430R00011
Enhanced Gob Gas Recovery
        June, 2000

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                            ACKNOWLEDGEMENTS
This report was prepared under Work Assignment No. 5-2 of U.S. Environmental Protection
Agency Contract 68-W5-0017 by Daniel Brunner of Resource Enterprises, Salt Lake City, Utah
under contract to Alernative Energy Development, Silver Spring, Maryland with editorial work
undertaken by Lee Schultz of Alternative Energy Development.  This report is a technical
document meant for information dissemination. The views represented here do not necessarily
reflect those of the U.S. Environmental Protection Agency. Mention of trade names or
commercial products does not constitute endorsement or recommendation for use.

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                                         INTRODUCTION

A gob, 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, as shown on  Figure 1, or after pillar removal with the room
and pillar mining method, as shown on Figure 2.  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 recovery from the gob is to reduce methane emissions into mine workings and assist the
mine's ventilation system in providing a  safe environment for coal exploitation activities.
                                       Longwall Gob Fracture Zone
                       Coalbed
                       Coalbed
                 120to
                 140 m
                      Coalbed
                      Coalbed
                     Coalbed
               20 to 40 m
                     Coalbed
          Figure 1. Side view of the effects of longwall mining on adjacent strata (Cervik, 1979).

At most mining operations, much of the gas emitted from the gob discharges to the atmosphere, either
directly from  the  recovery system or through the ventilation system.  Gob gas  recovery systems  may
produce  high-quality gas depending  on conditions, but generally produce  gas with lower methane
concentrations. Poor 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.  However, there is potential at
many mines to increase  recovery and decrease dilution levels by adopting  improved  degasification and
collection systems and by modifying operating practices.

There  are three primary methods of longwall gob degasification that are used  worldwide, and mining
operators  often adopt variations  of  these:  cross-measure boreholes,  superjacent methods (where
degasification takes place from overlying or underlying galleries and boreholes), and vertical gob wells, as
generally illustrated on Figure 3.   The purpose of this paper is to encourage improved gob gas recovery
and  collection with these methods by  presenting low-cost,  best practices  that may  increase methane
capture with less  intrusion of mine air.  Most of  these practices cost  little or  no more than traditional and
less  effective practices.
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                                                                   Cross-Cut
                          Panel Entry Developement
                                            nnnnoco
                         DDDDDDDDDDDDDDDDGf
                                        DDDDDDDDDOa
                                          DDDoooooan
                                           nnnnnncsao]
                                                          D1
                      Pillar Recovery
                                                            Entries
                     S - Curtain 1 - Stopping R - Regulator
                                                  Intake
                                                            Return
       Figure 2. Plan view of a typical room and pillar mining panel in the U.S. (Stefanko, 1983).
                                          Vertical Gob Wells-


                                                       n
                                                            Longwall Face



                                                    Cross-Measure Boreholes
                                         Longwall Panel
               Figure 3. Schematic showing the different gob gas recovery methods.
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                                 CROSS-MEASURE BOREHOLES
 Description
The cross-measure technique of longwall gob degasification is the dominant method used in Europe and
in the Commonwealth of Independent States (C.I.S.), where operators practice longwall mining in multiple
dipping coal seams, normally deeper  than  600 meters (m).  Several U.S. mines have tested  cross-
measure  boreholes  (Cervik  and  King,  1983)  and found them  effective where operators could not
implement vertical gob wells because of surface constraints or other conditions.

With the cross-measure technique, small diameter boreholes (50 to  100 millimeter (mm) diameter) are
drilled  at angles from gateroad entries  up into overlying or  down into underlying strata, in advance of the
longwall face, as shown generally on Figure 4.  In extremely gassy conditions, operators place boreholes
from the intake  as well  as the return entries surrounding the panel.  Site-specific conditions dictate the
angle,  length, and  spacing of the boreholes.  In order to maximize connectivity with the gob and minimize
inflow of mine air into the degasification system, operators insert and seal a pipe (commonly known as a
standpipe) into the initial borehole length.
                                               Tailgate
                                                                   Cross-Measure
                                                                   Boreholes

                                                                   Direction of
                                                             Face   Advance

                                                                   Packing
                              Single Entry Advance Mining
                                                  Cross-Measure
t_




Tailgate ^ 	 /
Face
Direction of *
Advance

/ Boreholes
?=^
1
GOB
|
I
                                Headgate
                               Single Entry Retreat Mining
     Figure 4.  Cross-measure boreholes for longwall gob gas recovery for advancing and retreating
               operations (Wisniewski and Majewski, 1994).

Single-entry gateroads that parallel the gob for advancing longwall operations retain their integrity because
operators always support them well. Therefore, it is fairly simple to maintain cross-measure boreholes
and the gas gathering system.  However, this becomes a concern with retreating longwalls, because the
gateroad entries adjacent to the gob collapse upon retreat.  For single  entry retreat mining (performed
outside the U.S.), there are two cross-measure borehole variations, as shown on Figure 5.  Both systems
provide access to the cross-measure boreholes and a protective environment for the gas gathering
system.  The upper figure shows  a  retreating system  where two gateroad entries  are  developed in
advance of mining along one side of the panel to accommodate the degasification boreholes.  The lower
figure shows an additional entry developed as the longwall panel retreats.
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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.
                           L
      Tailgate Entries
                                          Cross-Measure
                                          Boreholes
                                                 Direction of
                                                 Advance
                                                          Face
                                           Headgate
                  I
                  I    GOB
                  I
                                      Retreat Mining with Second Tailgate Entry
                           I   *        * Headgate

                             Retreat Mining with Second Entry Developed with Face Advance
   Figure 5. Cross-measure boreholes developed from a second entry for longwall gob gas recovery for
            retreating operations (Wisniewski and Majewski, 1994).


