
Other factors that may contribute to diminishing dis-
charge include biofouling or siltation of the material
filling the trench.
The vertical extent of the aquifer is an important consid-
eration when designing a trench system to establish
control of the site. If the vertical extent of the aquifer is
shallow, then the trench should fully penetrate the aqui-
fer thickness. In circumstances where it is impractical to
penetrate the full extent of the aquifer, however, con-
taminated fluids escaping beneath the trench may be a
concern (13). The severity of this concern depends on
the location and nature of the contaminants, which must
be determined prior to designing the recovery system.
The path of the dividing streamline (Figure 4-7), which
separates the area where the trench captures water
from the region where water escapes capture, depends
at least on the depth of the trench, pumping rate, hy-
draulic conductivity, regional gradient, and recharge.
Methods of estimating the position of the dividing
streamline are available in Zheng and others (13) and
have been validated with field tracer studies by Cham-
bers and Bahr (14).
Figure 4-7. Cross section of a trench partially penetrating an
aquifer in a regional flow, showing the vertical ex-
tent of aquifer captured by the trench.
4.2.3.3 Regional Flow
4.2.3.2 Aquifer Extent
The lateral extent of the aquifer affects the performance
of atrench, causing it to differ from the theoretical results
based on the infinite aquifer given above. In general,
aquifer boundaries that are within several trench
lengths, or perhaps longer, probably have a major effect.
Constant head boundaries, provided by a lake or river,
reduce the drawdown or increase the recovery rate in
Figure 4-6. No-flow boundaries, such as where the aqui-
fer terminates, have the opposite effect, increasing
drawdown or decreasing rate.
In addition to the operational considerations, lateral
boundaries of an aquifer should affect the decision of
where to locate a trench. For example, the boundary
between a contaminated aquifer and a surface water
body may be a critical point of exposure that a trench
could protect when installed parallel to the boundary.
Many applications of trenches involve installations nor-
mal to the regional gradient in an effort to cut off migra-
tion of a plume (12, 15) or to flush fluids through a zone
to enhance recovery (16). In these cases, the areas the
trench influences and the time required to reach the
trench are of interest. It is possible to estimate those
areas and times for various regional gradients and
trench orientations using relatively simple methods (11).
The most useful presentation of the results involves
combining the various parameters in dimensionless
groups. Because it is independent of actual aquifer pa-
rameters, this presentation allows analysis of many field
scenarios using just a few graphs. The following exam-
ple illustrates how it works.
Assume that the trench cuts completely through a con-
fined aquifer of thickness h and infinite lateral extent.
A regional gradient is present and flows either north
to south, northeast to southwest, or east to west
69
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(Figure 4-8). The dimensionless strength of the re-
gional gradient is
= x,aS/4hRn.
(4-2)
The variables of the equation are presented in Table 4-1.
A discussion earlier in this chapter pointed out that the
pattern of flow to a trench changes with time, so this
problem requires a transient analysis. Therefore, the
analysis must specify the strength of the trench relative
to aquifer properties, whereas a steady-state analysis
incorporates strength in the travel time. The dimension-
less strength is
= QSrtQKh2nRn.
(4-3)
Figure 4-8 presents a series of maps where the strength
of the trench is fixed at 0.01 and both the orientation and
magnitude of the regional gradient vary. Regional flow
sweeps away some particles. The areas where particles
are captured form U-shaped zones, with the principal
axis in the direction of regional flow. The shape of the
capture zone changes with increasing p by shortening
in the downgradient dimension and lengthening in the
upgradient dimension. Shortening of the capture zone
also occurs normal to the regional flow, but to a lesser
extent than shortening in the downgradient. The con-
tours of equal arrival time can change shape but not
area as a result of the changing direction of a uniform
and constant regional flow. This is because contours of
equal arrival time must circumscribe the same volume
₯=0.01
Areas Where Regional Flow
Sweeps Away Particles
= -0.003
Flow Is
East to
West
Flow Is
North to
South
Figure 4-8. Dimensionless arrival times as a function of location for various strengths and directions of regional flow (11).
70
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of water in a system operated at constant discharge
(17). It follows that the arrival time contours for a well
would be the same area as those for a trench, except
the zones for a well would be longer and narrower than
those fora trench.
To apply the maps of arrival times, assume a hypotheti-
cal situation in which a trench is installed in an aquifer
with the following parameters:
K = 10"4 meters per second
S=0.1
n = 0.1
h = 5 meters
x, = 50 meters
For ₯ = 0.01, it follows that Q = OAQnnKtf/S = 0.07539
cubic meters (20 gallons) per minute . Assuming the
regional gradient is 0.004, then p = -0.01, and the map
at the bottom of the middle column in Figure 4-8 repre-
sents the site conditions. The arrival times for specific
points can be estimated by obtaining fda from Figure 4-8.
For example, a point x = 50 meters, y = 50 meters,
where y is up the regional gradient (Xd = 1.0, yd = 1.0),
yields tda = 38. According to the equation td=4tKh/S%2,
this corresponds to 55 days in real time. The effect of
the regional gradient is illustrated by taking a point of
equal distance down the regional gradient (xj = 1.0, yd
= -1.0), where fda = 164, or 237 days. Alternatively, the
area affected after 6 months is circumscribed by the
contour of tda = 126.
The maps in Figure 4-8 can be applied to most aquifer
settings with valid limitations set by assuming confined
conditions and an absence of nearby boundaries. The
effect of regional gradient is negligible and probably can
be ignored for Ipl <0.001. At large values of regional
gradient (Ipl >0.05), a trench operating at a strength of
*P = 0.01 is unable to generate a sink strong enough to
effectively capture particles. The maps in Figure 4-8 are
limited to *P = 0.01, so other strengths require calculating
other maps.
The versatility and value that interceptor trenches may
offer is noteworthy. Interceptor trenches oriented at
acute angles to the regional flow may be valuable for
contaminant recovery. An interceptor trench oriented
parallel to the regional gradient is an effective sink for
areas upgradient of the trench. Moreover, in areas
where access or other factors prevent installing a trench
normal to the regional flow, it may be possible to achieve
the desired performance by installing the trench oblique
to the flow. Where the regional gradient is inclined 45
degrees to the axis of the trench, the arrival times in the
region upgradient of the trench are similar to those
where p = 90 degrees (Figure 4-8).
4.2.4 Trench Hydraulics
The hydraulics of the trench itself are relatively unimpor-
tant for dewatering applications but become increasingly
important when the trench must maintain control over
long periods. Because the objective of dewatering ap-
plications is to remove water as quickly as possible, the
trench is operated at maximum drawdown. Alternatively,
the objective during long-term control is to affect hydrau-
lic gradients at the site while removing as little water as
possible and still maintaining some factor of safety. As
a result, the traditional methods of designing trenches
for dewatering applications typically ignore trench hy-
draulics. Therefore, much of the detail of this process
has yet to be analyzed.
