u.s. EP;
Coalbed Methane
OUTREACH PROGRAM
www.epa.gov/cmop
IU
(9
Coal Mine Methane Recovery:
A Primer
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ACKNOWLEDGEMENTS
This report was prepared under Task Orders No. 13 and 18 of U.S. Environmental Protection
Agency (USEPA) Contract EP-W-05-067 by Advanced Resources, Arlington, USA. This
report is a technical document meant for information dissemination and is a compilation and
update of five reports previously written for the USEPA.
DISCLAIMER
This report was prepared for the U.S. Environmental Protection Agency (USEPA). USEPA
does not:
(a) make any warranty or representation, expressed or implied, with respect to the
accuracy, completeness, or usefulness of the information contained in this report, or that
the use of any apparatus, method, or process disclosed in this report may not infringe
upon privately owned rights;
(b) assume any liability with respect to the use of, or damages resulting from the use of,
any information, apparatus, method, or process disclosed in this report; or
(c) imply endorsement of any technology supplier, product, or process mentioned in this
report.
ABSTRACT
This Coal Mine Methane (CMM) Recovery Primer is a compilation and updating of five EPA
reports, written from 1999 - 2001, which reviewed the major methods of CMM recovery from
gassy mines. [USEPA 1999b, 2000, 2001a'b'c] The intended audiences for this Primer are
potential investors in CMM projects and project developers seeking an overview of the basic
technical details of CMM drainage methods and projects. The report reviews the main pre-
mining and post-mining CMM drainage methods with associated costs, water disposal
options and in-mine and surface gas collection systems. Updates from previous EPA reports
include advances in mining and CMM drainage techniques, directional drilling technologies
(from the surface and in mine), costs of the various drainage methods, and references to the
latest research papers and presentations covering CMM drainage issues. This report is
based primarily on examples from the two countries with the most developed CMM
industries, the United States and Australia.
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CONTENTS
Acknowledgements, Disclaimer, Abstract i
Glossary of Terms v
Abbreviations vii
1. Introduction 1
1.1 Coal mining practices 3
1.2 Methane generation, retention and migration in coal 4
1.3 Emissions of methane in coal mines 5
1.4 Control by ventilation 6
1.5 Overview of methane drainage practices 7
2. CMM drainage techniques which reduce in-situ gas content 8
2.1 Vertical wells 8
2.1.1 Planning and design 10
2.1.2 Well bore completion 11
2.1.3 Stimulation technologies 13
2.1.4 Gas content reduction and production 16
2.1.5 Costs 17
2.2 Horizontal in-seam boreholes 18
2.2.1 Short holes 19
2.2.2 Long holes 20
2.2.3 Superjacent boreholes 23
2.3 Surface-drilled directional boreholes 24
2.3.1 Borehole drilling techniques 25
2.3.2 Gas content reduction and production 28
2.3.3 Costs 29
2.4 Water disposal 30
2.4.1 Water disposal options 30
3. CMM drainage techniques which recover gob gas 33
3.1 Vertical gob wells 35
3.1.1 Planning and design 35
3.1.2 Gob well completion 36
3.1.3 Gob gas production and quality 38
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CONTENTS
3.2 Cross-measure techniques 40
3.2.1 Planning and design 40
3.2.2 Recovered gas quality and production 43
3.3 Superjacent techniques 44
3.3.1 Overlying or underlying galleries 44
3.3.2 Directionally drilled gob boreholes 46
4. Gas Gathering and Collection 49
4.1 Underground gas collection systems 49
4.1.1 Pipelines 49
4.1.2 Safety devices 51
4.1.3 Water separation 51
4.1.4 Monitoring and control 51
4.1.5 Underground gas movers 52
4.2 Surface gas collection systems 52
4.2.1 Pipelines 53
4.2.2 Compression 53
4.2.3 Gas processing 53
5. Summary 55
5.1 Benefits of CMM drainage for coal mines 55
5.2 Environmental benefits of CMM drainage 58
6. References 59
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EXHIBITS
Exhibit 1: Global methane emissions from coal mining (2005) 2
Exhibit 2: Retreat longwall mining 4
Exhibit 3: Methane migration in coal 5
Exhibit 4: Typical ventilation configuration at a U.S. longwall mine 6
Exhibit 5: Typical vertical well setup 9
Exhibits: Major U.S. CBM basins 10
Exhibit 7: Under-reamed CBM completion 13
Exhibit 8: Hydraulic fracturing schematic 14
Exhibit 9: Schematic plan view of short horizontal boreholes in longwall panels 19
Exhibit 10: Longhole drilling from within a mine entry 20
Exhibit 11: Plan view of horizontal methane drainage borehole patterns modeled for
d eg as ifi cation of a longwall panel (not to scale) 21
Exhibit 12: Schematic plan view showing in-fill drilling of in-seam boreholes between
hydraulically stimulated vertical wells 22
Exhibit 13: Superjacent boreholes reduce in-situ gas content and drain gob gas 23
Exhibit 14: Surface-drilled directional oil & gas well types defined by radius size 24
Exhibit 15: Schematic of multiple horizontal wells drilled to a single vertical well 25
Exhibit 16: Slant hole drilling 26
Exhibit 17: Dual well system 26
Exhibit 18: Top view of CDX Pinnate drainage pattern 27
Exhibit 19: Forced evaporation pond 31
Exhibit 20: Side view of the effects of longwall mining on adjacent strata 33
Exhibit 21: Schematic showing the different gob gas recovery methods 34
Exhibit 22: Profile of atypical U.S. vertical gob well 36
Exhibit 23: Cross-measure boreholes developed from a second entry for longwall gob gas
recovery for retreating operations 41
Exhibit 24: Cross-measure drilling 42
Exhibit 25: Cross-measure borehole wellhead configuration with monitoring provisions 43
Exhibit 26: A sealed superjacent gallery with drainage boreholes 45
Exhibit 27: Degasification of gob areas using the superjacent method in Eastern Europe 45
Exhibit 28: Layout of a horizontal borehole methane drainage system showing both in-mine
and surface facilities 49
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EXHIBITS
Exhibit 29: HOPE gas collection piping 50
Exhibit 30: Summary of gas collection pipe properties 50
Exhibit 31: Separation system at the base of a vertical collection well 51
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GLOSSARY OF TERMS
Casing: Sections of steel tubing, slightly smaller than the diameter of the wellbore, placed in
the hole and cemented in place to prevent collapse of the wellbore. Casing seals off any
water bearing strata that have been drilled through, protecting potential water sources and
preventing the wellbore from filling with water. Casing also seals any gas bearing strata,
preventing gas flow into the wellbore until it can be produced in a controlled environment.
Coalbed methane (CBM): Methane that resides within coal seams. The equivalent term in
Australia is "coal seam gas" and in the United Kingdom is "firedamp". In the U.S., CBM
production is defined as methane extraction from coal seams that have not been disturbed
by mining. Outside the U.S., methane production from undisturbed coal seams is often
referred to as "virgin CBM" or VCBM.
Coal mine methane (CMM): Methane released from coal and surrounding rock strata as a
result of mining activity. In some instances, methane that continues to be released from the
coal bearing strata once a mine is closed and sealed may also be referred to as coal mine
methane because the liberated methane is associated with past coal mining activity. This
methane is also known as "abandoned mine methane" (AMM).
Degasification system: A system that facilitates the removal of methane gas from a mine by
ventilation and/or by drainage. However, the term is most commonly used to refer to removal
of methane by drainage technology.
Drainage system: A system that drains methane from coal seams and/or surrounding rock
strata. These systems include vertical and directionally drilled pre-mine wells, gob wells, and
in-mine boreholes.
Fracturing: (frac, fraccing) In this report, fracturing refers to the process of pumping a gas or
liquid into a wellbore at high pressure, in an attempt to induce fracture creation in a gas
bearing geologic horizon. These fractures provide a conduit for gas flow from the reservoir
formation to the wellbore and then to the surface.
Gateroads: Access roadways (tunnels) in an underground coal mine, connecting the
longwall working face with the main roadways.
Gob (goaf): The area of unconsolidated rock behind an underground coalface, that forms
when overlying strata falls into the void left by mining of the coal seam.
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GLOSSARY OF TERMS
Headgate: An access tunnel to the longwall face. It usually contains the conveyor belt that
carries mined coal from the longwall face to the main roadways. It is also the intake airway
for ventilation air to the longwall face. Can also be termed the "maingate".
Methane drained: The amount of methane removed via a drainage system.
Methane emissions: This is the total amount of methane that is not used and therefore
emitted to the atmosphere. Methane emissions are calculated by subtracting the amount of
methane used from the amount of methane liberated (emissions = liberated - used or
destroyed).
Methane liberated: The total amount of methane that is released, or liberated, from the coal
and surrounding rock strata during the mining process. This total is determined by summing
the volume of methane emitted from the ventilation system and the volume of methane that
is drained.
Methane recovered: The amount of methane that is captured through methane drainage
systems.
Methane used: The amount of captured methane put to productive use (e.g., natural gas
pipeline injection, fuel for power generation, etc.).
Tailgate: An access tunnel to the longwall face situated on the opposite side of the coal
panel to the headgate. The tailgate commonly acts as the return airway from the coal face
and as a supply road to the face.
Ventilation system: A system that is used to control the concentration of methane within
mine working areas. Ventilation systems consist of powerful fans that move large volumes of
air through the mine workings to dilute methane concentrations to "safe" levels.
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ABBREVIATIONS
Unit Abbreviations
°C degrees Celsius
°F degrees Fahrenheit
$ United States Dollar
Bbl barrel
Bcf billion (109) standard cubic feet
Bcfd billion (109) standard cubic feet per
day
Bern billion (109) cubic meters
Btu British thermal unit
D (d) day
ft feet
in. inch
km kilometer
kPa kilopascal (103Pa)
m meter
m3 cubic meter
Mcf thousand (103) standard cubic feet
Mcfd thousand (103) standard cubic feet
per day
Mem thousand (103) cubic meters
Mcmd thousand (103) cubic meters per
day
md millidarcy (10"3D)
mm millimeter (10~3 m)
MMcf million (106) standard cubic feet
MMcfd million (10 ) standard cubic feet per day
psi pounds per square inch
scf standard cubic feet
Other Abbreviations
ARI Advanced Resources International, Inc.
CBM Coalbed Methane
CH4 Methane
CMM Coal Mine Methane
CO2 Carbon Dioxide
CO2eq CO2 Equivalent
ECBM Enhanced Coalbed Methane
HOPE High Density Polyethylene
ID Inner Diameter
IPCC Intergovernmental Panel on Climate
Change
MTCO2e Million tonnes CO2 equivalent
MSHA Mine Safety and Health Administration
NIOSH National Institute for Occupational
Safety and Health
U.S. United States of America
USDOE U.S. Department of Energy
USEPA U.S. Environmental Protection Agency
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1. Introduction
Coal mine methane (CMM) is gas released from coal or surrounding rock strata during and
after coal mining. As such, it is considered a mining hazard, a green house gas, and a
possible energy source.
CMM as a Hazard.
Methane is explosive in concentrations of 5-15% volume in air and has been the cause of
devastating mine explosions around the world throughout the history of coal mining. Modern
coal mine operators try to control methane concentrations at the working faces, and
throughout the mine, with the implementation of a well-designed ventilation system.
Over the past few decades, emissions of methane from coal mines have increased
significantly because of higher mining productivity; the trend towards recovery from deeper,
gassier coal seams; and greater pulverization of the coal product. When methane emissions
into the mine are greater than the ventilation system alone can dilute or remove, methane
concentrations may rise above mandated safety levels and production must be halted.
Adding additional ventilation capability is one solution to increased in-mine methane
emissions, but eventually, this becomes economically or technically infeasible.
To stay within mandated in-mine methane concentration limits, many coal mines develop a
degasification system to supplement the ventilation system. Drainage boreholes are drilled
from the surface, or from within the mine, to extract as much methane as possible from coal
seams and surrounding strata, before, during, and after mining, so as to lower methane
concentrations entering the mine workings.
CMM as a Greenhouse gas.
Methane released to the atmosphere is a significant greenhouse gas that contributes to
climate change and has a global warming potential 23 times greater than carbon dioxide over
100 years [IPCC, 2001]. The U.S. Environmental Protection Agency (USEPA) calculates that
coal mine methane contributes 8-10% of man-made methane emissions worldwide [USEPA,
20083]. Since 1994, the USEPA has been implementing a voluntary climate change program
to promote the profitable recovery and use of CMM (www.epa.gov/cmop).
As of early 2008, 14 countries have active mines employing some form of CMM drainage
system, with 12 of those countries having CMM recovery and utilization activities [USEPA,
20083]. Worldwide, there are more than 200 CMM drainage projects in place resulting in
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greater than 3.8 Bern (134 Bcf) of methane emissions avoided per year. China, USA,
Ukraine, Russia, Australia and India are the top six emitters of CMM as shown in Exhibit 1.
Methane Emissions
Coal Production
Country
China
USA
Ukraine
Russia
Australia
India
Rank
1
2
3
4
5
6
Emissions Volume
MMTCO2e Billion m3
135.7
55.3
26.3
26.2
21.8
19.5
8.7
3.5
1.7
1.6
1.4
1.2
Rank
1
2
11
5
4
3
Surface
mining %
10
67
1
56
80
85
Underground
mining %
90
23
99
44
20 (NSW 59)
15
Exhibit 1: Global methane emissions from coal mining (2005)
[USEPA, 2006]
CMM as an Energy Source.