Besf Practices in Cross-Measure Borehole Technique

The  objective  of cross-measure gob degasification is to provide a continuous low-pressure zone above
the mined coalbed in order to minimize the emission of methane into the mine openings. The continuous
low-pressure zone promotes not only downward migration of gas from overlying fractured strata, but also
upward migration  of mine  ventilation  air and gas released  from the rubble  zone in  the gob. Methane
capture efficiencies, defined as the ratio of the gas captured by the degasification system to that emitted
into the ventilation system plus that captured, 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  and  the connectivity between  the  boreholes and  the ventilation system.   The
following low-cost practices may improve gas production and/or recovered gas quality for this technique.

    1.   Site Boreholes to Maintain Integrity:  The cross-measure borehole collar location is critical to its
        performance and to recovered gas quality.  Because fracturing tends to occur along the entries
        adjacent to the longwall gob as the face passes, it is desirable to protect the integrity of cross-
        measure  boreholes developed in this region.  If connectivity exists  between the borehole and the
        mine  ventilation  system, wellhead suction  pressure  will  tend  to draw  in  air from  the  mine
        ventilation system,  rather than gas from the gob,  resulting in poor gas quality.  With  retreating
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        longwall systems, where multiple entries are used, researchers recommend initiating the borehole
        over the immediate adjacent pillar line to provide additional protection from the nearby fractured
        environment.  Operators should avoid cross-cuts as they are susceptible  to fracturing after the
        face passes. With advancing systems, operators should install additional roof supports, such as
        substantial packing walls, arches,  etc., at the immediate borehole location  to assist in protecting
        the initial  length  of the boreholes.   The practice results  in improved  borehole stability, thus
        preventing intrusion  of air and  improving the  gas quality.   Borehole integrity is  critical  for
        recovering high-quality gas.   For retreating systems, the additional incremental cost of this
        practice relates to the  added drilling required (i.e.,  between 20  and 40 percent) depending on
        projection  requirements and dimensions of entries and pillars. As drilling costs are approximately
        60 to 80 percent of the  cost of installing this system, the estimated incremental additional costs of
        this practice are  between 10  and 30 percent.  For advancing  systems,  operators may avoid
        incremental increases in costs by siting the boreholes near adequately supported packing walls or
        near supports. Benefits of this  practice are increased system efficiency and improved gas quality.
        Depending on baseline  conditions, this  practice  may  improve  recovery  efficiencies by  an
        estimated  20 to 40 percent and recovered gas concentrations by an estimated 10 to 20 percent.

    2.  Grout Borehole Collars:  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. This factor contributes significantly to  the  low qualities of
        gob  gas  recovered by  many cross-measure  installations.    Good sealing requires  proper
        construction techniques and materials.  Successful cross-measure  installations use good quality
        plastic or steel casing at least 6 m in length secured by grouting  in place. Results of U.S.  Bureau
        of Mines (USBM) studies suggest that standpipe grout annul! of 20 mm are  effective in the cross-
        measure borehole technique (Garcia and  Cervik, 1985).  Operators may pump the grout into the
        standpipe  and extrude  it through the annuli, or feed  it directly into the packed-off annulus with a
        system of  grout supply  and air relief tubing.  The practice results in a significant improvement in
        the prevention of intrusion of mine air near the collar.  In the case of advancing systems, where the
        life of a degasification  borehole is much longer, the practice would  also serve to increase the
        productive life  of the well.  Therefore, the overall benefits are increased gas  production duration
        and improved gas quality.  Estimates of the incremental cost of this  practice range from 10 to 15
        percent.   Improvements in recovery efficiency and  recovered gas quality depend on baseline
        conditions; however, operators may achieve  benefits similar to  those possible  by proper siting
        (Practice 1, above).

    3.  Increase Vertical Angle:   The cross-measure borehole should  target  the farthest known  gas
        source and be angled so that its farthest extent is situated in the  fractured zone above the rubble
        after the gob is formed.  If the borehole extends only to the rubble zone, it will be prone to drawing
        mine ventilation air, thus impairing its effectiveness and gas quality.  In trials of the cross-measure
        technique  in the  U.S. (three conducted  by USBM), vertical borehole inclinations ranged from as
        low as 21  degrees to as high as 37 degrees  (Cervik and King, 1983). The upper limit of  vertical
        inclination  depends on  the width of the longwall panel, depth below surface, thickness of the
        mined seam, site-specific  geomechanical  properties of the strata,  drilling  space available, and
        limitations  of the  drilling  equipment.   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, for every  gob there  is an optimum angle,  and exceeding it will  impair the
        borehole's performance. This optimum  must be determined and implemented. The incremental
        cost of implementing this practice  is minimal (relates to increased borehole length).  Benefits of
        this practice are  improved^ recovery efficiency and improved gas quality.   Again, depending  on
        baseline conditions, operators may realize improvements of between  20 to 40 percent in district
        recovery efficiency and  10 to 20 percent in recovered  methane concentration.

    4.  Increase Horizontal Angle: In European mines, as well as U.S. field trials, typical  cross-measure
        boreholes  in retreating longwalls angle toward the faceline on the horizontal plane. Studies have
        shown that most  of the  gob gas (up to 75 percent in USBM  tests - per Garcia and Cervik, 1985)
        emits from the newly fractured strata and stress relaxation zone  directly behind the faceline.


