By S. D. Maksirnsvir and Fred N. Missell 

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

Cecil D. Andrus, Secretary 

BUREAU OF MINES 

Lindsay D. Norman, Acting Director 
This publication has been cataloged as follows: 

7 

iClaksimovic, S D 

Three coal mine gob degasification studies using surface 

boreholes and a bleeder system. 

(Rr~~rt of investigations ; 8459) 

Supt. of Docs, no,: 1 28.21:8459. 

Coal mines and mining-Safety measures. 2* Mine gases, 3, ~ine 

ventilation, I. Kisscll, I-red Nu, joint author, 11. Title, 111. Series, 

TN23.U43 [~~295] 622s r6221.81 80-607966 
CONTENTS 

I~bstract ............................................................... Introduction ........................................................... First study: Degasification of pillared panels through a surface 

borehole ............................................................. Analysis .......................................................... Second study: Bleeder entry ventilation through a surface 

borehole ............................................................. Analysis .......................................................... Third study: Bleeder system for longwall gob ventilation .............. Analysis .......................................................... References ............................................................. 

1 ILLUSTRATIONS 1 

Map of the first study area ........................................ Detail of borehole completion ...................................... Borehole methane concentration. methane flow from borehole and 

all mined panels. borehole flow versus total flow. and gob 

area in acres ......e...e.ee...................................... 

Map of the second study area ....................................... Map of the third study area.. ...................................... Methane flow rate and longwalls gob area ........................... 

TABLES 

Methane emission survey from 61 West returns. 7 East entries. and 

borehole ......................................................... Concentrations in 7 East if borehole were unoperative .............. 
THREE COAL MINE GOB DEGASlFlCATlON STUDIES USING SURFACE 

BOREHOLES AND A BLEEDER SYSTEM 

by 

S. D. Maksirnovic ' and Fred N. Kissell 

I ABS TRACT 

The use of vertical surface degasification boreholes with bleeder systems 

hnd the use of a timbered bleeder system to degasify gob areas were studied by 

(the Bureau of Mines. This report describes three gob degasification studies 

,conducted in the Pittsburgh coalbed. 

One study was conducted to determine the number of panels degasified 

with one large borehole . The study showed that the borehole provided degasi- ( f ica tion for three panels . 

Another study was conducted during the retreat of a 35-acre panel with a 

single-entry and a two-entry bleeder system and a large surface borehole. A 

major portion of methane emitted from the panel was carried away by the borehole, leaving the bleeder entries relatively free of methane. 

The last study was made to determine the effectiveness of a timbered 

bleeder system. The bleeder entries successfully degasified the gob area when 

enough entries were available and adequate pressure was available to force 

the air through roof falls. 

INTRODUCTION 

The proper ventilation of coal mine gobs is becoming more difficult as 

deeper coalbeds are mined and restrictions on permitted methane levels become 

more stringent. The Bureau of Mines is conducting research into new ways of 

reducing methane in gobs, and most of this effort centers on the use of bore3 holes and better bleeders for gob degasification (L-3). One of the most 

important aspects of gob ventilation is the need to vent safely that portion 

of gas that flows into active workings when the barometric pressure falls. 

l~ining engineer. 2 Research supervisor. 

Both authors are with the Pittsburgh 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. 
LEGEND 

=C= Check curtain 

0 6'' 

Scale, feet 

=II= Permanent stopping 

=ID= Permanent stopping with door 

ZIR- Regulator 

=X= Overcast 

- - Intake air Return air 

FIGURE 1. - Map of the first study area. 
Casing 

Not to 

scale 

rC 

Pittsburgh coa lbed, 4 

FIRST STUDY: DEGASIFICATION OF PILLARED 

PANELS THROUGH A SURFACE BOREHOLE 

In this instance, the borehole was drilled 

before mining into the center of a panel so as 

to drain methane from the gob after the panel 

was retreat mined. The coal mine is located in 

Monongalia County, We Va., and the study area 

consisted of panels 5-9, right off 3 North 

Mains (fig. 1). Each panel was developed, 

then pillared and retreated before starting to 

develop the next panel. 