CMM is primarily composed of methane, a valuable, clean energy source. CMM may also
contain small amounts of nitrogen, carbon dioxide, ethane, propane and water in varying
quantities. When CMM is diluted by ventilation air, oxygen will also be present. Different
methods of CMM recovery produce varying concentrations of methane at the surface
collection points.
The quality of recovered methane is measured by its calorific (or heating) value, expressed in
kilocalories per cubic meter (kcal/m3) using the metric system, and British thermal units per
standard cubic feet (Btu/scf) using the British system. Pure methane has a calorific value of
approximately 8900 kcal/m3 (1000 Btu/scf), while a mixture of 50% methane and 50% air has
a calorific value of about 4450 kcal/m3 (500 Btu/scf).
There are a number of possible end uses for CMM depending on its methane concentration
(heating value). High quality gas, with a calorific value normally greater than 8455 kcal/m3
(950 Btu/scf), is acceptable for injection into natural gas pipelines, where it has many
domestic and industrial end uses. Lower quality gas, which is diluted with air, can be used at
the mine site in internal combustion engines, or gas turbines, for electricity production. It can
also be used to heat mine buildings or dry coal in a coal cleaning facility. These applications
require a caloric value of only about 2670 kcal/m3 (300 Btu/scf).
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1.1 Coal mining practices
Coal can be mined at the surface ("opencast mining") or underground, depending on the
depth of the seam, or seams, to be extracted. Approximately 60% of world coal production is
produced from underground mines, although in the United States and in Australia, the
second and fourth largest coal producing countries respectively, surface mining accounts for
over 65% of production. (Exhibit 1)
Surface mining is viable when coal is relatively near to the surface, typically less than 100 m
(350 ft) deep. The overburden of soil and rock is broken up and removed with large draglines
or by shovels and trucks. The exposed coal is drilled, fractured and excavated in a
succession of strips and then transported, via truck or conveyors to the coal preparation
facility. It is possible to extract coal seams as thin as 100 mm (4 in) and recover 90% or more
of the coal deposit. Opencast mines can cover an area of many square kilometers.
Underground mining is carried out by two principle methods: longwall mining, and room
and pillar mining. Almost all modern, high-production mines use a retreat longwall method of
mining.
Longwall mining involves the extraction of coal from a large 'panel' developed in the target
seam. Mining machines, called 'continuous miners', develop the sides of the longwall panel
by driving parallel tunnels, called 'entries' or 'gates', into the seam from the mine's main
entries. The outline of the panel is completed with a connecting tunnel between the 'gates'
which becomes the working face (Exhibit 2). In favorable geologic locations in the U.S,
longwall panels have been developed up to -440 m (1,450 ft) wide and -3,960 m (13,000 ft)
long [Karacan et al, 2007].
A mechanical shearer is mounted on a series of self-advancing, hydraulically powered ceiling
supports and shears coal, in repeated passes, from the longwall face. The coal falls onto a
conveyor and is transported to the surface. As the shearer moves forward to cut the next
swath of coal, the ceiling supports follow and the roof behind the supports collapses, forming
the gob (also known as goaf). Mining back towards the main entries in this way is termed
'retreat longwall mining' and over 75% of the coal in the deposit can be extracted with this
method.
Room and pillar mining is generally used at shallower depths and where the geology of the
coal seam is too complex for longwall mining. Coal is extracted using a continuous miner that
cuts a network of rectangular 'rooms' in the seam. Up to 60% of the coal can be recovered,
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with the remaining 40% forming 'pillars' which support the mined out rooms. These pillars
can be mined as the final stage in the extraction of the section.
Diagram courtesy of BMP Btlliton Illawara Coal
Exhibit 2: Retreat longwall mining
[World Coal Institute, 2005]
1.2 Methane generation, retention and migration in coal
This section briefly summarizes the key factors that influence methane's formation and
movement through coal seams.
Coal formation and methane generation
Coal seams form over millions of years from layers of plant material that decay in swamp and
marsh-like conditions to form peat. As the peat is covered with sediments and buried more
deeply, it is subjected to heat and pressure, which forces water, oxygen, nitrogen, carbon
dioxide, and hydrocarbon gases out of the organic matter, increasing its carbon content and
forming coal. Large volumes of methane are generated during this coalification process,
most of which escapes to the surface at shallow depths, but increased pressure at deeper
depths retains the methane within the coal.
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Coal cleat
Cleat is a coal miners' term for the natural system of vertical fractures generated by local
tectonic forces, and shrinkage of the source plant material, during the coalification process.
The dominant fracture orientation is called the "face cleat" and the secondary, perpendicular
fractures are termed "butt cleats". Face cleats can be spaced from one tenth of an inch to
several inches apart [Steidl, 1996] and are important pathways for migration of methane out
of the coal.
Methane retention
Methane is stored mainly in the matrix of the coal and partly in the fracture spaces (cleat).
Matrix porosity largely determines the ability of coal to retain methane [Steidl, 1996].
Methane molecules are packed tightly as a monolayer on the large internal surface area of
coal (adsorption) and are held there by hydrostatic pressure. A cubic foot of coal can contain
six to seven times the volume of natural gas that exists in a cubic foot of conventional
sandstone reservoir.
Methane migration
When the hydrostatic pressure in coal is reduced (i.e. during mining or by a drainage
borehole), the methane desorbs from the micropores of the coal matrix, diffuses through the
matrix and flows through the cleats (Exhibit 3).
Gas Desorption:
Internal coal surface
Gas Diffusion:
Matrix and Micropores
Gas and Water Flow:
Cleat and Fracture Network
Increasing Size
Exhibit 3: Methane migration in coal
[USDOE.2003]
1.3 Emissions of methane in coal mines
The pattern of methane release from the coal seam and surrounding strata is controlled
primarily by the mining method, the location of the gas in the seam and surrounding strata,
and the permeability of the relevant geologic materials. During room-and-pillar mining,
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methane is released from within the coal as the entries and crosscuts are developed. The
methane also emanates from the roof and floor during the pillar recovery process as the
overlying strata subside and the underlying strata heave. This process also occurs during
longwall mining with the geometry of the panels further affecting methane emissions.
In longwall mining, methane can be emitted directly from the longwall face and from mined
coal being taken to the surface. Lower pressures in the mining area, compared to the
surrounding strata, causes migration of gas from the surrounding strata into the mine
workings. A large source of emissions comes from the gob (or goaf) formed when overlying
strata collapses into the void left by longwall mining.
Methane emissions into a mine normally occur at a steady rate, but geologic discontinuities
such as faults, clay veins and igneous intrusions, along with other geologic features such as
floor feeders, sandstone paleochannels and localized folding, can all be responsible for
sudden, potentially dangerous, unusually high emissions [Ulery, 2008].
1.4 Control by ventilation
All the major coal-producing countries mandate maximum methane concentrations of 1.0-
1.25% at the coal face and within the mine workings [Thakur, 2006]. Coal mine operators try
to control methane emissions by using large fans to circulate large volumes of air throughout
the mine workings. The ventilation air dilutes methane concentrations and carries the
methane to the surface via 'bleeder entries' and ventilation shafts (Exhibit 4).
a
:> Fresh Air
Return/Bleeder Air
Face Air
Mining Direction
I
O)
a
Belt entry
1
-
Exhibit 4: Typical ventilation configuration at a U.S. longwall mine
[Schatzeletal.2008]
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1.5 Overview of methane drainage practices
In this report, methane drainage techniques are classified in two main groups - techniques
that reduce the gas content of the coal seam prior to and during mining, and those that
reduce the volume of gob gas entering the mine workings during and after mining. These
groups can be further categorized into techniques that originate from the surface and those
that originate from within the mine workings.
Techniques that reduce coal seam gas content include:
Vertical boreholes drilled from the surface
In-seam boreholes drilled from within the mine - "short hole" and "long hole"
Superjacent boreholes drilled directionally from within the mine
Horizontal in-seam boreholes drilled directionally from the surface
Chapter 2 describes each of these techniques to reduce coal seam gas content.
Techniques capturing gob gas include:
Vertical gob wells
Superjacent boreholes
Cross-measure boreholes
Chapter 3 summarizes each of the technologies that capture gob gas.
In practice, a combination of these methods is used to degasify coal seams as much as
possible before they are mined, and to decrease the amount of emissions from the gob into
the ventilation system during mining. The design of the methane drainage system should be
governed by the quantity of methane being generated, the geology of the coal seam and
surrounding strata, the pattern of emissions, the mining-related costs associated with the
methane, and the potential for obtaining a market income from the gas generated. The
drainage system design may require adjustment on a continuing basis to ensure that the
methane capture is optimal as the mine develops over time.
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2. CMM drainage techniques which reduce in-situ gas content
Decreasing methane flow into mine workings during coal production can be achieved by
reducing the gas content of the coal and adjacent gassy strata before mining occurs. Where
reservoir characteristics are favorable, for example where coals have sufficient permeability
and rapid diffusion rates, gas can be drained rapidly from large areas. With low permeability
coal and/or coals with slower diffusion rates, gas drainage should be started as far in
advance of mining as possible.
The main methods of pre-mining degasification are:
Vertical boreholes drilled from the surface
In-seam boreholes drilled from within the mine -"short hole" and "long hole"
Superjacent boreholes drilled directionally from within the mine
In-seam boreholes drilled directionally from the surface
2.1 Vertical wells
The term "vertical well" is generally applied to a well, drilled from the surface, through the
target coal seam or seams, which is then cased and hydraulically fractured to pre-drain as
much methane as possible prior to mining. Wells are placed in operation from 2 to 10 years
ahead of mining.
The water in the coal seams must be removed to lower hydrostatic pressure and allow
methane to desorb from the coal matrix and flow via the cleat system to the well. This water
is separated from the produced gas and then treated and/or disposed of in an
environmentally acceptable manner (see section 2.4). The gas passes through a separator
near the well head to remove water traces before being piped to a processing facility to be
compressed and dehydrated (Exhibit 5). The gas is then fed into commercial pipelines.
Vertical wells offer an advantage over other pre-mining drainage techniques in that they can
drain multiple seams of coal simultaneously. Under the right conditions, these wells can
produce pipeline quality gas with minimal processing and in sufficient quantities to make
them economically viable.
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Pumpjack
2-in, (50.8-mm) pipe
Gas
'sates
2-in. C50.8-mm)
Pipe Water
Disposal
CoalbedB
Coalbed C
Exhibit 5: Typical vertical well setup1.
In all of the major U.S. coal bed methane basins (Exhibit 6), vertical wells are used to
commercially extract methane from un-mined coalbeds (i.e., in projects that are not
associated with coal mining). In the context of coal mining, six of the twenty-three gassiest
underground mines in the U.S. use vertical wells at pre-mining degasification projects in
Alabama and Virginia (USEPA 2008b).
Vertical wells have had limited success in the rest of the world, where less permeable,
deeper and more geologically complex coal seams increase drilling costs and decrease
hydraulic fracturing success. Thakur [2006] notes that drilling, completion and fracturing
costs in Europe and Australia are three times those in the U.S., while permitting and site
preparation costs are also higher. High cultural development density and environmental
considerations make finding suitable surface drilling locations, in many worldwide coal mining
regions, more difficult than in the U.S. A lack of suitable drilling, completion and fracturing
equipment in many potential CMM regions is also a major hindrance to drilling successful
vertical methane drainage wells.
1 Source: Hartman et al., 1997. Copyright 1997, John Wiley & Sons, Inc. Reprinted with permission of John Wiley & Sons, Inc.
9
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Western
Washington
Powd er River
Cherokee & lllinois
Forest City
Pennsylvania
Anthracite fields
Greater
Green River
Uinta
Central
ppalachian
Valley
Piceance
San Juan
Raton
As lajKHUd hy LJM iS
Arkoma
Black Warrior Cahaba And Coosa
Exhibit 6: Major U.S. CBM basins
[Source: USDOE]
2.1.1 Planning and design
A detailed study of the coal geology of a mine, created with the use of coal thickness and
structure maps, is the first step in planning the location of pre-mining drainage well sites. An
optimized well pattern will take into account the mine development plan, the planned time
before mining intercepts the well, reservoir characteristics, completion effectiveness, well
stimulation effects, and drilling, completion and operating costs. [Rodvelt et al, 2008] The
final well pattern will be a compromise between the best theoretical plan and economic and
technical realities.
The area of the coal seam drained by a vertical well has been studied by Zuber, Kuuskraa,
and Sawyer [1990]. This study examined vertical wells in the Oak Grove field in Alabama,
U.S.A. and found that well spacing varying from 40 to 160 acres was optimal under the
assumed conditions with closer spacing being optimal for lower permeabilities (below 10.0
md). Typical well spacing in the Alabama coal fields is about 40 acres with 3 to 7 wells
placed in each projected longwall panel. Similar results were reported by Richardson,
Sparks, and Burdett [1991].
Spacing of vertical wells in CBM projects tends to be larger than spacing for CMM projects.