The hydraulics of a trench involve coupling flow in the
trench with flow in the aquifer. There is a point of lowest
potential, either head or air pressure, at the pump, and
then potentials increase along the trench until reaching
maximum values at the end. The flow per unit area, or
flux, into the trench is relatively large adjacent to the
pump, and then decreases towards the end. If the trench
terminates in an aquifer boundary the influx will be
lowest at the end of the trench (Figure 4-9). In the
absence of a boundary, however, the influx may in-
crease as it approaches the end of the trench. This is
because flow converges from distant points toward the
end of the trench. In this respect, the flow paths resem-
ble Figure 2-17.
In general, flowrate along the trench increases as flow
approaches the pump from the end of the trench. This
occurs unless fluid flows out of the trench at some point.
For example, isolated perched water that may flow into
a trench locally may be lost from the trench as it passes
an unsaturated interval. Another possibility is to lose
DNAPL out the bottom of a trench.
The magnitude of the variation of influx depends on the
permeability of the trench filling (including perforated
casing if it is used), permeability of the aquifer, amount
of drawdown, trench length, and location of pump. The
variation increases as:
Flux Into Trench
Pump'
V V V V V V V
Flow Along Trench
Figure 4-9. Map of the distribution of influx and flow along an
idealized trench.
71
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The permeability of the trench filling is reduced com-
pared with that of the aquifer.
The drawdown increases.
The trench becomes longer.
Heterogeneities in the aquifer result in local fluctuations
of influx that differ from the effects described above. In
highly heterogeneous aquifers, the heterogeneities,
rather than trench hydraulics, may dominate the vari-
ation of influx.
The distribution of influx becomes a concern when trying
to operate the trench to maintain control while recover-
ing a minimal amount of water. These efforts minimize
power as well as, possibly, treatment or disposal costs.
Currently, analytical methods of designing the hydrau-
lics of the trench are unavailable. Consequently, moni-
toring the heads within and adjacent to the trench is
recommended to ensure maintenance of the head gra-
dients toward the trench.
4.2.5 Contaminants
The type of contaminant mostly affects the method of
completing a trench. Trenches completed with methods
developed for dewatering can recover aqueous phase
contaminants (18), whereas specialized completions
can most effectively recover NAPLs.
NAPL completions may involve one pump and resemble
dewatering applications. This approach is commonly
used to recover LNAPL. Another possibility for comple-
tion involves the use of two pumps: one to recover water
and the other to recover the nonaqueous phase. Two
perforated pipes can also be effective, with each pipe
installed at the approximate depth of water and NAPL.
Recovery of water and NAPL simultaneously inhibits the
upward or downward migration of the NAPL/water inter-
face. This effect tends to pinch off the NAPL layer and
reduce the effectiveness of recovery.
Sealing the upper portion of the trench is a key consid-
eration when recovering vapors. Details of the vapor-re-
covery design may also include components to recover
water, depending on site conditions.
The depth of the trench is another factor that partially
depends on contaminant type. Trenches designed to
recover LNAPL must extend at least as deep as the
lowest seasonal depth of the water table; it may be
unnecessary for this trench to be much deeper. A design
for DNAPL recovery, however, must key trenches into
the formation on which the DNAPL has pooled. The
trench must not be cut too deeply and penetrate the
lower formation, because this could lead to loss of the
DNAPL from the trench. In cases where this could occur,
it may be advisable to place a liner along the bottom of
the trench.
Flammability of the contaminants is a consideration
when designing trench components, particularly the
pump and controller. Flammable vapors can accumulate
in the trench or access casing, then be ignited by a spark
from the pump. Design of the casing and pipe, as well
as the pump itself, must ensure they can withstand
contaminants that are corrosive compounds or vigorous
solvents.
4.2.6 Site Conditions
Site conditions need consideration before the start of
trench construction. Access for excavation equipment
along the path of the trench is important. Buildings can
pose a major obstacle, although trenches can be placed
inside some buildings. Prior to excavation, underground
structures (e.g., utility lines, sewers, or tanks) must be
located and marked. Space must be sufficient to accom-
modate storage of excavated material during construc-
tion, or provisions must be made to haul the material as
it is excavated.
In some locations, regulations prevent using contami-
nated material removed during excavation as backfill for
the trench. Regulations may require disposing of the
excavated material as contaminated waste, which can
significantly increase the cost of installation.
Some sites are covered with concrete, pavement, or a
building, which may act as a cover to confine air flow.
The presence of such structures and how they affect the
sources of air should be considered when trenches are
used for vapor extraction. Many older paved surfaces
are cracked and underlain by gravel, so they may pro-
vide a poor seal.
4.2.7 Trenches and Horizontal Wells
The geometry of a trench can resemble that of a hori-
zontal well. In fact, both trenches and horizontal wells
can be used for similar applications. In many applica-
tions, the decision between using a trench or horizontal
well hinges on economic rather than technical issues.
Depth can increase the cost of trench installation signifi-
cantly, and there is a maximum depth below which
trench installation is impossible. In contrast, depth has
less effect on the cost of a horizontal well (although
exceeding the 8-meter [26.2-feet] depth limit for elec-
tronic beacon locators may increase the cost). Rock can
present problems to both technologies, although blast-
ing can create trenchlike structures at shallow depths in
rock, and it is certainly possible to bore horizontal wells
in rock. In both cases, however, rock can increase in-
stallation cost substantially. Access problems, too, can
increase the cost of a trench or make installation impos-
sible, whereas access requirements for a horizontal well
are restricted to entry and possible exit points.
72
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Trenches present significantly more vertical exposure of
the contaminated area than horizontal wells. This is
particularly important in stratified formations, where a
trench can cut across many thin permeable layers while
a horizontal well directly intersects only a few layers.
Moreover, where contaminants are present over a large
vertical extent, trenches may be preferable, although the
increase in trench cost with depth should be considered.
For example, the vertical extent of trenches would make
them perform better than a horizontal well when recov-
ering LNAPL where large fluctuations in the water table
occur seasonally. Interception of a contaminated plume
migrating with regional flow is an application for which a
trench has no equal, particularly when excavation equip-
ment can reach the bottom of the aquifer.
Methods of creating both horizontal wells and trenches
for environmental applications are evolving rapidly, with
both improvements in capabilities and lower costs.
4.3 Applications
Interceptor trenches have a wide variety of environ-
mental applications, including hydrodynamic control and
dewatering, plume capture, NAPL recovery, and vapor
extraction. Trenches also have been central to an inno-
vative remedial application that decontaminates ground
water in situ as it flows through reactive material.
