Bureau of Mines Report of Investigations/l979 

Drainage of Methane 

From the Overlying Pocahontas 

No. 4 Coalbed From Workings 

in the Pocahontas No. 3 Coalbed 

UNITED STATES DEPARTMENT OF THE INTERIOR 
UNITED STATES DEPARTMENT OF THE INTERIOR 

Cecil D. Andrus, Secretary 

BUREAU OF MINES 
Finfinger, Gerald I, 

Drainage of methane from the overlying Pocahontas No. 4 

Coalbed from workings in the Pocahontas No. 3 Coalbed / 

by Gerald L- Finfinger and Joseph Cervik. [Washington] : 

U.S. Dept. of the Interior, Bureau of Mines, 1979. 

15 p. . ill., diagrs, ; 27 cm. (Keport of investigations - Brueau 

of hlines ; 8359) 

Bibliography. p. 13. 

1. Coal mines and mining. 7. Uinr ventilation. 3. Xlethanc. I. 

Cervik, Joseph, joint author. 11. United States. Bureau of Mines. 

111. Title. IV. Series: United States. Bureau of hlines. Report of 

investigations - Bureau of Mines, 8359. 

TN23.U7 no. 8359 622.06173 

U.S. IJept. of the Int. Library 
CONTENTS 

Page 

Abstract ................................................................ Introduction ............................................................ Acknowledgments ......................................................... Preliminary preparations ................................................ The study areao ......................................................... Preliminary tests ....................................................... Degasification program. ....................................*..*.e.*.*.. 

Results and discussion .................................................. Conclusions .. ...................................*..................... .. References .............................................................. Appendix.--Construction of mechanical packers ........................... 

ILLUSTRATIONS 

Surface venting facility ......................me.....m............ 

Location of study area ........................................... . Isopach map of interval between Pocahontas No . 3 and No . 4 

coalbeds ........................................................ Diagram of test holes ............................................. Location of tests ................................................. Pressure decline curve-- test 1 .................................... Pressure decline curve-- test 2 .................................... Pressure decline curve-- test 3 .................................... Diagram of degasification system .................................. Mechanical packer. released ....................................... Mechanical packer. compressed ..................................... Daily methane flows from degasification holes ..................... Methane volumes in the returns .................................... Locations of degasification holes in the working section (phases 1 

and 2) .......................................................... Locations of degasification holes in the working section (phases 3 

and 4) .......................................................... Cutaway view of mechanical packer ................................. 

TABLE 

1 . Comparison of gas compositions from the Pocahontas No . 3 and No . 4 

coalbeds ........................................................ 
DRAINAGE OF METHANE FROM THE 0.VERLYING POCAHONTAS NO. 4 

COALBED FROM WORKINGS IN THE POCAHONTAS NO. 3 COALBED 

by 

Gerald L. Finfinger and Joseph Cervik 

ABSTRACT 

The Bureau of Mines conducted an experimental degasification project in 

Island Creek Coal Co.'s Virginia Pocahontas No. 5 mine. Methane from the overlying Pocahontas No. 4 coalbed was entering mine workings in the Pocahontas 

No. 3 coalbed through fractures in the roof rock. Small-diameter (1-5/8-inch) 

holes were drilled into the overlying coalbed to reduce the gas pressure and 

intercept methane flows, which were piped to the surface through an underground 6-inch-diam steel pipeline. 

The total methane flow rate from the vertical holes averaged 150,000 

ft3/day for the first month. Methane flows into the mine were reduced by 

47 pct , thereby demonstrating the effectiveness of this form of degasification 

in reducing ventilation requirements and providing a safer working environment. 

INTRODUCTION 

The Bureau of Mines has been conducting research aimed at removing methane from coalbeds in advance of and during mining. One of the most effective 

methods of degasification of coalbeds has been the drilling of holes into 

coalbeds (1) .3 In the Pittsburgh coalbed, methane drainage from horizontal 

holes reduced methane flows at the working face 70 pct (2-3). - - 

The Virginia Pocahontas No. 5 mine was opened in an area where the 

Pocahontas No. 3 and the'overlying Pocahontas No. 4 coalbeds are separated by 

9 to 13 feet of sandstone. However, the interval between the two coalbeds 

increases in thickness as one moves away from the study area, and the overlying Pocahontas No. 4 coalbed decreases in thickness and in some areas 

pinches out completely. Gas from the Pocahontas No. 4 coalbed migrates downward through fractures in the sandstone and enteres the mine workings in the 

"~u~ervisor~ geophysicist. 

