By Steven J. Schatzel, Gerald L. Finfinger, 

and Joseph Cervik 

UNITED STATES DEPARTMENT OF THE INTERIOR 
Report of Investigations 8644 

Underground Gob Gas Drainage 

During Longwall Mining 

By Steven J. Schatzel, Gerald L. Finfinger, 

and Joseph Cervik 

UNITED STATES DEPARTMENT OF THE INTERIOR 

James G. Watt, Secretary 

BUREAU OF MINES 

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

Sc hatzel, Steven J 

Underground gob gas drainage during longwall mining. 

(Keport of investigations / United ,States Department of the Interior, Bureau of Mines ; 8644) 

Includes bibliographical references. 

Supt. of Docs. no.: I 28.23:8644. 

1. Coal Mines and mining-Safety measures. 2. Mine gases. 

3. Methane. I. Finfinger, Gerald L. 11. Cervik, Joseph. 111. Title. 

IV. Series: Report of investigations (Ilnited States. Bureau of 

Mines) ; 8644. 

TN23 .U43 [ TN2951 622s r622' -81 81-607572 
CONTENTS 

Page 

Abstract ....................ee..e..e...ee.............e......e................. 1 

Introduction .................ee..e........e.........e..e.......e............... 1 

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

Local geology .................................................................. 2 

Study area .................................e........................e.......... 3 

Drilling equipment and procedure .............e.............e......e............ 4 

Methane monitoring and safety system .....................e.e...e....e........e. 7 

Gas and water flows .................e....e...e........................e........ 8 

Cumulative flow data ..................e...e............................... 13 

Conclusion .............e...e................................................... 14 

ILLU STRATIONS 

Generalized stratigraphic column of Freeport and Kittanning Formations .... 1 

Bethlehem Mines Corp.'s Cambria 33 Mine (C' Coalbed) in Ebensburg. Pa ..... 3 

Study panel. cross-measure boreholes. underground pipeline. and vertical 

boreholes ................................e.................ee.....e...ee 4 

Post-mounted. electrohydraulic drill ..........................ee..e...e... 5 

Plastic standpipe with borehole seal^..............^..^.............^..... 6 

Underground data acquisition pipeline ..................................... 6 

Monitoring and safety system .............................................. 7 

Bureau of Mines vertical borehole and surface sight ..................em..e 8 

Methane production for day 16 .....................e.......e........e...... 9 

Methane production for day 17 ...........................ee.e.............. 10 

Methane production for day 22 ............................................. 11 

Methane production during a continuous 40-hour test on days 29 and 30 ..... 12 

Methane concentration during a continuous 40-hour test on days 29 and 30 .. 12 

Water production for hole 1 ............................e.................. 13 

Water production for hole 3 ..................................e............ 14 

TABLES 

1 . Hole parameters .................e.................e.........e.......e..... 

2 . Cumulative methane and water flow^................................^....... 
UNDERGROUND GOB GAS DRAINAGE DURING LONGWALL MINING 

by 

Steven J. Schatzel, 1 Gerald L. Finfinger(2 and Joseph Cervik3 

ABSTRACT 

Gas drainage through surface bnre- vactirlm on a pipeline parelleling the 

holes has been the conventional means of panel and draws the methane mixture out 

methane control for U.S. longwall gobs. of the mine. 

However, these vertical boreholes are 

becoming so costly, and the surface Auxiliary systems of gob gas drainrights so difficult to obtain, that the age during longwall mining will be essenBureau of Mines is developing underground tial for an increasing number of coal 

gob gas drainage as an alternate means of mines. Deep and gassy mines often find 

methane control for U.S. longwalls. ventilation insufficient for adequate 

Holes are drilled into the roof over the dilution of methane in bleeder entries. 

panel and on retreating longwalls, Although this cross-measure method of 

towards the working face from a location degasification has been used successfully 

inby the face. As the longwall retreats, in Europe, some of the European techan increasing portion of the hole inter- niques cannot be directly applied to U.S. 

cepts the fracture system over the caved mines. This is the first study of its 

gob. A surface exhauster maintains a kind in this country. 

INTRODUCTION 

Conventional U.S. gob gas control entries, the Bureau is exploring gob gas 

consists in ventilation of gob areas and drainage from underground locations as an 

drainage through surface boreholes. How- alternate method of methane control of 

ever, the mining of deeper coalbeds and u.'s. longwalls. 

the scarcity of suitable surface locations has increased the cost of surface Underground drainage of gob gas has 

boreholes. Irregular topography and the been used extensively in Europe, where 

inavailability of surface rights can coal basins tend to be deeper than those 

force some mines to rely on the normal in the United ~tates.4 Generally, Euroventilation system as the only means of 

reducing methane concentrations. However, since this may not be sufficient to 

adequately dilute methane in bleeder 

2~upervisory geologist. 