Vertical CBM wells in the San Juan Basin in the U.S. (see Exhibit 6) are typically drilled on a
160-320 acre spacing, with 160 acre spacing the standard in the Unita and Raton Basins and
10
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80 acres the typical spacing of wells in the Arkoma and Powder River Basins. Operators of
CMM vertical wells must strike a balance between the economics of the well and the main
aim of reducing the methane content of the target coal seam as much as possible in the time
available. If time before mining is relatively short, then wells should be spaced closer
together to drain gas faster, but this increases the number of wells needed for drainage and
the overall cost of the project.
Vertical wells drilled into virgin coal seams often produce large amounts of water and only
small amounts of methane during the first several months in operation. As more water is
removed, and the pressure in the coal seam is lowered, methane production increases.
Vertical wells are usually spaced on a regular grid pattern, such that drainage radii overlap,
to most efficiently enhance the dewatering process and reduce the coal seam hydrostatic
pressure. Adjustments to the grid pattern are made to accommodate any well site location
problems caused by surface topography or habitation.
Example
CNX Gas Corporation have considered the problem of balancing optimum well spacing, time
before mining, and costs, at the Consol Energy-owned Buchanan and VP 8 mines, in the
Oakwood coal field in Virginia. CNX have opted for an advance drainage time frame that
adequately balances the risk of investing in a vertical pre-mine drainage system with that of
Consol's mining plans. Thus, a three to five year advance degasification program is used to the
extent that this can be feasibly coordinated with overall mining strategies [USEPA, 2008b]. CNX
have drilled wells on 40 acre spacing in the Oakwood field and 60 acre spacing in the Middle
Ridge field, and they are currently evaluating results from drilling 53 wells on 30 acre spacing in
2007. The viability of drilling on 20 acre spacing is also being investigated [CNX, 2007]
2.1.2 Well bore completion
Once a well bore has been drilled, the hole is "completed" by lining it with steel casing. This
stops the well bore from collapsing and seals the well bore from potential water ingress.
Completions are broadly classified as either "open hole" or "cased hole".
Open-hole - The most basic type of wellbore completion is to drill through the target coal
seam and case to a point just above the seam. This is an "open hole" completion and can
only be used when the uncased well bore wall consists of competent geologic formations that
are unlikely to collapse.
11
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Cased hole - A typical vertical CMM drainage well is drilled through the target coal seam and
cased with steel pipe, with a section (joint) of fiberglass pipe used to case across the seam to
be mined. This joint maintains borehole stability in the coal during stimulation, but can be
mined through safely when production mining reaches the wellbore. After mine through, the
vertical well can, in some cases, continue to drain methane, operating as a gob web.
Final casing sizes and completion type depends on a number of factors including the
following:
Depth of the targeted coals
Number of seams to be stimulated
Maximum water production required to dewater the coals
Reservoir pressure of each drained coal seam
Example
Successful completions in the Appalachian Basin often cement 18 mm (7 inch) diameter
surface casing to depths of 30-90 m (100-300 ft) to protect shallow water sources. 114 mm
(4.5 inch) production casing is then set to the bottom of the borehole. The borehole is drilled
deeper than the target coal to produce a sump hole or "rat hole" which allows production
equipment to be installed below the target seam.
Under-reamed - If suitable geologic conditions exist, an additional stage to open-hole
completions can be added to widen the borehole where it intersects the target coal. After
casing is set and cemented to the top of the coal, a special reaming tool with rotating blades,
jets or drill cones, is used to ream out a cavity in the coal. Under-reaming is a technique that
can be applied to multiple seams. Once the wellbore has been widened at each seam,
slotted casing is inserted across the coal interval and, where needed, gravel is packed
between the walls of the cavity and the casing to keep the cavity open.
Examples
Under-reaming is a common completion method in coal bed methane projects in the Powder
River Basin, U.S.A., where boreholes are widened from 158 mm (6.25 in.) in diameter to ~355
mm (~14 in.) [Colmenares and Zoback, 2007]. After under-reaming, the well is cleaned out
with a fresh water flush. A down-hole submersible pump produces water up the tubing while
the gas that separates from the water is produced up the annulus (Exhibit 7).
Under-reaming also takes place in the shallow, high permeability coals of the Surat Basin,
Australia, where the completion of multiple seams has been demonstrated.
12
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a
i
'>J Cement
J^j"lo Surface
Exhibit 7: Under-reamed CBM completion
2.1.3 Stimulation technologies
Most vertical wells do not produce gas until the permeability of the reservoir coal seams is
enhanced through stimulation treatment. Stimulation can also help remediate any damage to
the reservoir caused by drilling and cementing fluids infiltrating the reservoir matrix, coal-
cleats and natural fracture system.
Hydraulic fracturing (often referred to as "fraccing" or "frac job") involves the creation of a
single, planar, vertical fracture (except in shallow zones where horizontal fractures can be
created) which extends in two wings (180 degrees apart) from a wellbore. The well casing is
perforated at the coal to be fractured and a frac fluid (such as water, gel, or nitrogen foam) is
pumped into the well. If subjected to sufficient pressure, the coal "cracks", forming a fracture
that is extended by continued injection of fluid. A solid proppant2, normally sand, is carried
with the fluid. When injection ceases and the fluid flows back to the wellbore, the fracture is
held open by the proppant left in-situ. Fractures can extend 60-150m (200-500 feet) from the
wellbore and they create highly conductive flow paths for water and gas to migrate to the
The term "proppant" refers to sized particles mixed with fracturing fluid to hold fractures open after a hydraulic
fracturing treatment. In addition to naturally occurring sand grains, man-made or specially engineered proppants
may also be used. Proppant materials are carefully sorted for size and sphericity to provide an efficient conduit for
production of fluid from the reservoir to the wellbore. (Schlumberger Oilfield Glossary-
www.glossary.oilfield.slb.com)
13
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wellbore, and be produced to the surface. Multiple layers of coal can be stimulated by
isolating each interval and running an individual frac job for each layer (Exhibit 8).
Exhibit 8: Hydraulic fracturing schematic
(Source: AGR Oil & Gas Services)
Cavitation - With this technique, the wellbore is completed using open-hole techniques, and
the target coal seam is under-reamed. Using compressed air, the exposed coal is repeatedly
pressurized and depressurized. The coal breaks up and is drilled out, forming a cavity
around the well bore. More expensive than hydraulic fracturing, this method has been used
in suitably permeable seams in the San Juan Basin, U.S.A., but has found little application in
other U.S. coal basins. Operators in Australia's rapidly growing CBM industry are
investigating this technique.
Potential problems - Water based fracturing fluid systems are not suitable for use on all
coal seams and in some cases have the potential to create damage to the reservoir, and also
introduce extra fluid into a system to be de-watered. Formation damage can take a variety of
forms, including gel and chemical residue blocking the pore spaces of the reservoir or water-
induced swelling of formation clays, both of which lead to a reduction in the relative
permeability of the coal. Methods to address these problems have been the topic of
considerable research, with focus on the use of carbon dioxide and nitrogen as fracturing
fluids.
14
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Carbon dioxide (CO2) fracturing - CO2 fracturing is discussed in this section, as a viable
fracturing method for CBM wells. However, it is not appropriate for use in CMM wells draining
active coal mines because high concentrations of CO2 in the mine workings are a serious
hazard to miners.
One approach to avoid formation damage associated with water-based fracturing systems
altogether is fracturing with liquid CO2, which is a non-aqueous, non-damaging fluid. In coal
seams, this technique can also provide a small amount of production enhancement through
the introduction of CO2. Liquid CO2 fracturing has a long track record in Canada.
The principal benefit of liquid CO2 fracturing for coal reservoirs is identical to that for natural
gas production wells - the elimination of formation damage and rapid cleanup. This may be
particularly significant since many CBM wells require six to nine months of de-watering after
fracture stimulation to clean-up and begin showing significant gas production.
Nitrogen (N2) fracturing - Like CO2, gaseous nitrogen is also a non-aqueous, non-
damaging fracturing fluid and is also a viable stimulation technique for formations potentially
sensitive to aqueous-based fracture fluid systems, such as coal seams. In this case, nitrogen
is pumped as a cryogenic liquid and then heated to form a gas prior to being injected into the
well. Fracturing mechanics occur as in any other hydraulic fracturing technique, the only
difference being that the fracturing fluid is a gas. Pumping nitrogen as a gas normally
eliminates the possibility of transporting proppants, and as such, nitrogen fracturing can be
classified as a proppantless, nonreactive stimulation technique.
After fracturing using an aqueous-based fluid and proppant technique, the well must be
cleaned of excess proppant and fluid that either did not enter the coal, or flows back out of
the fractured coal. The clean up process, involving pumping the excess material from the
well, usually takes a minimum of one week and up to a month. Some CMM/CBM operators
have indicated that the time for clean-up can be even longer -several months in some cases
- and it is in these environments that nitrogen fracturing may be of greatest benefit.
The use of nitrogen as a fracturing fluid may also assist in the production of CMM/CBM
through the enhanced production properties the nitrogen has with methane in the coal seam
reservoir. The dry coals found in the Alberta Basin in Canada contain very little or no water
and nitrogen is used extensively as a fracturing fluid to avoid adding water to the coal
reservoir.
15
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Coiled tubing fracturing - Coiled tubing is being increasingly used in the oil and gas
industry for a number of applications, including slimhole drilling, fishing operations, remedial
treatments3 and hydraulic fracturing. In coiled tubing operations, a continuous roll or "coil" of
small diameter pipe (19-114 mm, 0.75-4.5 in.) is used in place of drill pipe or tubing strings to
conduct the desired operation. Coiled tubing operations offer several advantages over
conventional methods of fracturing, including portability, a small well site footprint and speed
of operations.
In wells with multiple coal seams to be stimulated, coiled tubing can be used to isolate a
single perforated coal, fracture the coal and then move to the next seam. Hydraulic fracturing
operations that once required two to three days can now be completed in one day. Rodvelt
and others [2008], report that "for shallow CBM wells, as many as 24 intervals in two
separate wells have been fracture stimulated in a single day with the same crew and
equipment". The ability to complete multiple zones in a single trip mitigates the risk of
wellbore damage from the multiple well interventions and down-hole tool runs associated
with conventional fracturing operations. Cost savings are realized in several areas, including
the lack of need for work-over rigs and the elimination of bridge plugs for zonal isolation.
Manpower costs are also significantly reduced, as the time required for fracturing operations
can be more than halved.
2.1.4 Gas content reduction and production
The use of fractured vertical wells has proven to be an effective method for reducing the
methane content of coal seams in advance of mining, thereby ultimately lowering methane
emissions to the atmosphere and increasing mine safety and productivity. A study at the Oak
Grove mine in the Black Warrior Basin in Alabama [Diamond et al, 1989] documents that
twenty-three vertical, hydraulically fractured wells produced 73% of the original gas in place
in the Blue Creek Coalbed over a ten year period. Methane reductions of 79% and 75% were
achieved in the overlying Mary Lee and New Castle seams, respectively, over the same
period.
Six of the twenty-three gassiest mines in the U.S. use vertical, hydraulically fractured wells to
reduce coal seam gas content before coal is mined. The gas is injected into commercial
pipelines. Jim Walter Resources produces 212 million m3 per year (7.5 Bcf/year) of methane
3 "Slimhole drilling" refers to drilling wellbores with smaller diameters than conventional wellbores. "Fishing"
refers to the process of removing broken or stuck drilling equipment from the wellbore. "Remedial treatments"
refer to work done to repair any wellbore problems that occur after the initial completion of the well.
16
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from 400 vertical wells at its three Blue Creek mines in Alabama, U.S.A. at an average of
2.26 mcm/d (80 Mcfd) per well [JWR, 2008].
When compared to CMM projects, vertical wells drilled in U.S. CBM projects are typically
drilled on larger spacing sizes and have longer production lives because they are not mined
through (20-30 years for CBM wells compared to 5-10 years for CMM wells). Thus, CBM
wells generally have higher average production rates, ranging from 7 mcm/d (248 Mcfd) in
the Raton Basin, to 23 mcm/d (800 Mcfd) in the San Juan Basin [Greedy et al, 2001].
One of the main advantages of vertical degasification wells as a methane drainage method is
their ability to produce pipeline quality methane without the need for extensive processing.
The primary disadvantage to fractured vertical wells is that they are more expensive to drill
and maintain than in-mine boreholes or gob wells. The fracturing process can represent
one-third to one-half of total well costs.
2.1.5 Costs
The major variables in determining the cost of drilling and completing a vertical well are the
drilling depth, the method of completion, the number of coal seams completed, the size and
type of any hydraulic fracturing process used and the cost of building the well site
infrastructure. Vertical well costs therefore vary widely in the U.S, and around the world,
sometimes within the same coal basin, depending on the geology, topography, regulatory
constraints of the project area and, in new CMM areas, the availability of service companies
and drilling related raw materials.
Examples
Vertical wells drilled by CNX in Pennsylvania to an average depth of 305 m (1,000 ft) and
hydraulically fracturing multiple seams, have drilling and completion costs of about $200,000.
CNX vertical wells, drilled in Virginia to an average depth of 610-760 m (2,000-2,500 ft) and
fracturing a single seam, have drilling & completion costs of about $300,000.
In the Black Warrior Basin in Alabama, methane is drained from coals 150-1,000 m (500-
3,300 ft). Multiple seams are typically fractured and well costs range from $240,000 to
$350,000.
In Western Canada, multiple seams of dry coal, found 200-700 m (650-2,300 ft) deep, are
fractured with nitrogen only for costs between $75,000 and $200,000.
