Above the Lower Kittanning Coalbed 

UXt'ITED STATES DEPARTMENT OF THE INTERIOR 
UNITED STATES DEPARTMENT OF THE INTERIOR 

Thomas S. Kleppe, Secretary 

BUREAU OF MINES 

Thomas V. Falkie, Director 
I h~s publ~cat~on has been cataloged as follows: 

Moore, T D 

Longwall gob degasificat ion with surface ventilation 

boreholes above the Lower Kittanning coalbed / by T. D. 

Moore, Jr., Maurice Deul, and Fred N. Kissell. [washington] : 

Bureau of Mines, 1976. 

13 p. : ill,, diagrs. ; 26 cm. (Report of investigations - Bureau 

of Mines ; 8195) 

Bibliography: p. 13. 

1. Mine ventilation. 2. Methane. I. Deul, Maurice, joint 

author. 11. Kissell, Fred N., joint author. 111. Title. IV. United 

States. Bureau of Mines. V. Series: United States. Bureau of 

Mines. Report of investigations - Bureau of Mines ; 8195, 

TN23.U7 no. 8195 622.06173 

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

Abstract ............................................................... ~ntroduction ........................................................... Description of the study area .......................................... Mining plan ....................................................... Stratigraphy ...................................................... Equipment ........................................................ Ventilation ................,...................................... 

Test procedure ......................................................... Surface degasification boreholes .................................. Underground monitoring .. ......................................... Results ................................................................ Analysis ............................................................... Conclusions ............................................................ References ............................................................. 

ILLUSTRATIONS 

1 . Map of study area ................................................. 2 . Ventilation schematic of study area ............................... 3 Detail of . borehole 20 completion .................................. 4 . Borehole effectiveness . A, Coal production from section; 

B, methane flow from section and methane flow from borehole 20 .. - 5 . Methane flow from boreholes 20 and 21 ............................. 6 . Effluent methane concentrations-- boreholes 20 and 21 .............. 7 . Methane flow from borehole 21 .... ................................. 8 . Effluent methane concentrations-- borehole 21 ...................... 9 . Cumulative volume of methane from borehole 20 . .................... 10 . Cumulative volume of methane from borehole 21 ..................... 

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LONGWALL GOB DEGASIFICATION WITH SURFACE VENTILATION 

BOREHOLES ABOVE THE LOWER KITTANNING COALBED 

by 

Ta Dm Moore, Jr., ' Maurice DeulI2 and Fred N. Kissel I3 

Methane emission from two surface gob-degasification boreholes was 

measured by the Bureau of Mines. The investigation took place during mining 

of a 3,200-foot longwall panel in the Lower Kittanning coalbed in central 

Pennsylvania. The first hole was 500 feet from the start of mining, and the 

second hole was 2,200 feet. The first hole went on natural draft as soon as 

the longwall face passed it. This caused the methane flow in the return 

entries to drop 75 pct. In the 6 months required to complete the panel, the 

two holes emitted 69 MMcf of methane in varying percentages of concentration 

in air. In 3 years, the holes emitted a total of 150 MMcf of methane. 

Methane is contained under pressure within the micropores and fractures 

of coalbeds and in the adjacent strata. It is released into the mine atmosphere during working of the bed and must be removed either in advance of or 

during mining to prevent the formation of explosive gas-air mixtures 

4 (1, 5-5, 7-8). The removal of methane by conventional ventilation is particularly difficult in longwall mining of gassy coalbeds. As mining progresses 

in a longwall, the overburden continually fractures, and the immediate roof 

caves directly behind the roof support units. Thus, the gob is always near the 

coal face, and any methane-bearing material in the overburden adds to the 

methane released from the coal during mining. 

Air losses at the longwall face also create problems. A considerable 

amount of air can escape into the gob as it is coursed along the face, making 

ventilation of the face zone at the return end of the face especially 

difficult. To supply sufficient air, a considerable flow rate must be maintained at the intake end. 

1Mining engineer (now with Clinchfield Coal Co.). 