Both authors are with the Pittsburgh Mining and Safety Research Center, 

Bureau of Mines, Pittsburgh, Pa. 

3~nderlined numbers in parentheses refer to items in the list of references at 

the end of this report preceding the appendix. 
underlying Pocahontas No. 3 coalbed. The present report describes a procedure 

used for controlling methane flows from overlying coalbeds, as well as methods 

for experimentally determining the efficient spacing of holes drilled into the 

roof strata. 

ACKNOWLEDGMENTS 

The cooperation of the management of Is land Creek Coal Co. 's Virginia 

Pocahontas No. 5 mine, Grundy, Va., and management of Occidental Research 

Corp. , Keen Mountain, Va. , is greatly appreciated. 

PRELIMINARY PRE PARAT IONS 

Because methane levels in the Virginia Pocahontas No. 5 mine's returns 

were approaching 1 pct , methane from drainage holes could not be discharged 

into the returns. A 6-inch-diam waterline located in the return ventilation 

system and extending from the surface to the work area was converted to a 

methane pipeline. At the surface, the pipeline was extended horizontally 

about 100 feet from the shaft and then vertically 30 feet. The pipeline was 

grounded, and flame arresters were installed on the vent stacks (fig. 1). 

Methane could then be vented to the atmosphere free flow or through an 

exhauster. The exhauster increased flow from drainage holes and maintained 

a slight negative pressure in the pipeline. 

THE STUDY AREA 

The study area consisted of a set of four headings in Pocahontas No. 3 

coalbed advancing into virgin coal (fig. 2). Two weeks after the study began, 

the section was extended southward to intercept the skip shaft. In this area, 

the thickness of the Pocahontas No. 4 coalbed ranged from about 5 to 8 feet, 

6-in free-flow pipe between the two coalbeds 

increased to about 60 feet 

Electric ;r and=U 

turbo blower 

i 

6-in pipeline 

connected to 

shaft 100 ft I 1 I 

away 

FIGURE 1. - 

and the interval between the 

Pocahontas No. 3 and No. 4 

coalbeds ranged from 9 to 

13 feet. The interval 

Surface venting facility. 

as mining advanced southward 

toward the skip shaft 

(fig. 3). 

Shortly after an entry 

is advanced, the sandstone 

roof fractures with a noise 

like a thunderclap, These 

fractures, which may be 

caused by gas pressure in 

the Pocahontas No. 4 coalbed. conduct methane from 

the Pocahontas No. 4 to the 

entries in the Pocahontas 

No, 3 coalbed, 
Study area 7 

,;-7,--,F-7r-, 

I0 I, I ,; L-AL-dL-i; 

, Skip shaft ,;--~r--IT-3r-11 ,;~A~~~~~~L~~I 

(.--+ i;L:i ; 1 c-1: ,<L-:' ; [-+' I.-.' IL-;C-,L-;L ---- I r-1--,r-7r-7r-,: ;~=;~>;;Lj~:{~:,-j, 

~i;~ii=~Supply shaft 

, L-:L-:L-JL-~L-;~ 

r-Tr-7r-,r-7r-*l : L-:L-:L-:~--:L-:~ 0 400 ft , r-,r-,?-,r----J 

LL< &=j ;:;; t--+-i 

0 120 m 

Scale 

\ FIGURE 2. - Location of study area. FIGURE 3. - lsopach map of interval between 

Pocahontas No. 3 and No. 4 coalbeds. 

Numbers indicated distance in feet. 

PRELIMINARY TESTS 

A series of three tests was conducted to determine the number and spacing of holes required to effectively reduce methane emissions through fractures in the roof strata. Each test was conducted using three holes that 

were drilled upward through the Pocahontas No. 4 coalbed (fig. 4), using an 

air-powered stoper. The first 5 feet of each hole was drilled with a 

2-1/4-inch bit, and the remainder of the hole was drilled with a 1-5/8-inch 

bit. 

