By F. Garcia and J. Cervik 

UNITED STATES DEPARTMENT OF THE INTERIOR 

C 

MINES 75TH A~$ 
UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 

Robert C. Horton, Director 
Library of Congress Cataloging in Publication Data : 

Garcia, F, (Fred) 

Methane control on longwalls with cross-measure boreholes (Lower 

Kittanning Coalbed). 

(Bureau of Mines report of investigations ; 8985) 

Bibliography: p. 17. 

Supt. of Docs. no.: I 28.23: 8985. 

1. Coalbed methane drainage-Pennsylvania-Cambria County. 2. 

Longwall mining-Penn sy 1 vania-Cambria County-Safety measures. 3. 

Boring. I. Cervik, Joseph. 11. Title. 111. Series: Report of investigations (United States. Bureau of Mines) ; 8985. 

TN 23,U43 [TN 3051 622s [622'.334] 85-600 126 
CONTENTS 

. 

Page 

Abstract ....................................................................... Introduction ................................................................... Acknowledgments ................................................................ Study area ..................................................................... Drilling equipment and procedures ...............................m.............. 

Methane removal system ......................................................... Cross-measure borehole design .................................................. Surface gob holes .............................................................. Underground and surface instrumentation ........................................ Gas-water separator system ..................................................... Data analyses .................................................................. Longwall production .......................................................... Cross-measure borehole gas production ........................................ Methane migration through gobs ............................................... Borehole spacing ............................................................. Methane flow in the return air ............................................... Water production ......................e....................................e. 

Surface gob hole production ..................................m............... 

Migration of gas into a borehole ............................................... Summary and conclusions ...................................m.................... 

References ..................................................................... 

ILLUSTRATIONS 

Stratigraphic column above Lower Kittanning Coalbed ....................... BethEnergy Mines Inc.'s Cambria 33 Mine (Lower Kittanning Coalbed) ........ Longwall test panel ....................................................... Surface exhausters ........................................................ Cross-measure design characteristics .....................................e 

Surface gob hole locations for longwall panel ............................. Underground measurement instrumentation ................................... Dual exhauster arrangement ................................................ Gas-water separator ....................................................... Daily longwall coal production ............................................ Total gas flow from cross-measure system............................^^.^^^ 

Surface methane concentrations in gas flow............................^^.^ 

Methane flow from cross-measure system.....................^^.........^^^^ 

Methane concentrations in gas flow from hole 2 ............................ Partial vacuum measured at surface exhauster .............................. Interference test using hole 11 ........................................... Methane flow in return air ................................................ Effects of cross-measure system on methane flow in return air ............. Gas flow into boreholes ..........................................e........ 

TABLES 

1 . Production history of boreholes ........................................... 2 . Average methane and gas flows from boreholes .............................. 3 . Comparison of measured and calculated gas pressure differentials .......... 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

cm centimeter kPa kilopascal 

d/wk day per week ~/min liter per minute 

f t foot m meter 

ft3 cubic foot m3 cubic meter 

f t/d foot per day m/d meter per day 

f t3/min cubic foot per minute m3/s cubic meter per second 

f t/s foot per second mt/d metric ton per day 

ft/s2 foot per second per second Pet percent 

gal/min gallon per minute rad radian 

h hour ton short ton 

in inch ton/ d short ton per day 

in H20 inch of water (pressure) vol pct volume percent 

in Hg inch of mercury (pressure) 

A 
METHANE CONTROL ON LONGWALLS WITH CROSS-MEASURE BOREHOLES 

(LOWER KITTANNING COALBED) 

By F. Garcia ' and J. Cervik 

ABSTRACT 

The cross-measure borehole technique has been shown by the Bureau of 

Mines to be an effective method of controlling methane liberated by 

fracturing the roof strata in longwall gobs where overburden is less 

than 750 ft (229 m). Boreholes are drilled into roof strata before mining operations affect the roof strata. Longwall mining fractures the 

roof strata and releases methane from source beds. Surface exhausters 

or vacuum pumps are used to draw the gas to the surface through an underground pipeline and vertical exhaust hole. The captured methane is 

thus prevented from entering the mine's ventilation system. About 71 

pct of the methane produced by longwall mining in the Lower Kittanning 

Coalbed was captured by the cross-measure boreholes. 

Borehole spacing is an important factor affecting the performance of 

the technique. Interference tests and measurements of methane flows in 

return air indicate borehole spacing in the Lower Kittanning Coalbed 

should be limited to about 200 ft (61 m) except on the first 600 ft (183 

m) of the longwall, where spacing should be reduced to about 100 ft (30 

m) to capture the large quantities of methane released when the first 

large roof fall occurs. 

Comparisons between measured and calculated gas pressure differentials 

on boreholes indicate borehole length can be reduced from 280 to 140 ft 

(85 to 43 m), which reduces drilling costs significantly. Additional 

experimentation is necessary to verify the calculations. 

lnining engineer. 

2~upervisory geophysicist. 

Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 
INTRODUCTION 

Gob gas drainage through surface gob 

holes in the United States is an effective means of assisting a mine's ventilation system in controlling methane 

produced by longwall operations. Surface gob holes cannot always be drilled 

because of severe topography, surface 

right-of-way problems, and populated 

areas. An alternative method of controlling gob gas is necessary that is independent of these factors. 

In Europe, gob gas is controlled by 

drilling small-diameter holes into roof 

strata from underground locations (crossmeasure boreholes) and drawing the gas 

to the surface by vacuum pumps (1). 

This technique eliminates the factors 

affecting the drilling of surface gob 

holes. A test conducted by the Bureau 

and BethEnergy Mines Inc. in the Upper 

Kittanning Coalbed showed that approximately 50 pct of the methane produced by 

the longwall operation was captured by 

the cross-measure borehole system (2). 

The performance of the cross-measure 

system is affected by a number of design parameters, including location of 

methane-bearing strata in the roof, borehole length and angles with respect to 

the longwall panel (vertical and horizontal), pipeline diameter, borehole diameter and spacing, and operating characteristics of the surface pumping facility. 

In Europe, mathematical models have 

been developed for determining optimum 

design parameters. Unfortunately in the 

United States, "trial and error" is the 

only method currently available. This 

report describes further joint efforts by 

the Bureau and BethEnergy Mines Inc. to 

establish optimum design parameters that 

are necessary for controlling methane 

during retreat longwall mining in the 

Lower Kittanning Coalbed using the crossmeasure borehole technique. 

ACKNOWLEDGMENTS 

The authors thank the following person- Manager; R. F. Dodson, General Superinnel of BethEnergy Mines Inc., Cambria tendent; D. Weaver, Geological Engineer; 

Division, for their cooperation and N. Carpinello, Mine Foreman; and G. W. 

assistance in the study at the Cambria Moyer, General Assistant. 

33 Mine, Ebensburg, PA: E. J. Korber, 

STUDY AREA 

The study area was located in Pennsylvanian age strata of the Allegheny Group, 

specifically the Kittanning Formation 

(A). The Lower Kittanning Coalbed, locally called the B Seam, averages 36 to 

60 in (91 to 152 cm) in thickness in Cambria County. A detailed stratigraphic 

column was produced from the analysis of 

the core samples taken from a surface 

borehole drilled nearby the test longwall 

(fig. 1 Three coal groups are located 

about 15, 85, and 125 ft (5, 26, and 

38 m) above the roof of the Lower Kittanning Coalbed. A total of 9 ft (2.7 m) of 

coal lies within the first 130 ft (40 m) 

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

the end of the report. 

above the roof. These coalbeds are known 

sources of methane. 

The test retreating longwall is located 

between 16 Right and 17 Right off F West 

in BethEnergy Mines Inc.'s Cambria 33 

Kine (fig. 2). A detailed view of the 

test panel is shown in figure 3. All the 

cross-measure boreholes were drilled from 

the center entry and connected to an underground pipeline. A surface borehole 

was drilled between 16 Right and 17 Right 

to bring the gas to the surface. The 

center entry was supported with cribs to 

protect the underground pipeline and to 

prevent caving so that access to the 

pipeline and cross-measure boreholes 

could be maintained during life of the 

panel. 
Except for minor alterations, drilling equipment and procedures were similar to those developed in a prior study 

conducted in the Upper Kittanning Coalbed 

at the Cambria 33 Mine, Ebensburg, PA 

(2). The first 30 ft (9 m) of the crossmeasure borehole was drilled with a 4-in 

(10 cm) diamond core bit, and the cores 

were removed in 30-in (76 cm) sections. 

The remainder of the hole was drilled 

using a 1.9-in (5 cm) "stratapac" bit. 

The 1.6-in (4 cm) diameter drill rods 

were 15 in (38 cm) long, and no centralizers were used during drilling. 

After completion of the boreholes, a 

2.5-in (6 cm) diameter, 20-ft (6.1 m) 

plastic pipe was then grouted 18 ft (5.5 

m) into the hole using a cement-flyash 

slurry containing 50 vol pct flyash. An 

experienced two-person drill crew could 

set up the drill, drill the hole, and install and grout the plastic pipe in about 

ten 8-h shifts. 

METHANE REMOVAL SYSTEM 

Extraction of a coalbed by longwall 

mining fractures and breaks up the overlying strata and releases methane into 

the mine workings. By capturing and 

transporting methane through the crossmeasure borehole system, methane flows 

entering the mine's ventilation system 

can be reduced. 

Twelve cross-measure boreholes were 

drilled along the panel, and all holes 

were connected to a 6-in (15 cm) diameter 

polyethylene pipeline. The gas (air plus 

methane) was brought to the surface 

through a steel-cased, 8-in (20 cm) diameter borehole and discharged into the 

atmosphere. 
FIGURE 3. - Longwall test panel. 