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       Operators should angle the boreholes to intercept this zone  with the holes' farthest point of
       projection.  This orientation is especially important from 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.
       U.S. field trials of the cross-measure technique used horizontal angles  ranging from 45 to  60
       degrees. For advancing systems, cross-measure boreholes may be driven at less acute angles to
       the faceline.  The "relaxed" zone has higher permeability, and  in the case of retreating systems
       where the life of the well is somewhat limited, the practice  results in higher gas production. The
       incremental  cost of implementing this practice is minimal (also relates to increased borehole
       length). The benefits of this system are particularly significant for single-entry retreat systems
       where district  degasification efficiencies  may  increase  by  an estimated 20 to 50  percent,
       depending on baseline conditions.

    5.  Shorten Horizontal Projection:  USBM  tests of  cross-measure systems  along return gateroads
       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.   Implementing this practice may, in fact, result in some cost saving related to reduced
       hole length without any loss in gas production rates.

    6.  Tesf Adequacy of Well Spacing and Suction Pressure:  In order  to produce a continuous low-
       pressure zone over the gob using the cross-measure system, the boreholes must be spaced such
       that their influence zones overlap slightly.  If boreholes are too far apart, then the accumulated
       gases between boreholes will tend to migrate toward the nearest mine entry.  If  the boreholes are
       spaced too  close together,  they may  promote migration  of mine ventilation  air into the gob,
       reducing the quality of recovered gas.  USBM studies have  shown that for U.S.  conditions,
       borehole spacing of 60 to 75 m is adequate.  The spacing required, however, will also depend on
       the available suction pressure at the wellhead and the gob permeability.  Operators should test
       the adequacy of borehole  spacing  by shutting adjacent  boreholes and observing gas  quality
       changes and  impacts on methane concentrations  in  the district return.  This  test assists in
       determining  optimum  borehole spacings and suction pressures, the two most important  factors
       affecting gas production rates and gas quality with cross-measures.  The cost of implementing
       this practice would derive from the time and effort required to shut in the well and to monitor the
       gas  quality  and methane concentration  in  the mine return.   There may be  additional cost
       associated  with providing  the ability to vary the suction  pressure.   Optimized  cross-measure
       systems can recover up to 70 percent of gob gas emissions in a  district.

    7.  Decrease  Borehole Spacings at Ends of Panels:   Research studies  show that decreasing
       borehole spacing near the start and  ends of new panels (or adding new boreholes to an  existing
       configuration) to accommodate the gas generated in  these large strata tension zones significantly
       improves the production of gas.  Because the tension zones are more fractured, the permeability
       is higher and higher gas flow rates and capture efficiencies are  achievable.  Studies indicate that
       panel ends  contribute to  approximately  35 percent of total gob gas recovered by methane
       drainage systems (Diamond, 1995).  Operators must weigh the  cost of developing additional
       boreholes with the  benefits of increased recovery.  Depending  on panel length, halving  the
       borehole spacing at the start and ends of panels can  increase cross-measure  implementation
       costs by 30 percent.   If these efforts serve to recover 70 percent of the gob gas generated from
       panel ends, versus assumed baseline conditions of 50 percent, this practice can lead to  an
       increase in cumulative gas recovery of 15 percent for the longwall panel.

    8.  Provide for  Monitoring:  Because the degasification system operates in  conjunction  with  the
       ventilation system and mining operations, it  is important  to provide coordinated  management
       (using measuring instruments, monitoring, controls, and good communications) to optimize each
       of the three functions. Modern wellhead configurations enable  measurement of gas quality, gas
       flow rate, and pressure. Figure 6 shows low-cost provisions for suction control  and  pressure and
       flow monitoring for a  cross-measure wellhead.  Operators  measure the pressure differential


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        across the venturi to compute gas flow rate, they monitor the gauge pressure from the tap inby the
        venturi to determine static pressure, and they obtain gas samples through the taps via a vacuum
        to determine gas quality.  Operators may use hand-held manometers or diaphragm gauges for the
        pressure  measurements. This capability is useful for optimizing all of the factors that affect the
        performance of cross-measure systems, and for determining which boreholes to shut-in (shut-in
        values range from 25 percent methane in the U.S. to as high as 40 percent in some European
        operations (Thermie, 1994)).  The direct  benefit  of this practice is ensuring that the quality of
        recovered gas  is above the limiting value for the mine, while the indirect benefits are obtaining a
        reliable record of the gas quality and quantity and enhancing safety. Costs for hand-held, portable
        monitoring equipment and provisions  for pressure monitoring and  gas sampling are  minimal
        relative to the costs of implementing cross-measure systems mine-wide.
                                   Mine roof
                                                             Plastic pipe (stondpipe ),
                                                                 2.5 mlD-

                                                                   Not to scale
            Figure 6: Cross-measure borehole wellhead configuration with monitoring provisions
                     (Garcia and Cervik, 1985).

    9.   Provide for Water Separation at Wellhead: Water tends to collect in cross-measure boreholes
        developed over the mined seam directly above the wellheads and creates a need for a separation
        system.  The USBM developed a water removal system for cross-measure boreholes (Garcia and
        Cervik, 1985), as shown in Figure 7.  Water flows down the annulus between the production pipe
        and the standpipe and discharges from a one-way valve, mounted as indicated on the figure. The
        production pipe recovers gas through slots along  its upper extension and  delivers it to  the
        collection system.  An end-cap  assembly prevents  cascading water from  entering the gas
        production line.   Operators  applying this approach  may realize an estimated 5  to  15 percent
        increase  in implementation  cost,  depending  on  baseline practices  (such  as use  of grouted
        standpipes).  Operators  that  have problems controlling  water  accumulation  at wellheads may
        realize significant benefits in gas recovery with this practice.
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          Figure 7: Cross-measure boreholes water separation system (Garcia and Cervik, 1985).