4.3.1 Hydrodynamic Control
Interceptor trenches have been used extensively to con-
trol local, site-scale, hydraulic gradients (3,19, 20). One
site-scale application involves reducing the flow through
a contaminated area by pumping water from the up-
gradient side of the contaminated area and injecting the
water on the downgradient side (Figure 4-10). This es-
sentially produces a stagnant area in the region between
the trenches, which inhibits movement of contaminants.
Interceptor trenches can also be used in conjunction
with barriers around a site to control offsite migration
(19, 20). In this case, barriers placed around the site and
the interceptor trenches reduce piezometric levels within
the barriers. Vertical wells can supplement this applica-
tion, which is similar to traditional dewatering applica-
tions for trenches. The barriers should be keyed into
underlying an aquitard, if possible.
In some cases, using barriers can restrict flow into one
side of the trench (21), or can divert ground water mov-
ing under a regional gradient away from a contaminant
source (3). This configuration (Figure 4-11) is used when
clean surface water (e.g., lake or river) provides signifi-
cant ground-water recharge within the area influenced
by the trench. This application is most effective when the
barrier is keyed into a low permeability formation so that
ground water does not travel underneath the barrier.
Interceptor Trench
Barrier
Figure 4-11. Interceptor trench coupled with a hydraulic barrier
to minimize recharge from an adjacent surface
waterbody.
4.3.2 Plume Interception
The classic, common application of a trench involves
placing a trench perpendicular to the regional gradient
on the downgradient side of a plume to intercept con-
taminated water (3, 6,12,15). Figure 4-12 illustrates the
concept. The application serves the same function as a
line of wells but is more reliable, because water may
escape between the wells in the presence of permeable
zones unaccounted for during design or as a result of
other factors. This application, therefore, is useful when
high reliability is required, such as when protecting a
Well
O
Property Boundary
Trench
Figure 4-10. Controlling the flow through a contaminated area
by pumping from an upgradient trench and inject-
ing into a downgradient trench.
Figure 4-12. Trench used to capture plume and prevent migra-
tion to an offsite well.
73
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downgradient well. Installing the trench, however, by no
means guarantees that it will totally intercept the plume.
Monitoring enough locations of hydraulic heads in the
vicinity of the trench should ensure that the flow is
inward toward the trench. A barrier on the downgradient
side of the trench helps to ensure the reliability of plume
capture.
The idealized trench is a single straight feature, such as
in Figure 4-12, but in practice many applications involve
creating networks of trenches oriented to best achieve
remedial objectives and avoid obstacles to excavation.
Networks may have several segments oriented normal
to regional flow to arrest migration, or an irregular pat-
tern of contamination may dictate their location. In cases
where the ideal path for the trench is obstructed, a
trench network is used instead.
4.3.3 NAPL Recovery
The recovery of NAPL is one of the most widely used
applications of interceptor trenches, because trenches
offer an effective method of ensuring capture and arrest-
ing possible offsite migration of NAPL. In cases where
NAPL is relatively shallow (LNAPL in particular), com-
monly available excavation equipment can be used to
install the trench. Moreover, trenches can cut across a
significant depth range, making them well suited to
NAPL recovery at sites where seasonal hydrological
factors cause wide variations in piezometric levels.
The simplest trench design to recover DNAPL calls for
constructing a trench on top of an underlying imperme-
able unit. The product collects in drain lines within the
trench, then flows laterally to sumps, where it is pumped
to the surface (Figure 4-13a). Using two recovery (drain)
lines may enhance DNAPL recovery (22): one drain in
the DNAPL itself and the second drain in ground water
above the DNAPL drain (20). Pumping ground water
from the upper drain produces drawdown of the water
and upwelling or mounding of the product (Figure
4-13b). Pumping both drains results in drawdown in both
ground water and DNAPL (Figure 4-13c).
To enhance DNAPL recovery, water removed from the
upper drain of the two-drain system can be reintroduced
to the system through a nearby trench or drain (Figure
4-14). The reintroduction of the water to the aquifer
system enhances the hydraulic gradient, further driving
the mobile DNAPL (20).
Caution is required when pumping DNAPL recovery
systems; excessively high pumping rates can cause the
drawdown of the overlying ground water to "pinch-off'
the DNAPL plume (Figure 4-15). This results in a
marked decrease in the ratio of NAPL to water in the
recovery fluid, even though large volumes of NAPL re-
main relatively close to the trench.
a.
Ground Surface
b.
Ground Water
Ground-Water Drawdown,
DNAPL Mounding
Ground-Water Drawdown,
DNAPL Drawdown
Figure 4-13. Trench systems for DNAPL recovery: a) one-drain
system, b) two-drain system with upper drain
pumping, and c) two-drain system with both
drains pumping (22).
Trenches are also widely used to recover LNAPLs (2, 3).
This application may make use of a single pump, or it
can use a two pump system to reduce the upward
migration of the LNAPL/water interface. In this sense,
the LNAPL application resembles the approach de-
scribed above for DNAPLs.
4.3.4 Vapor Extraction
Interceptor trenches have been used successfully to
recover VOC, including petroleum hydrocarbons and
halogenated compounds (e.g., tetrachloroethane, 1,1,1-
trichloroethane, TCE). Vapor extraction with an intercep-
tor trench requires an impermeable trench cap. In
practice, low-permeability soils from trench excavation
have capped the trench. This requires compaction to
maximize the integrity of the seal. The combination of
Ground Wate
-~~»~
DNAPL
I^ISP*
Permeability U
Ground Water
Figure 4-14.
Enhanced recovery of DNAPL by reintroducing
water to increase the hydraulic gradient (20).
74
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Ground Water Drawdown,
DNAPL "Pinch Off
Figure 4-15. Excessive pumping rates from the ground-water
and DNAPL drains causing "pinch off" of the
DNAPL plume.
using an extremely low-permeability liner material (e.g.,
HOPE) along with the natural soils can result in a cap
with a sufficiently low permeability. Multiple soil-liner
layers also may be sufficient. Another technique in-
volves using a bentonite slurry in the upper few feet of
the trench. Liner material, such as HOPE, may also
supplement the bentonite slurry method.
To initiate vapor extraction, suction applied to perforated
casing within the permeable trench material induces air
flow from the adjacent soil into the trench. This results
in vaporization of the VOCs. Air and contaminant vapors
are collected and delivered to secondary treatment sys-
tems, just as in applications using conventional wells.