Gas flow occurs from a cross-measure 

borehole only after partial undermining of each hole and application of a 

partial vacuum by an exhauster or vacuum 

pump. In this study, two exhausters were 

used (fig. 4). The capacity of the 

smaller exhauster ranged to 450 f t3/min 

(0.212 m3/s) at a partial vacuum of 8.65 

in Hg (29 kPa). The larger exhauster 

requires a minimum flow of 350 ft3 /min 

(0.165 m3/s), and maximum capacity is 

1,300 ft3/min (0.614 m3/s) at a partial 

vacuum of 12.6 in Hg (42 kPa). The 

smaller exhauster was used initially until the gas flow from the undermined 

cross-measure boreholes reached about 400 

f t3/min (0.189 m3 /s) , and thereafter the 

larger capacity exhauster was used. 
FIGURE 4. - Surface exhausters. 

CROSS-MEASURE BOREHOLE DESIGN 

The design of the cross-measure borehole system was similar to the design 

used in the Upper Kittanning Coalbed 

studies (2, - 4): - 

Vertical inclination angle - 28' (0.49 rad) 

Hole length - 280 ft (85 m) 

Terminal height - 132 ft (40 m) 

Horizontal angle - 45' (0.79 rad) 

Spacing - 200 ft (61 m) on 

first half of panel 

and 300 ft (91 m) on 

second half of panel. 

All holes were surveyed after completion of drilling to determine the actual 

path of the bit through the roof strata. 

In general, the surveys show that the 

holes turned upward more steeply and to 

the right because of clockwise rotation 

of the bit. 

Generally, holes will turn upward because of difference in diameters of 

bit and drill rods. For the equipment 

used in this study, a 1.9-in (4.8 cm) 

diameter bit and 1.6-in (4.1 cm) diameter rods were used. Consequently when 

drilling, the first 5 ft (1.5 m) of drill 

string tends to be cocked upwards in the 

hole, and hole trajectory turns upward 

more steeply. Applying excessive thrust 

also tends to turn the hole upward more 
varied from 50 to 65 (0.87 to 1.13 

cause of debris in the holes. 

SURFACE GOB HOLES 

Surface gob holes have been used at 

this mine since 1968 to assist in controlling methane in gobs. Figure 6 shows 

a typical borehole layout on longwall 

panels. Shorter than normal longwall 

panels usually employ only two surface 

holes. Generally, a low-pressure exhauster is mounted on the surface gob 

holes to draw the methane from the gob. 

The cross-measure borehole study was 

conducted on a test panel where only one 

surface gob hole could be drilled about 

1,300 ft (396 m) from the start of the 

test panel because of a populated area on 

the surface. Even this borehole was displaced about 75 ft (23 m) from the center 

line towards the return side of the longwall to avoid surface buildings. 

UNDERGROUND AND SURFACE INSTRUMENTATION 

Figure 7 shows the underground measurement instrumentation used in connecting a 

cross-measure borehole to the underground 

pipeline. Gas flow rates (air plus methane) were calculated from differential 

pressure measurements with a U-tube manometer across the pressure taps of a venturi. A handheld, 0- to 100-pet methanometer was used for determining the 

concentration of methane in the gas 

flows, and the partial vacuum at each 

borehole was measured with a manometer at 

the venturi. 

The total gas flow from the crossmeasure system was determined by summing 

the flows from the individual boreholes 

and by measuring the flow at the surface 

through an orifice plate installed in the 

surface piping (fig. 8). The differential pressure across the orifice plate 
I 

LEGEND 

x Equidistant spacing @ Surface gob hole --- Centerline 

--$. --------- -$ --------- $- -- --- 

4 X+-'X~XDirection of retreat b 

bb, 3,000 to 5,000 ft ,~

FIGURE 6. - Surface gob hole locations for longwoll panel. 

Plastic pipe (standpipe 1, 

/AY/r(\Y/AV/~Y/AY/X\V/&Y/AV /XV/~V/AV//C\Y/X\Y/AV/~V/N/X\ Y/AV/x\' 

Not to scale 

FIGURE 7. - Underground measurement instrumentation. 
- - 47 /--- 

FIGURE 8. - Dual exhauster arrangement. 

was continuously recorded, and the gas concentration of methane in the gas flow 

flow was calculated from the differen- was measured continuously by drawing a 

tial pressure measurements. All gas small quantity of gas through a recording 

flow measurements were corrected to infrared gas analyzer. A pressure tap 

mine atmospheric conditions so that com- was used to measure the partial vacuum at 

parisons could be made between under- the inlet to each exhauster. 

ground and surface measured flows. The 

GAS-WATER SEPARATOR SYSTEM 

The strata overlying a coalbed may contain several water-bearing formations 

that are penetrated by cross-measure 

boreholes. Some boreholes produce water 

when initially drilled; others produce 

water when the strata are fractured by 

the longwall mining operation; and some 

boreholes do not produce water at any 

time. Consequently, boreholes need to be 

fitted with gas-water separators to prevent the water, if any, from entering and 

blocking the underground pipeline. If 

the underground pipeline can be sloped 

so that water runs freely to one end of 

the pipeline, then individual gas-water 

separators are not necessary. The water 

can be discharged directly into the main 

pipeline and subsequently removed through 

a gas-water separator at a convenient 

locat ion. 

