?/EPA
United States
Environmental Protection
Agency
EPA/540/SR-93/505
October 1993
SUPERFUND INNOVATIVE
TECHNOLOGY EVALUATION
Technology Demonstration
Summary
Hydraulic Fracturing Technology
Two pilot-scale demonstrations of the
hydraulic fracturing technology for en-
hancing the permeability of contami-
nated silty clays have been evaluated
under the Superfund Innovative Tech-
nology Evaluation (SITE) Program. The
technology was jointly developed by
the University of Cincinnati (UC) and
the U. S. Environmental Protection
Agency's (EPA's) Risk Reduction Engi-
neering Laboratory (RREL). Tests were
also conducted at UC Center Hill Solid
and Hazardous Waste Research Facil-
ity (Center Hill Facility) by UC. These
tests were conducted to determine the
factors affecting soil vapor flow through
sand-filled hydraulic fractures.
The hydraulic fracturing technology
was demonstrated in 1991 and 1992 at
a Xerox Corporation soil vapor extrac-
tion (SVE) site in Oak Brook, IL (the
Xerox Oak Brook site), and at a biore-
mediation site near Dayton, OH (the
Dayton site). The sites were contami-
nated with volatile organic compounds,
and the in-situ permeability of onsite
soils was less than 10'7 centimeters
per second. The technology created
multiple, sand-filled hydraulic fractures
at depths of up to 15 feet below ground
surface (ft bgs). At the Xerox Oak Brook
site, these fractures increased vapor
flow by one order of magnitude in an
area 30 times greater than the area
affected by wells in unfractured soil. At
the Dayton site, the groundwater flow
rate was 25 to 40 times higher in a well
screened in the fractured soil than in
one screened in unfractured soil.
This Summary was developed by
EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the SITE demonstration
that is fully documented in two reports
under one cover (see ordering informa-
tion at back).
Introduction
In response to the Superfund Amend-
ments and Reauthorization Act of 1986,
EPA established a formal program to ac-
celerate the development, demonstration,
and use of new or innovative technologies
that offer permanent, long-term cleanup
solutions at Superfund sites. This SITE
program is administered by EPA's Office
of Research and Development, RREL. One
component of the SITE program is the
demonstration program, which develops
reliable performance and cost information
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on innovative technologies so that poten-
tial users can assess a technology's suit-
ability for specific site cleanups. During
the demonstrations, hydraulic fractures
were created to increase the tn-situ per-
meability of contaminated soil. This per-
meability enhancement significantly im-
proved the rates of vapor and water flow
through the soil near the fractured wells.
Technology Description
The hydraulic fracturing technology is
designed to create sand-filled fractures up
to 1-in. thick and about 20 ft in radius in
tow permeability soils. These fractures,
when created at several depths from 5 to
40 ft bgs, increase the permeability of
contaminated soil. This increased perme-
ability promotes the flow of vapors and
liquid through the soil and enhances the
effectiveness of SVE, bioremediation, and
pump-and-treat remediation techniques.
The hydraulic fracturing equipment con-
sists of a fracturing lance, a tool to create
the initial notch, a continuous slurry mixer,
and a positive displacement pump
mounted on a trailer. A typical sequence
of operations for creating hydraulic frac-
tures is shown in Figure 1, and the slurry
mixer and trailer-mounted pump assembly
is shown in Figure 2.
A borehole is drilled using 6 or 8-in.
outside diameter, hollow-stem augers. In-
dividual segments of the rod and casing
are 5 ft long and are threaded together as
required by fracture depth. The tip of the
fracturing lance is then driven to a depth
where a fracture is to be created. The
lance is removed, leaving soil exposed at
the bottom of the casing (see Figure 1).
Steel tubing with a narrow orifice at one
end (the notching tool) is inserted into the
casing, and water is pumped through the
tubing to create a high-pressure water jet.
The water jet, which has a pressure of
about 3,500 pounds per square inch, is
rotated within the borehole and produces
a disc-shaped notch extending 4 to 6 in.
from the borehole (see Figure 1).
Sand slurry is then pumped into the
notch to create a hydraulic fracture. Sand
slurry is produced by mixing one part of
granular solid (coarse sand proppant) with
two parts of viscous fluid in a continuous
mixer. The viscous fluid consists of a mix-
ture of guar gum gel, water, and an en-
zyme that breaks down the gel after the
proppant has been deposited into the frac-
ture. A hydraulic fracture is created by
pumping a predetermined volume of slurry
at rates of 10 to 25 gals/min into the
notch. Lateral pressure from the soil on
the outer wall of the casing effectively
seals the casing and prevents leakage of
the slurry. The fracture nucleates at the,
notch and grows radially up to about 20 ft
from the borehole wall.