The path of air flow through the soil is critical to this
application. To estimate the path, it is best to evaluate
the location of a source of air as well as possible het-
erogeneities in the site. If the air flows in from the ground
surface, then much of the flow would follow a steeply
dipping path close to the trench. This effect is particu-
larly acute if the formation is homogeneous or contains
vertical fractures. Alternatively, if a naturally occurring
low-permeability unit overlies the contaminated area,
the effect is less serious. In the absence of such a unit,
it may be possible to add a cap to induce air through
more of the subsurface. A similar situation exists with
flow patterns of horizontal wells (e.g., Figure 2-19).
Using wells or another trench to provide a source of air
below the subsurface may augment flow through the
contaminated region. Perhaps the most effective imple-
mentation involves two parallel trenches, with suction
applied to one trench and atmospheric or positive pres-
sure applied to the other. Keep in mind, however, that
even with two trenches and a cap, heterogeneities be-
tween the trenches (such as fractures or sand lenses)
may cause an uneven distribution of flow.
One effect of vapor extraction may be the production of
water or other liquid phases, either by draining perched
zones or lifting the water table. The presence of water
reduces pneumatic conductivity, and thus performance,
during vapor extraction. Therefore, if the site may pro-
duce water, the trench design should aim to recover the
water as well as the vapor. Using a sump pump to
recover water while applying suction with a blower
should accomplish this goal. Alternatively, a dual-phase
extraction system (23) can recover both water and other
liquid phases using suction from the blower.
Adding air sparging can enhance a vapor extraction
system (24). Injecting air into VOC-contaminated
ground water enhances remediation by:
Stripping the dissolved-phase VOCs.
Increasing dissolved oxygen levels to promote bio-
degradation.
Design guidelines for installation of a coupled
sparge/vapor extraction system are similar to those for
standard vapor extraction and ground-water pump and
treat systems. The lateral and vertical extent of contami-
nation as well as the nature of the soil affect the sparging
radius of influence, which controls the overall scale of
the system. Individual design parameters that these
factors affect include depth and length of the screen,
well construction, and air pressure. All these variables
are necessary to predict rates of VOC removal and
project duration. For a coupled sparge/vapor extraction
system, a recommended vacuum-to-injection ratio is
10:1 (24). This should meet the objective of adequate
airflow into the contaminated ground water without in-
creasing the potential for offsite migration of the con-
taminant.
Operation of a combined sparge/vapor extraction sys-
tem should include monitoring of the following parame-
ters (24):
Injection: Air temperature, pressure, and air flow.
Extraction: Temperature, vacuum, air flow, and con-
taminant concentration.
4.3.5 Other Applications
Trenches have a variety of other environmental applica-
tions that are beyond the scope of this document be-
cause they are not directly related to delivery and
recovery. In many cases, trenches filled with low-perme-
ability materials form vertical barriers to flow (3, 19, 20).
Such barriers can enhance the control of fluids within a
site, in many cases in conjunction with recovery systems.
75
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Recently, trenches have been filled with reactive com-
pounds capable of degrading or adsorbing contami-
nants in situ (25, 26). This application involves placing
a trench filled with permeable material perpendicular to
a regional gradient so that natural flow sweeps contami-
nants through the trench. Ideally, compounds in the
trench remove the contaminants from the flow, and the
ground water is "clean" after it passes through the
trench. Several variations of this concept have been
proposed or are currently under development (16, 27-30).
4.4 Case Histories
This section provides an overview of several dozen
applications and describes five case histories in which
trenches have been used for various environmental pur-
poses.
4.4.1 Overview
Tables 4-2 through 4-4 summarize information on repre-
sentative applications of interceptor trenches from pub-
lished descriptions and case histories provided by
consultants. Table 4-2 lists the general site character,
including contaminant, site geology, and hydrologic
characteristics. Table 4-3 details the installed trenches,
including dimensions and fill material. Table 4-4 high-
lights operational parameters of the trenches and prod-
uct recovery.
4.4.2 Selected Examples
Following is a more detailed discussion of site imple-
mentations of interceptor trench technologies.
4.4.2.1 LNAPL and Dissolved-Phase Recovery,
Texas
At a former petroleum refinery located in southeastern
Texas (12), the predominant soil type was a massive,
laterally extensive marine clay with very thin silt and
sand interbeds. Slug tests from the site indicated hy-
draulic conductivity from 9.3 x 10"6 to 2.5 x 10"4 centime-
ters per second (0.0266 to 0.720 feet per day). The
saturated clay zone varied in thickness from 15.2 to 30.5
meters (50 to 100 feet). Depth to ground water typically
was no more than 3 meters (10 feet). Average annual
precipitation was 88.9 centimeters (35 inches), and the
regional ground-water gradient was 1.7 x 10"3. Contami-
nation consisted of petroleum hydrocarbons, which ex-
isted both in the dissolved phase and as free-product
phase on ground water. The total free-product plume
was estimated to contain more than 1,514 cubic meters
(400,000 gallons) and to cover approximately 8,094
square meters (87,120 square feet).
The trench was oriented normal to the regional ground-
water gradient and located near the downgradient end
of the free-product plume. It was constructed using a
backhoe and was backfilled with pea gravel. The trench
was 0.9 to 1.2 meters (3 to 4 feet) wide, 3.7 to 4.3 meters
(12 to 14 feet) deep, and 4.7 meters (16 feet) long. A
30.5-centimeter (12-inch) diameter recovery well was
located in the center of the trench, which was deepened
to a depth of 5.5 meters (18 feet) in the vicinity of the
well. Adding a low-permeability cover minimized infiltra-
tion of surface waters.
After 2 years of operation, approximately 9.8 cubic me-
ters (2,600 gallons) of free-product hydrocarbons were
recovered. With a product-to-water ratio of about 1:91,
the trench recovered approximately 53.9 liters (14.24
gallons) of petroleum hydrocarbons per day.
4.4.2.2 LNAPL and Dissolved-Phase Recovery,
Westminster, Colorado
At a municipal service center in Westminster (15), the
site geology consisted of 3 to 3.7 meters (10 to 12 feet)
of sandy clay underlain by interbedded sandstones and
claystones. Ground water occurred at depths between
3.7 to 6.1 meters (12 to 20 feet) during the operation. In
situ slug tests of the upper 3 meters (10 feet) of the
saturated zone indicated an average hydraulic conduc-
tivity of 7 x 10"5 centimeters per second. Regional
ground-water gradient was approximately 0.01. Con-
tamination at the site was free-product gasoline floating
on ground water. The gasoline plume covered an area
91.4 to 122 meters (300 to 400 feet) in width and 122
meters (400 feet) in length that was estimated to contain
189 to 379 cubic meters (50,000 to 100,000 gallons) of
product (Figure 4-16). A gasoline vapor plume extended
15.2 meters (50 feet) beyond the limits of the liquid
plume.