3~upervisory geophysicist. 

All authors are with the Pittsburgh 

Research Center, Bureau of Mines, 

Pittsburgh, Pa. 

pean coalbeds are steeply dipping with a 

high degree of tectonic disturbance. 

Outbursts of coal, methane, and rock are 

a serious problem. The first large-scale 

underground methane drainage and piping 

4~ervik, J. Methane Control on 

Longwalls--European and U.S. Practices. SME-AIME Ann. Meeting, New 

Orleans, La., Feb. 18-22, 1979; SMEPreprint COAL 79-06, 10 pp. 
Institute of Poland and the following 

personnel from Bethlehem Mines Corp.'s Lower Kittanning Coalbed (0) Cambria Division and Cambria 33 Mine: 

E. J. Korber, Manager; F. A. Burns, Gen- rlon era1 Superintendent; Dan Weaver, Mining 

Engineer; and Bill Radabaugh, Mine FIGURE 1. - Generalized stratigraphic columnof 

Foreman. Free port and Kittonn ing Formations. 

LOCAL GEOLOGY 

The Kittanning and Freeport Formations of Pennsylvanian age were the subject of this study (fig. 1). Deposition 

of these formations is thought to have 

taken place in a deltaic environment. 

The low accumulation rate of detrital 

sediment allowed vast swamp marshes to 

'~atuszewki, J., and W. Sikore. Report 

on the Technology of Degasification of 

Mines in Poland and Other European 

Countries. BuMines Project 14- 

01-0001-1447, January 1978, 117 pp.; 

available for consultation at Pittsburgh Research Center, Bureau of 

Mines, Pittsburgh, Pa. 

develop.6 The complexity of the depositional environment produced lithologic 

gradations and discontinuities. The 

Lower Kittanning Coalbed is generally 

thicker than the Upper Kittanning, but 

both are minable. Be thlehen Mines 

Corp. 's Cambria 33 Mine operates in both 

coalbeds simultaneously. On the study 

panel, the Upper Kittanning (C' Coalbed) 

varies from 29 to 45 in (74 to 114 cm) in 

6~uglio, D. G., and A. T. Iannacchione. 

Geology, Mining and Methane Content of 

the Freeport and Kittanning Coalbeds 

in Indiana and Surrounding Counties, 

Pa. BuMines RI 8406, 1979, pp. 20-25. 
FIGURE 2. - Bethlehem Mines Corp.'s Cambria 33 Mine (C'Coolbed) in Ebensburg, Pa. 
made by hole and the long axis of the 

longwall). All holes were drilled 

towards the face from a location inby the 

face. 

With holes oriented in this manner, 

the duration and volume of gob gas flows 

can be maximized. The fracture system 

first encounters the back of each hole. 

As the longwall retreats an increasing 

portion of the hole intercepts the fracture system, producing increased flows. 

With all other conditions remaining constant, relatively long holes increase the 

surface area for methane capture. Acute 

azimuth angles increase the component of 

the hole paralleling the axis of the 

longwall, thus increasing the duration of 

flow with respect to the progression of 

the working face. Also, acute azimuth 

angles mean that a large portion of hole 

is used in traversing the entries 

adjacent to the longwall where mine air 

general, hole trajectories are designed 

to pass over the panel at a minimum of 

five times the height of the coalbed. 

All holes in this study achieved enough 

height over the panel t o a t least penetrate the Lower Freeport horizon. 

Since surveying was not incorporated 

into the drilling cycle, true hole parameters were only approximated until a survey could be done once terminal length 

had been reached. Hole 1 maintained the 

20" (0.35 rad) inclination at the collar; 

hole 2 dropped below the initial 12" 

(0.21 rad) inclination; and hole 3, 

started at 22" (0.38 rad) above the horizontal, turned upward (table 1). Hole 3 

parameters were selected to compare U.S. 

and European coalbed characteristics with 

respect to abutment pressures and gas 

flows. Holes 1 and 3 intercepted a 

water-bearing horizon, the Mahoning 

Sandstone (fig. 1). 
FIGURE 4. - Post-mounted, ele~troh~droulic drill. 
Behind the core barrel, a combination of 

BX and EW flush joint casing in 5-ft 

(1.53) lengths made up the drill 

string. 

Normally, a core retriever with 

attached wire line must be inserted in 

the drill rod and driven by water pressure to the rear of the core barrel after 

each 4-f t (1.2711) interval drilled. However, with careful application of thrust, 

40 ft (12 m) could be drilled as portions 

of the core would break off and slip down 

the rods. When, eventually, the core 

became lodged against the inner walls of 

the drill rod or when the weight of the 

core interfered with drilling, the crew 

pulled tools and removed the core. Using 

this technique and with two shifts of 

drilling per day, each hole was completed 

in no more than 13 days. 