Most commercial gas-water separators 

are designed to operate in a positive 

pressure environment. When pressure is 

negative, the separators malfunction. 
Consequently, special gas-water separators must be designed and built to operate in an environment where partial 

vacuum conditions exist (5). These separators require expensivemachined parts, 

delicate adjustments, and maintenance. 

The Bureau has developed a separator that 

operates in a partial vacuum environment, 

contains only one moving part, and can be 

constructed of plastic pipe that is readily available (fig. 9) (&). This device 

requires only minimal maintenance. 

After a cross-measure borehole has been 

drilled, a 20-ft (6.1 m), 2.5-in (6.4 cm) 

diameter plastic pipe is grouted 18 ft 

(5.5 m) into the hole. About 2 ft (0.6 

m) of the pipe protrudes into the mine 

entry. A 2.5-in (6.4 cm) plastic "T" is 

fitted on the end of the pipe; this "T" 

provides support for a 1.25-in (3.2 cm) 

concentric gas pipe. Water flowing down 

the hole enters the annulus between the 

two concentric pipes and is discharged 

through a one-way check valve mounted on 

a pipe extension from the "T." Gas is 

drawn from the hole through the inner 

plastic pipe, which is fitted with an end 

cap and is slotted a short distance on 

its top surface only. The end cap prevents cascading water from entering the 

concentric pipe and accumulating in the 

underground pipeline. 

Typically, cross-measure boreholes are 

inclined about 30" (0.52 rad). Consequently, the vertical height from the 

roof to the end of the 2.5-in (6.4 cm) 

plastic pipe is about 9 ft (2.7 m). 

About 2 ft (0.6 m) is added for the extension pipe, which makes a total vertical height of 11 ft (3.4 m). When a 

2.5- in-l 0 PVC pipe 

height 

\ I 

Water discharge Not to scale 

Check valve 1 

FIGURE 9. - Gas-water separator. 
partial vacuum is applied to the hole, Small particles of roof rock may be 

water accumulates in the annulus until washed down the hole and accumulate at 

the vertical height of the water column the pipe extension just before the oneis balanced by the applied vacuum. Any way check valve. Eventually, the pipe 

further accumulation of water is dis- extension becomes blocked and water is 

charged through the one-way check valve. prevented from discharging through the 

For example, suppose the applied vacuum 

is 8 in Hg (27.2 kPa); then the vertical 

height of the water column is about 9.1 

ft (2.8 m). The water column extends 

about 14.2 ft (4.3 m) along the hole in 

the annulus above the roof line. The 

maximum partial vacuum that can be applied to the hole is about 9.7 in Hg (33 

one-way check valve. The water will accumulate in the annulus until the slot in 

the gas drainage pipe is reached and then 

discharge-into the underground pipeline. 

Consequently, the underground pipeline 

should be fitted on the bottom side with 

one-way check valves located in low spots 

along the pipeline. When the surface exkPa). In this case, the water column may hausters are shut off periodically, the 

reach the slot in the concentric gas one-way check valves open automatically 

drainage pipe, and consequently water and discharge the accumulated water in 

will discharge directly into the under- the pipeline. All cross-measure boreground pipeline instead of through the holes should be inspected to ensure that 

one-way check valve. In the event the rock particles are not accumulating in 

cross-measure borehole does not produce the pipe extension near the one-way check 

water, then the one-way check valve re- valve. 

mains closed and prevents mine air from 

entering the gas drainage line. 

DATA ANALYSES 

LONGWALL PRODUCTION Table 2 summarizes the average gas and 

methane flow rates from the boreholes. 

The test longwall is 585 ft (178 m) Flow rates are much greater where partial 

wide and 2,750 ft (838 m) long. The vacuum is low (holes 4, 9, and 12), indipanel was worked 3 shifts per day but in 

a predominantly 3-d/wk schedule because 

of the low demand for coal (fig. 10). On 

working days, the panel was retreated an 

average of 40 ft/d (12 m/d). Average 

coal 'production was 3,800 ton/d (3,447 

mt/d), which is above average coal production in a 4-ft (1.2 m) coalbed (7). 

The longwall panel was completely miTed 

in 69 working days, which were spread 

over a 240-day interval and included 

cating a highly permeable gob compared 

with those cases were partial vacuums are 

high (holes 3, 8, and 10) and flows lower. Approximately 71 pct of the methane 

produced by the longwall mining operation 

was captured by the cross-measure borehole system. 

three idle periods of about 73, 32, and 

10 days (fig. 10). 50 I 0 t ? 