    10. Maintain Access to Wellheads:  In order to optimize system pressure control underground, mine
       personnel must be able to access the gathering line and valve heads.  Retreat mining systems
       with single-entry gateroads do not  provide for this under normal circumstances, and  retreat
       systems with multiple entries may require additional supports along the pipeline to ensure access
       and pipeline integrity.   Operators should  consider this  practice if there is the potential for
       significant gas recovery over long periods of time after the face has passed  the borehole area.
       Research indicates that gob methane drainage systems may recover large volumes of gob gas (in
       some cases 50 percent of total  recovery) after  longwall mining is completed,  depending on
       conditions  (Diamond,  1995).  With  cross-measure systems, this  practice requires appropriate
       prior planning on the  part of operators  and may  require  costly measures that operators must
       weigh against the significant increase in overall gas production and improvement in gas quality.
                               THE SUPERJACENT TECHNIQUE
Description
Some gassy, deep underground operations in eastern Europe, the C.I.S., China, the U.S. and Australia
recover methane  using the superjacent technique.   The objective of this technique is to develop long
drainage boreholes or galleries in advance of mining in  overlying or underlying strata (rock  or coal).  In
some cases, operators also develop small-diameter,  short boreholes extending from galleries  into strata
overlying the  gob.  Overlying  galleries serve  as useful platforms for  targeting the fractured zones over
longwall gobs. Figure 8 illustrates two superjacent gob  drainage techniques used in eastern  European
mines.
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                                        A   Boreholes
                      ||	Overlying Gallery
                                                  B
             Tailgate

           Overlying
               Direction of
               Advance ^

                    Face
                                                           Headgate
                                                                        GOB
                           Section A-A'
                                                          Section B-B'
   Figure 8. Degasification of gob areas using the superjacent method in Eastern Europe (Wisniewski,
                                                1994).

Figure 9 illustrates the application of superjacent horizontal boreholes for initial premining degasification of
an underlying coalbed and subsequent use for gob degasification as implemented in Australia. Operations
in Japan, China, and the U.S. also use superjacent  boreholes.  The technique involves the use of in-mine
directional drilling equipment, usually to develop 75 to 100 mm diameter boreholes to lengths in excess of
1000m.
                                     Longwall face
                        Buili Coalbed
                                   Methane drainage
                                    hole
             Migrating gas
                        Balgownie Coalbed
                                               Migrating
                                                  gas
                        Upper  Wongawilli  Coalbed
                        Lower  Wongawilli  Coalbed
      Figure 9. Superjacent borehole drilled into an underlying coalbed in Australia (Diamond, 1993).
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Best Practices in the Superjacent Technique

The  objective of  gob degasification with  the superjacent technique is to provide  more immediate and
independent  access to  overlying  strata that will fracture as a result of longwall  mining.  The system
enables the  operator to place galleries and/or boreholes from non-production areas that are free  of
equipment- and production-related inconveniences. It also facilitates placement of  boreholes in advance
of the mining face for both  advancing and retreating systems.  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). As with cross-measure boreholes, gob gas
drainage efficiency and  gas  purity for  superjacent  systems  are  affected by  geologic and reservoir
conditions, orientation of the galleries and/or boreholes, size and spacing, gallery and borehole integrity,
suction control, and mine ventilation. The  following low-cost practices may improve gas production and/or
recovered gas quality for this technique.

    1.   Target Boreholes and Galleries Appropriately:  To determine appropriate  gallery and borehole
        placement in plan and  in profile, operators must consider the geomechanical characteristics of the
        strata, the fracture characteristics of the gob as it forms,  the  resulting gob permeability, the
        proximity  of source seams to the mined horizon, the system of ventilation, and the structure of the
        mining seam.  To improve gas  production (increase  production time), operators should target
        galleries and boreholes below the contributing source seams and above the rubble zone to take
        advantage of the connectivity provided  by the fracture network and  this region's stability (relative
        to the rubble zone).  Superjacent  boreholes and galleries should  target high-permeability tension
        zones along the perimeter of the panels, and consider gob gas migration patterns caused by the
        mine's ventilation system or the structure of the coal seam (grade), to improve gas recovery rates.
        There is no additional cost in  implementing this  practice.  Operators applying this  practice may
        realize substantial benefits in gas productivity and gas quality.  Research indicates  that gob gas
        recovery systems that target tension zones recover between 30  and 50 percent more gas than
        systems sited along the center of longwall panels (Diamond,  1995).

    2.   Monitor Suction Pressures and Gas Concentration:  Operators will need to  apply  a vacuum  to
        collection systems connected to superjacent galleries and boreholes to produce gob gas.  In  all
        applications  of the superjacent technique, operators  should carefully monitor  and adjust gas
        collection system suction  pressures and should  measure  the  methane  concentration of the
        collected  gas and that in the district returns to optimize  gob gas recovery and  quality.  Operators
        can perform this in a low-cost manner by monitoring pressures and gas concentrations with hand-
        held  equipment on a  scheduled  basis, as necessary.   Depending on baseline conditions, the
        primary benefit of monitoring and  controlling suction pressures is improved  gas quality; however,
        operators also may improve recovery using this practice.  Depending on baseline conditions, this
        practice may increase recovered gas quality by an average of 20 to 40 percent.

