UNITED STATES DEPARTMENT OF THE INTERIOR 
I INII'EII STATES IIEPARTMENT OF THE INTERIOR 

<;t.cil D. Andrus, Secretary 

F3I!KI.:AU OF MINES 
This publication has been cataloged as follows: 

. 

Maksimovic, S D 

Hydraulic stimulation of a surface borehole for gob degasification / by S. D. Maksimovic, C. H. Elder, and Fred 

N. Kissell. [Washington] : Bureau of Mines, 1977. 

17 p. : ill., maps, diagrams ; 26 cm. (Report of investigations - 

Bureau of Mines ; 8228) 

Bibliography: p. 17. 

1. Mine ventilation. 2. Methane. 3. Coal mines and mining - 

Safety measures, 4. Mine gases, I. Elder, Curtis H,, joint 

author. 11. Kissell, Fred N., joint author. 111. United States. 

Bureau of Mines. IV. Title. V, Series: United States. Bureau 

of Mines. Report of investigations - Bureau of Mines ; 8228. 

TN23.U7 no. 8228 622.06173 

U.S. Dept. of the Int, Library 
CONTENTS 

Page 

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

Introduction ............................................................. 1 

The study area ........................................................... 2 

Mining plan and equipment ........................................... 2 

Stratigraphy ........................................................ 3 

Ventilation ...................................................... 5 

Underground monitoring ................................................... 5 

Vertical gob-degasification boreholes ................................... 5 

Stimulated borehole 37 ............................................. 5 

Unstimulated boreholes 38 and 39 .................................... 8 

Results .................................................................. 8 

Borehole 37 ......................................................... 8 

Underground methane emission ........................................ 9 

Boreholes 38 and 39 ................................................. 9 

Conclusions .............................................................. 15 

Recommendations ......................................................... 16 

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

I LLUSTRAT IONS 

Map of the study area ............................................... 3 

Ventilation map with survey stations ................................ 4 

Detail of hydraulically stimulated borehole 37 completion ........... 6 

Lithology, gamma ray, porosity, and density logs of the strata 

overlying the Lower Kittanning coalbed ............................ 7 

Detail of borehole 38 completion ................................... 8 

Effluent methane concentrations for boreholes 37, 38, and 39 ........ 10 

Methane flow from section and from boreholes, and coal production 

from section ...................................................... 11 

Cumulative volume of methane from borehole 37 ....................... 12 

Cumulative volume of methane from boreholes 38 and 39 ............... 13 

Cumulative volume of methane from section and from boreholes 37, 

38, and 39 ........................................................ 14 
HYDRAULIC STIMULATION OF A SlJRFACE BOREHOLE 

FOR GOB DEGASlFlCATlON 

by 

S8 D8 Maksimovic,' C8 ti8 Elder,2 and Fred Na Kissel13 

ABSTRACT 

The Bureau of Mines evaluated the hydraulic stimulation of a gobdegasification borehole to determine if this procedure would aid in degasifying a longwall panel. The stimulated borehole did not degasify the longwall 

panel as expected either before or after mining of the panel started. It 

reduced the underground methane emission only 11 pct, considerably less than 

the reductions effected by two unstimulated boreholes in the same panel. 

Some probable reasons for the reduced methane flow from the hydraulically 

stimulated borehole are very low reservoir gas pressure, location of hydraulically stimulated zones within the grouted casing, a casing break caused by 

lateral shifting, possible vertical movement of rock strata during caving, 

grouting of a 4-inch pipe blocking three stimulated zones, plasticity of 

shales, and possible hole damage by swelling of water-sensitive minerals. The 

borehole was also located between two second-mined areas, which may have 

allowed partial degasification of the overburden and the coalbed in advance of 

mining. Finally, a period of 3 months is probably not long enough for the 

borehole to degasify the overburden before mining. 

INTRODUCTION 

Methane is stored under pressure in the micropores and fractures of coalbeds and is also present in adjacent strata. The rate of methane emission 

during coal mining may vary substantially from one mine to another. In some 

very gassy mines, a single section may produce several million cubic feet per 

day of methane (7).4 This creates problems even with the best conventional 

ventilation system that can be designed, particularly in longwall mining where 

the superjacent strata are caved as mining progresses. When caved, these 

l~inin~ engineer. 

2~eologist. 

3~hysical research scientist. 