CROSS-MEASURE BOREHOLE GAS PRODUCTION 

Table 1 shows all boreholes produced 

gas except hole 7. Other holes (holes 1, 

3, 4, 6, and 8) were shut in for short 

periods totaling 6 to 120 nonconsecutive 

days during the-life of the panel because o 40 

of excessive water flow. low methane conTIME, days 

centration, or blockage. FIGURE 10. - Daily longwall coal production. 
TABLE 1. - Production history of boreholes 

Comments 

Water problems and low methane 

Shut down 

periods, days 

Hole 

concentration. 

None. 

Blocked by debris. 

Water problems. 

None. 

Water problems. 

No gas production. 

Water problems. 

None. 

None. 

None. 

None. 

Production day 

Startup I Termination 

'NO productive life. 

TABLE 2. - Average methane and gas flows from boreholes 

Hole I Partial vacuum, Methane I Av. flow rate. ft3/min 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

conc, pct 

73 

3.7 

5.5 

1.9 

3.4 

3.1 

(9 

4.1 

1.6 

5.8 

3.9 

2.6 

I 

I Gas 

138 

112 

50 

118 

116 

84 

(9 

58 

121 

39 

136 

124 

2~o productive life. 

Methane 

100 

70 

30 

Most boreholes did not produce gas until a partial vacuum was applied and the 

longwall face passed 75 to 100 f t (23 to 

30 m) beyond the end of the borehole but 

before the face reached its collar. 

Holes 5 and 10 were exceptions. A free 

flow of methane [60 ft3/min (0.028 m3/s)] 

was measured from hole 5 before the face 

reached its collar. After the face 

passed beyond the collar, the free flow 

stopped and a partial vacuum was required 

to maintain flow. Gas production occurred from hole 10 only after the face 

passed well beyond its collar. 

Holes 10 and 11 were drilled from the 

same location; they terminated 167 and 

91 ft (51 and 28 m), respectively, above 

the mined coalbed (fig.. 5, column D). 

Table 2 shows that the average gas and 

methane flows from hole 11 were about 3 

times larger than flows from hole 10, 

suggesting that boreholes drilled lower 

in the gob are more productive. However, 

the production life of these boreholes 

was short because mining of the panel was 

completed and the study terminated shortly after the boreholes were undermined. 

If the boreholes had been drilled further 

from the end of the panel, long-term production history may have shown that the 

concentration of methane in the gas flow 

from hole 11 dropped significantly because of its low position in the gob. 

gob. More tests are needed to evaluate 

the effects of terminal height of boreholes on methane concentrations in the 

gas flow. 
The gas flow from the cross-measure 

system and the number of boreholes producing gas are shown in figure 11. Figure 12 shows the methane concentrations 

in the gas flow. During idle periods, 

the flow of gas from the cross-measure 

system remains relatively constant (fig. 

11) but the methane concentration in the 

gas flow drops abruptly (fig. 12). When 

mining resumed, the increase in methane 

concentration was as abrupt as the decline. The corresponding methane flows 

are shown in figure 13. Also shown are 

the times when each borehole started 

methane production, which is greatest 

when it first goes on production and is 

generally represented by a small peak in 

the graph. The dependency of methane 

flow rates on mining is clearly demonstrated by the prolonged idle periods. 

The first idle period (73 days) shows 

that methane flow declined from about 400 

to 100 ft3/min (0.189 to 0.047 m3/s) and 

when mining resumed, the flow increased 

again to about 400 ft3/min (0.189 m3/s). 

These data indicate that about 75 pct of 

the methane in the gob comes from newly 

fractured roof strata near the face. 

The variations in methane concentrations in the gas flows from individual 

cross-measure boreholes were very similar 

to methane concentration variations at 

the surface exhauster (fig. 12). Flow 

from hole 2 is typical of methane concentration variations in gas flow from individual cross-measure holes (fig. 14). 

The partial vacuum at the inlet to 

the surface exhauster averaged about 8.5 

in Hg (28.8 kPa) during mining and rose 

to above 11 in Hg (37.3 kPa) during 

idle periods (fig. 15). The decline in 

methane flows during idle periods (fig. 

13) is compensated by increased flow of 

air (fig. ll), which must be drawn from 

the mine upward into the gob and finally into the cross-measure system. This 

causes the exhauster to work harder to 

maintain a constant flow. Because methane is more readily available when mining 

resumes, the partial vacuum at the exhauster declines and air flow into the 

cross-measure system is reduced. 

NUMBER OF PRODUCING BOREHOLES 

1213 4 5 44 45 679 6 78 

i-7 TT1 

KEY 

- x Idle per~od 

TIME, days 

FIGURE 11. - Total gas flow from cross-measure 

system. 

+x* t+" lyt 

.- IIIIIIIII 

0 40 80 120 160 200 240 

TIME, days 

FIGURE 12. - Surface methane concentrations 

in gas flow. 

HOLE PRODUCTION STARTUP 

KEY 

- 

0 40 80 120 160 200 240 

TIME, days 

FIGURE 13. - Methane flow from cross-measure 

system. 
0 40 80 120 160 200 240 

TIME, days 

FIGURE 14. - Methane concentrations in gas 

flow ftom hole 2. 

a 

> 8 KEY 

-1 x Idleperiod 

0 40 8 0 120 160 200 240 

TIME, days 

FIGURE 15. - Partial vacuum measured at surface exhauster. 