In the Raton Basin, well depths average 610-760 m (2,000-2,500 ft) and multiple coal seams
are fractured for about $450,000.
Sources: Schlumberger, 2006. Oil & Gas Investor Magazine, 2008a
17
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2.2 Horizontal in-seam boreholes
Coal seams form as flat beds with a horizontal areal extent much larger than their vertical
section (1-1 Os of meters thick, but 1000s of meters in area). Therefore, vertical wells
intersect only a relatively small section of the coal seam to be drained, and almost always
need to be hydraulically fractured to create, or enhance, horizontal permeability paths in the
coal to allow gas to flow to the borehole.
An alternative method of pre-mining drainage is to drill horizontal4 boreholes, up to 1,525 m
(5,000 ft) long, within the coal seam, greatly increasing the volume of coal directly affected by
the drainage borehole and reducing or eliminating the need to hydraulically fracture the well.
Boreholes can be drilled directly into the coal seam from within the mine workings, or drilled
down from the surface, and turned though an arc, to drill horizontally through the coal. When
directionally drilled, horizontal boreholes can be positioned to perpendicularly intersect the
face cleats of the coal seam for optimum methane drainage.
The main types of in-seam boreholes described in the following sections are as follows:
"Short holes" - typically drilled parallel to the face of a longwall panel
"Long holes" - drilled longitudinally through the panel or can be drilled across
multiple panels.
Superjacent boreholes - used to pre-drain methane from over- and under-lying
gassy strata adjacent to the target coal seam
Directional surface boreholes - start at the surface and turn through varying radii
to drill horizontally through the coal
Of the twenty-three U.S. gassy mines identified by the USEPA as employing methane
drainage systems, seven of the mines use horizontal in-seam boreholes for methane
drainage prior to mining (USEPA 2008b). In Australian mines, in-seam boreholes are
extensively used for methane drainage and about 100km of in-seam holes are drilled each
year in the coal basins of New South Wales and Queensland. [Gray, 2002]
4 In reality, boreholes are never completely horizontal, as coal seams are rarely completely flat, but "dip" (slope)
upwards or downwards as local geology dictates
18
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2.2.1 Short holes
Short hole horizontal boreholes, drilled parallel to the coal face, drain methane from coal
seams shortly before mining, reducing methane flow into the mine workings. Short
boreholes, less than 305 m (1000 ft), can be drilled with relatively simple drills without the
steerable systems needed for long-hole drilling. Diamond [1994] provides an excellent
overview of the development of horizontal borehole drilling in the U.S., where methane
drainage using short boreholes is well established.
Short holes are normally 5-8 mm (2-3 inches) in diameter and spaced 30-122 m (100-400 ft)
apart. In longwall panels, they are drilled to within 15 m (50 ft) of the opposite side of the
panel. Boreholes are typically drilled from tail gate entries ('B' and 'C' in Exhibit 9) to
maximize drainage time from the future panel and reduce methane flow into adjacent
development entries as they are mined [Diamond, 1994].
{-Horizontal
\ hole
\ R
B Advance
drilling with
B development
Retreat
drilling with
adjacent
longwall
Developing
panel <«
L
D C7,a ^D a a a o?z7£^£3 a o D D CiW/^7 /~~i a n a a r\^~i C7
-------
Coal permeability, gas content, time to mining, and drilling economics are important factors in
determining borehole spacing. Several factors necessitate closer borehole spacing to
adequately degas a longwall panel, such as minimal time before mining, high seam gas
content, or low permeability. For example, a study by Aul & Ray [1991] in the Pocahontas #3
seam in Virginia found that 30% of in-situ gas could be removed by shorthole boreholes in
less than two months and 80% was removed after ten months, making possible a 79%
reduction in ventilation air volume. Produced gas quality from horizontal boreholes is typically
high and can be utilized as a pipeline product. Typical costs to drill boreholes using a rotary
drill and including utilities and logistical support are $50-65 per meter ($15-20 per foot).
Example
At the Blue Creek mines in Alabama, Jim Walters Resources reports producing 22.6
MMcm/year (800 MMcf/year) from short, across panel, in-seam degasification boreholes
[JWR, 2008].
2.2.2 Long holes
Long in-seam boreholes, drilled from existing mine entries into target coal seams (Exhibit
10), can significantly reduce the in-situ gas content of the coal, especially when drilled twelve
months or more before mining commences. Directionally drilled using down-hole motors,
long holes can be used to degas longwall panels months to years in advance of mining, and
to drain methane from coal in the vicinity of development entries as they are being mined.
These "shielding" boreholes reduce the volumes of methane entering the development entry.
Exhibit 10: Longhole drilling from within a mine entry
[JWR, 2008]
20
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Positioning - Advances in drilling technology over the last decade allow long boreholes over
1525 m (5000 ft) to be accurately, and rapidly, drilled in the coal seam. Stronger, more
powerful drilling equipment, coupled with precision, real time, drill bit navigation have
resulted in drilling accuracies of +/- 8 m (26 ft) over 915 m (3,000 ft). These advances have
led to reduced directional drilling costs and increased the opportunities for the use of this
technique in CMM drainage [Brunner and Schwoebel, 2005]
A study of in-seam borehole layouts by the National Institute for Occupational Safety and
Health (NIOSH) in the U.S. used a three-dimensional numerical simulator to model the
methane drainage of five different borehole layouts (Exhibit 11). NIOSH concluded that dual
and trilateral boreholes (layouts A and B) are more effective at decreasing emissions and
shielding entries compared to fewer, shorter, cross panel boreholes parallel to the face
(layouts C and D). Simulated reductions in methane emissions were 38.6% over 12 months
for the tri-lateral pattern, compared to 23% over 12 months for the cross-panel boreholes
[Karacan et al, 2007].
B
irr i i i i i i i i n
^A\ XJX xt\ ^ Tailgate entries
< Headgate entries
Mining direction
Exhibit 11: Plan view of horizontal methane drainage borehole patterns modeled for
degasification of a longwall panel (not to scale)
[Schatzeletal.2008]
Layout A in Exhibit 11 is a common in-seam borehole layout used in the U.S. As
development entries are mined to outline the longwall panel, a borehole is started from the
tailgate side of the panel and drilled parallel to the direction of the tailgate entry. A second
borehole branches from the first, across the panel and runs parallel to the headgate entry. In
21
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this manner, the development entries are shielded at the same time as the panel is being
drained [Karacan et al, 2007].
Long, in-seam boreholes can also be drilled into future longwall panels many months before
mining commences and, when directionally drilled, can be positioned in coal seams already
being degassed by vertical surface wells (Exhibit 12).
Existing Mains|
Vertical Wells
SO acre spacing /
Exhibit 12: Schematic plan view showing in-fill drilling of in-seam boreholes
between hydraulically stimulated vertical wells
[Brunneretal.2005]
Example
CNX Gas, at their southern Virginia mining operations, has drilled thirteen in-seam long holes,
the longest of which is 1,569 m (5,148 ft). These holes were directionally drilled into virgin coal
sections that were already being drained by hydraulically fractured vertical wells. The
boreholes were drilled roughly perpendicular to the axis of future longwall panels and
accurately placed to avoid the main fracture zones around the vertical wells. Hydraulic
fractures can cause borehole stability and fluid circulation problems while drilling [Brunner et
al, 2005]. The total drilled distance, including sidetracks, was 22,960 m (75,327 ft), and the
boreholes produced 31 MMcm (1.1 Bcf) of methane with no negative impact on the vertical
well production volumes. [CNX, 2007]
22
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Gas content reduction and production, costs - Long-hole degasification has been shown
to facilitate mine production. Brunner and Schwoebel [2005] report that at a mine in northern
Mexico, a 885 m (2,900 ft) shielding borehole reduced methane emissions into the adjacent
development entry by 30% after 2 months, reducing ventilation requirements by 30% and
increasing mining advance rates by 78%.
Up to 50% of in-situ gas can be drained by horizontal in-seam boreholes prior to mining in
the high permeability coals in the U.S. Total drainage is limited by the time available for
degasification. Shielding boreholes will be mined through once the development entries have
been completed and the longwall panel is ready for extraction, typically six months to a year.
Specialist, in-mine drilling contractors report that long, directionally drilled boreholes cost
$65-100 per meter ($20-30 per foot). A 1,370 m (4,500 ft) shielding borehole would cost
approximately $150,000 including wellhead and mine staff support costs.
2.2.3 Superjacent boreholes
Superjacent boreholes are directionally drilled from mine entries into coal seams, or other
gassy strata, above and below the target coal and can be up to 1,000 m (3,300 ft) long
(Exhibit 13). Their main purpose is to drain the gob area formed by longwall mining. As such,
they are generally considered a post-mining drainage technique, but depending on longwall
advance rates, they can drain gassy strata adjacent to the target seam for some time before
gob formation. In this case, Superjacent boreholes can also be considered a drainage
technique that reduces in-situ gas content. More detailed information on Superjacent
directionally drilled boreholes is provided in section 3.3.2.
_Coal Seam A
Exhibit 13: Superjacent boreholes reduce in-situ gas content and drain gob gas
[Brunner etal, 2005]
23
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2.3 Surface-drilled directional boreholes
Surface-drilled directional boreholes have been used extensively in conventional oil and gas
drilling for several decades. Drilling is started in the same manner as a vertical well (see
section 2.1) but at a predetermined "kick-off" point (KOP), the well is deviated from the
vertical, in an arc, so that the well bore enters the target formation roughly parallel to the
bedding plane. Surface-drilled directional holes are defined by the radius of their turn from
the vertical (Exhibit 14).
Drilling Method
Telescopic probe with hydraulic jet
Coiled tubing with hydraulic jet
Curved drilling guide with flexible drill
pipe; entire drill string rotated from the
surface
Steerable mud motor used with
compressive drill pipe; conventional
drilling technology can also be used
Conventional directional drilling
equipment used; very long curve
length of 850-1,350 m (2,800-4,400 ft)
needed to be drilled before achieving
horizontal
ladius Type
Zero
Ultra-short
Short
Radius
(m/ft)
0
0.3-0.6/1-2
1-12/3-40
Achievable
Lateral Length
(m/ft)
3/10
60 / 200
460/1,500
Medium
Long
60-300/200-1000
300-850+/1000-
2,500+
460-1,525+/
1,500-5,000+
600+/ 2,000+
(Record is over
12,000m/40,000 ft)
Exhibit 14: Surface-drilled directional oil & gas well types defined by radius size
[USDOE, 1993]
In the CBM and CMM industries, surface directional drilling was recognized as a way of
combining the best elements of vertical well and horizontal in-seam drilling. Drilling from the
surface is safer than from in-mine, does not hinder mining operations (for example, there is
no in-mine pipeline system), and can be carried out years in advance of mining. A long
horizontal borehole intersects a much greater volume of the coal seam than a vertical
borehole, negating the need for hydraulic fracturing in most cases and the borehole trajectory
can be controlled to take advantage of coal seam directional permeability. In addition, a large
area 2.6 km2 (640 acres), can be drained from a single surface site. This theoretically
replaces 16 vertical wells drilled on 0.16 km2 (40 acres) spacing and greatly reduces the
environmental impact of the methane drainage project and results in drilling, infrastructure,
and maintenance, cost savings.
24
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2.3.1 Directional borehole drilling techniques
Medium radius boreholes are the most common type of surface directional boreholes
currently drilled for methane drainage from coal seams. Over the last 10 years, the technique
has been refined and seen increasing use in the CBM and CMM industries in the U.S. and in
Australia. Early attempts at horizontal drainage boreholes drilled from the surface had
problems with the removal of produced water. The curved configuration of the wellbore made
conventional pumping techniques difficult, and more complex, solutions were prohibitively
expensive. U.S. and Australian drilling companies introduced new drilling techniques
involving the directional drilling of a horizontal well to intersect a standard vertical well that
produces gas and water.
In Australia, a commonly used technique is to directionally drill multiple boreholes to the
same vertical well. The technique is referred to as surface to in-seam drilling, or SIS. The
boreholes usually drain the same coal seam, but can target multiple coal seams at different
depths (Exhibit 15).
Sll-Ullm
SINGLE PRODUCTION BORE
^WELLHEAD
WATEROUT-
-GASOUT
COAL SEAM
UNDER REAMED TO 016"
6" I.D. CASING
6" I.D. CASING
COAL SEAM ft3
COAL SEAM
UNDER REAMED TO 016"
--185m
-220m
-27Im
'6" ID CASING
6" SUMP FOR PUMP
5!mDEEP
CONVERTED MINERAL RIG WITH MAST SET AT 60'.
>5 3/4" HOLE DRILLED TO 60m AND CASED WITH 4" I.D. STEEL CASING.
>096mm HOLE DRILLED STRAIGHT TO WITHIN 125m OF THE TARGET COAL SEAM.
>096mm RADIUS BEND DRILLED TO LAND HORIZONTAL AND INTO THE COAL SEAM.
BEND RATIO IS 7' PER 38m.
>096mm HOLE STEERED TO STAY WITHIN THE TARGET SEAM AND TO INTERSECT THE VERTICAL PRODUCTION WELL. - DRILLED DOWN DIP.
LIMITED "ROOF TOUCH" BRANCHES TO ACCURATELY POSITION THE INSEAM HOLE IN THE SEAM AND TO CONFIRM SEAM DIP ANGLES.