2 Supervisory geologist. 3 Physical research scientist. 

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

the end of this report. 
In most coal mines, the methane dilution requirements of the Federal Coal 

Mine Health and Safety Act of 1969 can generally be met by providing bleeder 

entries with adequate pressure differentials across the gob. However, where 

emissions of gob methane are unusually high, the bleeder system must be supplemented with one or more surface gob-degasification boreholes drilled prior to 

mining (5). Some earlier work on the subject of degasification has been 

reported by Ferguson (2-3). - - 

This Bureau of Mines report details results from two such boreholes 

drilled over a mine located in the Lower Kittanning coalbed in Cambria County, 

Pa., where gob-degasification holes must be used on a regular basis. 

DESCRIPTION OF STUDY AREA 

Mining Plan 

The block of coal to be mined by a retreat longwall system was bounded on 

each side by a set of panel entries (7 Left and 8 Left) and on the ends by 

D West and E West mains (fig. 1). The block was 570 feet wide, 3,200 feet 

long, and 58 inches thick. The three entries of 7 Left and 8 Left panels were 

mined on 80-foot centers, with cross entries on 100-foot centers. All entries 

were 18 feet wide. The middle 7 Left entry was used as a trackless haulway 

for workers, supplies, andequipment to the longwall face, and as a return when 

the longwall face was retreated past the first cross entry connection to the 

gob. 

At the start of the study on September 20, 1971, the longwall face had 

been retreated 360 feet and was 140 feet from the first surface borehole 

(borehole 20, fig. 1) . At the end of the study on September 30, 197 1, the face 

was 85 feet past this borehole. 

Stratigraphy 

The overburden at the study area was 600 to 700 feet thick and consisted 

of carbonaceous shales, argillaceous carbonaceous sandstones, some argillaceous limestones, and a number of coalbeds. The immediate roof strata comprised 7 inches of bone and coal and 9 feet of dark, sandy shale. The 

immediate 4 feet of floor was fire clay with coal and shale partings. 

McCulloch and Deul (5) have reported that the amount of gas emitted 

directly from the Lower Kittanning coal into the mine working is known to be 

small. During development and in preparation for installation of longwall 

mining equipment, methane emission rates are very low. In one methane 

emission study in a development section, only 23 cfm of methane was measured. 

In contrast,when longwall mining was being conducted, methane emission rates 

rose to 1,500 cfm from a single section. This additional methane mostly originates in the overburden. Four other coalbeds, Upper and Middle Kittanning 

and Upper and Lower Freeport, are in close proximity to the mined Lower Kittanning coal, and these are known to be gassy. The result is a much higher proportion of methane from the overburden than is common in most mines. 
Masonry stopping with 3-ft 

wide by seam height opening 

Masonry stopping 

Stopping with mandoor 

--#T- Stopping with double door 

----- Belt entry - Intake air 

Return air 

LEGEND 

R Regulator 

0 300 

Scale, feet 

c Check curtain 

/-4 Survey stations 

/ Recording methanometer station 

2-3 Air pressure and volume station 

/, 3, and 4 Hand held methanometer statlb~ 

3 Recording barograph station 

FIGURE 1. - Map of study area. 

Equipment 

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

58-inch-thick coalbed. A chain conveyor received the coal along the face and 

discharged it at the head end onto a mother-chain conveyor in the closest 

7 Left entry. From there the coal was discharged onto a rope-frame, 36-inch 

belt conveyor that loaded into mine cars. 
The roof was supported by hydraulic self-advancing steel props. Each 

unit consisted of four 42-ton-capacity steel props mounted on a rigid base and 

topped with shielded steel beams. The units were paired to operate as an 

eight-prop chock, spaced on 42-inch centers across the face. Coal production 

ranged from 2,100 to 3,800 tpd. 

Ventilation 

The flow distribution, the range of air and methane volumes, and the pressure differential across the gob are shown in figure 2. The primary air split 

that ventilated the section was conducted across the face from the tail end at 

8 Left to the head end at 7 Left. 

A regulated secondary intake of approximately 3,000 cfm was conducted in 

the middle entry of 7 Left to the head end. Most of the return air flowed to 

D West through the middle entry of 7 Left and the 7 Left edge of the gob, 

which was cribbed. The D West return air split joined the longwall return air 

in the main return entries at 7 Left and 8 Left. The maximum pressure differential between intake station 3 and return station 2 was 0.17 inch of water. 

