Bureau of Mines Report of Investigations/l982 

Degasification Study From an Air 

Shaft in the Beckley Coalbed 

By Tobias W. Goodman, Joseph Cervik, 

and George N. Aul 

UNITED STATES DEPARTMENT OF THE INTERIOR 
Report of Investigations 8675 

Degasification Study From an Air 

Shaft in the Beckley Coalbed 

By Tobias W. Goodman, Joseph Cervik, 

and George N. Aul 

UNITED STATES DEPARTMENT OF THE INTERIOR 

James G. Watt, Secretary 

BUREAU OF MINES 

Robert C. Horton, Director 
This publication has been cataloged as follows : 

Goodman, T. W. (Tobias W.) 

Degasification study from an a i r s h a f t in the Beckley Coalbed. 

(Iieport of investigations ; 8675) 

Bibliography: p. 17. 

Supt. of Ilocs. no.: I 28.23:8675. 

1. Coal mines and mining-Safety measures. 2. Methane. 3. Mine 

ventilation. I. (Icrvik, Joseph. 11. Aul, G. N. (George N.). 111. Title. IV. 

I Series: Iicport of investigations (United States. Rureauof hlincs) ; 8675. 

TN23.U43 ITN2951 622s 1622l.81 82-600098 AACR2 

I 

I 
CONTENTS 

-.. 

Page 

Abstract ....................................................................... 1 

Introduction ................................................................... 2 

Acknowledgments ................................................................ 2 

Experimental program ........................................................... 2 

Description of study area ..........................m......aa.....a........ 2 

Drilling procedure ........................................................ 2 

Underground pipeline ...................................................... 7 

Gas analysis .............................................................. 7 

Relationship between data obtained from horizontal drilling and the geology of 

the coalbed .................................................................. 8 

Patterns of gas and water flows ................................................ 8 

Effectiveness of degasification ........................................m....... 16 

Summary ........................................................................ 17 

References ..................................................................... 17 

Appendix A*-- Gas pressure tests ..............................m....m.a.m.....a.. 18 

Appendix B.--Mining through degasification holes ...............em...........emm 19 

ILLUSTRATIONS 

Degasification site ...................................................... 3 

Stratigraphic column above Beckley coal bed...............^....^...^^^.... 3 

Geological features in Beckley Coalbed ..............aa.....m....mm....mm. 4 

Electrohydraulic drill ................................................... 5 

Drilling assembly. standard ............................................a. 5 

Drilling assembly. modified .............................................. 6 

Location and length of holes and projected main entries .................. 6 

Contour map of coalbed from vertical boreholes ........................... 9 

Vertical sections of horizontal boreholes 1. 2. 3. and 4 ................. 9 

Vertical sections of horizontal boreholes 5. 6. 7. and 8 ................. 10 

Contour map of coalbed from horizontal bore hole^............^..^......... 11 

Contour map of coalbed from level surveys ................................ 12 

Composite contour map .................................................... 13 

Gas and water flows. holes 1. 2. 3. and 4 ................................ 14 

Gas and water flows. holes 5. 6. 7. and 8 ................................ 14 

Vertical section through holes 1 and 8 ................................... 15 

Total gas and water flows ....................m...........m...m.m......... 15 

Gas flow from section faces .........................m.............a.a.... 16 

Gas flows versus in situ pressure. holes 6 and 8 ......................... 18 

TABLES 

1 . Comparative gas analysis ................................................. 7 

2 . Average gas and water flows ............................m....aa........... 8 

B-1 . Cement slurry design ..................................................... 19 
DEGASlFlCATlON STUDY FROM AN AIR SHAFT IN THE BECKLEY COALBED 

By Tobias W. Goodman, 1 Joseph Cervik,2 and George N. Au13 

ABSTRACT 

The Bureau of Mines conducted research to degasify a virgin block 

nf cnal in the Beckley Coalbed using long horizontal holes drilled in an 

array from the bottom of a ventilation air shaft. Eight horizontal 

holes and an in situ pressure hole were drilled in a radial pattern to 

depths ranging from 418 to 910 ft (127 to 277 m) and 205 ft (62 m), 

respectively. 