Each test was conducted in the same way, although the orientation and 

spacing of the holes varied. In each test, three holes were drilled in a 

straight line within 5 feet of the rib. The first 5 feet of each hole was 

packed with a 42-inch-long, inflatable packer that was inflated by hand pump 

with water to a pressure of 600 psig. The holes were shut in for 24 hours to 

stabilize the gas pressure. After 24 hours, the two outside holes were opened 

to flow, and gas pressure at the middle hole was monitored. Methane from each 

hole was piped to the underground pipeline and vented at the surface. The 

objective of this test, called an "interference test," was to determine the 

time required for the drainage radius of the outside holes to reach the center 

hole. When the drainage radius reaches the middle hole, gas pressure begins 

to decline; this is called an interference effect. If interference occurs 

promptly, spacing of holes can be increased, thus reducing the number of holes 

to be drilled. A similar interference test is conducted for each new spacing, 

until a spacing is found that minimizes the number of holes to be drilled and 

for which interference occurs in a reasonable length of time. 
FIGURE 4. - Diagram of test holes. A = inflatable packer; B = packer inflation valves; 

C = outlet for n~easurin~ industrial flow; D = control valves. 
After a satisfactory spacing has been determined, holes are drilled 

upward through the Pocahontas No. 4 coalbed along the left and right return 

entries. Each hole develops an interference effect with the adjacent holes. 

No methane can migrate from virgin coal through the line of holes, and, consequently, flow of methane through fractures into the entries in the Pocahontas No. 3 coalbed declines . 

The holes should be drilled immediately following a cut of coal. In the 

present study, however, the holes were drilled with a stoper, which interfered 

with the miner's production cycle. Theref ore, drainage holes were not drilled 

at the face following a cut of coal. 

In the first test, holes 1, 2, and 3 were oriented north-south and the 

spacing between holes 1 and 3 was 24 feet (fig. 5). The shut-in gas pressures 

in holes 1, 2, and 3 stabilized at 50, 45, and 37 psig, respectively. The 

difference in pressure is probably due to different permeabilities of the 

joints and fractures in the roof strata at each hole. The initial flows from 

holes 1 and 3 were 16,300 and 11,500 ft3/day, respectively. In 30 minutes, 

gas pressure at the center hole (hole 2)declined to 36 psig (fig. 6). After 

5 hours, pressure declined to 31 psig, while the flows from holes 1 and 3 

declined to 15,900 and 10,450 ft3/day, respectively. For the next 19 hours, 

the pressure at hole 2 remained steady at 31 psig, and the flows from holes 

1 and 3 declined to 10,000 and 2,000 ft3/day, respectively. 

In the second test, holes 4, 5, and 6 were oriented east-west (fig. 5) 

and the spacing between holes 4 and 6 was 24 feet. The stabilized gas pressures in holes 4, 5, and 6 and 37, 45, and 50 psig, respectively. Initial 

flows were 15,500 ft3 /day from hole 4, and 27,360 ft3 /day from hole 6. The 

gas pressure in hole 5 declined from 45 to 38 psig in 30 minutes (fig. 7) and 

to 35 psig in 5 hours. Flows from holes 4 and 6 decreased to 14,460 and 

24,500 ft3/day, respectively. 

Tests 1 and 3 The pressure in hole 5 

/ declined further to 18 psig 

FIGURE 5. - Location of tests. 

looool after 7 days, and gas flows 

from holes 4 and 6 declined 

nn~unno~ O~ODODO narlononn oono ooo 

OODO00OO00ODC7Oq 

to 5,400 and 15,000 ft3/day, 

~0000~ The third test was conducted in the same location 

as the first test (fig. 5); 

the distance between the two i outside holes (holes 1 and 7) 

was 48 feet. Hole 3 was the 

gas-pressure monitoring hole. 

The stabilized gas pressures 

in holes 1, 3, and 7 were 

36, 35, and 30 psig, respectively. Initial flows from 

holes 1 and 7were 12,500 and 

6,300 ft3/day, respectively; 
(I: 

a 

FIGURE 6. - Pressure decline curve-test 1. 

I 2 3 4 5 6 7 8 9 10 

HOURS 

I 2 3 4 5 6 7 8 9 10 

HOURS 

FIGURE 7. - Pressure decline curve-test 2. 

W 

(I: 

3 

V) 

V) 

FIGURE 8. - Pressure decline curve-test 3. 

50 

40 

- 

I I I I 1 I I 

- - 

30 

20 

------------ ---------- ------------ - 

I 1 I I 1 1 I I I 

0 I 2 3 4 5 6 7 8 9 10 

HOURS 
FIGURE 9. - Diagram of degasification system. 
Drainage holes were drilled upward with an air-powered stoper to the top 

of the Pocahontas No. 4 coalbed. Holes were drilled with a 1-5/8-inch bit and 

sealed with mechanical packers fabricated by the mine's machine shop 

(appendix A). These packers are relatively inexpensive, easily constructed, 

and simpler to use than inflatable packers (figs. 10-11). 