Exhibit 15: Schematic of multiple horizontal wells drilled to a single vertical well
[Mitchell Drilling, 2005]
25
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A magnetic guidance tool, lowered down the vertical well to the target coal seam, helps direct
the horizontal drillers to intersect the production well [Mitchell Drilling, 2005]. In Australia,
directional drilling from the surface using standard oil field equipment proved to be too
expensive when applied to shallow, relatively low producing coal seams. Australian drilling
companies use small, modified mineral drill rigs to
reduce costs, and practice "slant-hole" drilling
where the borehole is drilled from the surface
starting at angles of between 60-90 degrees to
the horizontal (Exhibit 16). Slant hole drilling
reduces the angle that needs to be turned through
to achieve horizontal drilling and allows the
targeting of shallower coals compared to drilling
starting vertically and turning through a 90 degree
arc.
Exhibit 16: Slant hole drilling
[Mitchell Drilling, 2005]
In the U.S., the vertical production well is situated close to the point where the directional
borehole first becomes horizontal. The horizontal borehole intersects the vertical well, or a
lateral leading from it, and then continues for lengths up to 1,525 m (5,000 ft). (Exhibit 17)
Lateral holes can then be drilled from the first borehole, in various layouts, to increase the
areal extent of coal drained. The laterals are drilled such that produced water drains to the
vertical well for pumping to the surface.
Drilling Rig
Horizontal and Service
We 11 bore
Air Compressor
Vertical and Producing
Cavity
Wellbore
* 7^ Coal Seams
-"^
Exhibit 17: Dual well system
[CDX Gas, 2005]
26
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Production we
CDX Gas in the U.S. has used this dual well technique to drill laterals in a "pinnate" drainage
pattern (Exhibit 18). Four sets of main laterals with associated side laterals can be drilled,
forming a 360 degree drainage pattern that can drain 1,280 acres and replace 16
standard 80 acre locations. Successful
Mainia<,,rai pinnate configurations, drilled in suitable
geologic environments in the Appalachian
and Arkoma Basins in the U.S., have large
sid.toi.rais jnjtjg| proc|Uction figures and have
dewatered the coal very quickly, resulting in
drainage of 80-90% of in-situ methane
within two to three years.
Exhibit 18: Top view of CDX Pinnate drainage pattern
[CDX Gas, 2005]
Modeling multi-lateral drainage patterns, Maricic et al [2005] concluded that the optimum well
configuration can be determined by considering the total horizontal length, the spacing
between laterals and the number of laterals. Longer horizontal length increases the contact
with the coal seam and increases yields for more gas recovery, but at the same time
increases drilling costs and drilling risks. Balancing these factors has led operators to more
commonly drill a simple three to four lateral pattern per horizontal well.
Example
CNX Gas, at its Mountaineer CBM field in southern Pennsylvania and northern West Virginia,
targeting the Freeport coal seam, drilled a total of 176 horizontal wells in 2007 and 2008 at
average depths of 180-240 m (600-800 ft). The drilling technique was changed from using a
simple 3 lateral design draining 2.6 km2 (640 acres), to an asymmetrical quad design ("turkey
foot") which resulted in more uniform methane drainage and a decreased well spacing to 1.9
km2 (480 acres). Consequently, drilling times were reduced from 21 to 15 days, greatly
improving well economics. The use of a gamma detector close to the drill bit to more
accurately steer the horizontal borehole in the coal, further reduced drilling times to 10 days.
One of the first wells brought on line produced at 25 Mcmd (900 Mcfd). [CNX, 2008]
The vertical section of the wells, and in some cases the arc of the well, are cased to ensure
borehole stability and prevent any potential water ingress from shallow water bearing rock.
The main laterals can be lined with slotted pipe to prevent borehole collapse. While
directionally drilling the horizontal laterals, if the wellbore intersects the roof or floor of the
27
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coal seam, the drill string (pipe) can be pulled back, and drilling continues at an angle away
from the coal boundary. This is known as "sidetracking" and ensures that the lateral stays
within the coal for its entire length.
Surface-drilled horizontal borehole techniques have seen little application in other countries,
often because of the relatively lower coal permeabilities and more complex geology of many
coals compared to those found in the U.S. and Australia. One exception is China, where
twenty-five multi-branch horizontal wells have been drilled through 2007. [Qiu. 2008]
2.3.2 Gas content reduction and production
Horizontal wells drilled from the surface into relatively high permeability coals, several years
before mining takes place, are able to drain over 80% of in-situ methane. This is similar to
the drainage efficiencies of vertical wells, but in general, horizontal wells degas coal seams
at higher production rates. Gas is drained from virgin coal seams with no dilution by mine
ventilation air and, after any necessary processing to remove excess carbon-dioxide,
nitrogen, or water, is usually of good enough quality for injection into a commercial pipeline.
Production examples
Target Drilling report average initial gas production of 18-21 Mcmd (650-750 Mcfd) from 1220+ m
(4000+ ft) horizontal wells in relatively high cleat permeability wells in Pennsylvania, with continued
production of 11 Mcmd (400 Mcfd) after two years.
Kreckel [2007] reports that "between 1998 and 2002, six operators drilled 110 horizontal wells in
the Hartshorne coal in the Arkoma Basin, Oklahoma. Laterals reached up to 1,615 m (5,300 ft) at
depths of 230-915 m (750-3000 ft). Initial production from half of these wells performed at better
than twice the average of vertical wells, between 5-11 Mcmd (200-400 Mcfd). Seven came in at
well over 28 Mcmd (1,000 Mcfd). The highest initial production of 32 Mcmd (1,152 Mcfd), came
from a horizontal lateral of 489 m (1,604 feet) length."
Green Dragon Gas Ltd has drilled two horizontal wells in China using the Australian surface-to-
inseam technique with lateral lengths of 816 m (2,676ft) and 1,280 m (4,200ft). The initial well
produced at an average of over 7 Mcmd (247 Mcfd) in its initial six months of production [OilVoice,
2008]
CDX Gas has used their pinnate drilling pattern to drain coal seams at the Pinnacle Mine in West
Virginia, and reports that 80-90% of all in-situ gas is recovered in a two to three year period. In
2006, the Pinnacle mine recovered and sold approximately 130 Mcmd (4.6 MMcfd) of gas from its
pre-mine drainage wells. (USEPA, 2008b)
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2.3.3 Costs
Drilling costs for surface-drilled horizontal wells are dependent on the depth of the target coal
seam or seams, the number of laterals drilled and the length of those laterals. Operators are
constantly looking for ways to minimize costs, resulting in innovative drilling methods such as
using modified mineral drilling rigs or experimenting with lateral layout patterns.
While drilling horizontal wells tends to be two to three times more expensive than drilling
vertical wells in the same area, faster gas recovery times, higher initial gas production and
larger ultimate production recoveries can result in lower dollar per produced volume of gas
values compared to vertical wells. Horizontal wells have a significant cost advantage
because they do not require hydraulic fracturing, which can constitute 30% of the cost of a
vertical well completion. Also one horizontal well replaces several vertical wells, with
resultant multiple savings in infrastructure capital costs (location and access road
construction, gathering pipeline, etc.) and operating costs.
Examples
Maricic et al [2005], in discussion with industry experts, estimates horizontal drilling costs at $30/m
($100/ft). This is approximately in line with reported costs from several operators.
In their Mountaineer field in northern West Virginia and southern Pennsylvania, CNX Gas have
drilled horizontal wells, consisting of four laterals, each approximately 915 m (3,000 ft) in length, at
depths of 180-240 m (600-800 ft). CNX has had to build new gathering and processing
infrastructure in the area. Drilling and completion costs per well, total about $800,000, with another
$100,000 forgathering and processing. [Oil & Gas Investor, 2008a]
Target Drilling estimate that a typical horizontal well with three to four laterals, each approximately
1,370 m (4,500 ft) in length, might cost in the range of $1.5 million, including surface gas drying
and compression equipment. [American Longwall magazine, 2007]
CDX Gas report costs of $2.2 million for wells targeting coals 275-395 (900-1000 ft) deep in their
Hillman field in West Virginia. Laterals are drilled in a pinnate pattern for a total drilled length over
6,100 m (20,000 ft) and drain 2.4 km2 (600 acres). Wells have initial production of over 14 Mcmd
(500 MMcfd). In 2008, 21 wells were producing in the Hillman field at a rate of 595 Mcmd (21
MMcfd). The average estimated ultimate recovery per well is about 28 MMcm (1 Bcf) per well. [Oil
& Gas Investor, 2008b]
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2.4 Water disposal
As is the case with CBM well drilling, pre-mining drainage of CMM usually involves the
drainage of water from the coal seam to lower reservoir pressure, so that methane will
desorb from the coal and flow via the wellbore to the surface. The volumes of water involved
vary among coal basins around the world, depending primarily on reservoir thickness,
porosity, permeability, well spacing, pump rates, proximity to aquiferous sandstones or
intrusions, and proximity to meteoric recharge.
In the U.S., average daily water production rates from CBM wells vary from 2-5 m3 (17-42
bbl) per day to over 60 m3 (500 bbls) per day [Greedy et al, 2001]. Total production from a
CMM drainage project involving a large number of wells can be considerable and must be
carefully managed to meet local environmental requirements.
The quality of produced coal seam water varies widely among and within coal basins. In
some regions, the water is of good enough quality to be used for beneficial purposes such as
irrigation, drinking water, or industrial use. In poor quality water areas the water contains high
concentrations of salt (up to 5 times that of seawater) and must be intensively treated before
use, or disposed of by reinjection into a suitable aquifer.
2.4.1 Water disposal options
There is no established technology for reducing water production without adversely affecting
gas production rates. Consequently, mitigation technologies have focused either on
disposing produced water using underground injection or surface evaporation, or by surface
treatment of produced water for disposal or utilization.
In the U.S., produced water from CBM/CMM operations is disposed of using several different
approaches. The most appropriate method depends on many variables, including water
volume, salinity levels and chemical composition, as well as on non-reservoir factors such as
local climate, surface drainage, and environmental regulations. Water disposal technology is
highly site specific and must be determined for each individual application.
The most commonly used water disposal options include:
Surface discharge
Impoundments (or evaporation pits)
Shallow and deep re-injection
Active treatment using Reverse Osmosis (RO)
30
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Surface discharge - Surface discharge is the least expensive of the water disposal options.
Uses for surface discharged water may include crop irrigation or animal watering, depending
on water quality. However, these options will most likely be secondary to any beneficial use
at the mine or in other industrial applications where potable water is not required. These
applications include ore washing, power plant cooling, drilling/fracturing fluid, and dust
suppression. Depending on the end-use, some degree of clean-up of the water may be
required.
Impoundment / evaporation - Disposal of produced water in evaporation ponds is a simple
process, involving constructing and maintaining a shallow, impermeably lined pond with a
large surface area, introducing produced water into the pond, and allowing the water to
evaporate. Depending on the salinity of the produced water and evaporation rates, the
accumulated salt deposits within the pond must be removed. In the San Juan Basin, this
accumulation amounts to approximately 5 cm per 20 years of continuous operation.
Evaporation rates can be significantly
enhanced in active evaporation ponds
through the use of a pump-and-spray system,
reducing the required surface area to dispose
of a given volume of water, although at higher
operating cost. (Exhibit 19)
Exhibit 19: Forced evaporation pond
If produced water is of sufficient quality, impoundment ponds can also be used for beneficial
uses such as fishponds, livestock and wildlife watering ponds or recreation.
Underground re-injection - In the U.S., water must be re-injected to a depth at which the
re-injected water's salinity matches that of the aquifer into which it will be pumped. For
example, CBM produced water in the Powder River Basin of Wyoming and Montana is
relatively fresh, so shallow re-injection wells are typically only 90-300 m (-300-1,000 ft) in
depth.
Downhole gas/water separation - A relatively new method of water disposal is downhole
gas/water separation. Downhole gas/water separation requires well boreholes to be drilled
deeper than originally designed in order to inject water into a permeable horizon below the
coal seams. A pump below the coal seams draws water down, while allowing gas to flow to
the surface. Downhole water separation may be economically viable under certain
conditions, actually increasing gas flow rates and eliminating water transportation costs. This
31
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technique, however, requires an adequately permeable zone located below the coal that can
take substantial volumes of fluid.
Reverse Osmosis - Reverse osmosis (RO) of brackish produced water involves the use of a
permeable membrane to separate fresh product water and waste brine streams. Each pass
through the membrane can half the salinity of the product water, thus the performance of an
RO system depends on the requirements for product water chemistry. A typical RO system
involves processing produced coal seam water to generate fresh product water and a small
waste stream of highly saline water that can be injected in a conventional underground
disposal well or trucked to a permitted disposal location.
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3. CMM drainage techniques which recover gob gas
A gob (also known as "goaf"), or gob area, is a region of fractured geologic material from
overlying strata that has settled into mined-out areas after coal recovery. The overlying and
underlying material relaxes after shortwall or longwall mining operations have passed by
(Exhibit 20), or after pillar removal with the room and pillar mining method.
Gas volumes liberated by gob areas into the mine ventilation system depend on the method
of mining, the number and proximity of overlying and/or underlying gas-bearing strata, their
reservoir characteristics, and other geological factors. The primary motive for gas drainage
from the gob is to reduce methane emissions into mine workings and assist the mine's
ventilation system in providing a safe environment for coal exploitation activities.
