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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June, 2000
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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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June,2000
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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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June, 2000
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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
Enhanced Gob Gas Recovery
June,2000
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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.
Enhanced Gob Gas Recovery June, 2000 5
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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
Enhanced Gob Gas Recovery June, 2000 6
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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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June, 2000
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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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June, 2000
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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
Enhanced Gob Gas Recovery June, 2000 10
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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.
Enhanced Gob Gas Recovery June, 2000 11
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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
Enhanced Gob Gas Recovery
June, 2000
12
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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
Enhanced Gob Gas Recovery June, 2000 13
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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
Enhanced Gob Gas Recovery June, 2000 14
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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,
Enhanced Gob Gas Recovery
June, 2000
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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.
Enhanced Gob Gas Recovery June, 2000 16
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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.
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