Holes were connected to the main underground pipeline with plastic pipe. 

Holes 8 through 17 required a gas-water separator. Initial waterflow from 

these holes was 3 gal/hr and declined to about 0.1 gal/hr within 24 hours. 

Holes 18 through 37 produced no water and were connected directly to the pipeline. Gases from both the Pocahontas No. 3 and No. 4 coalbeds contain a high 

percentage of methane (table 1). 

TABLE 1. - Comparison of gas compositions from the Pocahontas 

No. 3 and No. 4 coalbeds 

Ethane (C2 I& ) ........... Carbon dioxide (C% ) .... Oxygen (Oz). ............ Nitrogen (N,). .......... Methane (CH, ) ........... 

Composition, pct 

Pocahontas No. 3 

1.3 

.26 

.18 

1 .O 

97.2 

Pocahontas No. 4 

0.14 

1.6 

.12 

.46 

97.7 
0 10 20 30 40 50 60 70 80 90 100 

TIME, days 

FIGURE 12. - Daily methane flows from degasification holes. 

Figure 2 shows the location of the underground, 6-inch-diam, steel 

pipeline. The length of the underground segment is 1,700 feet, followed 

by a segment of about 1,600 feet rising vertically in the return air shaft. 

When methane flow through the pipeline was 150,000 ft3/day, gas pressure in 

the pipeline near the working face was -5.2 inches water gage. Negative 

pressure is due to buoyancy of methane as it rises in the vertical section 

of the pipeline. However, as the section advanced, the pipeline was extended 

horizontally 500 feet with 2-inch-diam plastic pipe. Because of frictional 

resistance to flow in the plastic line, gas pressure in the line became 

positive and some methane began flowing through fractures in the roof strata. 

An exhaust fan, installed on the pipeline at the surface, eliminated the 

positive pressure underground and increased the flow of methane from the 

drainage holes. 

Daily flow measurements were made at the end of the 6-inch pipeline on 

the surface (fig. 12) using an anemometer and converting to a volume flow. 

Laboratory tests with air flowing through a 6-inch pipe showed that the 

velocity measured with an anemometer at the end of the pipe is about 24 pct 

higher than the true average velocity, which was determined with a venturi. 

Because the anemometer is calibrated for air measurements, the pipeline flow 

measurements were also corrected for density of methane (4). Over 12 million 

cubic feet of methane was drained from the Pocahontas NO. coalbed over 

96 days. 

RESULTS AND DISCUSSION 

Methane surveys were conducted in the section before the drilling and 

drainage phases started for use as a comparison basis. Air velocities were 

measured with anemometers. Methane levels were determined in the laboratory 

from bottle samples. Stations 1 and 2 are located in the immediate returns 

from the working faces, and stations 3 and 4 are located in returns about 
1,000 feet outby the working faces (fig. 2). Because a split system of 

ventilation is employed on the section, the methane flow from the faces of the 

section and the methane entering the intake air entries through fractures and 

joints in the roof strata are determined by adding the methane flows measured 

at stations 1 and 2. The total methane flow from roof strata, working faces, 

and the two outside ribs is determined by adding the methane flows at 

stations 3 and 4. 

The effects of the roof hole drilling and drainage program on methane 

flows in the returns are shown in figure 13, which is divided into four phases. 

Curves A and B show methane levels in the return at the inby (stations 1 plus 

2) and outby (stations 3 plus 4) locations, respectively. Base level of 

methane in the immediate return was 1,700 ft3/min and about 2,350 ft3/min at 

the outby station (phase 1). During phase 1, holes were drilled into the roof 

strata; however, these holes were shut in (fig. 14, phase 1). 

During phase 2, the 23 holes drilled and shut in during phase 1 were 

connected to the underground pipeline and opened. Five additional holes were 

drilled during the 22-day period (fig. 14 phase 2). Curves A and B (fig. 13) 

show a steady decline in methane levels in the returns over 22 days; curve A 

shows a 47-pct reduction and curve B shows a 43-pct reduction in methane flows. 

Phase 3 corresponds to a 5-day period during which no degasification 

holes were drilled although the section was advanced (fig. 15, phase 3). 

