Bureau of Mines Report of Investigations/l986 

Comparisons Between Cross- Measure 

Boreholes and Surface Gob Holes 

By T. W. Goodman and J. Cervik 

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

Donald Paul Hodel, Secretary 

BUREAU OF MINES 

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

d 

b 

Goodman, Tobias W 

Comparisons between cross-measure boreholes and surface gob 

holes. 

(Report of investigations ; 90 13) 

Bibliography: P. 14. 

Supt. of Docs. no.: I 28.23: 9013. 

1. Coalbed methane drainage-Pennsyl vania-Ebensburg RegionTesting. 2. Longwall mining-Pennsy lvania-Ebens burg Region. 3. 

Boring-Pennsylvania-Ebensburg Region. I. Cervik, Joseph. 11. Title. 

111. Series: Report of investigations (United States. Bureau of Mines) ; 

90 13. 

TN23,U43 [TN305] 622s [622',4] 85-6003 10 
CONTENTS 

Page 

Abstract ...........eeeeeeeeee.eeeeeeeeeeeeeeeeeeeeeee.eeeeeeeeeeeeeeeeeeeeeeee. 

Introduction .......eeeeeeee.eeeeeeeeeeeeeeeeeeeeeeeeee.eeeeeeeeeeeeeeeeeeeeeee. 

Acknowledgments ....eeeeee.ee..eeee..eee.ee..e.eeee...e..eeee.eeeeeeee.ee.ee.e.e 

Study area ..................................................................... Cross-measure borehole system .................................................. Drilling equipment and procedures ..........e..eeee..eee.e.ee.eeeeee.eeeee.e.e 

Underground instrumentation ....eeee.eeee.ee.ee......eeeeee.eee.eeeeeee.ee.eee 

Methane removal .............................................................. Gas-water separator .......................................................... Cross-measure borehole design ................................................ Surface gob holes .............................................................. Data analysis .................................................................. Panel A ...................................................................... Cross-measure system-- gas production ....................................... Surface gob hole-- gas production ........................................... Panel B ..................e.eeee.e.e.e.ee.e.eee..ee.eeeeeeeeeeeee.e..e.eee.eee 

Cross-measure and surface gob holes-- gas production ........................ Capture ratios .............eeeeeeee.eeeeee.ee.e...e.e.ee.e.eeeeeeee..ee.eee 

Gob gas migration into a borehole .............................................. Summary and conclusions .........eee...eeee.ee.e.e..e.e..ee.....ee...e.e......ee 

References ...........eeeeeee.ee....ee.ee..eee.....e.eeeeeeee.eee.eee...eee..... 

ILLUSTRATIONS 

Stratigraphic column above Lower Kittanning Coalbed ....................... BethEnergy Mines Inc., Cambria 33 Mine .................................... Longwall test panels ...................................................... Underground measurement instrumentation ................................... Surface exhausters ........................................................ Gas-water separators ...................................................... Surface gob hole location plan ............................................ Daily longwall coal production (panel A) .................................. Methane flow from cross-measure system (panel A) .......................... Methane concentrations in gas flow from hole 2 (panel A) .................. Metharfe flow in return air (panel A) ...................................... Daily longwall coal production (panel B) .................e.ee..eee.e.e.eee 

Partial vacuum measured at surface pump (panel B) ......................... Partial vacuum measured at hole 1 (panel B) ............................... Gas and methane flows from cross-measure boreholes (panel B) .............. Methane flow from surface gob holes (panel B) ............................. Methane concentration in gas flow from the cross-measure system (panel B) . Methane flow in return air (panel B) ...................................... Effectiveness of cross-measure system and surface gob holes (panel B) ..... Migration of gas into a borehole .......................................... Comparison of observed and calculated partial vacuums on hole 3 (panel B) . 

TABLES 

1 . Design parameters for cross-measure bore hole^..........^^^^^^^.^^.^.^^..^^ 

2 . Panel A and B capture ratios .............................................. 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

centimeter kPa kilopascal 

degree lb/f t3 pound per cubic 

foot 

day per week 

m meter 

foot 

m3 cubic meter 

foot per second 

m3 /s cubic meter per 

foot per square second second 

cubic foot per minute Pet percent 

hour rad radian 

inch st/d short ton per day 

inch of mercury 

(pressure) 
COMPARISONS BETWEEN CROSS-MEASURE BOREHOLES 

AND SURFACE GOB HOLES 

By T. W. Goodman ' and J. ~ervi k 

ABSTRACT 

The Bureau of Mines conducted studies to compare the effectiveness of 

the cross-measure borehole system with that of surface gob holes on two 

successively mined retreating longwalls in the Lower Kittanning Coalbed. 

Only the cross-measure system was in operation on panel A; on panel B, 

both systems were in operation. In addition, the cross-measure boreholes on panel B were drilled parallel to the face, whereas those on 

panel A were drilled at a 45O angle vith respect to the longwall axis. 

The studies show that the cross-measure system captured 71 pct of the 

methane generated by longwall mining on panel A. On panel B, the surface gob hole dominated over the cross-measure system for about 700 ft 

(213 m) after it was intercepted. Beyond that point, the cross-measure 

system dominated. About 93 pct of the methane generated on panel B was 

captured by the two systems. 

Cross-measure boreholes that were drilled parallel to the face were as 

effective as boreholes oriented 45" with respect to the longwall axis. 

In addition, they are shorter and require less drilling time. Comparison between observed gas pressure differentials in boreholes and calculated values indicate that boreholes can be shortened further to 140 

ft (43 m). Additional studies are needed to verify these calculations. 

'13ining engineer. 