The gel-to-sand ratio in the slurry is
adjusted to propagate the fracture and to
move the sand into the fracture. The
amount of the gel is reduced when a pos-
sibility exists of the fracture venting to the
surface. In cases where the fracture propa-
gates horizontally, the sand content is in-
creased during pumping to increase the
thickness and length of the fracture. The
gel-to-sand ratio in the slurry is adjusted
from fracture to fracture, depending on
depth and site-specific soil conditions. For
a fracture created at 15 ft bgs, about 150
gal of gel and 14 cu ft of sand are used.
The direction and distance of propaga-
tion of the fracture is measured by moni-
toring the uplift of the ground surface.
Several stakes are placed along radial
1
Casingr—
Cap
•>"Rod
'Lance tip
Extension
rf"' rod
/-Steel tubing.
,/- Cutting jet.
Removal
of lance
Sand slurry
; Pressure from soil
seals casing
.:,- Notch
Flgura 1. Sequence of operations for creating hydraulic fractures.
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Figure 2. Slurry mixing and pumping equipment mounted on trailers.
directions around the borehole before frac-
turing. After fracturing, a leveling telescope
can be used to measure the change in
elevation of preexisting marks on the
stakes to determine the location and net
uplift of the ground surface resulting from
the fracture. A laser system called the
Ground Elevation Measurement System
was developed by DC (patent pending) to
measure real-time uplift data during hy-
draulic fracturing. The system uses a la-
ser and an array of sensors to track the
displacement of each point in the array
with time.
Factors Affecting Technology
Effectiveness
Several factors affect the performance
of the hydraulic fracturing technology.
These include various site characteristics
and the communication between horizon-
tal fractures. These factors, as applied to
the SITE demonstrations, are discussed
below.
Site Characteristics
Hydraulic fracturing is a permeability-
enhancement technique used in conjunc-
tion with other soil remediation methods.
Therefore, overconsolidated silty clays that
have low in-situ permeabilities are best
suited for the use of hydraulic fracturing.
In overconsolidated clays, the horizontal
, stress is greater than the vertical stress,
and this stress condition permits fractures
to propagate in a horizontal direction. Frac-
tures that remain horizontal can grow to
significant lengths, thereby enhancing flow
in the subsurface.
Hydraulic fracturing is ineffective in nor-
mally consolidated clays where the hori-
zontal stress is less than the vertical stress.
Demonstrations of hydraulic fracturing in
such clays created fractures that were
steeply dipping and vented to the surface.
Also, the presence of water decreases
the efficiency of SVE; hence, the use of
hydraulic fracturing to enhance SVE should
be limited to unsaturated clays.
Communication Between
Horizontal Fractures
Horizontal fractures increase the per-
meability of soil in their immediate vicinity.
However, the permeability of soil between
the fractures is not affected unless verti-
cal or inclined fractures are created be-
tween the horizontal fractures. Further work
at UC is being planned to study this. In
SVE applications, changes in soil vacuum
(suction head) applied to horizontal frac-
tures may induce communication between
the fractures.
Overview of SITE
Demonstrations
The SITE demonstrations had the fol-
lowing objectives:
• to establish the viability of creating
sand-filled hydraulic fractures in low
permeability silty clays,
• to study the factors that affect the
performance of the fractures over
time,
• to compare the vapor flow rates in
wells in fractured and unfractured
soils,
• to compare the water flow rates and
moisture content in fractured and
unfractured soils, and
• to develop information required to es-
timate the operating costs of the tech-
nology.
The Xerox Oak Brook site contained
silty clays contaminated with ethylbenzene;
1,1 -dichloroethane; trichloroethene;
tetrachloroethene; 1,1,1 -trichloroethane;
toluene; and xylene. Two out of four wells
used for two-phase SVE were fractured at
depths of 6, 10, and 15 ft bgs. Over a
period of 1 yr, the soil vapor flow rates,
suction head, and contaminant removal
rates were measured and compared for
the fractured and unfractured wells. The
soil vapor flow was measured with vari-
able area flow meters; the suction head in
the vicinity of the wells was measured
using pneumatic piezometers; and the con-
taminant concentration in the soil vapor
was measured with a gas chromatograph.