Longwall Gob Fracture Zone
Coalbed
Coalbed
120 to
140 m
Coalbed
Coalbed
Coalbed
20 to 40 m
Coalbed
Exhibit 20: Side view of the effects of longwall mining on adjacent strata
[Cervik, 1979]
At most coal mining operations, much of the gas emitted from the gob discharges to the
atmosphere, either directly from the drainage system or through the ventilation system. Gob
gas drainage systems may produce high-quality gas depending on conditions, but generally
produce gas with lower heating values between 2,670 and 7,120 kcal/m3 (300 and 800
33
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Btu/cf) [EPA, 2008b]. Poor methane gas recovery may be due to the nature of the resource
itself, or may result from focusing attention on minimizing gas emissions into the mine
ventilation system, resulting in the dilution of gob gas with ventilation air. However, there is
potential at many coal mines to increase gob methane recovery and decrease dilution levels
by adopting improved degasification and collection systems and by modifying operating
practices.
The three primary methods of longwall gob degasification, used worldwide, are as follows
(depicted in Exhibit 21):
vertical and deviated gob wells - drilled from the surface
cross-measure boreholes - drilled from mine entries adjacent to the longwall panel
superjacent methods - degasification takes place from overlying or underlying
galleries and boreholes
To maximize gob degasification, CMM drainage systems often use a combination of these
techniques.
Vertical Gob Wells
Degasification Gallery
Longwall Face
Cross-Measure Boreholes
Longwall Panel
Exhibit 21: Schematic showing the different gob gas recovery methods
(Source: REI Drilling)
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3.1 Vertical gob wells
Wells drilled from the surface to just above the working coal seam are the predominant gob
degasification technique applied in the U.S. Gob wells are normally drilled prior to mining, but
are operated only after the longwall face mines past the wellbore and the gob is formed.
Methane emitted from fractured strata in and above the gob then flows into the well and up to
the surface. Vertical gob wells are the most effective method of reducing methane content in
shallow, rapidly moving longwall faces in the U.S. They are less widely used in the rest of the
world, where deeper, less permeable coals and greater surface access problems make other
gob degasification methods more applicable.
3.1.1 Planning and design
The number of vertical gob wells on a longwall panel varies considerably, with the number
being a function of the rate of mining, the length of the longwall and the gas content of the
caved strata. The first borehole is typically sited 50-170 m (150-500 ft) from the longwall face
and, during mining of a typical 3000 m (10,000 ft) longwall, between 3-30 gob wells may be
needed for adequate degasification. [Thakur, 2006] A higher density of wells at the beginning
of the longwall is often used to drain the higher methane emissions encountered in the initial
stages of the longwall caving operation.
Examples
At their Virginia operation in the U.S., CNX Gas typically drill 6-7 gob holes in the first 305 m (1000
ft) of the panel and continue on a 150 m (500 ft) spacing [CNX, 2007].
Jim Walter Resources in Alabama, typically drill 5-6 gob holes per 3,660 m (12,000 ft) longwall
panel [JWR, 2008].
Studies by the U.S. Bureau of Mines (Diamond, Jeran, and Trevits, 1994; Diamond, 1995)
indicated that wells located in the zone of tension along the margin of the longwall panel5,
produced 77 percent more gas than wells drilled on the traditional centerline location, which
is in compression. Often, operators select a location for the first gob well on a panel and
evaluate its production record to help locate the subsequent wells.
5 When a coal seam is mined, the overlying strata subside into the void left behind forming the gob (Exhibit 20).
Maximum subsidence occurs along the centerline of the gob, where gob material is "pushed" together in
compression. At the edges of the gob, the overlying strata are partially supported by unmined rock below it and
are "stretched" over this support and into the gob. This zone of "stretched" strata is in tension, which is surmised
to enhance fracture permeability and gas production. [Diamond et al, 1994]
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3.1.2 Gob well completion
Vertical gob wells are drilled in advance of mining to a depth 6-28 m (20-90 ft) above the
working coal seam. Gob wells are normally cased and cemented to a point just above the
uppermost coal seam or gas-bearing stratum believed capable of liberating gas as a result of
longwall mining. The lower portion of the well is either left uncased as an open hole
completion (see section 2.1.2), or is lined with slotted casing to maintain borehole integrity
while allowing gas flow to the well, as shown in Exhibit 22.
The slotted liner is not cemented in place, but hung from the bottom of the casing. Gob well
completions in the different coal basins in the U.S. vary depending on depth, anticipated gas
and water flows, and geomechanical characteristics of the overlying strata.
TO GOB WELLHEAD
METERS
5
FEET
16
30-90
6-28
CO.
100-300
§
SURFACE CASING
CEMENT
CASING
SLOTTED CASING (OPTIONAL)
OPEN HOLE
20-90
COAL SEAM
Exhibit 22: Profile of a typical U.S. vertical gob well
36
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A number of aspects of gob well completion must be carefully considered, including vertical
placement of the well within the gob, maintaining well integrity and productivity after
undermining, ensuring connectivity with the fracture zone to enable gas flow, and isolating
shallow, water-bearing strata from the well and mine workings by proper well-casing
cementation practices. In some cases, significant water inflow is unavoidable and may
worsen after mining because of increased conductivity exhibited by fractured strata
(Diamond, 1995). Water collection will impede gas production.
Completion examples
West Elk Mine in Colorado, USA, operated by Mountain Coal Company [Peacock, 2006]. A
borehole, 311 mm (12 1/4 inch) in diameter, was drilled to a casing point depth of 610 m (2000
ft). The hole was cased from the surface with 244 mm (9 5/8 inch) steel casing and then a 222
mm (8 3/4 inch) diameter hole drilled for another 91 m (300 ft), stopping 8 m (25 feet) above
the seam to be mined. 178 mm (7 inch) slotted steel casing was hung in the hole to maintain
borehole integrity, but not cemented in place, so as to allow gas to flow from the gob and
surrounding fracture zone into the wellbore.
In the Pittsburgh coalbed in Pennsylvania, USA, where average overburden depths range from
152-274 m (500-890 ft), gob wells were drilled to 9-14 m (30-46 ft) of the top of the coal and
completed with 178 mm (7 inch) casing and 61 m (200 ft) of slotted pipe [Karacan et al, 2007]
Gob wells 311 mm (121/4 inch) in diameter have been drilled as deep as 915 m (3000 ft) in
Virginia, USA, [Atlas Copco, 2007].
Some non-U.S. coal operators have had little success with gob well degasification due
primarily to their completion practices. Wells that are completed mostly "open hole" and
extend into the rubble zone may encounter water problems or shear after undermining, which
limits the productive life of the well. Proper casing of the well above the gob area, to isolate
surface water-bearing zones avoids water accumulation. At the same time, protecting the
borehole in the gob area with slotted casing minimizes the potential of well bore shearing and
resultant short production life.
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3.1.3 Gob gas production and quality
As with all gob degasification techniques, the methane quality and quantity produced from
vertical gob wells vary depending upon many factors including:
Site-specific geological and reservoir characteristics,
Mining characteristics,
Well siting,
Completion practices,
Wellhead operations, and
Degasification and ventilation practices at the mine.
Intrusion of mine ventilation air is common because of connectivity in the gob between the
borehole and ventilation system. Typical U.S. gob well capture efficiencies6 are in the 30-
70% range [USEPA 1999b], depending on geologic conditions and the number of gob wells
within the panel. Some operators who use vertical gob wells in favorable geologic and
reservoir settings have claimed high methane capture efficiencies up to 80%.
The flow rates of gas from the gob to the well are controlled by the permeability of the
fracture zone, the natural pressure differential created by low-density methane gas rising in
air, and the amount of suction produced by vacuum pumps (exhausters) at the surface.
Vertical gob wellhead operators have attained significant increases in gas production, and
methane reductions in the mine ventilation system, with just slight vacuum pressures. In
some cases, operators have noted a three-fold increase in production rate with application of
6.9 kPa (1 psi) suction pressure [Mazza, Mlinar, 1977] to gob wellheads.
Vertical gob well performance records indicate that well productivity is also linked to the
dynamic creation of the gob and is dependent on the volume of coal extracted. At many U.S.
operations, gob gas production rates depend on the longwall face advance rate. Operators
have reported a two- to three-fold increase in gob gas production rate with increased
longwall face productivity.
In a vertical gob well system, wellhead operations on the surface, ventilation controls situated
in the mine offices, and mining operations underground are widely separated. Therefore, it is
important to have effective tools to manage the impacts of each system on the others.
Because of the gob wells' effect on the mine ventilation system (particularly if gob ventilation
6 Methane capture efficiency is defined as the ratio of the gas captured by the degasification system to the sum of
(the gas emitted into the ventilation system plus that captured).
38
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is required, as in the U.S.), operators must closely coordinate these systems with continuous
monitoring. Continuous monitoring is particularly imperative to maintain gob gas quality for
projects that use the recovered gas. At some mining operations, excellent production rates
and high gas qualities are maintained with suction and proper monitoring and control.
Example
In the Jim Walter Resources mines in Alabama, gas-bearing strata with high gas content over
lie the mined seam. Gob permeabilities are typically very high. Methane production and mining
activities are closely coordinated and a system to carefully monitor gob gas collection and
process it for pipeline injection has been implemented. The company reports that gob wells
initially produce at rates in excess of 56 Mcmd (2 MMcfd), declining to approximately 2.8
Mcm/d (100 Mcfd). [USEPA, 2008b]
Data from 2006 show that of the twenty-three U.S. mines employing a methane drainage
system, all of them use surface vertical gob wells as part of their methane control plan
[USEPA, 2008b]. Thirteen of the mines are selling the recovered gob gas into pipelines, with
three mines using the recovered gas for medium-quality applications such as on-site
electrical power generation and heating applications. The rest of the mines are venting the
gob gas to the atmosphere because of a lack of an identified economic use for the gas.
Some U.S. operators recover large volumes of gob gas (up to a half of a gob well's
cumulative production) after they complete longwall mining. These operators seal the mined
gob areas from main mine ventilation air courses to minimize leakage and to increase
ventilation system efficiency. Effective sealing of gob regions from the active portion of the
mine can improve recovered gas quality.
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3.2 Cross-measure techniques
The cross-measure technique of longwall gob degasification is the primary technique
employed in Europe and Russia, where operators practice longwall mining in multiple dipping
coal seams, normally deeper than 610 meters (2000 feet). Several U.S. mines have tested
cross-measure boreholes [Cervik and King, 1983] and found them effective. However, the
general ease of using vertical gob holes in the U.S., and their relative cost effectiveness
when compared to cross-measure boreholes, has resulted in minimal use of cross-measure
boreholes in U.S. mines. Cross-measure systems may be more attractive in the U.S. in
deeper, gassier mines and where the siting of vertical gob wells at the surface is impossible.
Example
At the West Elk Mine in Colorado, Mountain Coal Company compared methane drainage from
cross-measure (CM) holes and surface gob wells [Peacock, 2006]. Both methods were used to
degas a longwall in a heavily faulted area, where, even with new, larger airshafts, the ventilation
system could not adequately dilute in-mine methane concentrations to safe levels, resulting in
production slowdowns. After an initial learning process in drilling both types of drainage borehole,
the gob wells were found to be much more effective at methane drainage than CM boreholes.
CM holes were more prone to be affected by air ingress from the mine ventilation system
decreasing the amount of methane captured by the system.
Gob wells were able to drain up to 10 times the amount of methane per hole compared to CM
holes at a cost approximately that of 5 times a CM hole.
Gob wells were effective over a wider range of panel than the CM holes and the decrease in
gas concentrations around the longwall face was immediately noticeable, with a subsequent
increase in mining production levels.
3.2.1 Planning and design
Borehole positioning
Cross-measure boreholes are drilled in advance of the longwall face, at varying angles into
the roof or floor of gateroad entries (Exhibit 23). Their purpose is to pre-drain over- and
under-lying strata, and then capture gas from the gob area once the longwall face has
passed. Boreholes are normally placed in the return entry, but in extremely gassy conditions,
boreholes can be sited in both the intake and return entries surrounding the coal panel. The
angle, length, and spacing of the boreholes are all dependent on site specific conditions such
as the width of the longwall panel, depth below the surface, thickness of the mined seam,
40
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geomechanical properties of adjacent strata, available drilling space and drilling equipment
limitations.
In retreating longwall mining, keeping the cross-measure boreholes and their gas gathering
pipelines intact is complicated by the gateroad entries adjacent to the gob collapsing upon
retreat. For single entry retreat mining (performed outside the U.S.), boreholes are drilled
from an extra gateroad, developed along one side of the panel (Exhibit 23), which provides
access to the degasification boreholes and a protective environment for the gas gathering
system.
t
Tailgate Entries
Cross-Measure -
Boreholes
I
Headgaie
r
Retreat Mining with Second Tailgate Entry
Direction of
Advance
Exhibit 23: Cross-measure boreholes developed from a second entry for longwall
gob gas recovery for retreating operations
[Wisniewski and Majewski, 1994]
Borehole sizing and spacing
Cross-measure boreholes are small in diameter, between 50 to 100 mm (2-4 inches) and are
drilled at angles varying from 20-50 degrees from horizontal. U.S. studies and European
experience indicate that boreholes at higher vertical angles have a longer productive life and
tend to produce purer methane. However, each gob has an optimum drilling angle and
exceeding it will impair the borehole's performance.