A feature of the ventilation system was a row of timber cribs at the edge 

of the gob that were installed one by one at the head end of the longwall face 

as it retreated. Their purpose was to provide a high-permeability path for 

the return air at the gob edge (fig. 1). This allowed ventilation air to 

dilute gob methane before it entered accessible areas of the mine. In this 

way, the methane concentrations in accessible areas near the gob could be kept 

low enough to meet regulations. 

Airflow across the face and through the row of cribs was controlled by 

alternately opening and closing stoppings adjacent to the head entry (fig. 1). 

The closed stoppings near the gob in 7 Left confined the gob gasses and permitted bleeding through the cribbed entry. The four open stoppings in the same 

row allowed ventilation of the active face and the adjacent gob to the middle 

7 Left entry. As the panel was retreated to cross entry B, the opening in 

cross entry A stopping was closed, and stopping C was opened to airflow. This 

procedure was followed, with the system as shown in figure 1, until the panel 

was mined out. 

The masonry stoppings were constructed with 3- by 5-foot openings into 

which cement blocks could be easily fitted. The blocks were removed and 

replaced as needed. 

TEST PROCEDURE 

Surface Degasification Boreholes 

Before longwall mining was started in the study panel, surface borehole 20 was drilled to a depth of 783 feet, which was 1 foot below the bottom of 

the Lower Kittanning coalbed (fig. 3). Because of the rough surface topography, borehole 20 was located 500 feet from the back edge of the gob instead of 

the usual 300 to 400 feet. An 8-inch-ID steel casing was installed in the 
7 LEFT 99000 to (-1 0. I 7 in watdr 1 

13,000 cfm 

LEGEND 

-.- Masonry stopping with opening 

+ Masonry stopping with door 

--&- Masonry stopping 

-& Masonry stopping with double door -+ ,+ Regulator 

- Check curtain Intake air 

s--( Return air 

*/,2,3,4 Survey stations 

FIGURE 2. - Ventilation schematic of study area. 

hole to 760 feet. The lowermost 119 feet of the casing was ungrouted in the 

lower portion of the hole and served to prevent hole collapse as the mine roof 

wzs caved. It was slotted to provide easy access to gas emerging from all 

levels within the 119-foot interval. Most of the gob methane entering the 

mine probably originated in this region. Both the Middle and Upper Kittanning 

coalbeds, located 25 and 100 feet above the mined Lower Kittanning coal, were 
here. One object in the borehole completion procedure was to try to locate 

the top of this open portion as high as 

possible in order to bleed off methane 

from the highest possible point in the 

Casing cavity created by caving in the mine. 

However, at the same time, it was necesFirm rock -- 

sary to stay below water bearing strata. 

Above the open portion, 150 feet of 

the hole were grouted to maintain the 

position of the casing and protect 

against intrusion of water. Still 

higher, the upper portion of the casing 

was not grouted. This was to avoid the 

casing breakage that can occur when the 

mine roof is caved and the strata around 

the borehole subsides. The standpipe of 

the hole was equipped with a check valve 

to prevent airflow from the surface to 

the mine. Surface borehole 21 (565 feet 

deep) was drilled 1,700 feet from 

hole 20 and completed in a similar manner. 

Underground Monitoring 

A recording methanometer installed 

at station 1 continuously measured the 

total methane from the longwall section 

and from D West development (fig. 2). 

At stations 2 and 3, ventilation pressures were measured with an altimeter, 

and air velocities were measured with an 

anemometer. A recording microbarograph 

located at station 3 during each monitoring shift recorded barometric changes 

and underground pressure changes caused 

'4 I by i n t e r f e r e n c e s to normal air£ lows. 