All authors are with the Pittsburgh Mining and Safety Research Center, Pittsburgh, Pa. 

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

the end of this report. 
strata emit methane in addition to that already released by the mined coal. 

This additional gas cannot always be removed easily by a conventional mineventilation system. Additional shaft installations and airway maintenance to 

provide greater amounts of air for methane dilution are very costly. 

To aid the conventional ventilation system of some gassy mines, surface 

gob-degasification boreholes are drilled into the overburden in advance of 

mining. When the longwall face passes the holes, roof subsidence causes 

cracks in the overburden, and methane accumulating in these cracks flows to 

the borehole rather than to the mines. 

Gob degasification through surface boreholes has proved to be a safe and 

effective method of removing large quantities of methane from the Lower 

Kittanning (B) coalbed (12) and from the Pocahontas No. 3 coalbed (13). It 

was found that some surface boreholes exhausted up to 80 pct of theethane 

emitted from the pillar area (8). However, other holes were not as successful, 

so the average methane liberation was only 45 pct. 

Hydraulic stimulation is a conventional technique used to improve the 

productivity of gas and oil wells. More recently, hydraulic stimulation of 

coalbeds has yielded increases in gas flow from fivefold to twentyfold (4-5). - - It seems reasonable to assume that the productivity of gob-degasification 

boreholes might also be improved by hydraulic stimulations. 

The Bureau of Mines evaluated one such hydraulic stimulation conducted by 

a mining company to improve the productivity of their gob-degasification boreholes. The emission from the stimulated borehole was compared with emissions 

from two unstimulated boreholes in the same panel. Underground methane emission was also measured. 

THE STUDY AREA 

Mining Plan and Equipment 

The study area was located in Cambria County, Pa., in the Lower Kittanning coalbed. The block of coal, mined by retreat longwall system, was 570 

feet wide, 5,150 feet long, and 52 inches thick. The block was circumscribed 

by a set of three panel entries (5 Left and 6 Left) on each side, and E West 

and F West Mains on each end. The longwall panel was the fifth to be mined 

between E West and F West Mains. The three entries of 5 Left and 6 Left panel 

were mined on 80-foot centers. All entries and cross entries were approximately 18 feet wide. Figure 1 shows the location of the study area, the portion of the longwall panel mined during the study period, and the location of 

boreholes 37, 38, and 39. The roof was supported by units of four 40-toncapacity props mounted on a rigid base and topped by shielded steel beams. 

A single-drum, bidirectional shearer was used to mine 52 inches of the 

coalbed. The mined coal moves by a face chain conveyor and is discharged at 

the head end onto a chain conveyor in the closest 5 Left entry. From there, 

the coal is discharged onto a rope-frame, 42-inch belt conveyor that dumps it 

into mine cars. One pass of the shearer across the face produced 
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LEGEND 

. No 3ENTRII 

I$ # 

- . No2 ENTRYPermanent stopp~ng - Intake alr Stopping w~th mandoor - Return alr Regulatw X - Overcast 

Check curtaln 2. Air survey sta

- 1 No 3 ENTRY 

Scale, feet 

Study 

, D~rect~on of 

lonaw0lI retre

FIGURE 2; - Ventila
is about 500 feet thick. It consists mostly of carbonaceous shales, sandy 

shales, a few coalbeds, and some thin argillaceous limestone. The immediate 

roof strata are bone, coal, and 3 to 10 feet of dark to dark gray shale. The 

immediate 2 feet of floor is clay with coal partings. When the Lower Kittanning coalbed is mined, most of the methane comes from the fractured overburden. 

The amount of methane emitted directly from the coalbed into the mine working 

is relatively small (10). Methane is known to be present in the Upper Freeport (E) , Lower Freeport (D) , Upper Kittanning (C '), and Middle Kittanning (C) 

coalbeds, all of which overlie the mined Lower Kittanning (B) coalbed (3). - 

Ventilation 

The study area was ventilated by two air splits (fig. 2). The primary 

intake air of about 29,000 ft3/min from No. 1 entry of 6 Right panel and 

No. 2 entry of 6 Left panel was conducted from the tail end (6 Left) to the 

head end (5 Left) of the longwall system. The middle (No. 2) entry of 5 Left 

panel was used first as a trackless haulway for men, equipment, and supplies, 

and later as a return airway. A secondary regulated air intake of approximately 8,000 ft3/min was conducted from the 5 Right panel in the middle (No. 2) 

entry of 5 Left panel to the head end, where it was mixed with air from the 

6 Right and 6 Left panel entries mentioned previously. The first 400 feet of 

5 Left entry adjacent to the longwall panel were supported with wood cribs to 

permit the flow of air along the right edge of the gob after caving. The airflow at the edge of the gob is controlled by opening the stoppings in cross 

entries between No. 2 and No. 3 entries of 5 Left panel (figs. 1-2). 