The trench was located nearly normal to the contami-
nant plume and was constructed using a backhoe with
a sliding trench box. The trench was 0.9 to 1.2 meters
(3 to 4 feet) wide, 5.5 to 6.1 meters (18 to 20 feet) deep,
and 183 meters (600 feet) long. The base of the trench
contained 10-centimeter (4-inch) perforated pipe, which
sloped at a grade of 0.01 toward a corrugated metal pipe
sump in the middle of the trench. The trench was back-
filled with 1.9-centimeter (3/4-inch) gravel to within 2.4
meters (8 feet) of the ground surface (688 cubic meters
[900 cubic yards] required); the remainder of the trench
was filled with materials excavated from the trench. In
addition to the trench, two 12.2-meter (40-foot) deep
recovery wells were installed elsewhere in the plume.
The thickness of the original plume varied from 0.6 to
1.8 meters (2 to 6 feet). After 1 year of trench operation,
most monitoring wells showed less than 15 centimeters
(6 inches) of product. Total flow of product and ground
water from both the trench and recovery wells was
approximately 7.6 cubic meters (2,000 gallons) per day,
and after 1 year total product recovery was estimated to
be 114 cubic meters (30,000 gallons). It will take an
76
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Table 4-2. Summary of Applications of
Reference Site Name
Hydrodynamic Control
Piontek and Simpkin,
1992
Day and Ryan, 1992
Laramie Tie
Ohio
Trenches for Delivery or Recovery: Site Characteristics
Hydraulic
Contaminant Soil Type Conductivity
Creosote/pentachlorophenol
mixture (DNAPL)
Mixed-waste landfill
leachate
Alluvial sands
Glacial till/bedrock
0.25-1 0 cm/sec
Gradient Depth
<3 m
Ground-Water Remediation
Mast, 1991
Mast, 1991
Mast, 1991
Ganser and Tocher,
1988
Rawl, 1994
Rawl, 1994
Day and Ryan, 1992
Vapor Extraction
Barrera, 1993
Barrera, 1993
Barrera, 1993
Barrera, 1993
Site A
Site B
Site C
Westminster
Rinker Concrete
Langley AFB
Northern
California
Case #1
Case #2
Case #3
Case #4
Free product and dissolved
petroleum hydrocarbons
Dissolved petroleum
hydrocarbon
Free product and dissolved
petroleum hydrocarbons
Free product and dissolved
gasoline
Dissolved-phase PAHs
Free product and dissolved
jet fuel (JP-4)
"Spilled processing
chemicals"
VOCs (PCE, TCE, TCA)
BTEX, petroleum
hydrocarbons
Toluene
VOCs (acetone, MEK,
toluene, xylene, e-benzene)
Marine clay
Clay/sand/clay
Deltaic clay/sand
Sandy
clay/sandstone
Limestone/sandstone
Sands and silts
Clays, silts
Clay/sand and gravel
Clay/silt and fine
sand
Sand and silt
Silt and clay/sand
0.0001-0.00001
cm/sec
<0.0001 cm/sec
0.003-0.04
cm2/sec (T)
0.00007 cm/sec
>0.1 cm/sec
0.00001-0.0001
cm/sec
0.0001 cm/sec
0.0017 <3m
0.0625 1 m
0.0645 1-3 m
0.01 4-6.5 m
1 m
2.5 m
0.04 5-6 m
5-7 m
1-2 m
Combined Ground-Water Remediation and Vapor Extraction
Rawl, 1994
LNAPL Recovery
Hanford and Day,
1988
Day and Ryan, 1992
DNAPL Recovery
Meiri et al., 1990
Rawl, 1994
Sale and Applegate,
1994
Flow Barrier
Rawl, 1994
Law Center
San Jose
South Texas
Rock Creek
Gray Iron Works
Laramie Tie
Star Lake
Free product and dissolved
BTX
Free product diesel fuel
Free product waste oil
TCE
TCE
Creosote/pentachlorophenol
mixture
Free product diesel fuel
Sand and
clay /granite
Sands, silts, clays
Glacial till/bedrock
Silty clay glacial till
Alluvial sands
Sands, silts, clay
0.000007-0.03
cm/sec
0.00002-0.00008
cm/sec
<0.0003 cm/sec
0.25-1 0 cm/saec
9-11 m
Shallow
0.02 1-2 m
<3 m
4.5 m
77
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Table 4-3. Summary of Applications of Trenches for Delivery or Recovery: Trench Parameters
Reference Site Name Shape Length Width Depth
Fill Material
Hydrodynamic Control
Piontek and
Simpkin, 1992
Day and Ryan,
1992
Laramie Tie
Ohio
Closed loop 5,100 m
Linear (2) 1 m
14 m
Drainline with
bentonite barrier wall
Sand, geotextile,
bentonite barrier wall
Ground-Water Remediation
Mast, 1991
Mast, 1991
Mast, 1991
Ganser and Tocher,
1988
Rawl, 1994
Rawl, 1994
Day and Ryan,
1992
Vapor Extraction
Barrera, 1993
Barrera, 1993
Barrera, 1993
Barrera, 1993
Combined Ground-Water
Rawl, 1994
LNAPL Recovery
Hanford and Day,
1988
Day and Ryan,
1992
DNAPL Recovery
Meiri et al., 1990
Rawl, 1994
Sale and
Applegate, 1994
Flow Barrier
Rawl, 1994
Site A
Site B
Site C
Westminster
Rinker
Concrete
Langley AFB
Northern
California
Case #1
Case #2
Case #3
Case #4
Remediation
Law Center
San Jose
South Texas
Rock Creek
Gray Iron
Works
Laramie Tie
Star Lake
Linear 5m 1m
Linear:
N-S 48 m 1m
E-W 45 m 1m
Linear 9m 1m
Linear 180 m 1m
Linear 15m 0.3 m
Linear (19) 1,000 m 0.3 m
Linear 0.75 m
Linear (5) 150 m
Linear (8)
Linear (9)
Linear (8)
and Vapor Extraction
Linear (7) 360 m 0.3 m
Linear (2) 180 m 1-1.5 m
Linear 400 m
Model only
Linear (4) 250 m 0.3 m
Linear (12) 670 m
Curvilinear 400 0.3 m
3-4 m
3 m
3 m
3.2 m
5-6 m
3 m
6 m
9 m
2.5 m
3 m
5 m
1 m
5-7 m
1 3-1 4 m
6 m
7 m
4.5 m
Pea gravel
Pea gravel
Pea gravel
Pea gravel
3/4-in. gravel
Sand
Sand
Gravel with
geotextile, slurry wall
Pea gravel
Pea gravel
Perforated sheet
piling
Pea gravel
Gravel over sandy
gravel
Geomembrane panels
Sand
Perforated drain only
40-mil HOPE
78
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Table 4-4. Summary of Applications of Trenches for Delivery or Recovery: Product Recovery
Reference Site Name Pump/Well Type Pump/Well Location Pump Rate
Product Recovery
Hydrodynamic Control
Piontek and
Simpkin, 1992
Day and Ryan,
1992
Laramie Tie
Ohio
Recovery wells
In trench
5 gal/min per
trench
Ground-Water Remediation
Mast, 1991
Mast, 1991
Mast, 1991
Ganser and
Tocher, 1988
Rawl, 1994
Rawl, 1994
Day and Ryan,
1992
Vapor Extraction