The surface degasification borehole was 

located in panel 5, adjacent to a coal barrier f. ) It was drilled ahead of the 

approaching pillar line to a depth of 616 ft, 

34 ft above the Pittsburgh coalbed. It was 

28 inches in diameter4 and was cased with 

24-inch-OD casing to a depth of 602 ft. The 

casing interval between 398 and 602 ft in 

depth was slotted, with the remaining unslotted casing cemented from 398 ft to the surface (fig. 2). 

Underground methane flow was measured 

during periodic ventilation surveys using the 

difference in methane concentrations between 

intake and return airflows. During retreat 

mining, the volume of the intake air varied 

from 22,000 to 40,000 ft3/min. Roughly half 

of this air was conducted across the gob to 

the bleeder entries and the surface borehole. 

The borehole was intersected by the 

pillar line in March 1972. However, for the 

first 7 weeks, the methane concentration in 

the hole did not build up enough to provide 

a natural draft. Since no exhaust fan was 

installed on the surface, the hole would not 

produce. By May, the methane concentration 

FIGURE 2. - Detail of borehole had risen sufficiently to provide a natural 

completion. draft, and the initial production was 300 

ft3/min, which was 60 pct of the total 

methane flow from panel 5 (fig. 3) . When 

panel 5 was completed, the borehole flow dropped together with the - underground 

flow. As panel 6 was pillared, borehole flow increased to 200 ftd/min, and 

40rdinarily such a large-diameter hole would not be drilled, but it was a test 

hole for a mine rescue project. 
Development - - - Pillar mining - 

7 Half month period for 

-Weighted average 

k-~xhauster - 

(av concentration 25 pct) 

during the life of this 

panel, roughly 50 pct of 

the total methane released 

from panels 5 and 6 combined 

emerged through the borehole 

(fig. 3). 

Borehole effectiveness 

dropped considerably during 

the pillar mining of panel 7, 

producing only 30 pct of the 

total from all three panels. 

However, there is evidence 

that about one-third of the 

gas from panel 7 was flowing 

to the borehole. As panel 

7 was mined, the borehole 

flow increased about 50 

ft3/min, whereas the total 

flow (borehole and return) 

increased about 140 f t3 /mine 

There was no increase in 

borehole flow as panel 8 

was pillared, indicating 

that no measurable gas from 

panel 8 gob was flowing to 

the borehole. 

O~~IIIIIIII I II'IIIII IIIIIIIIII11 11 I During all of this 

\AMJJASoNLyjJFMAMJ JAso NDAJ~F-Y~M~~/ time, the methane concent-a19 72 19 73 1974 tion in the borehole effluent 

varied widely. For the first FlGUR E 3. - Borehole methane concentration, methane flow few months as panel 5 was 

from borehole and all mined panels, borehole pillar mined, it averaged 

flow versus total flow, and gob area in acres. over 50 pct but fluctuated 

considerably on a day-t o-day 

basis (fig. 3). After panel 

5 was completed, the concentration occasionally dipped into the 20-pct range, 

with the hole losing its natural draft and beginning to intake. When pillar 

mining of panel 6 started, an exhauster fan was installed on the surface, but 

its effect was to lower the effluent concentration5. It was removed 6 months 

later (fig. 3). The mining of panel 7 resulted in some higher concentrations 

for a short period, with the hole also frequently intaking. After panel 7, the 

exhauster was reconnected, and the effluent concentration averaged about 25 

pct . 

5Previous experience with degasification boreholes indicates that the larger 

hole or the exhauster fan does not necessarily increase the quantity of 

methane flow from the gob area. In some cases, these only increase the 

total effluent flow at a lower methane concentration. 
A large-diameter borehole was drilled to the 

intersection of bleeder 

entries located at the back 

end of a large panel. The 

purpose of the hole was to 

serve as a ventilation supplement to the conventional 

bleeder system. The back end 

of the panel was adjacent to 

a property line and was 

difficult to ventilate. 

The panel, denoted 61 

Butt, was located in a coal 

mine in Washington County, LEGEND 

P SloPDlnq Pa. It was bounded on the SR Rapulamr 

- Intakmair sides by 7 East entries and - bNrn 08, 

~P,IIo,II.. a coal barrier. At the back 

was a property line. Access 

was from 61 West entries 

located on the north side 

\ " (fig. 4). 