UNDERGROUND MONITORING 

Hand-held instrument readings of methane concentrations and air volume 

were taken during the study period on a weekly basis and were supplemented 

with readings from continuously recording methanometers. During the weekly 

survey, bottle samples for laboratory analysis were taken to check the two 

types of methanometer data. Ventilation and methane concentration were monitored from September 4, 1974, when the longwall face had been retreated 70 

feet, to November 26, 1974, when the longwall face had been retreated 2,230 

feet. Continuously recording instruments were normally in operation 24 hours 

each day. The monitoring stations, and flows of methane and air, are shown in 

figure 2. 

VERTICAL GOB -DEGASIFICATION BOREHOLES 

Three degasification boreholes were drilled from the surface into the 

longwall panel in advance of mining. 

Stimulated Borehole 37 

Borehole 37 was located in the middle of the panel 400 feet from the 

starting end. A 16-inch surface hole was drilled 23 feet to firm rock and 

cased with 14-inch-ID casing. From this point, the hole was continued 12 

inches in diameter to a depth of 538 feet, 10 feet below the base of the Lower 

Kittanning coalbed (fig . 3) . 
, 

. 

L - 

-0 

.- 436 ft, bottom of 4-in pipe 3 

2 439-451 ft, P 8 S FI, 

468-475 ft, P 8 S 

496-503 ft, P 8 S . . 507 ft, bottom of 8-in pipe 

-z 'i" 528 ft, bottom of B coal 

538 ft, bottom of 12-in hole 

selected gas-bearing strata 

(fig. 3). A total of 42 

feet of casing was perforated with 84 charges 

spaced 2 per foot in each 

of the 6 zones. 

The perforated zones 

were treated with 4,000 gallons of 7-1/2 pct hydrochloric acid (HC1) 

FIGURE 3. - Detail of hydraulically stimulated borehole containing 2 PC~ potassium 

37 completion. (P & S indicates perforation chloride. This treatment 

and stimulation, Perforations were 0.45 in. cleaned the perforation 

in diam, two per foot.) holes of cement to assure 

that the strata would break 

down and reduced the formation damage from cement and water invasion of the porous rock strata. 

Following this, the rock strata in the perforated zones were hydraulically stimulated in stages with 20,000 gallons of gelled water. The treatment 

fluid contained 2 pct potassium chloride to inhibit clay swelling, 40 pounds 

of gelling agent (guar gum) per 1,000 gallons, 2 pounds friction reducer per 

1,000 gallons, and 1/2 pound breaker per 1,000 gallons. Then 20,000 pounds of 
LEGEND 

E]~andy shale hale ml~une and shale 

1-7 clay ~CO~I 

API UNITS DENSITY, g/cm3 

FIGURE 4. - Lithology, gamma ray, porosity, and density logs of the strata 

over lying the Lower Kittann ing coo lbed. 
366 feet to 273 feet deep, 

and was held in place by a 

seal ring and cement basket 

attached to the bottom of 

the casing. This left 110 

feet of open hole below the 

grouted casing free for gas 

flow from the developing gob. 

Borehole 39 was located 

369 ft, bottom of 6-in pipe in the same panel, outside 

the study area, 1,600 feet 

from borehole 38. It was 

drilled and completed in a 

similar manner to borehole 38. The lower 120 feet 

of the hole was open. 