Barrera, 1993
Barrera, 1993
Barrera, 1993
Barrera, 1993
Site A
Site B
Site C
Westminster
Rinker Concrete
Langley AFB
Northern
California
Case #1
Case #2
Case #3
Case #4
Combined Ground-Water Remediation and
Rawl, 1994
LNAPL Recovery
Hanford and Day,
1988
Day and Ryan,
1992
DNAPL Recovery
Meiri et al., 1990
Rawl, 1994
Sale and
Applegate, 1994
Flow Barrier
Rawl, 1994
Law Center
San Jose
South Texas
Rock Creek
Gray Iron Works
Laramie Tie
Star Lake
12-in. recovery well
12-in. recovery well
24-in. recovery wells
Metal sump
8-in. recovery well
8-in. recovery well
12-in. recovery well
Vacuum
Vacuum
Vacuum
Vacuum
Vapor Extraction
8-in. recovery well
12-in. recovery well
Recovery wells
with skimmers
N/A
8-in. recovery well
N/A
Middle of trench
Bend in trench
Middle of trench
End of trench
End of trench
328-ft intervals in
trench
Out of trench
Out of trench
Out of trench
Out of trench
100-ft intervals in
trench
Wells in trench
N/A
End of trench
N/A
9 gal/min 14 gal/day
0.5 gal/min
0.033 gal/min
1 .4 gal/min 30,000 gal, first
12 months
0.05-0.16 gal/min None, below MCL
Not yet on line
1 5 HP/1 ,000 cfm 1 00 Ibs, first 30
days
15 HP/500 cfm 270 Ibs/day
10 HP/250 cfm 25 Ibs, first 120
days
30 gal/min
N/A N/A
5 gal/ min
280,000 gal
(1992-1993) (50%
after 30 days;
90% after 90 days)
N/A N/A
79
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Underground Gasoline Storage Tanks
Extent of Free Product
Monitoring Wells
Interceptor Trench
100ft
Figure 4-16. Plan of the Westminster gasoline recovery site (15).
estimated 5 years to recover 50 to 70 percent of the
product (15).
4.4.2.3 DNAPL Recovery, Laramie, Wyoming
At a former wood treating facility (31) in Laramie, the site
geology consisted of approximately 2.4 meters (8 feet)
of alluvial sediments underlain by fine-grained sand-
stone. Surficial alluvial sediments of sands, silts, and
clays graded downward to coarse sands and fine grav-
els at the base. The lower several feet of the alluvial
aquifer were contaminated with wood-treating oil (Figure
4-17), a DNAPL that sank below the surface of the
ground water. The initial residual oil concentration
ranged from 5,000 to 100,000 ppm in soil boring sam-
ples (32).
An initial field demonstration was conducted in 1988 to
test the effectiveness of product recovery. The demon-
stration used horizontal drains for waterflooding fol-
lowed by chemical flooding. Two 4.6-meter (15-foot),
parallel horizontal drains were installed 4.6 meters (15
feet) apart, and a sheet piling wall separated them from
the surrounding alluvial aquifer. During this demonstra-
tion, approximately 6.1 cubic meters (1,600 gallons) of
oil were recovered. This was estimated to be 94 percent
of the oil in the test cell.
In 1989, a second field demonstration was conducted.
A 39.6-meter (130-foot) square sheet pile wall isolated
this test cell from the surrounding aquifer. Three 10-cen-
timeter (4-inch) horizontal drains were installed; these
drains were oriented parallel, spaced 18.3 meters (60
feet) apart, and located at the aquifer-bedrock contact.
The initial volume of wood treating oil was estimated to
be approximately 259 cubic meters (68,500 gallons). An
initial waterflood removed the mobile phase of the oil;
recovery from this step was 151 cubic meters (40,000
100 meters
Sheet Pile
Figure 4-17. Plan of the Laramie DNAPL recovery site (20).
gallons.) Chemical flooding removed the residual phase
oil; this step removed an additional 90.8 cubic meters
(24,000 gallons) of oil. In total, the horizontal drain sys-
tem coupled with water and chemical flooding removed
242 cubic meters (64,000 gallons) or 93.5 percent of the
initially estimated product reservoir.
80
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Full field implementation and system operation began in
1991 with installation of 12 oil recovery units. Drainlines
in the system were 54.9 meters (180 feet) long, and
spaced 27.4 meters (90 feet) apart. During 1991, three
units were operated that recovered 227 cubic meters
(60,000 gallons) of oil. From the summer of 1992 to the
summer of 1993, all 12 units were operated, recovering
1,136 cubic meters (300,000 gallons) of oil. Approxi-
mately 3.79 x 10s cubic meters (100 million gallons) of
water circulated through the alluvial aquifer.
Production records from the 1992-1993 pumping period
indicate that 50 percent of the mobile oil was recovered
after approximately 30 days of system operation, and 90
percent recovery was achieved in 90 days. Pumping
beyond 130 days produced only minor increases in oil
recovery.
Total recovery from all pilot studies and the first 2 years
of system operations was approximately 3,407 cubic
meters (900,000 gallons) of oil.
4.4.2.4 Vapor Recovery, Illinois
In northeastern Illinois (24), the site geology consisted
of clayey surface sediments underlain by a plastic clay
that contained trace coarse-grained materials. A 0.6-
meter (2-foot) thick sand and gravel unit occurred near
the water table at a depth of about 2.4 meters (8 feet).
Contamination at the site included VOCs and semivola-
tile organic compounds (SVOCs) in the soil and ground
water (Figure 4-18).
Initial investigation indicated that dissolved phase VOCs
could be removed from the aquifer by air sparging;
followup investigations concluded that because of lat-
eral geologic heterogeneities, sparging was not a viable
option unless the extraction wells could be placed very
near the seasonal high water level. The sand and gravel
layer located at the water table would serve as a path-
way for VOC removal by vapor extraction.