P' 

FIGURE 5. - Plastic standpipe with borehole seals. 

,Fitting for 

methanometer Steel-reinforced sensing head 

,suction hose -Vacuum tao 

bwatw separator 

.Cway check 

valve 

Main 

methane 

pi pel ine 

FIGURE 6. - Underground data acquisition pipeline. 
FIGURE 7. - Monitoring and safety system. 
FIGURE 8. - Bureau of Mines vertical borehole and surface sight. 
and fabricated by Bureau personnel. Water flows of 3 gpm (11.3 l/min) were measured on hole l during this period. The 

pipeline was drained, and the water separators were installed on holes 1 and 2. 

Day 16 was the first day of gas production on hole 2. Only hole 2 was on 

stream during this test with borehole 33- 

12 operational. Cross-measure borehole 2, surface borehole 33-12, the 

Bureau's surface borehole, and the evaluation point were monitored (fig. 3). The 

relatively low volume of methane 

the test to 0.4 in (1.0 cm) Hg near the 

end of the test. 

On day 17, holes 1 and 2 were on 

stream and borehole 33-12 was shut-in. 

The lowest volume of methane at the evaluation point corresponds to the peak 

methane production from the Bureau's surface borehole (fig. 10). Under a negative pressure of 2.5 in (6.4 cm) Hg, 

100 x 103 cfd (2.8 x 103 m3/day) of 

100 pct methane were produced on day 17. 

Methane concentrations were no problem, 

although borehole 33-12 was shut-in. 

extracted during this test does not conclusively establish the effectiveness of With boreholes 33-12 shut-in and 

underground gob gas drainage during long- holes 1 and 2 producing gas on day 22, 

wall mining (fig. 9). The negative pres- the bleeders again were essentially free 

sure at hole 2 was gradually changed from of methane. Concentrations of methane at 

3.0 in (7.6 cm) Hg at the beginning of the evaluation point never exceeded 

0 I 2 3 4 5 

TI ME, hours 

FIGURE 9. - Methane production for day 16. 

160 

140 

120 

100 

80 

60 

- I I I I - 

- 

- 

- U - 

- Borehole 33-12 data 

- 

- 

- 

- 

- 

- - 

- 

- - 

- 

- 

- 

USBM borehole data 

- 

m 

Evaluation point data 
TI ME, hours 

FIGURE 10. - Methane production for day 17. 

0.2 pct anywhere in the mouth of the 

entry. Note the correspondence between 

the peak production measured on the surface at 1.75 hours and the lowest methane volume underground at 1.85 hours 

(fig. 11). 

The gas being removed by the holes 

drilled into the roof is either derived 

from or added to the gas released from 

the caved area. The holes communicate 

with gob gas through the gob fracture 

system, but the holes also penetrate 

methane-bearing horizons. Before turning 

off the Bureau's borehole exhauster at 

6.25 hours, the 33-12 borehole exhauster 

was restarted corresponding to a reduced 

amount of methane at the evaluation 

point. 

A continuous 40-hour test was begun 

at 8:00 a.m. on day 29. Again, borehole 33-12 was shut-in and holes 1 

and 2 were on stream (fig. 12). The partial vacuum at each hole was held 

constant at about 2.6 in (6.6 cm) Hg. 

Methane at each hole underground and at 

the surface did not change appreciably 

(fig. 13). During test hour 28, holes 1 

and 2 were shut-in underground to drain 

water from the main pipeline. With 

borehole 33-12 shut-in, the volume of 

methane at the evaluation point rose 

steadily until hour 30 of the test, when 

flow was restored. It can be seen from 

figure 13 that there was little change in 

concentration during the nonproducing 

period. The temporary shut-in of the 

holes underground was unsuccessful at 

removing water and increasing pipeline 

flow. Therefore, the fluctuations in 

methane production at the surface and 

methane volume underground between 

hours 28 and 33 are a function of changes 

in the volume of the gas reservoir near 

the gob. 
I I I I - 

L 

- - 

- 

USBM borehole data 

- - 

- 

- 

- 

- 

- 

- Evaluation point data 

- 

- 

- 

w I I I I I L 

0 I 2 3 4 5 6 

TIME, hours 

FIGURE 11. - Methane product ion for day 22. 