Handheld methanometer readings were 

taken at 15-min intervals at stations 1 

and 3, and methanometer and anemometer 

readings were taken occasionally at station 4 in D West. The accuracy of the 

FIGURE 3. - Detail of borehole 20 methanometers was checked by laboratory 

completion. analysis of bottle samples. All monitoring was performed on the day shift 

(8 a.m. to 4 p.m.) for 5 days prior to mining under borehole 20 and for 3 days - - 

after the hole was intersected and had become productive. 
FIGURE 4. - Borehole effectiveness. A, Coal production 

from section; B, methane flow from section 

and methane flow from borehole 20. 
RESULT S 

A surface borehole becomes useful only after mining passes the hole. 

Until that time, all the methane must be handled by the mine ventilation 

system. On September 28, borehole 20 was uncapped after the longwall face had 

reached it (fig. 4), and underground methane flow from the longwall section 

dropped 75 pct. Underground monitoring could not be continued beyond September 30 because a strike began on October 1. However, surface monitoring of 

the borehole continued (figs. 5-6). 

During October and November, flow from the borehole was by natural draft 

alone. Methane flow started at about 1,400 Mcfd and drifted down to about 

300 Mcfd over a 5-month period. The methane level in the effluent drifted 

from an initial 100 pct to about 45 pct. 

KEY 

Borehole 20 

Borehole 2 1 

FIGURE 6, - Effluent methane concentrations-boreholes 20 and 2 1. 

0 I I I I 1 

, Sept. Oct. Nov. Dec. /\ Jan. Feb. , v V 

197 1 1972 
I FIGURE 7. - Methane flow from borehole 21. 

I5The exhauster was a centrifugal blower, and the vacuum was generally between 1 

At the end of November, 

the strike ended. Mining of 

the longwall face resumed 

and, at the same time, an 

exhauster5 was connected to 

the borehole at the surface. 

This combined action raised 

the borehole flow for a week, 

but it soon returned to 

earlier levels (fig. 5). 

The methane concentration in 

the borehole effluent 

continued to drift downward 

(fig. 6) . Borehole 20 valve 

was closed at the end of 

January when the effluent 

concentration dropped below 

25 pct methane. Periodic 

testing during February and 

March still indicated low 

methane concentrations in 

the effluent, so this hole 

was permanently sealed. 

At the end of January, 

the longwall face intersected the second borehole 

(borehole 21), which was 

then uncapped. The flow 

from this hole was initially 

1,000 Mcfd at 80 pct methane, 

but it soon began to drift 

downward (fig. 5-6). 

10,ooor~~ 

1,000 

D 

I't; 

1: 

W 

I- 9 

1 ' z I A 

LL I JJ Z 

a 

I 

IW 

r 100- 

lo 

17 and 35 inches of water. 

1972 1973 

- IIIIII IIIII 11 11 1- - - - Borehole 21 - - - - - 

- - 

- - 

- - 

X 

- - - - - 

- x 1 - xi^ - - - 

- X 

x - 

- - 

X X xX 

xx 

- X "; - 

Xxx ,( xxx 

x"x 

X 

Xx X x Xxxx x $ - xx r"xx x xTxx xXY 

- X X x - - X XX x - - - - X - 

- X - 

- - 

7 - 

111111111111~~111~1~~1, 

,JFMAMJJASONPdFMAMJJASON/ v V 
Mining of the panel was completed in March 1972, only 5 weeks after the 

hole (borehole 21) had been put into service. Nevertheless, the borehole 

continued to emit methane for the next 2 years. Flow and concentrations 

during 1972 and 1973 are shown in figures 7 and 8. The hole was equipped 

with an exhauster for the entire period. In 1972, methane flow dropped to 

200 Mcfd, and the effluent concentration was about 35 pct; in 1973, the flow 

dropped and the concentration rose. 

Cumulative methane amounts for both boreholes are given in figures 9 and 

10. While in service, borehole 20 yielded 40 MMcf methane; up to December 

1973, borehole 21 yielded 110 MMcf methane. In 1974, borehole 21 declined 

substantially, and by July 1974, the flow was only 32 Mcfd. 

olllllllllllllllllll1l1 I11 

,J FMAMJ JASONQdFMAMJ JASONQ, V V 

80 

60 

4- 

U 

a 

wi- Z, 40- 

I 

k 

W 

I 

20 

FIGURE 9, - Cumulative volume of methane from borehole 20. 