The implemented system consisted of nine air sparging
wells and five vapor extraction trenches with a cumula-
tive length of 168 meters (550 feet). The trenches, which
were excavated by backhoe, were 0.9 to 1.2 meters (3
to 4 feet) wide, and 2.1 meters (7 feet) deep (Figure
4-18). Installation of the sparging wells and extraction
trenches required about 3 weeks. On the basis of air-
flow analysis, a 10 horsepower, 5.7 cubic meters (200
cubic feet) per minute blower was selected for
sparging, and a 15 horsepower, 28.3 cubic meters
(1,000 cubic feet) per minute blower was selected for
vacuum extraction. Cost for the system as described
was less than $190,000.
The sparging/vapor extraction system removed more
than 45.4 kilograms (100 pounds) of VOCs in the first
30 days of operation and reduced VOC concentrations
by 75 percent in the first 4 months of operation
VOC Plume Limit
Regional Flow
30ft
Building
Monitoring Well
Figure 4-18.
Access Pipe
Compacted Natural Soil
4-in. HOPE Slotted Casing
With Filter Sock
Naturally Washed, Rounded
Pea Gravel
Map of the northeastern Illinois vapor extraction
site and detail of trench completion (24).
4.4.2.5 TCE Recovery, Modeling, and Field
Results, Ohio
Modeling of ground-water flow at a site can be a valu-
able tool during the design of a delivery or recovery
system. Therefore, this case study presents site charac-
terization, predictive numerical modeling to support the
design of an interceptor trench system, field data from
the installed system, and a comparison of the predicted
and observed results. The material presented here has
been taken from the following references: Krishnan and
Siebers, (33); Meiri and others, (34); Weston, (35-37);
and Woodward-Clyde Consultants (33-38).
Geology. The site, located in northeastern Ohio, in-
cluded two adjacent properties totalling approximately
13 acres (33). The site geology (Figure 4-19) consisted
of an upper fractured till unit (upper aquifer 0 to 2.1
meters [0 to 7 feet] below ground surface), a lower till
aquitard (2.1 to 3.7 meters [7 to 12 feet] below ground
surface), weathered shale bedrock (lower aquifer, 3.7 to
8.5 meters [12 to 28 feet] below ground surface), and
competent shale bedrock (34). Depth to ground water in
81
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Shallow Aquifer
Aquitard
Lower Aquifer
28ft
I Competent Shale =
Figure 4-19. Typical subsurface profile (34).
the upper unconfined aquifer ranged from 0.6 to 1.5
meters (2 to 5 feet).
Regarding the site's hydraulic profile, slug tests of wells
screened in the upper aquifer indicated hydraulic con-
ductivities of 2 x 10"5 to 8 x 10"4 centimeters per second
(0.065 to 2.4 feet per day); porosity was estimated to be
0.30. Regional ground-water gradient in the upper aqui-
fer, determined from water levels in screened wells,
averaged about 0.02, with flow to the west. Depth to the
piezometric surface of the lower confined aquifer was
slightly more than 0.6 meters (2 feet). Pump tests on a
well screened in the lower aquifer indicated a transmis-
sivity of about 4.1 square meters (13.5 square feet) per
day, or a hydraulic conductivity of about 0.3 meters (0.85
feet) per day (assuming an aquifer thickness of 4.9
meters [16 feet]). Lower aquifer storativity was esti-
mated to be 3 x 10"4. Regional gradient in the lower
aquifer, based on data from three deep wells, was cal-
culated to be 0.01, also to the west. Pump tests using
adjacent wells screened in the upper and lower aquifers
indicated that the two aquifers were separated hydrau-
lically; the vertical hydraulic conductivity of the aquitard
was calculated to be 3 x 10"8 centimeters per second (8
x10'5feet per day) (34).
Distribution of Contaminants. Contamination at the site
included TCE and heavy metals in soil. The ground
water contained TCE, tetrachloroethene, trans-dichlo-
roethene, 1,1-dichloroethene, vinyl chloride, 1,1,1-
trichloroethane, ethylbenzene, and xylene (33). TCE
was the principal ground-water contaminant. The con-
taminants apparently leaked from concrete silos (Figure
4-20).
As of 1984, TCE in the upper till aquifer had migrated
45.7 to 76.2 meters (150 to 250 feet) from the probable
source area (Figure 4-21). It was assumed that the
migration had occurred over an 8- to 10-year period,
suggesting an average plume front velocity of about 4.6
to 9.1 meters (15 to 30 feet) per year. Average ground-
water flux (based on an average hydraulic conductivity
of 0.37 meters [1.2 feet] per day, a regional gradient of
0.02, and a porosity of 0.30) was estimated to be 9.1
Regional Gradient
Shallow Aquifer
Monitoring Well
Storage Silos
Probable Source Area
Figure 4-20. Site map showing locations of monitoring wells,
storage silos, and probable contaminant source
area (34).
Regional
Flow
400ft
Probable Source
1984 Plume Limit
1986 Plume Limit
Prescribed Head Model Boundary
No Flow Model Boundary
Monitoring Wells
Figure 4-21. Extent of TCE plume in 1984 and 1986; also shown
is the ground-water model domain, with pre-
scribed (fixed) head and no-flow boundaries (34).
82
-------
meters (30 feet) per year. This indicated a retardation
factor between ground-water velocity and TCE plume
front velocity of 1 to 2. Comparison of plume front posi-
tion between 1986 and 1984 indicated a retardation
factor of 1.2 to 2. Based on the 1986 data, the TCE
plume covered an area of approximately 12,077 square
meters (130,000 square feet). Assuming a uniform
plume thickness of 1.5 meters (5 feet) and a porosity of
0.30, the volume of the plume was approximately 5,522
cubic meters (195,000 cubic feet) (34). Data from the
deep weathered-shale aquifer were insufficient to define
the extent of the TCE plume.
System Design. Vertical extraction wells and trenches
were considered as remedial alternatives. The vertical
extraction well system was deemed impractical because
modeling indicated that discharge rates would be on the
order of 0.4 liters (0.1 gallons) per minute per well. This
low rate would limit the radius of influence to a few feet and
would require several thousand wells for remediation.
Drains installed in shallow trenches also were consid-
ered as a remedial alternative. Two-dimensional
ground-water flow and contaminant transport models
were used to predict water table drawdown, trench dis-
charge, and TCE concentration in the extracted water.
The two-dimensional coupled flow transport models
used both advective and dispersive contaminant migra-
tion and retardation. Model boundary conditions were
based on surface topography and configuration of the
water table surface, with boundaries selected beyond
the expected region of influence of the trench system
(Figure 4-22). The base of the uppertill was selected as
the bottom of the aquifer. A finite element mesh was
established, with 212 node points and 204 elements. All
existing shallow monitoring wells were located at nodes.