The investigation showed the shaft was in a basin saturated with 

water, which resulted in low gas flows from holes. The in situ pressure 

at 200 ft (61 m) into the coalbed was 79 psig (544.6 KN/m2). The average gas and water flows for the eight holes were 94,100 ft3/day 

(2,665 m3/day) and 13 gal/min (49.3 l/min) . After 514 days of degasification, 18.7 million ft3 (0.53 million m3) of gas had been drained. 

Methane face emissions, which were measured as a section advanced toward 

the shaft, were reduced about 77 pct, emphasizing the value of drainage 

by this technique. All holes were grouted before the area around the 

shaft was mined. 

'~ining engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, Pa. 

2~uperv isory geophysicist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, Pa. 

3~eologist, Occidental Research Corp., Keen Mountain, Va. 
INTRODUCTION 

Commercial quantities of methane 

have been produced by using horizontal 

holes drilled into virgin coal from 

the bottom of shafts in the Pittsburgh 

Coalbed; average flow rates were between 

120 and 150 ft3/day (11.1 and 13.8 m3/ 

day) per foot (meter) of hole for 

2-1/2 to 3 years (3-4) -- .4 The drilling 

techniques and equipment used at these 

sites were described earlier (2). - 

The Pittsburgh and Beckley Coalbeds 

are similar. Both are blocky-type coals 

having well-developed cleats and high 

fracture permeability. A geological 

study of the Beckley Coalbed (1) showed 

that coalbed water that collected in the 

regional syncline at the Maple Meadow 

Mine might fill the cleat spaces and prevent natural degasification. 

The study described herein was carried out by the Bureau of Mines to degasify a portion of the Beckley Coalbed and 

to determine the effects of degasification on methane flows during mining. 

ACKNOWLEDGMENTS 

The cooperation of Doug Tolley (chief engineer) of the Maple Meadow 

(Manager of Mines, Cannelton Industries, Mining Co., Fairdale, W. Va., is greatly 

Inc., Montgomery, W. Va.) and of Dick appreciated. 

Busick (superintendent) and Arnold Keaton 

EXPERIMENTAL PROGRAM 

Description of Study Area 100 ft (30.5 m) are common. The genesis 

of these structures is attributed to a 

The degasification site was the depositional environment. 

No. 2 shaft located in a projected set of 

five entries at the Maple Meadow Mine in The dominant regional geological 

the Beckley Coalbed (fig. 1) . The structure, the Pineville Syncline, is 

concrete-lined shaft, 24 ft (7.3 m) in oriented in a NE-SW direction. The No. 2 

diameter, is divided for intake and shaft is located in the bottom of the 

return air. At the shaft, the coalbed is synclinc (1). Local bedding folds are 

7 ft (2.1 m) thick and overburden is common. 

800 ft (244 m). 

The stratigraphic column is sandstones (sometimes crystallized) and 

shales (fig. 2). Geological features 

reported during mining include sand channels, washovers, and splits (fig. 3). 

Grades of 20 to 30 ft (6.1 to 9.1 m) per 

---- -- %Jnderlined numbers in parentheses 

refer to items in the list of references 

preceding the appendixes. 

Drilling Procedure 

A 30-hp (223.8 x 102 W) electrohydraulic drill (fig. 4) was used to drill 

the holes. The drilling hardware 

included 3-112-in (8.9-cm) drag and rollercone bits, a one-way check valve, an 

18-ft (5.5-m) NQ drill collar, spiral 

centralizers, and BQ drill rods (figs. 5 

and 6). 
3,000 ft 

900 m 

SCALE 

FIGURE 1. - Degasification site. 
A Stream channel 

B Washover 

C Parting 

D Split 

E Concretion 

F Sandbar 

FIGURE 3. - Ge
FIGURE 4. - Electrohydraulic drill. 

Centralizers 

Length- 10 in, 25.4cm 

7 Diam - 3g in, 87cm 

{BQ drill casing 1 rill collar, / I / Bit, 32 in, 

18 ft, 55 m 8.9 cm 

FIGURE 5. - Drilling assembly, standard. 
Centralizer, 

Drill collar, 

BQ drill casing 18 ft, 55m 

Length- I0 - in, 254cm 

FIGURE 6. - Drilling assembly, modified. 