Interference tests were conducted to gob into hole 2 and through the main 

quantify communication between holes. No pipeline. Sampling was conducted at the 

discernible diversion of methane flow surface site and at borehole 33-12 for 

could be made. Also, sulfur hexafluoride 3 hours following the release of the 

(SF6) tracer gas was released into bore- tracer. Although SF6 is detectable in 

hole 1. While liberating the gas, hole 2 concentrations of 1 ppb, no tracer was 

was on stream at full vacuum in an found in any of the gas bottle samples. 

attempt to pull the tracer through the 
- 

- 

- 

100 - USBM borehole data 

s 

0 - * - 

IQ 

0 80- 

* 

W 

Z Evaluation 20 $- 

Q: point data a 5 I 

Is I5 9 

40 

10 

TI ME, hours 

20 

FIGURE 12. - Methane product ion during a continuous 40-hour test on days 29 and 30. 

0 2 4 6 8 10 12 14 16 18 20 22'24 26 28 30 32 34 36 38 40 

- 

FIGURE 13. - Methane concentration during a continuous 40-hour test on days 29 and 30. 

- Day 29 

90 

80- 

70 

60- 8 

=m- Day 30 

~I~III~III~I~IIIIIIIIIIIIIIIIIII~III

% 00 q 

m 0 Do 0 000 

8 8 o 00 000 00 0 0 Oo0o 0 00 

- 0 0 0 0 - A A eHole I underground measurements - 

AAAAAA AAAAA AA~A~AAAAOAA A 

A 

\surfme data AAAAAAA- 

- 5 

- z 

- 

lI1IIIIII111III 

a 

+ I 40 

W 

30- 

20 

Illl.llllllllIllI11IIl I1 

CQ 0 

- o 0 

- 

- - - Day 29 

10 - Day 30 - 

11111lrl1l1l1l1 rl~l~l~l~l~l~l~l

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 3032 3 36 38 40 

TIME, hows 
Cumulative Flow Data hole 3 showed a trace of water too small 

to be accurately measured, less than 

Table 2 shows the total volume of 0.5 qt (0.47 1) per 5 minutes. Flows 

methane removed from each hole during from holes 1 and 3 were comparable 

this study. Hole 3 produced gas on one between days 85 and 90, after 

day only (day 29). which hole 3 flows dropped steadily. 

Cumulative water flows are given in 

Following the initial surge of table 2. Hole 2 produced no water 

12 qt/min (11.32 l/min), water flow from because it did not penetrate the Mahoning 

hole 1 remained fairly consistent at 4 to Sandstone. 

6qt/min (figs. 14and 15). On day36, 

TABLE 2. - Cumulative methane and water flows 

Hole 1 ....... Hole 2.. . . . . . Hole 3 ....... 

12 

II 

10 

. 

c - 

9 

E 

$8- 

g7- 

g 6 

5 

4 

3~ 

I 1 I I 1 I 1 I 1 -3 

10 20 30 40 50 60 70 80 90 100 

TIME, days 

FIGURE 14. - Water production for hole 1. 

Cumulative time 

of production, 

- hours 

69.33 

114.8 

1 

Average methane 

concentration, 

pc t 

7 9 

44.5 

66 

Cumulative methane 

production 

II 

10 

-9 

c 

=+ 

i 

W 

5 

4 

A 

ft3 

274,450 

311,850 

1,700 

v 

- 

- 

- 

m3 

7,766 

8,825 

48.1 

Cumulative ~20- 

production 

I I I I 1 I I I - 

KEY 

Flow under vacuum - 

o Flow at atmospheric 

pressure 

gal 

180,100 

0 

36,600 

-8 

-7 

-6 - B 

-5 

- 0 0 

U - 

- 0 

R 

682,000 

0 

138,500 
KEY 

o Flow at atmospheric 

pressure 

5 - 

. 

t - 

E 

\ 4- 

5 

$3- 

s 2- 

I - 

1 1 CL 1 

0 I0 20 30 40 50 - 60 70 

TI ME, days 

FIGURE 15. - Water production for hole 3. 

CONCLUSIONS 

Underground gob gas drainage during 

longwall mining can make invaluable contributions to the U.S. coal mining industry. Future studies may benefit from 

information that has been presented here. 

The authors stress that no methane was 

encountered during drilling; little or no 

methane was found until the gob fracture 

system intercepted the cross-measure 

boreholes, and gas was available from the 

cross-measure boreholes only when 

extracted under a negative pressure. Gas 

production from holes 1 and 2 was more 

than sufficient to maintain safe methane 

concentrations in bleeder entries while 

the conventional means of gob gas drainage was not in use and during longwall 

mining. Communication between crossmeasure boreholes could not be induced 

with the chosen hole spacing. The concentration of the gas drained was essentially contant over a 40-hour period. A 

cost analysis will be included in future 

studies in this field. 

lNT.-BU.0 F MlN ES,PGH.,P A. 25960