11111111111111111111111~ 

Borehole 21 - X 

- - 

x xx 

- x x~*~x2x~xor~,":h - x 

"x X X 

X X x - xi' x x ;r - Xx X - xx i'*x 

x xxx , Xxxwx,x 

- - 

60 

40 

20 

FIGURE 8, - Effluent methane concentrationsborehole 21, 

I I 1 1 I 

- Borehole 20 - 

- . . . . . - 

. . - . . . - 

. . - . . . - . . . - 

0 4 I I I I 

\ Sept. Oc t. V Nov. Dec. /\ Jan. V Feb. , 
120 11111111111111111111111 

- Borehole 2 1 X 

0 t;IlIIIIIIIIIIIIIIIlIII: x 

dFMAMJJASON94FMAMJ JASOND, 

V V 

1972 1973 

The effectiveness of 

surface boreholes in draining off methane that otherwise would have to be removed 

by the ventilation air is 

evident in figure 4, where 

uncapping borehole 20 produced an immediate 75 pct 

decrease in the underground 

methane flow from the section. 

Methane levels were too high 

to allow us to shut off a 

hole temperari-ly as an 

experiment to see what 

increase in underground flow 

would result ; however, when 

borehole 20 was finally shut 

down, the section had to be 

idled for 5 shifts due to 

high methane levels in the 

returns. These levels 

occurred notwithstanding 

the fact that borehole 21 

was already producing 

1 MMcfd (fig. 5). 

FIGURE 10, - Cumulative volume of methane from Had the surface borelborehole 21, holes not been in operation, 

the ventilation air would have had to be boosted from the 20-30 Mcfm shown in 

figure 2 to as much as 75 Mcfm across the face. Under those conditions, 

allowing for the normal overhang and the considerable obstruction created by 

the self-advancing hydraulic jacks, the face conveyor,and the coal-shearing 

machinery, the air velocities at times across part of the face would exceed 

2,000 fpm. This would be intolerable for the workers and would disperse 

excessive amounts of coal dust. 

The company usually locates the first surface gob-degasification borehole 

about 300 to 400 feet from the start of longwall retreat. However, because of 

the terrain, hole 20 could not be located closer than 500 feet. This delayed 

the time until hole 20 became productive and resulted in some very high underground methane flows between September 21 and 27 (fig. 4), but subsequently 

the methane control system functioned successfully. 

In general, both holes started with high methane flows and high methane 

percentages in the effluent; within a few weeks, both flow and concentration 

declined substantially. Large fluctuations in flow and concentration took 

place on a week-to-week or even day-to-day basis. 
CONCLUSIONS 

Gob degasification through surface boreholes permits high-production 

mining of the Lower Kittanning coalbed. Without the surface drainage 

described here, an additional 150 MMcf of methane would have had to be diluted 

and carried out of just one section of the mine by the underground ventilation 

system. This would have necessitated increasing airflows to unreasonably 

high levels. 
REFERENCES 

1. Deul, M. Structural Control of Methane Migration Through Bituminous 

Coalbeds. Abs. Geol. Soc. Am. Bull., v. 3, No. 7, 1971, p. 542. 

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

1972, pp. 76-78. 

3* - Degasification of a Coal Seam--Longwall Gob Area in Methane 

Control in Eastern U.S. Coal Mines. BuMines IC 8621, 1973, pp. 58-61. 

4. Kissell, F. N., C. M. McCulloch, and C. H. Elder. The Direct Method of 

Determining Methane Content of Coalbeds for Ventilation Design. 

BuMines RI 7767, 1973, 17 pp. 

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

and ethane Emission Problems. The Lower Kittanning Coalbed, Cambria 

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

6- 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 pp7. Price, H. S., R. G. McCulloch, J. C. Edwards, and F. N. Kissell. A 

Computer Model Study of Methane Migration in Coalbeds. CIM Bull., 

September 1973, 10 pp. 

8. U.S. Bureau of Mines. Methane Control in Eastern U.S. Coal Mines. 

BuMines IC 8621, 1973, 58 pp.