Dayton site contamination included ben-
zene, toluene, ethylbenzene, and xylene
(BTEX) and petroleum hydrocarbons. One
out of two wells was fractured at depths of
7, 8, 10, and 12 ft bgs. Water containing
hydrogen peroxide and nutrients was grav-
ity fed into these wells intermittently for
about 6 mos. The site operator was re-
sponsible for this activity, and UC was
responsible for monitoring the progress of
bioremediation near the fractured and
unfractured wells. Two rounds of sam-
pling were conducted at locations 5, 10,
and 15 ft north of the fractured and
unfractured wells after bioremediation was
in progress for 1 and 6 mos. Soil samples
were obtained and analyzed for moisture
content, microbial metabolic activity, num-
ber of colony forming units, BTEX, arid
petroleum hydrocarbons.
The Center Hill Facility tests were con-
ducted in uncontaminated ground during
the winter and summer of 1992. Perfor-
mance of three fractured wells (CHF1,
CHF2, and CHF3) was compared with
that of two conventional vapor extraction
wells (CHC1 and CHC2). Well No. CHF1
intersects hydraulic fractures created at
depths of 5 and 10 ft bgs, and Well Nos.
CHF2 and CHF3 intersect fractures at a
depth of 5 ft bgs. At Well No. CHF2, the
fracture intersected the ground surface
(vented). Air flow from these wells was
measured with variable area flow meters,
suction head near the wells was mea-
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sured with pneumatic piezometers, and
tha intensity of rainfall was measured with
a rain gauge.
Demonstration Results
The pifot-scale demonstration at the
Dayton site and tests at the Center Hill
Facility were conducted under the Quality
Assurance Project Plan (QAPP), Project
Category IV, developed by UC Center Hill.
Testing at the Xerox Oak Brook site was
conducted by Woodward-Clyde Consult-
ants (WCC) under a QAPP developed by
WCC. Center Hill Facility test results and
Xerox Oak Brook and Dayton site demon-
stration results are discussed below.
Center Hill Facility Tests
Parameters measured during the Cen-
ter Hill Facility tests included air yield,
suction head, and rainfall intensity. Air yield
was obtained as a function of time and
rainfall intensity for three fractured wells
and was compared with air yield data from
two unfractured wells. Suction head was
measured near the fractured and
unfractured wells. The average and maxi-
mum yields (in cubic feet per minute [cfm])
and average zone of pneumatic control
for the five wells are summarized in Table
1.
The results demonstrate that air yield
from fractured wells is one order of mag-
nitude higher than that from unfractured
wells. The zone of pneumatic control of
the fractured well was 15 to 30 times
greater than that for unfractured wells.
Rainfall intensity affected the performance
of fractured wells, decreasing the yield
and increasing the suction head.
Xerox Oak Brook Site
Demonstration Results
Vapor discharge from, and suction
heads in, two fractured wells (Nos. RW3
and RW4) and one unfractured well (No.
RW2) were obtained over a period of 6
mos. The vapor discharge readings (in
actual cfm [acfm]) from these wells are
summarized in Table 2.
The vapor discharge from fractured wells
was about 15 to 30 times greater than
that from the unfractured well. The water
recovery rate from the wells fluctuated
widely, ranging from 20 to as much as
500 gallons per day. Water recovery rates
and vapor discharge rates were inversely
related. The cumulative mass of contami-
nants recovered from fractured wells was
consistently one order of magnitude
greater than that for the unfractured well.
The suction head near the fractured well
extended radially about 25 ft, compared
with less than 1 ft from the unfractured
well. These data correlated well with the
radius of fractures in Well Nos. RW3 and
RW4.
Dayton Site Demonstration
Results
Water flow rates and soil sampling re-
sults near a fractured well (SAD2) with
fractures at 7, 8, 10, and 12 ft bgs were
compared with those from an unfractured
well (SAD4). The flow rate of water en-
riched with hydrogen peroxide and nutri-
ents was about 25 to 40 times greater in
the fractured well than in the unfractured
Table 1. Performance of Wells at the Center Hill Facility
Wall No.
CHF1
CHF2
CHF3
CHC1
CHC2
Average Yield
(cfm)
3.7
6.7
3.4
0.33
0.59
Maximum Yield
(cfm)
6.1
7.2
4.05
0.38
1.25
Average Zone of
Pneumatic Control* (ft)
25 to 30
20 to 25
15 to 20
Less than 1
Less than 1
'Zone in which tha pressure distribution can be controlled by varying the applied suction head.