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Exhibit 24: Cross-measure drilling
Studies by the U.S. Bureau of Mines (USBM) have shown
that up to 75 percent of the gob gas emits from the newly
fractured strata and stress relaxation zone directly behind the
face (Garcia and Cervik, 1985). Cross measure boreholes in
European mines are typically angled toward, and above, the
longwall face, to intercept this zone (Exhibit 24). This
orientation is especially important for single-entry gateroads
used with retreat mining because of the need to maximize
production within the borehole's finite life span. Once the face
passes the wellhead, the access to the borehole is lost, and
the well is normally shut-in. Boreholes are typically inclined to
the longwall axis at 15-30 degrees. [Thakur, 2006]
In order to produce a continuous low-pressure zone over the gob using the cross-measure
system, the boreholes must be spaced such that their influence zones overlap slightly. If
boreholes are too far apart, then the accumulated gases between boreholes will tend to
migrate toward the nearest mine entry. If the boreholes are spaced too close together, they
may promote migration of mine ventilation air into the gob, reducing the quality of recovered
gas.
Borehole spacing can vary from 25-60 meters (80-200 ft) apart and is dependent on
available suction pressure at the wellhead and the gob permeability. Spacing may be
decreased near the start and end of new coal panels, to capture increased gas flows
generated in these tension zones, which are more fractured and have higher permeabilities
than in the rest of the panel. [Garcia and Cervik, 1985, Diamond, 1995]
USBM tests of cross-measure systems along return gate-roads show that a borehole's
horizontal projection over the longwall rib does not need to be very long. In fact, 30 m
horizontal projections were sufficient to obtain capture efficiencies of 71 percent with this
system [Garcia and Cervik, 1985], and data indicated that shorter holes would have been as
effective. Thakur [2006] quotes lengths varying from 18-152 m (60-500 ft).
Horizontal and vertical placement considerations
The point where a borehole is drilled into the mine roof, or floor, is called the "collar location"
and is critical to the borehole's performance and to recovered gas quality. The collar location
is normally situated close to existing pillars or other roof supports, in an attempt to minimize
42
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fracturing near the initial length of the borehole as the longwall face passes. In order to
maximize connectivity with the gob, minimize inflow of mine air into the degasification
system, and prevent the borehole from closing, a steel or plastic standpipe, up to 10 m (33 ft)
long, can be inserted and sealed into the initial borehole length.
3.2.2 Recovered gas quality and production
Monitoring
Because the degasification system operates in conjunction with the ventilation system and
mining operations, it is important to provide coordinated management (using measuring
instruments, monitoring, controls, and good communications) to optimize each of the three
functions.
Modern wellhead configurations
enable measurement of gas
quality, gas flow rate, and
pressure. Good monitoring
practices ensure that the quality
of recovered gas is above the
limiting value for the mine. Exhibit
25 shows low-cost provisions for
suction control, pressure and flow
monitoring, and water separation
for a cross-measure wellhead
Exhibit 25: Cross-measure borehole wellhead configuration
with monitoring provisions
[Garcia SCervik, 1985]
Gas quality
Cross-measure boreholes connect to a gas collection manifold that is normally under suction
induced by a vacuum pump. Suction (negative pressure) is the primary means of controlling
the gas flow rate from cross-measure boreholes. Maintaining standpipe integrity is crucial to
borehole productivity and recovered gas quality, because system suction will promote
intrusion of mine ventilation air through inadequate standpipe seals.
Methane capture efficiencies6 range from 20 percent to 70 percent with the cross-measure
borehole technique [McPherson, 1993]. Lower gas purities are typical with this system
because of the number of boreholes drilled, and the connectivity between the boreholes and
the ventilation system. A typical flow from an individual borehole is 815 m3/day (28 Mcfd), but
can occasionally reach over 4000 m3/day (141 Mcfd) for deeper holes. [Thakur, 2006]
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3.3 Superjacent techniques
Superjacent techniques involve the creation of low pressure zones in strata immediately
above (and occasionally below7) the rubble zone of the gob, into which gob gas migrates
instead of into mine workings. The low pressure zones can be formed by using an existing
mined gallery (roadway), driving a new one, or by directionally drilling a series of boreholes.
The zone is sealed and subjected to vacuum pressure.
Superjacent drainage systems have the advantage of being initiated away from production
mining operations, which facilitates placement of boreholes in advance of the mining face for
both advancing and retreating longwall systems. Gob degasification with Superjacent
techniques is applicable in mines that cannot implement surface-drilled gob wells, or
effectively control gob gas emissions using only these wells, and as a cost effective
alternative (implementation and operation) to cross-measure boreholes. [Brunner and
Schwoebel, 2001]
3.3.1 Overlying or underlying galleries
Superjacent techniques involving the use of drainage galleries (roadways) developed in
advance of mining in overlying or underlying strata are used at some of the deeper and
gassier mining operations in Eastern Europe, Russia, and China. This method was
developed in highly gassy coal seams in French and German mines in the late 1940's.
In these techniques, a gallery is developed 20-35 meters (65-120 ft) above the seam to be
mined. Development costs can be partially offset if the gallery is driven in a coal seam. It is
sealed and connected directly to a gas collection system operating under high vacuum,
forming a low pressure sink into which gob gas migrates. Small diameter, short boreholes,
can be drilled into surrounding gassy strata from the gallery to improve gas migration (Exhibit
26).
Using pre-existing galleries makes this technique more economically viable. Generally,
operators using Superjacent methods claim methane capture efficiencies of up to 80 percent
(efficiencies as high as 90 percent have been reported [Liu and Bai, 1997] with Superjacent
galleries). Thakur [2006] reports methane flow rates averaging 28-40 Mcmd (1-1.4 MMcfd)
from Superjacent galleries.
7 Although the term "superjacent" literally means "lying above", drainage galleries and boreholes developed under
the working seam are also included as superjacent techniques.
44
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Methane Extraction System
Over Lying Coal Seam
3
Exhibit 26: A sealed superjacent gallery with drainage boreholes
[Thakur, 2006]
Exhibit 27 shows two superjacent gob drainage techniques used in eastern European mines.
A Boreholes
II Overlying Gallery
B
4
|
I
,£
s i
§
6
i
Sub-Main
t. +.
Tailgate
Overlying S *
Gallerv />^
XX
^
Direction of
Advance N=
Face
Headgate
Y//
^
GOB
1
r -*
Overlying
Oallery
' GOBN
Kace
Section B-B1
Exhibit 27: Degasification of gob areas using the superjacent method in Eastern Europe
[Wisniewski, 1994].
45
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3.3.2 Directionally drilled gob boreholes
Over the past two decades, superjacent techniques involving in-mine directionally drilled
boreholes, placed over or under the mining seam in advance of longwall operations, have
been applied in Japan, China, Australia, Germany, and in the U.S. This technique uses state-
of-the-art, in-mine directional drilling equipment normally used to develop long in-seam
methane drainage or exploration boreholes.
In-mine gob boreholes, 76-152 mm (3-6 inches) in diameter, are drilled into the strata
overlying or underlying un-mined panels, to lengths up to 1,000 m (3,280 ft) as previously
shown in Exhibit 13.
Positioning
Ideally, overlying boreholes should be positioned taking into account as many of the following
factors as possible:
In or below the lowest producing source seam,
To intersect the fracture zone above and below the rubble zone after the gob
forms,
Over the tension zones near the edges of the panel,
To take advantage of gob gas migration patterns caused by the mine's ventilation
system and geometry of the gob - gob gas will accumulate toward the low
pressure side of the gob and higher elevations
To consider water accumulation - either slope upwards from the collar to allow
water to drain back to the wellhead for separation, or, once the target horizon has
been reached, drill downgrade so that water drains down the borehole and back
into the gob
To remain intact following undermining and produce gob gas over the entire length
of the borehole
The optimal vertical placement of the borehole is typically determined by trial and error and
requires properly monitoring gas flow and quality, longwall face production and controlling
vacuum at the well heads.
46
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Superjacent directionally drilled boreholes have several advantages over the cross-measure
method, namely:
The boreholes can be developed in advance of mining, away from mining activity
for either advancing or retreating longwall systems,
Fewer, longer boreholes can produce an effective low pressure zone over the gob,
Strategic placement may allow borehole collars to remain intact (protected from
the effects of local stress redistribution) and allow boreholes to remain productive
after longwall mining is completed
The system may be more effective and less costly to implement and easier to
operate than a system of cross-measure boreholes.
Relative to a system employing galleries, horizontal gob boreholes will be less costly to
implement, particularly if the galleries are developed specifically for degasification purposes
and mined in rock or uneconomic coal seams.
Borehole diameter and spacing
The volume of longwall panel gob gas emissions determines the number of boreholes
required per panel. At least three boreholes per panel appear to be necessary for adequate
gas capture and to provide redundancy in the case of borehole failure. Long boreholes, in
excess of 500 m (1640 ft), 100 mm (4 inches) in diameter and subject to a high vacuum (100
mm Hg) can recover approximately 15 Mcmd (530 Mcfd) of gob gas. Short deviated
boreholes can be drilled from the main borehole to enlarge the zone of reduced pressure
over the gob.
As is the case with cross-measure boreholes, developing a continuous low-pressure zone
over the gob requires borehole influence zones to overlap slightly. If boreholes are
insufficiently sized and spaced too far apart, gob gases will tend to migrate to the mine entry.
If boreholes are over-designed and too close together, they may promote migration of mine
ventilation air into the gob. Fewer, farther-spaced, larger-diameter (150 mm or 6 inch)
boreholes may recover more gas at lower pressure losses than smaller-diameter, closely-
spaced holes. This would result in less drilling, fewer wellhead connections and minimized
leakage, leading to improvement in gas production rate and quality, increasing the system
efficiency and ease of maintenance. The extra cost incurred in drilling larger-diameter holes,
which might even require different equipment and/or larger galleries, needs to be weighed
against the incremental benefit of increased gas production and improved gas quality.
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Recovered gas quality and production
Gob gas drainage efficiency and gas purity for superjacent systems are affected by geologic
and reservoir conditions, orientation of the galleries and/or boreholes, borehole size and
spacing, gallery and borehole integrity, suction control, water accumulation, and mine
ventilation.
Superjacent borehole drilling is technically more complicated than in-seam drilling, and this is
reflected in higher drilling costs of $100-130 per meter ($30-40 per foot).
Example
Test studies developed by REI Drilling, Inc., in mines in the U.S., Japan, China and Germany
resulted in average borehole production figures ranging from 8,300 mcm/d to 15,000 mcm/d
(293-530 Mcfd). Boreholes were 500-800 m (1640-2624 ft) in length. Gob gas quality varied
between 35-90 percent and was directly affected by longwall face advance rates, requiring
constant monitoring and vacuum control [Brunner and Schwoebel, 2001]. REI Drilling tested
the effects of drilling larger (150 mm, 6 in.) diameter boreholes for high capacity gob gas
recovery in very gassy conditions; lining the borehole with perforated steel casing to improve
borehole integrity; and the development of parabolic boreholes to simulate effective surface-
drilled angled gob wells.
48
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4. Gas Gathering and Collection
An integral component of a mine degasification system is the gas collection and transport
infrastructure. Underground, this infrastructure serves to move coal mine methane collected
from degasification boreholes up to the mine surface. On the surface, gathering infrastructure
can include gob wells, pipelines, compression and processing facilities (if the methane is to
be used commercially), flare stacks and exhausters.
4.1 Underground gas collection systems
Underground gob gas collection systems are typically more difficult to control and maintain
than surface systems because of mining activity and the complex subsurface environment.
Gas collected from underground degasification boreholes comes to the surface via a network
of pipes fitted with safety devices, water separators, monitors and controls, and vacuum
pumps (Exhibit 28).
Sales
system
Discharge
stack
Compressor
Centrifugal
blower
1000 cfm at
Ipsi
Horizontal
boreholes
Working
section
Flame
arresters
'* -4
X
«* V*
Ain. transmission line
8in. mai
Exhibit 28: Layout of a horizontal borehole methane drainage system
showing both in-mine and surface facilities8.
4.1.1 Pipelines
In-mine methane drainage boreholes normally connect to a collection line via flexible hoses.
Collection lines are either suspended or laid on the mine floor (Exhibit 29), and transport
Source: Hartman et al., 1997. Copyright 1997, John Wiley & Sons, Inc. Reprinted with permission of John Wiley
& Sons, Inc.
49
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drained methane to a main gas line, which leads to a vertical collection well that may be
freestanding or affixed onto the lining of an exhaust shaft. In the U.S., guidelines stipulate
that a methane drainage pipe should be in return airways, visible along its entire length, not
submerged at any location, and pressure tested during installation.
Air leakage into a negative pressure gas collection system affects recovered gas quality and
system performance. Fewer leaks at pipe joints and fittings leads to less dilution of drained
methane and allows for greater system suction pressures, resulting in higher gas quality and
volumes gathered at the surface.
Pipelines are steel or, where permitted, high-density polyethylene (HOPE). Steel lines are
preferred for mechanical strength, especially for the underground to surface connections, but
HOPE is easier to work with and is non-corrosive.
Steel pipes are joined by threaded connections, or by gasketed, flanged connections, and
both types corrode and leak over time, particularly if frequent pipeline moves are necessary.