    3.   Develop Fewer,  Larger-Diameter Superjacent Boreholes or Boreholes from Galleries: To develop
        a continuous low-pressure zone over the gob, operators should develop superjacent boreholes  at
        appropriate sizes and spacings so that borehole influence  zones overlap  slightly.  As with the
        cross-measure system, 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  boreholes  may recover more gas  at  lower  pressure losses than smaller-diameter,
        closely-spaced holes.  Operators  should use  the Weymouth formula  to assist  in  selecting the
        optimum borehole diameter (i.e., the capacity increases as a function of the diameter raised to the
        exponent of 2.667) for any given production rate and pressure differential (anticipated gob gas
        pressure  minus  inlet  suction  pressure).  Applying this practice results in  less drilling, fewer
        wellhead connections,  minimized leakage, and improved system operation and control if there is a
        direct tie  between the wellheads  and the collection system.  The  benefit  of the practice  is an
        improvement in gas production rate and quality, increasing the system efficiency  and ease  of
        maintenance.  Mine operators should  weigh the extra cost incurred  in drilling larger-diameter


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        holes,  which might  even  require different equipment  and/or  larger  galleries, against the
        incremental benefit of up to an estimated 20 percent in gas production and improved quality. .

    4.   Maximize Use of Directional Steering Capability to Minimize Water Problems  and Increase
        Borehole Surface Areas with Superjacent Boreholes:   Collected water from  upper  strata and
        drilling  fluids can inhibit gas production, particularly for boreholes  developed to target seams
        below the mining elevation.  Similar to cross-measure boreholes developed below the working
        horizon, these holes  may not produce gas until  the water migrates through fractures that will
        develop during mining without water-lifting provisions (compressed air lift, for example).  With
        longer horizontal  superjacent boreholes, as shown on Figure 9, deviations in borehole trajectory
        can produce water collection areas ("U" shaped low-elevation zones) that impede gas flow. Using
        the steering capability of the directional drilling equipment, operators can minimize such water
        problems.  They  can develop a multitude of deviated,  tangential  boreholes from  a single initial
        superjacent  borehole.   They  can vertically  control  borehole  undulations to  avoid water
        accumulations in  low areas, or they can  develop tangential down-grade water control branches
        from the main borehole.  Once the main  borehole  has  reached the  desired  horizontal  level,
        operators should (1) develop horizontal  boreholes in  a general  downgrade direction, and  (2)
        develop deviated holes in order to enlarge the zone of reduced  pressure over the  gob.  This
        practice increases the  extent of the  low-pressure horizon over  the gob, and mitigates water
        accumulation problems that may impair gas production.  Because this practice may avoid some
        subsequent  parallel  boreholes,  it  may offset the additional  costs  of drilling to control water
        accumulation.  Operators that experience water accumulation problems will benefit significantly
        from this practice, as this problem  is otherwise only resolved when adequate gas  pressures can
        move the column of water. This practice provides  for earlier use of the borehole to reduce in-situ
        gas contents or produce free gas, and may increase cumulative production by an estimated 10 to
        15 percent,  depending on conditions.


                                    VERTICAL GOB WELLS

Description

The predominant gob degasification technique  applied in the U.S. involves vertical wells developed from
the surface in advance of mining.   As with all gob degasification techniques, the methane quality and
quantity  produced  from  these wells vary and  depend  upon  site-specific  geological  and reservoir
characteristics,  and  upon  mining, degasification, and ventilation practices at the mine.

The usual  practice  is to drill large-diameter (up to  300  mm) vertical gob  wells in advance of mining  to
within 10 to 30 m above  the working  coal seam. Operators case and cement the wells to a point just
above the uppermost coal seam or gas-bearing stratum believed capable  of liberating gas as a result  of
longwall mining. They leave the lower portion of the well open or complete  it with slotted casing as shown
on Figure 10.

Operators  apply a vacuum to the  gob wellhead to enhance production.   At some mining operations,
excellent production rates and high gas qualities are maintained with suction  and proper monitoring and
control.   An example of such  an  operation is the Jim  Walter Resources  mines in  Alabama,  where
overlying gas-bearing strata of high gas content are present, and where gob permeabilities are very high.
Methane production and mining activities are closely coordinated. The Jim Walter Resources  facility has
implemented a system to carefully monitor gob gas collection and process it for pipeline injection.  At most
U.S. mines, however, the gas is not  pipeline  quality and vents from  vertical gob wells directly to the
atmosphere.
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             Figure  10:  Profile of a typical U.S. vertical gob well (dimensions in U.S. units).
Best Practices in Vertical Gob Well Technique

Operators who use vertical gob wells in favorable geologic and reservoir settings  have claimed high
methane capture efficiencies.   Mining operations with overlying gas-bearing strata exhibiting high  gob
permeabilities have been able to obtain nigh gas purities with this system. As with all gob gas recovery
methods,  intrusion of mine  ventilation air is common because of connectivity in the gob between the
borehole and ventilation  system.   Several  factors (e.g.,  geologic and reservoir  conditions,  mining
characteristics, well siting, completion  practices, wellhead operations, and the mine  ventilation system)
affect gas capture efficiency and gas purity.  The following low-cost practices may improve gas production
and/or recovered gas quality for this technique.

    1.   Install Wells in Advance of Mining:  Operators  have not had much success with vertical wells
        drilled into gobs developed after longwall mining (Li et al., 1997).  Operators that drill in advance
        of mining encounter  better conditions to properly drill and complete an effective gob well.  Benefits
        are considerably higher gas production rates at no or lower implementation costs, depending on
        baseline conditions.