FIGURE 4. - Map of the second study area. 
Ventilation was provided with intake and return airways to 61 West 

entries. The side and back were provided with bleeder entries, with the 

bleeder air passing to the borehole and 7 East entries. 

The borehole was drilled 640 ft deep, 28 inches in diameter, and cased 

with 24-inch casing. Two exhausters located on the surface served to move 

an average of 9,000 ft3/min up the borehole. 

Retreat mining of 61 Butt started in October 1972 and was completed in 

November 1973. During this period, the Bureau of Mines made periodic ventilation surveys to measure the methane emission and determine what fraction was 

exiting to 61 West returns, 7 East entries, or the borehole. The ventilation 

picture was complicated somewhat by the introduction of methane from the adjacent 60 Butt (fig. 4). However, in the survey, it was possible to distinguish 

the methane amounts contributed by 60 Butt and 61 Butt, respectively. 

The results of these surveys are shown in table 1, Methane emerging from 

the front of the panel passed to 61 West returns, and from the side and rear 

to 7 East and the borehole. Generally, the borehole carried most of the load 

at the sides and rear, leaving 7 East free of large amounts of methane. This 

indicates that the borehole worked well. However, the major question to be 

resolved is whether 7 East could have handled the methane load without the 

borehole, Table 2 gives methane concentrations that would have been found in 

7 East had the borehole not been present. Three different airflows were used: 

the actual measured airflow from table 1, an assumed airflow of 5,000 ft3/min, 

and an assumed airflow of 10,000 ft3/min. A glance at table 2 shows that if 

the borehole had not been present, an airflow of 10,000 ft3/min or more would 

have been consistently required in 7 East to maintain safe methane levels. 

TABLE 1. - Me thane emission survey from 61 Wes t returns, 

7 East entries, and borehole, ft3/min 

Dateof 

survey 

10-02-72 

11-02-72 

11-21-72 

12-06-72 

12-20-72 

01-11-73 

02-06-73 

02-22-73 

03-15-73 

04-12-73 

05-17-73 

08-09-73 

10-30-73 

Front of Panel 

61WestReturns 

Air 

(thousands) 

42 

51 

44 

37 

45 

52 

53 

52 

48 

44 

42 

39 

7.1 

Methane from 

7 East and 

borehole 

25 

111 

70 

82 

124 

147 

140 

294 

291 

166 

291 

210 

281 

Sides and rear of panel 

Methane 

55 

32 

30 

49 

63 

36 

18 

53 

10 

22 

23 

19 

0 

7East 

Air 

(thousands) 

12 

2.5 

1.9 

1.7 

2.2 

1.6 

1.7 

9.4 

3.6 

0 

8.5 

11 

8.2 

Rorehole 

Methane 

25 

26 

18 

17 

18 

22 

14 

160 

69 

0 

89 

74 

66 

Air 

(thousands) 

0 

6.8 

6.5 

7.2 

8.5 

8.6 

10 

11 

10 

10 

11 

10 

11 

Methane 

from 61 

Butt 

0 

85 

52 

65 

106 

121 

95 

0 

73 

141 

185 

128 

207 

Methane 

from 60 

Butt 

0 

0 

0 

0 

0 

4 

31 

134 

149 

25 

17 

8 

8 
TABLE 2. - concentrations1 in 7 Eas t if borehole were inoperative, pc t 

'calculated by F. N. Kissell. 

Analysis 

Date of 

survey 

10-02-72 

11-02-72 

11-21-72 

12-06-72 

12-20-72 

01-11-73 

02-06-73 

02-22-73 

03-15-73 

04-12-73 

05-17-73 

08-09-73 

10-30-73 

A major fraction of the methane emitted from 61 Butt was carried away by 

the horehole, leaving 7 Fast relatively free of methane, Without the borehole, 

7 East could not have safely handled the methane without major changes in the 

ventilation plan. 