HOPE pipe sections are non-corrosive and can be
fused together, greatly reducing mine air leakage into
the pipeline system. HOPE pipe is lighter and easier to
handle than steel pipe, reducing installation and
maintenance costs. Depending on conditions,
increases in recovered gas quality as high as 50
percent may be realized with HOPE systems versus
,, _ WJr ,t - ".''I,,,;- -,:*>
>"&.'-:", ,";' :''<"':.! '' . flanged steel pipe networks.
Exhibit 29: HOPE gas collection piping
Steel pipe
High-density polyethylene pipe
Advantages
Superior mechanical
strength
Disadvantages
Connections can corrode
and leak overtime
Heavy and difficult to
move
Advantages
Non-corrosive - resistant to H2S,
does not rust
Lighter and easier to handle
than steel, reducing installation
and maintenance costs
Connections can be fused
together minimizing leaks
Exhibit 30: Summary of gas collection pipe properties
Disadvantages
Less mechanical
strength than steel
Some concern
about static
electricity issues
50
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4.1.2 Safety devices
Safety devices installed along the pipeline network serve to protect the infrastructure from
leakage during pipe ruptures. Operators typically install automatically activated safety shut-
off valves at each borehole and at regular intervals along the pipe network. This sectionalizes
the system and minimizes methane liberation into the mine ventilation system should a
breech in the pipeline occur. The valves are activated pneumatically or electrically by means
of methane sensors in the airway, pressure sensors, or, most commonly in the US, protective
monitoring tubing devices.
4.1.3 Water separation
Water traps, or separation devices, installed at low elevations along a methane drainage
network, prevent the accumulation of water (condensate, or formation water) which would
otherwise impede gas production. Large separators are placed at wellheads and the base of
vertical collection wells (Exhibit 31). These devices subject drained methane to a sudden
expansion that reduces its velocity, dropping any entrained water.
Exhibit 31: Separation system at the base of a vertical collection well
4.1.4 Monitoring and control
Gas collection system monitors sense three parameters: pressure, flow rate, and
concentration of gas constituents. Valves comprise the control system. They are activated
either manually or remotely by pressure, gas quality, or flow sensors. Negative pressure
applied at the wellhead affects gas production and quality. High suction pressures tend to
introduce mine ventilation air, while insufficient suction may impair production and increase
51
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methane emissions into the ventilation system. Proper pressure control in the drainage
system is achieved through strategic placement of control valves within the system,
employing sufficient wellhead monitors, and properly designing the vacuum pump and
gathering system. Pressure responses are specific for each drainage borehole. Frequent
monitoring at critical junctions underground can optimize system performance and provide
warning of increased system demands. Benefits are improved system performance and
increased recovered gas quality.
4.1.5 Underground gas movers
There are three types of extractor pump systems that are most prevalent for supplying
negative line pressures to a degasification pipe network: water seal extractors; centrifugal
blower/exhausters; and rotating pumps. Water seal extractors are preferred for installations
underground because of the inherent safety features of producing a vacuum, since these do
not significantly increase gas temperature and do not require contact between stationary and
moving parts (McPherson, 1993).
4.2 Surface gas collection systems
At the surface, gas is collected from vertical frac wells, surface-drilled horizontal wells, gob
wells and centralized vacuum stations, which collect the gas produced by in-mine boreholes.
Ideally, all CMM collected at the surface would be used commercially. Depending on
produced gas quality and volumes, CMM can be used for a number of purposes:
Fed into to a natural gas pipeline,
Used to power electricity generators for the mine or local region,
Used as a an energy source - co-firing in boilers, district heating, coal drying, use
as a vehicle fuel, and manufacturing or industrial uses such as ammonia
production.
In many CMM drainage projects worldwide, commercial CMM use is currently not technically
or economically viable. As a result, the drained gas is vented directly to the atmosphere, via
an exhauster/well head blower. One option to reduce the environmental impact of direct
venting, is to burn the vented methane in a controlled flare system [USEPA, 19993]. While
the byproduct of burning methane is carbon dioxide, itself a green house gas, about the
global warming potential is reduced since carbon dioxide is 23 times less potent than
methane. CMM flaring has been used successfully in the U.K. and Australia, but has yet to
gain acceptance in the U.S. coal mining industry.
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The main components of a surface gas collection system comprise of the well head
equipment, gathering pipelines, any necessary gas processing equipment and compressors.
4.2.1 Pipelines
Gas is transported from individual wells, via an in-field gathering system, to a central
processing facility, where the gas is treated and compressed to meet transmission pipeline
specifications. The pipeline gathering system requires various diameters of pipe at different
intervals to be efficient.
A system of relatively small diameter, low pressure pipelines, referred to as "flowlines", is
designed to move gas or water from the wellhead to a larger diameter pipe that moves the
fluid from the field to a central treatment facility. Flowlines are typically made of high-density
polyethylene and are 100-200 mm (4-8 inches) in diameter (water flowlines can be as small
as 50 mm (2 inches) in diameter).
The larger diameter pipe, made of steel, is known as a "trunkline". Intermediate lines
between the trunk and flowlines, sometimes referred to as "gathering lines", are necessary
as the system grows with field development. Once processed and compressed, a large, high
pressure steel pipeline, operating at 4,480-8,620 kPa (650-1250 psi) and referred to as a
transmission pipeline moves the gas from the project area to a marker.
4.2.2 Compression
CMM is collected from the wellbore at relatively low pressures and is compressed to attain
the necessary pressure requirements for injection to a transmission pipeline. The number of
stages needed for compression will depend on the suction and discharge pressures needed
to produce the wells and compress the gas into the transmission line, and the compression
ratios of the equipment. Three to four stages of compression are common in CBM/CMM
projects in the U.S. due to the low suction pressures required to maintain gas production and
the high pressures (see above) required for interstate transmission lines. A low suction
pressure of between 70-210 kPa (10-30 psi) is typical for the network of flowlines taking gas
from the well sites to the central treatment facility. Depending on engineering requirements,
some operators will locate compressors at each well site, while others will situate
compressors at a central facility.
4.2.3 Gas processing
Gas drained from vertical frac wells, horizontal wells and in-seam boreholes is usually of
sufficient quality (greater than 90% methane) for injection into natural gas pipelines with
53
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minimal processing. Gas from gob wells and cross-measure boreholes is more variable in
quality (30-80% methane), depending on the amount of dilution caused by air infiltration into
the gob and boreholes. An integrated processing plant can be installed at a central facility to
remove contaminants and increase the quality of the gas to pipeline specifications. The CMM
is treated in a series of connected processes which first removes any hydrogen sulfide
present, followed by excess oxygen, carbon dioxide, water vapor, and nitrogen. In the U.S.,
pipeline quality gas must contain less than 0.2% oxygen, less than 3% nitrogen, less than
2% carbon dioxide and less than 112 kg/MMcm (7lbs/MMcf) of water vapor, while having a
heating value of greater than 967 Btu/scf. [USEPA, 2008C]
Example
Jim Walter Resources, at its Blue Creek Coal mines in Alabama, installed a low quality
methane recovery plant in 2000 and processes 230 Mcmd (8 MMcfd) of 60% methane gas,
producing 115 Mcmd (4 MMcfd) for injection into a sales pipeline. [JWR, 2008]
54
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5. Summary
There are significant benefits to the mining operation and to the environment of optimizing
methane drainage systems at coal mines.
5.1 Benefits of CMM drainage for coal mines
Many benefits accrue from a methane drainage system. An efficient methane drainage
system can achieve the following:
Improve mine safety resulting from lower methane contents in the face, returns,
gobs and bleeders;
Enhance coal productivity because of less frequent downtime or production
slowdowns caused by high methane concentrations in the mine;
Decrease fan operating costs because of reduced ventilation air requirements for
methane dilution;
Reduce shaft sizes and number of entries required in the mains;
Increase tonnage extracted from a fixed-size reserve as a result of shifts of
tonnage from development sections to production sections;
Decrease dust concentrations and improve worker comfort through reduction of
ventilation air velocities at the working face; and
Reduce mining problems caused by water.
Each of these benefits is described below.
Improved mine safety
The effect of a methane drainage system on the safety of a mining system will certainly result
in positive benefits [Ely and Bethard, 1989]. Any high-methane operation will incur a higher
level of hazardous operating conditions than an equivalent mine with a methane drainage
system in place.
Increased coal production
Enhanced coal productivity is a significant benefit obtained from the installation of methane
drainage systems. The value of such a benefit can be extremely large when one considers
that the value of coal that comes off a longwall in a shift averages about $100,000 to
$200,000 for an average modern longwall. Any lost production caused by excessive methane
levels in the mine workings results in a sizable cost - around $200 to $400 per minute of
downtime in this case. The significance of this cost can be realized when it is considered that
55
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downtime of up to 11,000 minutes per month for a single longwall have been reported in the
literature [Aul and Ray, 1991] and that many longwalls will experience slowdowns in
production as well as times where the longwall is completely down due to high methane
concentrations. The economic benefit of having a methane drainage system will thus be
substantial in such a case.
A similar economic advantage will occur in room-and-pillar operations that have the
possibility of production interruptions due to methane emissions in the working sections. With
continuous miner productivities continually rising, the downtime cost can be in the range of
$50 - $100 per minute of downtime averted by a well-designed gas drainage system.
Ventilation power cost savings
Several papers have outlined costs associated with ventilating high-methane mines [Mills
and Stevenson, 1989; Kim and Mutmansky, 1990; Aul and Ray, 1991]. Aul and Ray [1991],
cite situations where a methane drainage system reduced the ventilation requirements for
methane dilution to about half, thus greatly reducing the ventilation power costs. Cost
savings are dependent on mine size, the ventilation plan, electrical power costs, and the
actual air quantities saved in a particular mine ventilation network. Wang [1997] has verified
the significant nature of ventilation power cost savings, especially if gas released during
mining is 10 m3/tonne (400 ft3/ton) or more. The study by Wang also concluded that potential
power cost savings in continuous mining operations were even more significant than they are
in longwall operations.
Reduced development costs and increased reserves
The installation of a methane drainage system can significantly reduce mine ventilation
requirements and allow for the extraction of wider longwall panels. Reduced ventilation
requirements may make possible a reduction in the size, and number, of shafts and other
development openings connecting the coal seam to the surface. Extracting wider longwall
panels also reduces the number of development entries in a mine.
Longwall mining can extract 85-95% of the coal under optimal conditions, while in
development sections only about 50% of the coal is recovered. The coal produced from
development sections is generally more costly to extract, on a dollars per ton basis, than the
coal produced on a longwall panel. Therefore, increasing panel width and decreasing the
number of development entries not only leads to an increase in mineable coal reserves, but
56
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also lowers the extraction cost per ton of those reserves. These cost differences can be
significant as shown in previous studies [Kim and Mutmansky, 1990].
Reduced dust problems and increased worker comfort
The level of comfort of work in a mining environment deteriorates if high air velocities are
required to keep methane concentrations below the regulatory limits. Air velocities above 180
meters per min (600 ft/min) can generate more dust and ordinary tasks become more
difficult. In some longwall sections, for example, the high velocities downward of the shearer
result in the transported dust creating a "sand blasting" effect on the exposed skin of workers
that is both unpleasant and a hazard to their eyes. While the number of personnel working
downwind of the shearer is generally small, the hazards involved are both significant and
avoidable.
Reduced water problems
The presence of water in coalmine roof strata can be a costly source of delays in some
underground mining operations. Generally, the most sizeable delays will be encountered in
the development sections of the mine and will be quite variable depending upon the geologic
parameters of the roof strata. The water in the roof, when occurring in conjunction with high
methane contents, can be mitigated by a methane drainage system. The statistics of
downtime reductions in such mines may vary, but the reduction in water downtimes may be
of notable economic value. Reese and Reilly [1997] have outlined one description of such a
benefit for a Pennsylvania longwall mine. In this operation, the utilization of gas drainage
wells achieved a 63% reduction in water downtimes and a 16% reduction in methane
downtimes.
57
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5.2 Environmental benefits of CMM drainage
The major environmental benefit of CMM drainage and utilization is a reduction in the
amount of methane entering the atmosphere and contributing to anthropogenic greenhouse
gas emissions. When methane is captured and either flared, or used as an energy source,
the combustion process destroys the methane and produces CO2, which is twenty-three
times less potent as a green house gas than methane [IPCC, 2001].
USEPA [2008a] estimates that there are more than 200 CMM projects worldwide, which
through draining, capturing and utilizing methane, reduce emissions to the atmosphere by
more than 3.8 Bern (134.1 Bcf) methane a year, equivalent to 59.1 MTCO2e.
USEPA [2008b] has profiled fifty of the gassiest mines in the United States and concludes
that only about 35% of the total estimated methane liberated from the profiled mines is being
utilized. At thirty-six of the fifty mines, there are no methane drainage and utilization projects
in place and 1.3 Bern (46.5 Bcf) of methane per year is estimated to be liberated to the
atmosphere. If methane recovery projects were implemented at these mines and assuming a
20-60% range of recovery efficiency (i.e. the portion of total methane liberated that is
recovered and utilized), an estimated 264-791 MMcm/yr (9-28 Bcf/yr) of methane emissions
would be avoided. This is equivalent to about 4-12 MTCO2e. Significant potential also exists
for increased methane recovery at many of the mines that currently have operating recovery
projects.
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