- 499 ft, bottom of B coal 

, 

500 ft, bottom of lo-in hole 

RESULTS 

Borehole 37 

Borehole 37 was opened 

FIGURE 5. - Detail of borehole 38 completion, for flow on ~uly 1, 1974, 11 
weeks before the longwall face reached the hole. During this period, the 

effluent contained 88 pct methane (fig. 6) and the methane flow was 11 Mft3/ 

day (fig. 7). On September 16, the borehole was intersected by the retreating 

longwall face. Methane emission from the borehole increased as the overburden 

began to subside and crack. However, on the following day, lateral shifting 

and a casing break caused a water leak. These were fixed by grouting a 4-inch 

pipe inside the 8-inch-ID casing (fig. 3), but the methane production was 

below 100 Mft3/day and the methane concentration in the effluent was low. An 

exhaust fan" was connected to increase the flow; however, the effluent concentration dropped closer to the upper explosive limit of 15 pct, making it necessary to shut off the fan. During the first week of October, the hole began 

intaking due to a loss in natural draft, so the exhaust fan was reconnected. 

By this time, the face was 400 feet beyond the borehole and the effluent methane concentration remained above 35 pct (fig. 6). The flow peaked at 250 

Mf t3 /day shortly thereafter and then began to decline steadily (fig. 7) . The 

cumulative volume of methane from borehole 37 is shown in figure 8. 

Underground Methane Emission 

Underground methane emission from the longwall panel gob area during the 

study period started at about 170 ~ft~/day, reached its peak of 1,540 Mft3/day 

10 weeks later (fig. 7), and then declined sharply. This decline was due to 

two factors. First, a strike shut down the mine between November 10 and 

December 9. Since the methane emission from a longwall panel is closely tied 

to production (6, 8-9, G), a halt in production would sharply reduce methane 

emission. Second, the longwall panel passed borehole 38, and much of the gas 

began to emerge through it. 

Borehole 37 reduced underground methane emission during the study period 

about 11 pct, a considerably lower percentage than those obtained with the 

other boreholes. A survey conducted by mine personnel during April 15-May 15, 

1975, showed that 17 boreholes at the mine, including boreholes 37 and 39, 

emitted an average of 2.5 ~~ft~ of methane, accounting for 30 percent of the 

total methane emission of 8.2 M?Ift3/day. 

Boreholes 38 and 39 

At the beginning of November, the longwall face intersected borehole 38. 

The hole was open 110 feet below the grouted casing. With an exhauster fan" 

on, methane flow started to increase and reached 650 ~ft~/day (fig. 7). During the strike, the flow declined, but when coal production resumed, the flow 

increased again. This demonstration of buildup and drop in methane flow is 

repeated whenever there is a significant change in coal extraction--after a 

lag-time of 2 to 6 weeks. Flow from borehole 38 dropped again when borehole 39 started production. It gradually decreased to about 60 ~ft~/day over 

a period of 2 months (fig. 7). The methane concentration in the effluent from 

borehole 38 averaged 42 pct during its production life (fig. 6). 

5The exhaust fan was a centrifugal blower, and the vacuum was generally 

between 16 and 29 inches of water. 

 he fan was similar to that used at borehole 37. 
1 I 

Borehole 38 

I I I I I 1 I 

100 

80 

Borehole 37 

I I I I I I 1 I I 

I I I 1 I 

- Borehole 39 

KEY 

\July Aug. Sept. Oct. Nov. Dec7 ,Jan. 

I/ 

1974 1975 

daily reading - 

during the week 

40 - - 

-Weighted weekly average 

Lowest daily reading - 

during the week 

FIGURE 6, - Effluent methane concentrations for boreholes 37, 

38, and 39, 
2 6,000 11 I 'I'll I'I

0 z-. FS 0, ', 5,000 - a g 2 a .- 4,000- Z 

-10 

a- 8 3,000- 

m 

W 

0 2 5 2,000 - WE 

>k 

1,000 1 I 

llll~lIII~I

Underground e

Borehole 37 , 

v 

1974 

FIGURE 7. - Methane flow from sec
Sept. Oc t. Nov. Dec. /\ Jan. ~v Feb. 

FIGURE 8, - Cumulative volume of methane from borehole 37. 
Borehole 39 started methane production 2 months before mining of the 

panel was completed. Methane flow from this hole was initially 1,500 ~ft~/da~ 

at 93 pct concentration, but 1 week later it dropped to about 340 Mft3/day and 

continued at this rate over a 7-week period (fig. 7) at a methane concentration of 71 pct (fig. 6). Borehole 39 continued to emit methane during 1975, 

but declined from 329 Mft3/day in April to 187 Mft3/day in January 1976, at a 

55-pct concentration. 