FIGURE 7. - Location and length of holes and ~roiected main entries. 
First, a 20-ft (6.1-m), 4-in (10.2- 

cm) diameter steel pipe was grouted into 

the coalbed. Then, a Bureau-designed 

stuffing box and a 4-in (10.2-cm) fullport valve were attached to the steel 

pipe. The stuffing box separates gas 

frorn water and drill cuttings during 

drilling. Flexible tubing ducts the 

methane frorn the separator to a 4-in 

(10.2-cm) diameter vertical steel pipe in 

which the rear centralizer is removed 

(fig. 6) from the standard assembly was 

used to start holes 1, 2, and 5 (5). - 

Underground Pipeline ---------.------- 

As each hole was completed it was 

connected by 2-in (5.1-cm) steel pipe to 

the vertical steel pipeline in the return 

side of the shaft. The piping arrangethe return side of the shaft. The meth- ment included a valve, a gas-water sepane was discharged into the atmosphere at arator tank, a venturi meter (for making 

the surface. The 30-ft (9.1-m) venting gas flow readings), and a second gasstack, equipped with a flame arrestor, water separator directly beneath the verwas grounded. tical steel pipeline. 

A Sperry-Sun single-shot survey 

instrument was used to measure vertical 

bit inclination every 20 to 40 ft (6.1 to 

12.2 m) during drilling. After each hole 

was completed, its location was determined in the azimuth plane and then 

plotted on mine maps. Figure 7 shows the 

location and length of each hole drilled 

from the shaft. 

Several drilling techniques were 

used during the study to vary the bit 

response. These include changes in 

drilling parameters (thrust and rotational speed) and in the drilling 

assembly. The standard assembly (2), bit 

plus centralizer plus 18-ft (5.5-m7 drill 

collar plus centralizer (fig. 5), was 

used to start holes 3, 4, and 6. A fourwing drag bit with a 10-ft (3.0-m) NW 

drill collar, substituted in the standard 

assembly to drill hole 7, was found to be 

uncontrollable. A modified assembly in 

A methane-monitoring system (6) was 

installed to detect leaks that might 

occur in the pipeline. Sensors were 

positioned at the top and bottom in the 

return side of the shaft. Air-actuated 

shutoff valves were connected to each 

hole and pressurized by a 1-ft3/min 

(0 .028-m3 /min) compressor located at the 

surface. The valves close automatically 

when the methane monitors detect more 

than 1.5 pct methane. 

----- 

Gas Analysis 

Tests conducted in the Pittsburgh 

Coalbed at the Federal No. 2 Mine (3-4) 

-- have shown that coalbed gas can be substituted for natural gas. Table 1 compares the average composition of gas 

liberated from the eight holes drilled at 

the No. 2 air shaft with that of the 

previous studies. 

Table 1. - Comparative gas analysis, pct 

Coalbed 

~ittsbuFgh (multipurpose borehole) 

Pittsburgh [18-ft (5.5-m) diam. 

shaft]....... .................... Beckley (No. 2 air shaft). -- ........ 3.12 96.71 
RELATIONSHIP BETWEEN DATA OBTAINED FROM HORIZONTAL DRILLING 

AND THE GEOLOGY OF THE COALBED 

A contour map of the Beckley Coalbed, based on data obtained from surface 

exploratory diamond drill holes irregularly spaced at 500- to 2,000-ft (152.4- 

to 609.63) intervals, was available 

when drilling started. This generalized 

structure map (fig. 8) shows that the 

No. 2 shaft is located in a synclinal 

trough which trends in the NE-SW direction. The maximum dip of the coalbed 

along the flanks of the syncline is less 

than 2" (0.035 rad). 

Figures 9 and 10 show the hole 

trajectories in the vertical plane for 

the eight holes drilled. A contour map 

was drawn based on these sections by 

evaluating the differences in elevation 

between the collars of the holes and contacts with the roof and floor rock along 

the lengths of the holes (fig. 11). This 

map, which adds refinements to figure 8, 

shows several rolls in the area east of 

the shaft. The trough of the syncline is 

rotated and trends in a N-S direction, in 

contrast to figure 8. Dips northeast of 

the shaft are greater than 2" 

(0.035 rad). 

After the areas around the shaft had 

been mined, a contour map based or1 the 

elevation surveys in these entries was 

constructed (fig. 12). Close agreements 

exist between figures 11 and 12. Figure 13 shows final refinement of geological structure around the shaft. 