METHANE MIGRATION THROUGH GOBS 

The numbers shown in figure 14 represent the distances between hole 2 and 

the moving face. During the first idle 

period, the concentration of methane in 

the gas flow from the hole declined from 

about 90 to 30 pct in 20 days. When 

mining resumed on day 140, the methane 

concentration in the gas flow began to 

increase and reached 100 pct on day 178, 

when the face of the panel was 1,630 ft 

(497 m) fromhole 2. During the second 

idle period (day 180 to 210), the methane 

concentration again declined and then increased and reached 90 pct after mining 

resumed. At this time the distance between face and hole 2 was 2,120 ft (646 

m). These data clearly indicate that 

methane migrates through the gob for distances of at least 2,120 ft (646 m). A 

Bureau study in the Pittsburgh Coalbed 

showed that an increase in gas flow from 

a vertical gob hole was attributable to 

an increase in mining rate at the face, 

which was 2,800 ft (853 m) from the gob 

hole. 

BOREHOLE SPACING 

Application of a partial vacuum to a 

cross-measure borehole creates a lowpressure zone around the borehole in the 

gob. If boreholes are properly spaced, 

these low-pressure zones overlap and create a continuous low-pressure zone within the gob. Methane migrating downward 

and some mine air and methane from lower 

levels in the gob will be drawn upward 

toward the low-pressure zone and captured 

by the cross-measure system. If the lowpressure zones around each hole do not 

overlap, which indicates boreholes are 

spaced too far apart, then methane from 

higher levels in the gob will migrate into the mine opening between boreholes. 

Borehole spacing depends upon gob permeability and operating characteristics 

of the surface pump or exhauster. Because little is known about gob permeability, borehole spacing is best determined from underground interference 

tests. During this test, all boreholes 

are shut-in and the gas pressure in the 

gob is allowed to stabilize. The boreholes are then opened to flow, except for 

one borehole (test borehole) which is 

monitored for changes in gas pressure. 

If no pressure change occurs, adjacent 

boreholes are spaced too far apart. If a 

slight pressure change occurs, spacing is 

adequate. 

Interference tests were conducted on 

hole 1 near the start of the panel, on 

hole 5 near the center of the panel, and 

on hole 11 near the end of the panel. 

All test results were similar to the interference test on hole 11 (fig. 16). 

When all holes were shut-in, the negative 

gas pressure in the gob around hole 11 

quickly dissipated, and in 5 min, gas 

pressure in the gob reached a positive 

0.28 in H20 (0.07 kPa) with respect to 

the mine environment. Then holes downstream from hole 11 (hole 2, 4, 5, 6, and 

9) were open to flow, and within a few 

moments, gas pressure at hole 11 declined 

and stabilized at a partial vacuum of 

0.15 in H20 (0.04 kPa). Hole 9 is spaced 

220 ft (67 m) from hole 11; the test 
indicated that the spacing is adequate. 

When hole 12 (located 280 ft (85 m) upstream from hole 11) was opened to flow, 

the partial vacuum at hole 11 increased 

further to 0.5 in H20 (0.12 kPa). These 

data appear to indicate that a borehole 

spacing of 220 to 280 ft (67 to 85 m) is 

reasonable for the fractured strata above 

the Lower Kittanning Coalbed. 

METHANE FLOW IN THE RETURN AIR 

Figure 17 shows the flow of methane in 

the return air from the longwall and the 

times each cross-measure borehole started 

gob gas production. Measurements taken 

at the tailgate of the longwall indicate 

a methane flow due to mining of 30 ft3/ 

min (0.014 m3/s) across the face and no 

measurable flow after mining stops. 

The effect of each borehole on methane flow in the return air is clearly 

demonstrated by the large decrease in 

flow in the return immediately after 

each borehole starts gob gas production. 

These peak flows range from about 200 to 

450 ft3/1nin (0.142 to 0.212 m3/s) and 

suggest that borehole spacing should be 

reduced. 

After mining of the panel was started, 

the first large roof fall was accompanied 

by large flows of methane and mining was 

suspended for short periods because sufficient air was not available to maintain 

methane concentrations in the returns at 

permissible levels. Generally only one 

borehole is on production at this time, 

and the quantity of gob gas that can be 

drawn through a 1.9-in (4.8 cm) diameter 

borehole is small in comparison to the 

tremendous quantities generated in the 

gob. Consequently, gob gas begin to 

spill into and overload the return air 

system. Additionally, boreholes drilled 

between holes 1 and 2 and between holes 2 

and 3 would prevent large peak flows and 

would lower the general level of methane 

flow in the return. Therefore, boreholes 

should be spaced 100 ft (30 m) apart 

along the first 600 ft (183 m) of the 

longwall and thereafter 200 ft (61 m) 

apart. Although the data presented in 

figure 16 appear to indicate that a borehole spacing of 220 to 280 ft (67 to 85 

m) is possible, the smaller spacing 

should prevail to prevent or to reduce 

0 

N 

I 

W 

E 

3 - cn -1 cn Hole 12 turned on W 

. J 

0 10 20 30 40 50 60 70 

TIME, min 

FIGURE 16. - Interference test using hole 11. 