Cumulative methane volume for boreholes 38 and 39 is given in figure 9. 

Comparative cumulative methane emission underground and from boreholes is 

given in figure 10. While in operation and during the mining of the study 

section, borehole 37 yielded 15.5 ~~ft~ of methane, borehole 38 yielded 35 

MMft3 of methane, and borehole 39 yielded 25 MMft3 of methane. Methane flow 

from-boreholes 37 and 39 continued after the longwall panel was completed at 

the end of March 1975. 

\ Nov. V Dec. /\ Jan. Fe b. Mar. / 

1974 1975 

FIGURE 9. - Cumulative volume of methane from boreholes 38 and 39. 
FIGURE 10. - Cumulative volume of methane from section and from boreholes 37, 38, and 39

70 

P 

Y2 

a= 60- 

w" 

z 

a 

50- 

W 

2 

% 40- 

W 

5 

6 30- 

> 

W 

2 20- Ia 

A 

1) 

5 lo- * 

Week 

- 

Underground survey-1 

completed Nov.26 i _-- - 

- 

Underground methane emission 

- 

- 

- 

--Borehole 38 Underground survey- - began emission 

started Sept. 4 

- 

-Borehole 37 - began emission 

O,,I -I*! JY~Y,~, -. -. =! 

7 I4 21-28~4-1','18'25 ( 

ending July Aug . Sept . Oct. Nov. Dec. ,, Jan. Feb. 

I/ 

1974 1975 v 
CONCLUSIONS 

Borehole 37 did not degasify the overburden as expected. It started to 

relieve methane from the gob area about 1 month after the longwall-face intersection. In contrast, hole 38 started to relieve methane within 24 hours 

after intersection. The methane flow from borehole 37 reduced the methane 

emission from the gob area 11 pct during the study period, or about one-third 

the average 30-pct effectiveness of 17 boreholes drilled over the same coalbed. 

The reduced methane flow from this hole is probably due to the following 

factors : 

1. Borehole 37 was cased and grouted to 10 feet below the bottom of the 

Lower Kittanning coalbed. All six hydraulically fractured zones were located 

in the grouted casing. Because the gas pressures were very low, the casing 

perforations may have restricted gas flow. The six zones were located so no 

roof above any coal seam would be fractured. Hydraulic fracturing of the open 

hole would have provided maximum exposure of fractured strata for gas flow 

with a minimum chance of formation plugging. The hole was cased and grouted 

because it was not intended to fracture the coal seams or roof above the coal 

seams. 

2. Lateral shifting, which broke the casing, was probably followed by 

vertical movement of the formation during caving. This may have caused the 

perforated casing and stimulated fractures to become misalined, further 

restricting the flow. 

3. Another major gas flow restriction was caused by the casing break. 

To fix this, it was necessary to grout new 4-inch-ID pipe inside the 8-inch-ID 

casing, to a depth of 436 feet, blocking the upper three hydraulically fractured zones. 

4. All of the fractured zones were in shales. Here, the extension of 

fractures may be limited because of formation plasticity (I), - eventual hole 

damage, and swelling of water -sensitive minerals. Thus, methane could not 

flow freely from the gas-bearing strata to the fractures created by stimulation and caving. 

5. Borehole 37 was located between two mined areas, one pillared and the 

other partially mined (figs. 1 and 2). This may have allowed partial degasification of overburden and coalbed in advance of mining. Figure 7 shows that 

the methane flow from the hole increased more rapidly during the first half of 

October when the longwall face passed from the partially mined areas. Underground methane emission followed a similar trend. At this time, a 4-inch pipe 

was grouted inside the 8-inch pipe and an electric pump was installed. 

6. A lead time of 3 months before mining of a longwall panel appears to 

be insufficient for the overbcrden to degasify before mining. More time would 

allow the hole to produce more gas at the same rate. In West Germany, for the 

very gassy Luisenthal mine, the minimum recommended time interval between 
completion of the hydrofractured holes and the beginning of mining operations 

in the section was 5 years (2). - 

7. None of the four coalbeds above the Lower Kittanning coalbed were 

fractured. Four fractured zones were in shale, one was in sandy shale, and, 

one was in coal and shale. Three of these zones were in subjacent strata of 

coalbeds that are normally less gassy than the coalbed above (fig. 4). 