PATTERNS OF GAS AND WATER FLOWS 

Gas and water flows from the indi- hole 6. Average water flows ranged from 

vidual holes are shown in figures 14 0.2 gal/min (0.8 l/min) from hole 7 to 

and 15. Average gas flows ranged from 4.5 gal/min (17.1 l/min) from hole 1 

3,400 ft3/day (96.3 m3/day) from hole 2 (table 2). 

to 26,100 ft3/day (739.4 m3/day) from 

Table 2. - Average gas and water flows 

1 Hole I Average gas I Average water 

Hole 

-- 

l................. 

2................. 

3................. 

4................. 

5................. 

6................. 

7................. 

- 8................. 

length 

f t 

910 

710 

660 

660 

490 

460 

418 

545 

m 

277.4 

216.4 

201.2 

201.2 

149.4 

140.2 

127.4 

166.0 

production 

f t3/day 

4,800 

3,400 

19,500 

8,400 

8,200 

26,100 

5,500 

18,200 

production 

m3/day 

136.0 

96.3 

552.4 

238.0 

232.2 

739.4 

155.8 

515.6 

gal/min 

4.5 

2 .O 

1.4 

1.8 

1.4 

1.0 

.2 

.7 

l/min 

17.1 

7.6 

5 .3 

6.8 

5 .3 

3.8 

.8 

2.7 
O Diamond drill hole I- 1-r-i 

k::-: Mine workings I-*-,a 

FIGURE 8. - Contour map of coalbed from vertical boreholes. 

HOMZONTAL DEPTH, m 

,8 20 40 60 80 106 120 140 160 180 

12 - Hole I 

Ilrl'l1lll1lll

0 80 160 240 320 400 480 560 

HORIZONTAL DEPTH, fl 

FIGURE 9. - Vertical sections of horizo

3, and 4. 
HORIZONTAL DEPTH, m 

0 20 40 60 80 100 120 140 160 

121',1,'1,1'1 l~l]l]~~ 

'0 80 160 240 320 400 480 560 

HORIZONTAL DEPTH, ft 

FIGURE 10. - Vertical sections of horizontal boreholes 5, 6, 7, and 8. 
FIGURE 11. - Conto
FIGURE 12. - Co
FIGURE 
Hole 4 

300 1,,,111IIII111111 w 

; 'I 

KEY I :& Me3 v L 

-60 

- Worn ---- L-, 

'b<-,*--e-, , -40 

lA &, - 20 

a 

W 0 'llllllllll-n-' 0 

-20 

---,- J 

TIME. dg 

FIGURE 14. - Gas and water flows, hoies 1, 2, 3, and 4. 

P/ min gal/min lo3 f t3/day k m3/day 

150~ 101 I I I I I I I I I I I I I 140 

KEY Hole 8 

5- Water 

---- '--,----, ---,-- -- 

50 Gas -\ JI Grouted 

I I 1 0 

I I) \ Hde 7 

0 2o I" 

15, I I I I I I I I I I I I I - 60 

Hole 6 

Hole 5 

I I I ! 

-20 1 .8 

.4 

- - ___---------__ 

0 0 

360 400 440 480 520 

TIME, days 

FIGURE 15. - Gas and water flows, holes 5, 6, 7, and 8. 
The coalbed surrounding the shaft 

appears to be saturated with water 

because of the high average water flows 

for all holes except hole 7. Water is 

migrating down the flanks of the syncline 

toward the shaft site (fig. 13). A 

flooded abandoned mine is located about 

5,700 ft (1,737 m) southwest of the shaft 

site, and this mine may be the source of 

some of the water migrating toward the 

shaft. Hole 1, whi.=h was drilled toward 

the abandoned mine, produced water at an 

average rate of 4.5 gal/min (17.1 l/min) 

(table 2). 

Migration of water toward the shaft 

from the east is prevented by a roll in 

the coalbed (fig. 13). Within 400 ft 

(121.9 m) of the shaft in an easterly 

direction, the coalbed rises 23 ft 

(7.0 m) and then rolls over (fig. 16). 

Northeast of the shaft site, water is migrating down the flanks of the syncline 

toward the intake side of the shaft. 

Water production from hole 6 averaged 

1.0 gallmin (3.8 l/min). 