HOLE PRODUCTION STARTUP 

C - 12 3 456 89 11 10,12 

I I1 I I I I 

KEY 

TIME, days 

FIGURE 17. - Methane flow in return air. 

Open boreholes 

100 o Shut ~n boreholes 

50 o 500 1,000 1,500 2,000 2,500 

TIME, min 

FIGURE 18. - Effects of cross-measure system 

on methane flow in return air. 

the severity of peak methane flows (fig. 

17). The large peak methane flow around 

day 180 (fig. 17) was caused by a partially water blocked pipeline and a nonproductive cross-measure borehole (hole 

7). Hole 7 was undermined around day 160 

(fig. 17). 

When the cross-measure system is shutin, methane gradually refills the gob 

area and then begins t o s p i l l over into 

the return airways. Methane flows in the 

return gradually increase and more than 

double in a 20-h period (fig. 18). When 

the system is back in operation, methane 

flows declined to pretest conditions in 

about 4 h. These tests could only be 

conducted during nonmining periods or 
over weekends. This test was 

on day 78 when the first five 

had been intercepted and were 

gob gas (fig. 17). 

WATER PRODUCTION 

All boreholes were equipped 

water separators. No water 

curred from holes 2, 3, 5, 1 

conducted 

boreholes 

producing 

with gasflows oc1, and 12 

during mining of the test panel. Initial 

water production from holes 4 and 7 was 

27 and 10 gal/min (102 to 38 L/min), respectively. When undermined, water flow B 

from hole 4 declined to a negligible 

quantity and that trom hole 7 declined 

to a steady 1.5 gal/min (5.7 L/min). 

All other boreholes (hole 1, 6, 8, 9 and 

12) produced water at rates from 0.25 to 

1.5 gal/min (0.95 to 5.7 L/min) after 

undermining. 

SURFACE GOB HOLE PRODUCTION 

The surface gob hole produced gob gas 

for about 6 days and was finally shut-in 

because of low methane concentrations in 

the gas flow. The hole vented about 

1,200,000 ft3 (34,000 m3) of methane during its short productive life. Gas production started on day 156; no indications exist that the gob hole affected 

methane flows in the return air from the 

longwall (fig. 17), in spite of the fact 

that the end of hole 7 (nonproductive) is 

in the same area as the surface gob hole - 

(fig. 3). FIGURE 19. - Gas flow into boreholes. 

MIGRATION OF GAS INTO A BOREHOLE 

Because little is known about the migration behavior of methane in the gobs 

or the inflow of gas into a borehole, gas 

pressure differentials associated with 

flow of gas from boreholes were calculated for two widely separated cases and 

then compared with the measured values. 

In the first case, gob gas was assumed to 

flow into the borehole nearest the pillar line (fig. 19~). The remainder of 

the borehole in the gob is nonproductive. 

In the second case, gob gas is assumed 

to flow into the borehole uniformly 

along the length of borehole in the gob 

(fig. 19B). The following equation was 

used to compute pressure differentials 

for the two cases (5): - 

where Ap = pressure differential (in 

Hg) , 

1 for case 1 (fig. = 113 for case 2 (fig. 

A = coefficient of resistance 

(dimensionless) , 
L = pipe or hole length (ft), 

d = pipe or hole diameter (ft), 

p = gas density (lb/ft3), 

B = average gas velocity (ft/s), 

and g = accelerations of gravity 

(f t/s2) . 

Table 3 summarizes the measured and 

calculated pressure differentials. In 

most instances, the calculated pressure 

differentials for case 1 (fig. 19A) appeared to match the measured data more 

closely than case 2 (fig. 19B). Holes 4, 

9, and 10 were exceptions. 

The measured pressure on hole 10 on day 

241 (table 3) is much greater than the 

computed pressure, because the gob around 

hole 10 at this time is tight and gas 

does not readily enter the hole. On day 

260, the measured and calculated pressures (case 1) agree reasonably well and 

indicate the gob is more permeable and 

gas flows easily into the borehole. 

For both holes 4 and 9, the measured pressures are much less than the 

calculated pressures (cases 1 and 2). 

However, if one assumes that gob gas enters the segment of the borehole over the 

pillar (fig. 19C), then the measured and 

computed pressures are more nearly in 

agreement. 

The data presented in table 3 appear 

to indicate that borehole length can be 

reduced from 280 to 140 ft (85 to 43 m), 

because gob gas enters the boreholes 

only near the pillar line. The remainder 

of the borehole in the gob is nonproductive. Reducing the length of boreholes 

makes the cross-measure system more cost 

effective. 