    2.   Provide Flexibility in  Plans for Subsequent Wells: Gob well performance records indicate that well
        productivity is 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. Typically, operators  select a location for the first well on
        a panel and evaluate its production record to help locate the  subsequent wells.  To successfully
        apply vertical gob wells, operators should ensure that there is flexibility to decide  upon the exact
        well locations shortly before drilling. Benefits are higher gas production and improved gas quality
        at low or no incremental cost.

    3.   Target Stress Relief Zones: USBM studies indicate that gob well location significantly impacts gas
        production (Diamond, 1995).  Records typically show that the wells positioned at the start and end
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        of panels produce the most gas.  This occurs because these wells intercept the strata within the
        zone of tension between the barrier pillar (at the start and end of the panels) and the two adjacent
        pillar lines.  Because of the aspect ratio (the ratio of length over width) of the  longwall panel, the
        tension zones  at the ends of the  panel are typically much greater than those alongside the pillar
        lines paralleling the panel.  It is in  the tension zones where the fractures generated  by longwall
        mining activity  tend to maintain their aperture, and where gob permeabilities are the highest.  Gob
        wells located within the tension zones produce gas at higher flow  rates and for greater durations
        than wells positioned along the centerline of panels. As indicated by tests conducted in the Lower
        Kittanning Seam  in Pennsylvania, offset wells produced 30 to 55 percent more gas than centerline
        wells on a cumulative basis. Operators can typically achieve this benefit with no incremental cost.

    4.   Experiment with Shorter Completions:  U.S. mine operators  normally complete gob wells in
        advance of mining at depths to within 10 to 30 m of the mining seam.  Experimentation is normally
        required to optimize vertical placement of each well. Some U.S. operators have noted that shorter
        completions, say 30  m above the  mined  seam, produce as  effectively as  completions that are
        much closer to the mined seam.  In a case where the Pittsburgh seam is mined  in West Virginia,
        measurements of gas inflows within the boreholes indicated that the intervals directly above the
        mined coal produced essentially  no gas (Mazza and Mlinar, 1977).  The benefit is cost savings
        without compromising gas production rates or gas quality.

    5.   fse Surface and Slotted Casing:  A high-performance gob well must maintain wellbore integrity as
        well as  connectivity with  the fracture zone.   Gob well completion concerns include vertical
        placement of the well within the gob, maintaining well integrity and productivity after undermining,
        and isolating water-bearing horizons.   Water-bearing strata must be isolated 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.  Some non-U.S.
        coal operators  have not  had much  success with this technique because  of 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 surface
        casing to isolate surface water-bearing zones avoids water  accumulation, while  slotted casing
        minimizes the  potential  of shearing and  a short  productive life.   The benefit is a  significant
        increase in gas recovery at estimated incremental costs of between 30 and 40 percent, depending
        on baseline conditions.

    6.   Use Active Extraction with Local Monitoring: Vertical gob wells will generally produce gas without
        applying suction due to their great elevation difference, but usually not until they are approached
        by the mining face.  Some shut-in wells may exhibit positive pressure increases. Values as  high
        as 40 kPa  gauge  have been recorded  in Appalachian  operations  (Mazza and  Mlinar,  1977).
        However, 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 a -
        6.9  kPa (1  psi) suction  pressure (Mazza, Mlinar, 1977)  to gob  wellheads.  Operators should
        determine (by  monitoring) an optimum suction pressure that achieves underground gob  gas
        drainage objectives and  minimizes the introduction of mine ventilation air.  This requires careful
        monitoring of gas quality and the ability to adjust the vacuum pressure promptly. The result of this
        practice  is a significant increase  in gas production; however,  if the system is not implemented
        correctly, it may result in poor gas quality.  Depending on local drilling and completion costs, this
        practice  will increase gob well costs by an estimated 30  to 50  percent (with  local  monitoring
        provisions).

    7.   Monitor  Continuously to Achieve  High  Recovery and Gas  Quality:   Wellhead  operations,
        ventilation controls, and mining operations are widely separated with the vertical gob well system.
        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 is required, as in the U.S.), operators must closely coordinate these two systems  with


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       continuous  monitoring.   Continuous monitoring is  particularly imperative  to  maintain gob gas
       quality for projects that use the recovered gas.  Operators should employ systems that indirectly
       measure methane content, as these are less costly. This involves obtaining continuous oxygen
       monitoring data from the wellhead, calibration data from nitrogen samples taken periodically at
       gas gathering points, and a formula to extrapolate the results. Oxygen sensors are accurate, less
       costly, and readily obtainable.  A central control facility can collect data by FM transmission, and
       computers with programs updated with calibration data can conduct the extrapolations.  Ideally,
       methane concentration information collected by sensors in the longwall districts would supplement
       the surface information,  and operators would have the ability to make adjustments whenever
       necessary.  The benefits of this practice are improved gas quality and increased gas production;
       however, it is not a low-cost practice and is presented herein only for completeness.

    8.  Effectively Seal Mined-Out Areas:  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. This practice  also mitigates the occurrence of "heatings" in
       cases  where  operators are concerned with spontaneous combustion.  Effective sealing of gob
       regions can improve recovered gas quality. Operators can more effectively seal mined-out areas
       by reducing the number of seals necessary to isolate the gob from active workings through use of
       large coal pillars (barrier  pillars) or by implementing more effective  sealing methods.  Operators
       would need  to incorporate this  practice into their mining plans and must weigh the implementation
       costs with the benefits of improved gas quality.  In  cases where  lower coal extraction ratios  are
       the result, or  a large number  of "effective  seals" are required, operators may not consider this
       practice low in cost.