Assuming airflow 

of 5,000 ft3/min 

in 7 East 

0.5 

2.2 

1.4 

1.6 

2.5 

2.9 

2.8 

5.9 

5.8 

3.3 

5.8 

4.2 

5.6 

Using measured 

airflow in 7 

Eas t 

0.2 

4.41 

3.6 

4.8 

5.6 

9.2 

8.2 

3.1 

8.3 

0 

3.4 

1.9 

3 04 

THIRD STUDY: BLEEDER SYSTEM FOR LONGWALL GOB VENTILATION 

Assuming airflow 

of 10,000 ft3/min 

in 7 East 

0.25 

1.1 

0.7 

0.8 

1.2 

1.4 

1.4 

2.9 

2.9 

1.6 

2.9 

2.1 

2.8 

A more common approach to gob ventilation is to depend entirely on the 

bleeder system, without the use of supplementary boreholes. If not badly 

blocked by falls, such a system is capable of safely carrying a considerable 

amount of methane. 

One effective conventional bleeder system is located in a mine in the 

Pittsburgh coalbed in Monongalia County, W. Va. It illustrates what volumes 

of methane can be handled without the use of supplementary boreholes. 

The area (fig. 5) included four adjacent longwall panels that were mined 

in sequence. Each was circumscribed by three panel entries on the side, South 

Bleeders at the back, and 2 South Mains. Daily production ranged from 1,000 

to 4,000 tons of coal. Intake air totaling 40,000 to 90,000 ft3/min was conducted from 2 South Mains through the panel entries and to the South Bleeders. 

The South Bleeders carried an additional 20,000 to 40,000 ft3/min to 1 Left 

Gob on the other side. 

In the South Bleeders, entry 3 was timbered down the center of most of 

its length with closely spaced cribs. Most of the time, entries 3 and 4 served 

as bleeders for the long&ll panels, with entries 1 and -2 serving as intakes 

for 1 Left Gob. A typical air volume in entries 3 and 4 totaled 65,000 ft3/min. 
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As panel 4 was mined, 

all of the South Bleeder 

a. entries were converted to 

returns , allowing a boost 

in the air to 95,000 ft3/min. 

700 70 The methane flow rate from \ 

Longwalls \\ 

gob area 

Underground 

, methane emission 

/ 

/ 

/ 

the entire four panel area 

60 f (fig. 6) increased steadily 

" over a 3-year period to 

5 850 ft3/min and then 

50 declined, the decline due m 

o to slow mining of panel 4. (3 

$400[ Panel 1 'panel 2 ,-:,A,,A,/ =;-~ane13+ pane14 I4O2 a Analysis a 5 300 

E 

30 Z 

z x 9 Conventional bleeder 

entries can safely carry 

200k ;' / 20 away considerable amounts of 

me thane, provided enough 

lo entries are available and 

enough pressure is available 

to force the air through I I I Ill 1IIIII 

-1973~1974~1975 /\ 1976~1977 roof falls. Because of 

heavy timbering, entry 3 did 

FIGURE 6, - Methane flow rate and longwalls gob area, not provide the completely 

unobstructed ventilation 

aircourse that was hoped for; however, it was still possible to move 65,000 

ft3/min through entries 3 and 4 with a pressure loss of 2 inches water gage in 

the bleeder. The underground emission peaked at about 850 f t3/min of methane, 

which was much more gas than was measured in the two studies where boreholes 

were used. The bleeder system effectively handled this larger gas emission, 

although four bleeder entries were required. 

In practice, it is not often easy to drain methane adequately using this 

type of bleeder, because of the difficulties involved with maintaining the 

necessary air pressure differentials. 
REFERENCES 

1. Mazza, R. L., and M. P. Mlinar. Reducing Methane in Coal Mine Gob Areas 

With Vertical Boreholes. Res. Contract H0322851. BuMines Open File 

Rept. 142-77, 1977, 146 pp.; available for consultation at the Bureau 

of Mines facilities in Denver, Colo., Twin Cities, Minn., Bruceton and 

Pittsburgh, Pa., and Spokane, Wash.; at the National Library for Natural 

Resources, U.S. Department of the Interior, Washington, D.C.; and from 

National Technical Information Service, Springfield, Va,, PB 272 768/~~. 

2. 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, 14 pp. 

3, Moore, T. D., Jr., and M. G. Zabetakis. Effect of a Surface oreh hole on 

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

7657, 1972, 9 pp. 

-i?U.S. GOVERNMENT PRINTING OFFICE: 1980-603-1 02/78