An experienced drill crew requires 

about eight shifts to drill a 280-ft 

(85 m) cross-measure borehole. If hole 

length could be reduced significantly without affecting its performance, 

a considerable savings in labor costs 

can be effected. For example, if the 

drilling time is cut to about four shifts 

per hole, then about 768 person-hours 

are saved in drilling 12 cross-measure 

boreholes into rock strata above a retreating longwall. Assuming an hourly 

rate of $10, about $8,000 can be saved 

per longwall. 

TABLE 3. - Comparison of measured and calculated gas pressure 

differentials 

Hole 

1 

2 

4 

5 

6 

9 

10 

11 

12 

Pressure 

Measured 

3.8 

3.5 

4.8 

3.8 

2.7 

1.6 

1.6 

4.7 

3.2 

3.6 

3.2 

1.2 

1.3 

5.4 

3.9 

3.9 

4.3 

2.9 

3.0 

Day 

24 

49 

23 

247 

79 

207 

220 

122 

220 

207 

220 

220 

260 

241 

260 

220 

233 

234 

247 

Gas flow differential, in Hg 

rate, ft3/min 

142 

105 

133 

96 

109 

116 

123 

129 

116 

95 

90 

138 

119 

3 1 

113 

135 

119 

122 

101 

Case 1 

4.8 

3.5 

5.5 

3.9 

3.0 

5.0 

3.9 

5.0 

3.1 

3.2 

2.1 

4.2 

4.9 

.2 

4.4 

3.7 

3.7 

3.2 

3.4 

Methane 

conc., pct 

92 

45 

100 

52 

95 

43 

100 

50 

100 

48 

100 

100 

28 

100 

2 1 

100 

48 

95 

20 

Calculated 

Case 2 

8.2 

6.1 

8.0 

5.6 

5.0 

8.3 

6.5 

8.5 

5.3 

5.1 

3.2 

7.8 

9.0 

.3 

7.8 

6.1 

6.1 

5.0 

5.4 
SUMMARY AND CONCLUSIONS 

The cross-measure borehole technique 

is an effective method of controlling 

methane in gobs in the Lower Kittanning 

Coalbed. About 71 pct of the methane 

produced by the longwall mining operation was captured by the cross-measure 

boreholes. 

Interference tests and measurements of 

methane flows in return air indicate 

that borehole spacing should be limited 

to 200 ft (61 m) except on about the 

first 600 ft (183 m) of the longwall, 

where spacing should be reduced to 100 ft 

(30 m). More boreholes are necessary 

near the start of the longwall to capture 

the large quantities of methane that are 

released when the first large roof fall 

occurs and to prevent it from overloading the return air system. 

About 75 pct of the methane in the gob 

emanates from fractured roof strata near 

the face. Prolonged idle periods showed 

that low methane concentrations in the 

gob located over 2,000 ft (610 m) from 

the face increase to above 90 pct after 

mining resumes. 

Comparisons between measured and calculated gas pressure differentials in boreholes indicate that gob gas enters the 

boreholes close to the pillar line, which 

is about 140 ft (43 m) from the collar of 

the borehole. The remainder of the borehole is nonproductive. Reducing borehole 

length from 280 to 140 ft (85 to 43 m) 

will lower drilling cost per longwall by 

about $8,000 provided the shorter boreholes are as effective as the longer 

boreholes. Additional experimentation is 

necessary to verify these calculations. 

REFERENCES 

1. Cervik, J. Methane Control on 

Longwalls - European and U.S. Practices. 

Ch. in Longwall-Shortwall Mining, Stateof-the-Art. Soc. Min. Eng. AIME, 1981, 

pp. 75-80. 

2. Campoli, A. A*, J. Cervik, and 

S. J. Shatzel. Control of Longwall Gob 

Gas With Cross-Measure Boreholes (Upper 

Kittanning Coalbed). BuMines RI 8841, 

1983, 17 pp. 

3. Iannacchione, A*, and D. G. Puglio. 

Geology of the Lower Kittanning Coalbed 

and Related Mining and Methane Emission 

Problems in Cambria County, PA. BuMines 

RI 8354, 1979, 33 pp. 

4. Schatzel, S. J., and G. L. Finfinger, and J. Cervik. Underground Gob Gas 

Drainage During Longwall Mining. BuMines 

RI 8644, 1982, 14 pp. 

5. Boxho, J., P. Stassen, G. Mucke, 

K. Noack, C. Jeger, L. Lescher, E. J. 

Browning, R. Dunmore, and I. He Monis. 

Firedamp Drainage. Verlag Gluckauf GmbH, 

Essen, 1980, pp. 130-133. 

6. Campoli, A. A*, and F. Garcia. 

Self -Activating ~as/~iquid Separator. 

U.S. Pat. 4,500,329, Feb. 19, 1985. 

7. Peake, C. J. Longwall Productivity 

Shows Solid Growth. Coal Age, v. 89, No. 

8, Aug. 1984, pp. 61. 

a U.S. GPO: 1985-505-019/20,108