            ENHANCED UNDERGROUND GOB GAS COLLECTION AND TRANSPORT

An integral  component of a mine degasification system is the gas collection and transport infrastructure.
Underground, this infrastructure  serves to move methane collected from degasification boreholes up to
the mine surface or to a dedicated underground dilution area.  On the surface, gathering infrastructure
ranges from vertical in-seam gas collection wells to compression and processing facilities.  This section
focuses on  underground gob gas  collection systems that are typically more difficult to control and maintain
than surface systems  because of mining activity and the complex subsurface environment. Gas collected
from underground gob degasification boreholes comes to the surface via a network of pipes fitted  with
safety devices, water separators,  monitors and controls, and  vacuum pumps.

Components of an Underground Gas Collection and Transport System

Pipes:  A gob borehole normally connects to a collection line via a flexible  hose. Collection lines transport
the gas 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. Pipelines are steel or high-density polyethylene (HOPE), where permitted.
Steel lines  are  preferred for mechanical  strength, especially for the underground to surface connection,
but HOPE is easier to handle and is non-corrosive.  Pipes are either suspended or laid on the mine floor.
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.

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 along the piping network at enough locations to sectionalize the system during failure
conditions.  The valves are activated pneumatically or electrically by means of methane sensors in  the
airway, pressure sensors, or protective monitoring tubing devices (which are common in the U.S.).

Water Separators: Operators install water traps, or separation devices, at  low elevations along a methane
drainage  network so that  water (condensate, or formation water)  will not impede gas  production.
Operators typically use large water separators at wellheads  and at the base of the vertical collection well


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as shown on Figure 11.  These devices subject the gas to a sudden expansion that reduces its velocity,
dropping the entrained  water  droplets.  After separation  from entrained water  at the wellheads, gas
mixtures are typically saturated with water vapor.
                                     Water Separator
                                                             Gas Collection Line
                                                                 Float Trap
                 Figure 11. Separation system at the base of a vertical collection well.

Monitors and Controls: Gas collection system monitors sense three parameters: pressure, flow rate, and
concentration of gas constituents. Valves activated either manually or remotely by pressure, gas quality,
or flow sensors comprise the control system.

Gas Movers: Gas moving equipment includes extractor pumps and compressors.  There are three types
of 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).
Best Practices in Underground Gob Gas Collection Systems

The following are practices in  underground gob gas collection systems that would improve safety and
assist in improving gas production rates and gas quality.

    1.  Use High  Density  Polyethylene  (HOPE)  Piping:   Air  leakage  into  a  negative-pressure gas
       collection system affects  recovered gas  quality  and system performance.   Leaks are  most
       common with carbon steel pipes because they are susceptible to corrosion and require threaded
       or gasketed connections.   Gasketed, flanged fittings  tend to leak over  time, particularly when
       exposed to the  mine environment, or when operators  frequently  move  or strike the pipeline.
       Welding the pipe sections and fittings in  advance of placement reduces leaks, but inevitably,
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       because of access limitations, welding constraints underground, and handling difficulties, flanged
       connections are required.  The primary arguments against the use of HOPE piping in the U.S. are
       related  to material strength  and static electricity discharge.  These  reasons have limited its
       acceptance in underground mines worldwide. The material strength concern was overcome in the
       U.S. after rigorous testing (Energy Applications, 1976).  To prevent static electricity problems, all
       HOPE lines in U..S. mines must be grounded with a wrapping of 10 gauge copper wire along the
       entire pipe length and by appropriate connections to ground.   In this way, operators may use
       HOPE piping, where permitted, and take appropriate precautions. Use of HOPE prevents leakage
       as operators may fuse rather than flange together pipe sections as  well as pipe fittings.  Its lighter
       weight facilitates handling, and it is noncorrosive and requires less  maintenance than steel pipes.
       The benefits  of this  practice are improved  production  through  increased suction pressures
       underground and improved gas quality by mitigating leakage at pipe joints and fittings. Operators
       may offset the increased capital cost of HOPE (costlier for larger diameters) with cost  savings
       gained in installation and maintenance. Depending on conditions, operators may realize increases
       in recovered  gas quality as high  as  50  percent with HOPE systems versus flanged steel  pipe
       networks.

    2.  Monitor Pipe  Integrity:  Impact tubing makes it possible to monitor the integrity of the entire
       gathering line. It is a small diameter, mechanically weak tubing that is affixed to the gas collection
       line and pressurized  by an inert gas such as  nitrogen.  Operators  should connect this  line to
       actuators fitted on isolation valves at boreholes and along collection piping. The monitoring tubing
       would break on impact to the collection line and would activate appropriate isolation valves.  This
       practice makes the operation safer and avoids unforeseen accidents resulting from rupture of
       tubing.  This practice  would increase the installed cost of an underground gas gathering system
       by 30 to 40 percent, depending on baseline conditions.

    3.  Sectionalize  the  Gathering  System: Operators  should sectionalize  their  underground  gas
       gathering systems to limit methane  release after pipeline breaches.  U.S. guidelines suggest
       sectionalizing  to limit  methane releases to 28.3 m3 per 4.7  m3/s of  ventilating  air  flowing in the
       entry containing the pipeline (U.S. Department of Labor, 1978).   Operators  should apply this
       practice with Practice 2 above.