Teblo 2. Summary of Well Discharge Readings at Xerox Oak Brook Site
Week No.
RW2
RW3
RW4f
RW4*
Range of Rates
0.1-4.6
2.2-22.0
27.9-42.7
17.1-29.7
Average
Discharge
(acfm)
1.1
14.3
34.2
22.6
Discharge
Percentage in
6 ft bgs Zone
46.3
61.2
36.0
not applicable
Discharge
Percentage in
10 ft bgs Zone
27.3
8.4
41.0
not available
Discharge
Percentage in
15 ft bgs Zone
23.2
30.4
23.0
not available
'Thae-tt-daepfractureat WeHNo. RW4 ventedto the surface. Data for this well include the discharge average
when suction was applied to all three of the fractures.
f Wet discharge average when suction was applied to the 10- and 15-ft-deep fractures only. Hence, well
discharge was smaller than when suction was applied to all three of the fractures.
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well (see Figure 3). This increased flow
resulted in doubling the moisture content
near the fractured well. The demonstra-
tion did not provide reliable quantitative
data on contaminant removal rates at the
site.
Costs
Economic data obtained from the SITE
demonstrations were analyzed to estimate
the cost of using this hydraulic fracturing
technology at a hazardous waste site
remediation. Twelve cost categories were
examined, including capital costs, labor
costs, and supplies and consumables
costs.
The capital equipment costs include
costs for an equipment trailer on which
the slurry mixer, pumps, tanks, and hoses
are mounted; a fracturing lance with well-
head assembly; pressure transducer and
display; and uplift survey instruments. The
capital equipment cost was $92,900. A
capital equipment daily rental cost, as-
suming 30 rentals/yr and a depreciation
life of 3 yrs, was $1,000. A crew of four to
five can operate the fracturing and moni-
toring equipment. Labor costs were esti-
mated to be about $2,000/day. Supplies
and consumables include sand, guargum
gel, enzyme, and diesel or gasoline for
operating the pumps. The cost for sup-
plies and consumables was about $1,0007
day. The daily cost for creating about four
to six fractures was about $5,700 and
included site preparation ($1,000) and cost
of installing pneumatic piezometers ($700)
near the fractures. Hence, the cost per
fracture can vary from about $950 to
$1,425.
Conclusions
The tests conducted at the Center Hill
Facility and the pilot-scale demonstrations
completed at the Xerox Oak Brook and
Dayton sites lead to the conclusions pre-
sented below.
1. Sand-filled hydraulic fractures, up to
1 in. thick and 25 ft in radius, can be
created in overconsolidated, low per-
meability silty clays.
2. The fractures remained effective for
a period of more than 1 yr. Rainfall
decreased vapor yield and increased
suction head of fractured wells.
Unfractured wells were not affected
by rainfall.
3. Fractured wells yielded vapor flow
rates 15 to 30 times greater than did
unfractured wells. This increased flow
was obtained from a distance of 25 ft
away from the fractured well.
4. The water flow was about 25 to 40
times greater in a fractured well than
in an unfractured well.
5. The cost of creating a fracture can
vary from about $950 to $1,425.
The technology demonstrations have
established the increased permeability in
silty clays resulting from hydraulic frac-
tures and the radius of influence of these
fractures. Further enhancements of in-situ
permeability can be achieved if communi-
cation can be created between horizontal
fractures.
0.4
0.3
.65
I 0.2
CC
iŁ
I
0.1
20
*~r-
40
60
Time (days)
~^- Well No. SAD2 (fractured) —•— Well No. SAD4 (unfractured)
Figure 3. Flow volumes of injected water in wells no. SAD2 and SAD4.
&V.S. GOVERNMENT PRINTING OFFICE: 1993 - 7«MW«0079
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The EPA Project Manager, Naomi Berkley (see below), is with the Risk
Reduction Engineering Laboratory, Cincinnati, OH 45268.
The complete report, entitled "Hydraulic Fracturing Technology—Technology
Evaluation and Applications Analysis Reports," (Order No. PB94 -100 161'/
AS^ Cost: $27.00, subject to change), will be available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Manager can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGES FEES PAID
EPA
PERMIT No. G-35
Official Business
PenaKy for Private Use
$300
EPA/540/SR-93/505
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