Figure 17 shows total gas and water 

flow rates for all holes. The peak gas 

flow was 136,000 ft3/day (3,853 m3/ 

day), compared with 1,000,000 f t3 /day 

(28,329 m3/day) from a similar study in 

the Pittsburgh Coalbed (4). Cumulative gas and water production after 

514 days was 18,700,000 ft3 (529,745 m3) 

and 5,705,000 gal (21,621,9501), 

respectively. 

Hole 8- ..,# I,. 

I Hole 1 

'NO. 2 shaft 

FIGURE 16. - Vertical section through holes 1 and 8. 

grouted 

grouted 

,/-, 

; '/Water flow -4 , 1 

I, ) 

I ',& - - - r-L- 

- - - 5 holes 

grouted 

R/ 

min 

TIME, days 

FIGURE 17. - Total gas and water flows. 
EFFECTIVENESS OF DEGASIFICATION 

Methane emission measurements were was about 550 ft (167.6 m) from the shaft 

made periodically in the section advanc- (fig. 18). To avoid an explosion hazard 

ing toward the shaft site to determine when a hole is intercepted by mining, 

the effects of degasification on mining. holes in the easterly direction were 

Gas flows from the faces of the section grouted. Gas flows through the section 

declined from 260 to 60 ft3/min (7.4 to faces gradually increased to 150 ft3/min 

1.7 m3/min), or 77 pct, when the section (4.2 m3/min) after grouting. 

DISTANCE BETWEEN COAL FACE AND NO. 2 SHAFT, m 

C .- 

E \ *+ + 

i 

300 - 5 

W 

V) 

LL 

0 

V) 

W 

0 

3 200 - 

z 

E 

LL 

Z 

2 

V) 

V, 

E looW 

z 

a 

x + 

W 

L 

Holes 6,7, and 8 

t - 

drilled 

\ 

Degosificotion line frozen, 

holes shut in 

Holes 6 and 7 i5 E 

q= 

Hole 

grouted 

0 I I I 1 I I I I I I I I 

2,400 2 ,OO 0 1,600 1,200 800 400 0 

DISTANCE BETWEEN COAL FACE AND NO. 2 SHAFT, ft 

FIGURE 18. - Gas flow from section faces. 
SUMMARY 

Although commercial quantities of prevented dewatering of the coalbed, 

gas were not produced, methane emissions and consequently, gas flows from 

during mining were lowered in the section the drainage holes were low. 

advancing toward the shaft. The gas flow Total gas and water producinto the mine increased after the tion was 18,700,000 ft3 (529,745 m3) 

drainage holes were grouted. Water and 5,705,000 gal (21,621,950 l), 

migrating toward the shaft site from the respectively. 

northerly and westerly directions 

REFERENCES 

1. Aul, G. N., and J. Cervik. Grouting Horizontal Drainage Holes in Coalbeds. BuMines RI 8375, 1979, 16 pp. 

2. Cervik, J., H. H. Fields, and 

G. N. Aul. Rotary Drilling Holes in 

Coalbeds for Degasification. BuMines 

RI 8097, 1975, 21 pp. 

3. Fields, H. H., J. Cervik, and 

T. W. Goodman. Degasification and Production of Natural Gas From an Air Shaft 

in the Pittsburgh Coalbed. BuMines 

RI 8173, 1976, 23 pp. 

4. Fields, H. H., J. H. Perry, and 

M. Deul. Commercial-Quality Gas From a 

Multipurpose Borehole Located in the 

Pittsburgh Coalbed. BuMines RI 8025, 

1975, 14 pp. 

5. Finfinger, G. L., L. J. Prosser, 

and J. Cervik. Influence of Coalbed Characteristics and Geology on Methane Drainage. Proc. 1980 SPE/DOE Symp. 

on Unconventional Gas Recovery, 

Pittsburgh, Pa., May 18-21, 1980, 

pp. 319-324. 

6. Irani, M. C., P. W. Jeran, and 

D. H. Lawhead. Methane Analyzer 

System To Record Continuously the 

Methane Content of Coal Mine Ventilation Air. BuMines RI 8009, 1975, 14 pp. 

7. Popp, J. T., and C. M. McCulloch. 

Geological Factors Affecting Methane in 

the Beckley Coalbed. BuMines RI 8137, 

1976, 35 pp. 
APPENDIX Am--GAS PRESSURE TESTS 

A 205-ft (62.5-m) long gas-pressuremonitoring hole was drilled on the 

intake air side of the shaft after 

holes 1 to 5 were drilled on the return 

side (fig. 7). The initial pressure, 

79 psig (544.6 kN/m2) (fig. A-1), f e l l t o 

37 psig (255.1 kN/m2) after holes 6 and 8 

were drilled nearby. 