Both curves A and B (fig. 13) show an increase of methane levels in the 

returns . 2,600~ 1 I I III~III~III~III~III~I 1111 

Ill' 

- 

I I - 

- 

I 

I- - W 

z 

1,000 - I 

- I - 

,--Phasel- - Phase 2 -.-Phase 3 ---Phase4 + 

600 'I1 III~III~I~I~IIIIIIIII 1111 1111 

0 4 8 12 16 20 24 28 32 36 

TIME, days 

FIGURE 13. - Methane volume in the returns. 
Phase I 

(Days 1-41 

KEY 

0 Holes drilled into 

Poco hontas No.4 

coalbed Phase 2 

(Days 4-26) 

FIGURE 14. - Locations of degosification holes in the working section (~hases 1 and 2). 

Phase 3 

(Days 26-31) 

KEY I 

o Holes drilled into 

Pocahontas No. 4 Phase 4 

coalbed (Days 31 -96) 

FIGURE 15. - Locations of degasification holes in the working section (phases 3 and 4). 

Phase 4 is a 3-day period during which six degasification holes (fig. 14, 

phase 4) were drilled. The effect of these holes was an abrupt decrease of 

methane levels in the returns (fig. 13). 
CONCLUSIONS 

This investigation showed that roof hole drilling and drainage is an 

effective method of controlling methane flows from overlying coalbeds. 

Methane levels were reduced significantly (47 pct) in the immediate returns 

from the section in the underlying mine and, more importantly, intake-air 

methane levels were also reduced. Generally, safety conditions were improved 

throughout the mine. In addition, the mine's existing ventilation system was 

extended and a costly shutdown of the mine was avoided. 

This method of methane control is applicable to any gas-bearing stratum 

that discharges methane into an underlying mine opening. Holes are drilled 

through the gas-bearing stratum, and packers are set immediately below the 

stratum. These packers seal against the wall of the hole and conduct the gas 

to the underlying mine opening where it is discharged into an underground 

pipeline and piped to the surface. 

Gas pressure in overlying roof strata is suspected to be the cause of 

roof instability in some cases, but documented case studies could not be found 

in the published literature. The technique presented in this study can be 

used to reduce gas pressure and perhaps alleviate roof instability. 

Strata overlying coalbeds may contain methane under pressure in the pore 

system. If the pore system is not interconnected, the strata lack permeability and, consequently, roof drilling and drainage will not be effective. 
REFERENCE S 

1. Cervik, J., H. H. Fields, and G. N. Aul. Rotary Drilling Holes in Coalbeds for Degasification. BuMines RI 8097, 1975, 21 pp, 

2. Deul, M. and J. Cervik. Methane Drainage in the Pittsburgh Coalbed. 

Proc. 17th Internat. Conf. of Mining Safety Research, Varna, Bulgaria, 

Oct. 3-7, 1977. 1977, pp. 9-15. 

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. her, E , The Measurement of Air Flow. Chapman and Hall Ltd. , London, 

3d ed., 1949, pp. 146-184. 
ppp

() 

1 Standard black pipe bushing (1/2 inch by 1 inch welded to 1/2-inch pipe) 

1 Nut 

,./ 

Flat steel washers. 5/8 in ID, In OD (6) 

,Standard black pipe, I in ID /' 

.- 

'k2- in-10 standard black pipe /' 

with mach~ne threads and nut coupler 

Pipe bushing 02 In by 1 in) brazed 

to &-in pipe 

FIGURE A-1. - Cutaway view of mechanical packer. 
The packer is constructed as follows (fig. A-1) : 

1. Cut approximately 4 inches of standard bolt thread on one end of the 

1/2-inch pipe and 2 inches of pipe thread on the other end. Screw 

the matching nut onto the bolt threaded end. 

2. Weld or braze the pipe bushing on the bolt threaded end of the 

1/2-inch pipe. 

3. Slip a washer over the other end of the 1/2-inch pipe. 

4. Slip the 1-inch pipe over the 112-inch pipe. 

5. Alternately add washers and rubber seals to the 1/2-inch pipe until 

the pipe threads are reached. 

6. Screw the pipe coupler onto the 1/2-inch pipe. 

The packer can now be expanded by tightening the nut which cumpresses the 

rubber seals. 

*US. GOVERNMENT PRINTING OFFICE: 1979-603-002~23 INT.-BU.OF MINES, PGH.,PA. 23730