8. Gas flow from boreholes 38 and 39 was much higher than from 37. The 

main reason for the higher flow apparently was the lack of casing in the lower 

part of the holes. 

RECOMMENDATIONS 

A hydraulically stimulated gob-degasification borehole yielded less gas 

than two other unstimulated boreholes located over the same longwall panel. 

This experiment shows that oilfield and gasfield completion procedures applied 

to gob-degasification boreholes must be designed especially for that purpose. 

To avoid the problems that resulted in reduced effectiveness of gob 

degasification when using the hydraulic stimulation procedure described in 

this report, a different approach should be used. We recommend the following: 

1. The zone believed to be the major source of gas in the overburden 

should be left uncased. 

2. This zone should be hydraulically stimulated. 

3. After dewatering, a secondary string of casing, appropriately slotted, 

should be installed so that the casing is suspended from the upper part of the 

hole where lateral shifting and vertical separation are minimized. 

4. This procedure should be instituted as early as possible, certainly 

more than 1 year ahead of mining, and probably as soon as plans for mining the 

panel have developed. 

5. Foam stimulation should be considered because one-third to one-fifth 

the volume of fluid is used, simplifying water removal and cleanup. This 

alone may justify the slightly higher cost of the foam. 

6. The formations considered for stimulation should be tested for swelling so that zones with an adverse mineral composition will be avoided. 
REFERENCES 

1. Atkinson, C. H. Rio Blanco Massive Hydraulic Fracturing Demonstration. 

BuMines TPR 88, 1975, 11 pp. 

2. Commission of European Communities. Studies of the Influence of Geology 

and of the Type of Formation on the Origin, the Presence and the Control of Firedamp in the Coalbearing Strata of the Saar. Collection of 

Reports on Research Relating to Coal, No. 55, Luxembourg, 1974, SMRE 

Trans. No. 6479, 5 pp. 

3. Elder, C. H. Use of Vertical Boreholes for Assisting Ventilation of 

Longwall Gob Areas. BuMines TPR 13, 1969, 13 pp. 

4. Elder, C. H., and M. Deul. Degasification of the Mary Lee Coalbed Near 

Oak Grove, Jefferson County, Ala., by Vertical Borehole in Advance of 

Mining. BuMines RI 7968, 1974, 21 pp. 

5. . Hydraulic Stimulation Increases Degasification Rate of Coalbeds. 

BuMines RI 8047, 1975, 17 pp. 

6. Flugge, F. Die Methanausgasung wghrend der Gewinnung und der Forderung. 

(Methane Emission During Production and Haulage). Gluckauf, v. 108, 

May 25, 1972, pp. 425-431. 

7. Ferguson, P. A. Methane Control by Boreholes. Coal Age, v. 77, No. 1, 

January 1972, pp. 76-78. 

8. . Degasification of a Coal Seam--Longwall Gob Area. Paper in 

Methane Control in Eastern U.S. Coal Mines. Proceedings of the Symposium of the Bureau of Mines/Industry Technology Transfer Seminar, 

Morgantown, W. Va., May 30-31, 1973. BuMines IC 8621, 1973, 

pp. 58-61. 

9. Joliffe, G. V. The Emission of Methane From Rapidly Advancing Coal Face. 

Min Engr., v. 129, February 1970, pp. 325-341. 

10. McCulloch, C. M., and M. Deul. Geologic Factors Causing Roof Instability 

and Methane Emission Problems. The Lower Kittanning Coalbed, Cambria 

County, Pa. BuMines RI 7769, 1973, 25 pp. 

11. Merritts, W. M., W. N. Poundstone, and B. A. Light. Removing Methane 

(Degasification) From the Pittsburgh Coalbed in Northern West Virginia. 

BuMines RI 5977, 1962, 39 pp. 

12. Moore, T. D., Jr., M. Deul, and F. N. Kissell. Longwall Gob Degasification With Surface Ventilation Boreholes Above the Lower Kittanning 

Coalbed. BuMines RI 8195, 1976, 13 pp. 

13. Moore, T. D., Jr., and M. G. Zabetakis. Effect of a Surface Borehole on 

Longwall Gob Degasification (Pocahontas No. 3 Coalbed). BuMines 

RI 7657, 1972, 9 pp. 

U S GOVERNMENT PRINTING OFFICE 1977-703-001 62 INT.-BU.OF MIN ES,PGH.,PA. 22142