    4.  Provide for and Maintain Adequate Water Separation: Accumulation of water is common either at
       the wellhead or along gas gathering lines, and this is a major cause of  poor gas production.
       Uncontrolled accumulation of water occurs when entrained water separators, traps, or scrubbing
       devices cannot properly drain, or if the  pipelines are aligned without consideration for water
       drainage. Operators should remove water accumulation at wellheads, at lower elevations along
       the pipeline,  and at the base of vertical collection wells with  the use of  automatic water trap
       systems.  Operators should use float traps (modified to account for negative pressure) to release
       water while preventing air intrusion, or water dumping systems if operating at high vacuum.  This
       practice would increase the installed cost of an underground gas collection system, but would
       systematically reduce the cost of operations and greatly enhance system performance.

    5.  Monitor the Gathering System: 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.    Pressure
       monitoring and control capabilities at the wellhead are critical to proper production control for the
       entire system because  pressure responses are  specific for each  borehole.   Operators must
       achieve proper pressure control through  strategic  placement of control valves within  the system,
       employing sufficient wellhead monitors, and properly designing the vacuum pump  and gathering
       system.  Frequent monitoring of static pressure and orifice  plate flow meters  installed near gas
       movers and at critical junctions underground will help operators to optimize system performance
       and will inform them of increased system demands. Operators could implement this practice with
       daily inspections for minimal incremental costs.  Benefits are improved system  performance and
       increased recovered gas quality.
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                                          SUMMARY

This paper presents best practices for gob degasification to increase gob gas production and improve
recovered gas quality. Specific effective, low-cost practices can improve productivity and gas quality at
existing systems.  The  practices cover a wide range, including borehole (well) siting or placement
considerations, borehole integrity, water separation, gob sealing, pipe materials, and system monitoring
and  control.   Monitoring and control  systems are essential for  improving,  verifying,  balancing,  and
quantifying gob degasification system performance.

Gas quality, gas flow rate, and  system pressure are the  three characteristics of a gob gas drainage
system that operators should monitor.  The  paper recommends that  mine operators  provide facilities
suitable for  use  of  hand-held measuring devices at each underground wellhead,  gas moving plant,
collection  area, and  vertical gob wellhead.  Monitoring  provisions (e.g., static  pressure tappings, in-line
orifice plates or venturi systems, and gas sampling ports) are simple, inexpensive, and easy to install and
use. Operators can easily take pressure readings with manometers or diaphragm gauges and assess gas
quality samples with  infrared analyzers, interferometers, or acoustic methanometers.  Quick, indirect gas
quality measurements are also  possible by measuring oxygen concentration and inferring  nitrogen
concentrations.  The paper also discusses continuous monitoring, which is more costly and requires data
transmission to a central  location  for analysis  and  system  surveillance.    However, for large-scale
commercial operations that are concerned with gas quality, such a system can prove to be an asset.

                                        REFERENCES

Cervik, J., 1979, "Methane Control on  Longwalls - European and U.S. Practices",  Longwall-Shortwall
Mining, State of the Art, Chapter 9, pp. 75-80.

Cervik, J., King,  R.,  1983, "Control of Methane in Gobs and  Bleeders by the Cross-Measure Borehole
Technique", Conference on Coal Mining  Health, Safety and Research,  August 23-24,  1983,  Virginia
Polytechnic Institute and State University, MSHA and USBM.

Diamond,  W.P.,  1993, "Methane Control for  Underground Coal  Mines" in  Hydrocarbons from Coal,
editors: Law, B.E. and Rice, D.D.,  publisher: AAPG, pp: 237-267.

Diamond,  W.P., 1995, "The Influence of Gob Gas Venthole Location on Methane Drainage: A Case Study
in the Lower Kittanning Coalbed, PA., Proceedings of the 7th U.S. Mine Ventilation Symposium, June 5-7,
1995, Lexington Kentucky.

Energy Applications,  Inc.,  1976,  "Design and Recommended Specifications for a Safe Methane Gas
Piping System", U.S. Dept. of Commerce National Technical Information Service, PB-259 340.

Garcia, F., Cervik, J., 1985,  "Methane Control on Longwalls With Cross-Measure Boreholes (Lower
Kittanning Coalbed",  Report of Investigations 8985, U.S.  Department of the Interior.

Li, J., Schwoebel. J., Brunner, D., 1997, "Vertical Gob Well  Gas Recovery in Tiefa,  China", Proceedings,
International Coalbed Methane Symposium, Tuscaloosa, Alabama, pp. 271 - 281.

Liu, Z., Bai,  W., 1997, "Coalbed Methane  Development, Utilization and  Prospect in the Yangquan Coal
Mining Area", China Coalbed Methane.

Mazza, R.L., Mlinar,  M.P., 1977, " Reducing Methane in Coal  Mine Gob Areas with Vertical Boreholes",
U.S. Bureau of Mines Open File Report 142-77.

McPherson,  M.J., 1993, Subsurface Ventilation and Environmental Engineering. Chapman and Hall, 2-6
Boundary  Row, London SE1 8HN.
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Stefanko, R., 1983, Coal Mining Technology. Society of Mining  Engineers of the American Institute of
Mining, Metallurgical, and Petroleum Engineers, Inc. NY, NY.

Thermie  Programme Action C 15, 1994, "Utilisation of Colliery Methane in  Central and Eastern Europe",
European Commission Directorate-General for Energy DG XVII.

U.S. Department of Labor, 1978, "Piping Methane in  Underground Coal Mines", Mine Safety and Health
Administration, Informational Report (IR) 1094.

Wisniewski B.,  Majewski,  J.,  1994, "Information on  Coalbed Methane  Recovery and its Utilization in
Poland",  State Hard Coal Agency.
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