u.s. EP;
Coalbed Methane

        Coal Mine Methane Recovery:
        A Primer

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.
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
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.


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


  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


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


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

                           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.

                           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
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
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.

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

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

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

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

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

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
Emissions Volume
MMTCO2e Billion m3
mining %
mining %
20 (NSW 59)
                      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).

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
     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
     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,

     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.

     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
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,

     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).
:> Fresh Air
 • Return/Bleeder Air
  Face Air
  Mining Direction
                                                           Belt entry
                          Exhibit 4:  Typical ventilation configuration at a U.S. longwall mine

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.

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.

                                                   2-in, (50.8-mm) pipe
                                                             2-in. C50.8-mm)
                                                             Pipe    Water

                                                 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.


Powd er River
     Cherokee &  lllinois
     Forest City
Anthracite fields
        Green River
                         San Juan
             • As lajKHUd hy LJM iS
                                                         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

     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.

       •  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.

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
       •  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

  •  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.

  •  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.

                                                          '>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-


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

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

     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
     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.


     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.

        •  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
        •  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

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


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].
                                                        \   hole
                                                        \       R
                                                                B  Advance
                                                                   drilling with
                                                                B development
                                                       drilling with
                                                          panel  <«
             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).

       •  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]

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].
                              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)
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

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
  •  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]

     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]

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
                                                              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
ladius Type
Lateral Length
60 / 200

 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.

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).
                                                                           SINGLE PRODUCTION BORE
                                                                        COAL SEAM
                                                                 UNDER REAMED TO 016"
                                                                            6" I.D. CASING
                                                                            6" I.D. CASING
                   COAL SEAM ft3
                                                                        COAL SEAM
                                                                 UNDER REAMED TO 016"


                                                                           '6" ID CASING
                                                                                      6" SUMP FOR PUMP
                        Exhibit 15:  Schematic of multiple horizontal wells drilled to a single vertical well
                                              [Mitchell Drilling, 2005]

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
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

* 7^ Coal Seams
                                  Exhibit 17:  Dual well system
                                       [CDX Gas, 2005]

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.

  •  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

      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,
      •  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)

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.

      •   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]

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)

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
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
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

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.

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
          120 to
          140 m
         20 to 40 m
                       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

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
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
                                        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)

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.

     •  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]


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



                                                     SURFACE CASING
                                                   SLOTTED CASING (OPTIONAL)
                                                    OPEN HOLE
                                    COAL SEAM
                                Exhibit 22: Profile of a typical U.S. vertical gob well

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.

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).


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.

  •  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.

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.

       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,


geomechanical properties of adjacent strata, available drilling space and drilling equipment
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
  Tailgate Entries
                                  Cross-Measure -
                            Retreat Mining with Second Tailgate Entry
Direction of
                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.

  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
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

     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
     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
                                                           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]

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
     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.


                                        Methane Extraction System
                             Over Lying Coal Seam
                         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
s i

t. +.

Overlying S *
Gallerv />^
Direction of
Advance N=

r -*
                                                                          '  GOBN
                                                            Section B-B1
                Exhibit 27: Degasification of gob areas using the superjacent method in Eastern Europe
                                            [Wisniewski, 1994].

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.
     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
            • 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.

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.

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
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).

  •  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.

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).
1000 cfm at
                                                           '*      -4
                                                          «*•      V*
                                              A—in. transmission line
                              8—in. 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.

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
   Superior mechanical
Connections can corrode
and leak overtime
Heavy and difficult to
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
Less mechanical
strength than steel
Some concern
about static
electricity issues

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


     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
     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.


     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

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]

  •  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]

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


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

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
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

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

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