A gas pressure test was run on 

hole 3 immediately after its completion. 

When the hole was shut-in, the pressure 

increased from 9 to 18.5 psig (62.0 to 

127.5 k~/m~) in 3-112 hr. The test was 

terminated because methane concentration 

in the immediate area of the shaft had 

increased to 1.5 pct. This test indicates that the gas pressure in the pipeline to the surface venting stack will 

build to about 18.5 psig (127.5 k~/m2) if 

the pipeline or stack is blocked by 

debris or frozen water. 

300 320 340 360 380 400 420 440 460 

TIME, days 

FIGURE A-1. - Gas flows versus in situ pressure, holes 6 and 8. 
APPENDIX 8.--MINING THROUGH DEGASIFICATION HOLES 

Mining into a degasification hole 

releases large quantities of gas into the 

cutting head of a miner, and enormous 

amounts of air would be required to 

dilute the methane rapidly to below 

1 pct. Because such large-volume flows 

of air are not available at the interception point, drainage holes are sealed 

with cement before interception by 

mining. 

Procedures for sealing drainage 

holes at the Maple Meadow shaft site were 

similar to sealing procedures used in the 

Pittsburgh Coalbed (1). A 1.5-in 

(3.8 cm) ID, schedule 40 polyvinyl chloride (PVC) pipe was inserted to the backend of each hole. The pipe was coupled 

using a PVC cement and manually pushed 

into the hole. The cement slurry was 

pumped through the PVC pipe to the backend of the hole. When the cement slurry 

returned through the annulus to the 

collar of the hole, the hole was shut-in 

and the slurry was pressurized to 200 to 

300 psig (1,378 to 2,068 k~/m2). The 

slurry consisted of cement, flyash, and 

water (table B-1). 

Table B-1. - Cement slurry design ------- ---- 

Component --- Quantity 

Class A portland cement...... 1 sack (94 lb) (42.6 kg) 

Flyash............ ........... 1 sack (75 lb) (34.0 kg) 

Water........................ 10 gal (37.9 1) 

Friction reducer (CFR-2) l.... 2.5 lb (1.1 kg) 

Teference to specific products does not imply endorsement by the Bureau of Mines. 

Hole 8 was grouted (day 415) when 

the section advancing toward the shaft 

was about 200 ft (61.0 m) from the end of 

hole 8 (fig. 7). This hole was subsequently used to guide the mining machine 

into the shaft. Holes 6 and 7 and the 

gas-pressuremonitoring hole were grouted 

on day 450. Holes 4 and 5 were sealed 

on day 555 and holes 1, 2, and 3 on 

day 644. 

Five of the seven drainage holes 

were intercepted by mining, and methane 

problems developed when hole 1 was intercepted. Water flow from hole 1 immediately before grouting was about 5 gal/min 

(18.9 l/min), which diluted the slurry; 

subsequently water separation from the 

slurry created voids. A mechanical 

packer was placed in the hole to seal a 

small leakage of methane from the hole at 

the interception point. Voids were also 

observed when hole 2 was intercepted by 

mining, but no gas flow was detected from 

the hole. Water flow from hole 2 before 

sealing was about 2 gal/min (7.6 l/min). 

Hole 5 was sealed after the hole 

had been inadvertently intercepted by 

mining. Methane flow from the hole when 

intercepted was about 3.5 ft3/min 

(0.1 m3/min), which caused no methane 

problems. No methane problems developed 

when holes 3, 4, and 8 were intercepted. 

Examination of holes 3 and 8 at the 

interception points indicated the slurry 

filled the holes; no voids were observed. 

Holes 6 and 7 were not intercepted by 

mining. 

Generally, where water flow from 

a drainage hole before grouting is 

greater than 2 gal/min (7.6 l/min), 

excessive dilution of the slurry occurs 

with subsequent formation of voids. When 

intercepted by mining, the excess water 

can be forced from the voids by gas 

pressure, followed by a flow of methane. 

Where water flows are 0.5 gal/min 

(1.9 llmin) or less, holes are 

satisfactorily sealed.