The flow model was calibrated using empirical data with
an assumption of steady-state conditions. Calibration
was completed by varying precipitation recharge, initial
boundary head values, and hydraulic conductivity. The
predicted heads in the final calibrated model differed from
the observed field values by 0.3 meters (1 foot) or less.
Proposed trench locations were near the plume front
and along the plume center at a depth of 2.1 meters
(7 feet) below ground surface (Figure 4-23a). Operation
of the trench was simulated as a transient (non-steady-
state) system for 420 "model" days. Beyond 420 days,
water level and flow rate changes with time were insig-
nificant. Predicted trench discharge rates dropped from
an initial value of approximately 37.9 liters (10 gallons)
per minute to a steady-state rate of about 3.8 liters
(1 gallon) per minute.
Then, a preliminary transport analysis used the flow
model to estimate changes in TCE concentration
resulting from operation of the trench system. Assumed
TCE-related transport model parameters were:
Model Domain
t^__A/ Site Boundary
Figure 4-22. Model domain showing location of the interceptor
trench, the 1986 TCE plume limit, and extent of the
capture zone (34).
Observed -1986
Calculated -130 months
0 400 ft
Calculated - 286 months
TCE>10ppb
Interceptor
Trench
Capture
Zone
Figure 4-23. TCE concentration distribution through time
(Note: TCE 10 ppb outside of capture zone.)
retardation factor = 1.5, longitudinal dispersivity = 9.1
meters (30 feet), and transverse dispersivity = 3 meters
(10 feet). Distribution of TCE in 1986 was used as the
initial concentration for the model. The model simulated
83
-------
TCE movement for 8,580 days (23.5 years). Results of
the preliminary simulation indicated an overall reduction
in TCE concentration and predicted locations of con-
tamination during the recovery system operation (Figure
4-23). The results also indicated, however, that some
marginal areas of the TCE plume could avoid capture
as advective flow and dispersion moved it beyond the
trench capture zone.
The transport modeling indicated that the operation
would require approximately 24 years to decrease aver-
age discharge TCE concentrations to about 8 ppb (from
an initial value of about 1,550 ppb). This would require
the removal of approximately 9 pore volumes of con-
taminated ground water. Because the analysis assumed
rapid TCE desorption from soil, no TCE transformation
or degradation, and other simplifying factors, the esti-
mates presented are rough and to be used only for
planning purposes.
Implementation. Plans for constructing and implement-
ing the interceptor trench (Figure 4-24) were developed
in 1987 (38). Designs called for all trench segments to
be approximately 2.1 meters (7 feet) deep to fully pene-
trate the shallow aquifer. The initial trench system was
approximately 565 feet in total length and consisted of
two trench segments 2.1 meters (7 feet) deep, with
separate sumps (designated Martin and Henfield sumps
on Figure 4-24).
In addition to a depth of 2.1 meters (7 feet), each trench
segment was 0.9 meters (3 feet) wide. A filter fabric
(geotextile) lined the excavation. Then, 15 centimeters
(6 inches) of 1.9-centimeter (3/4-inch) gravel was placed
at the bottom of the trench followed by a 15-centimeter
(6-inch) diameter, continuously perforated PVC drain
pipe (Figure 4-25). The trench was backfilled with
1.9-centimeter (3/4-inch) gravel to within about 0.3 me-
ters (1 foot) of the ground surface. The remaining 0.3
meters (1 foot) of backfill consisted of natural soils to
minimize infiltration of surface waters.
Results. The ground-water extraction system began op-
eration in January 1989. Figure 4-26 summarizes the
ground-water extraction from the two interceptor
trenches in the shallow, upper till aquifer from Septem-
ber 1989 through September 1993. In general, several
observations could be noted for this period:
Data from monitoring piezometers indicated that the
regional flow in the shallow aquifer was north-north-
west, rather than to the west as assumed during the
modeling phase of the project.
Temporal variations were observed in the concentra-
tion of total VOCs (monthly averages typically varied
from 1,000 to 6,000 ppb), but no consistent trend of
decreasing concentration through time seemed to oc-
cur. On the basis of an initial "best fit" first-order rate
constant, it was estimated that lowering VOCs to a
Martin Sump
Regional Gradient
Shallow Aquifer
Monitoring Wells
Storage Silos
Probable Source Area
Figure 4-24. Site plan showing locations of initial interceptor
trench segments (38).
Figure 4-25. Schematic cross section of trench construction
(38).
residual concentration of 20 ppb would take approxi-
mately 15 years (an extraction of approximately 1.2
x 10s cubic meters [32 million gallons]).
The property containing the storage silos appeared
to be the principal region of contamination of the
shallow (upper till) aquifer, but insufficient data ex-
isted to generate a valid spatial model.
84
-------
80,000
Sept89 Mar90 SeptQO Mar91 Sept91 Mar92 Sept92 Mar93 Sept93
Key: a Martin Sump
* Henfield Sump
The actual trench system is recovering 155 percent of
the predicted value. These differences probably result
from parameter estimates that the modeling used,
including:
Rate of recharge by precipitation (model value ap-
pears to underestimate the actual value).
Hydraulic conductivity of the shallow aquifer (model
value appears to underestimate the actual value).
Regional gradient (a different direction as indicted by
the observed data).
Additional examination of the data could possibly yield
additional causes for this difference.
Figure 4-26. Monthly extraction from interceptor trench sumps. 4.5 References
The initial interceptor trench system was insufficient
to capture the VOC plume. Consequently, in Decem-
ber 1991 additional trench segments were consid-
ered, followed by a recommendation in December
1992 to construct these segments on an adjacent
property. In October 1993, the design of the additional
trench segments was completed, and the trenches
were installed during the winter of 1994.
As of September 1993, approximately 40.8 kilograms
(90 pounds) of VOCs were treated at the site.
For the first year of operation of the interceptor trench
system, the predicted extraction (from modeling) ex-
ceeded the actual extraction. After the first year, the
actual extraction exceeded the predicted volume. Figure
4-27 compares the actual and predicted volumes. A
linear regression through the observed data and fixed at
the origin yielded an average flow rate of about 67,000
gallons per month. In contrast, a linear model through
the steady-state portion of the predicted data (after the
first 8 months) yielded an average flow rate of approxi-
mately 164 cubic meters (43,200 gallons) per month.
3,500,000
3,000,000
2,500,000
(A
_g 2,000,000
"ro
CD
1,500,000
1,000,000
500,000
Observed Discharge
»Tl I I I I
Sept89 Mar90 Sept90 Mar91 Sept91 Mar92 Sept92 Mar93 Sept93
Figure 4-27. Observed and predicted discharges from trench
system.
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87
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