2~upervisory geophysicist. 

Pittsburgh Research Center, Bureau of ~ines, Pittsburgh, PA. 
INTRODUCTION 

The methods of controlling methane 

during longwall mining in the United 

States are ventilation and surface gob 

holes. Generally, three to four surface 

gob holes are drilled over a 5,000-ft 

(1,5249) panel. Gob holes cannot always 

be drilled because of rough topography 

and/or right-of-way problems, or because 

mining may be under populated areas. 

The cross-measure system has been indispensable in degasifying longwall gobs 

in Europe since Germany first introduced 

methane drainage using piping systems in 

1943 (1) .3 The European coal basins tend 

to be- deeper and steeper dipping, and 

more frequently are located under populated areas than are those in the United 

States (2). The single-entry longwalls 

in ~uro~e are usually advanced, in contrast to the predominantly retreated 

longwalls in the United States. This 

prevents immediate technology transfer 

and generates the need for research in 

this area. 

The Bureau of Mines conducted several 

studies structured to develop design 

criteria for the cross-measure technique. 

Two of the preliminary studies (3-4) were -- sited in the Upper Kittanning Coalbed, 

where the European technology underwent 

feasibility tests. Adaptations, experimentally determined in those tests, have 

been introduced in two studies conducted 

on adjacent panels in the Lower Kittanning Coalbed. The objectives of the 

studies in the Lower Kittanning Coalbed 

were to compare the effectiveness of the 

cross-measure technique and surface gob 

holes in controlling gob gas during retreat longwall mining and to further modify European procedures to meet the needs 

of the U.S. mining industry. 

ACKNOWLEDGMENTS 

Special thanks are due to the follow- Korber, manager; R. F. Dobson, general 

ing officials of BethEnergy Mines Inc., superintendent; D. Weaver, geological enCambria Division, for their coopera- gineer; N. Carpinello, mine foreman; 

tion and help in this study at the G. W. Moyer, general assistant; and 

Cambria 33 Mine, Ebensburg, PA: E.J. W. Miller, longwall foreman. 

STUDY AREA 

The studies were conducted in the Lower 

Kittanning Coalbed, which is about 48 in 

(1.2 m) thick. The stratigraphic column 

shows there are three coal groups located 

about 15, 85, and 125 ft (5, 26, and 38 

m) above the roof of the Lower Kittanning 

Coalbed (fig. 1). A total of 9 ft (2.7 

m) of coal lies within the first 130 ft 

(40 m) above the roof. These coalbeds 

are known sources of methane. Overburden 

is about 645 ft (197 m). 

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

the end of this report. 

The test retreating longwalls are located between 16 Right and 18 Right off F 

West in BethEnergy Mines, Inc., Cambria 

33 Mine (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 entries were supported with cribs 

to protect the underground pipelines and 

to prevent caving so that access to the 

pipelines and cross-measure boreholes 

could be maintained during the lives of 

the panels. 
FIGURE 3. - Longwoll test panels. 
partial vacuum at each borehole was 

measured with a manometer at the inlet venturi tap 1. Return air from each 

longwall was channeled through a single 

entry (evaluation point) before it entered the main returns (fig. 3). Ventilation surveys were made in these locations using a methane detector and an 

anemometer. The total gas flows from 

Plastic pipe (standpipe ). 

-in main pipeline 

Not to scale 

FIGURE 4. - Underground measurement instrumentation. 

min (0.19 m3/s), ranged to 450 ft3/min 

(0.21 m3/s) with partial vacuums up to 

8.65 in Hg (29 kPa). The larger exhauster, used thereafter, requires a minimum 

flow of 350 ft3/min (0.17 m3/s). Maximum capacity is 1,300 ft3/min (0.61 m3/s) 

with partial vacuums up to 12.6 in Hg 

(42 kPa) . 

GAS-WATER SEPARATOR 

Each borehole was equipped with a 

Bureau-designed gas-water separator (6). 

Figure 6 shows the design of the separator used on each borehole on panel A. 

In some instances, water blocking 

problems developed when rock particles 

washed down the hole and accumulated in 

the annulus between the gas drainage pipe 

and the standpipe or in the pipe extension near the one-way check valve. When 

this occurred, the water accumulated in 

the annulus until the slot in the gas 

drainage pipe was reached and then discharged into the underground pipeline. To circumvent this problem, the 

first 30 ft (9 m) of each cross-measure 
FIGURE 5. - Surface exhausters. 

borehole on panel B was enlarged from 

4 to 6 in (102 to 152 mm) so that the 

standpipe diameter could be increased 

from 2.5 to 4 in (64 to 102 mm) and the 

gas drainage pipe diameter from 1.5 to 2 

in (38 to 51 mm). Thus the annulus size 

was increased by a factor of 2 over the 

holes on panel A. No debris accumulated 

in the annulus of boreholes on panel B, 

and little downtime occurred due to water 

bypassing the separator. The separators, 

however, should be checked periodically 

to ensure that no rock particles are accumulating in the pipe extension near the 

one-way check valve. 

Not to scale CROSS-MEASURE BOREHOLE DESIGN 

The design parameters for the crossmeasure boreholes on panel A are similar 

to the design used in earlier Bureau FIGURE 6. - Gas-water separators. studies (3-4). - - These boreholes were 
TABLE 1. - Design parameters for cross-measure boreholes 

drilled 45' with respect to the axis on mined coalbed, and even though the borethe longwall, which is a common practice holes of panelB are 55 ft (17 m) shorter 

in Europe. On panel B, the cross-measure than those of panel A, the penetration 

boreholes were drilled perpendicular to depth of the boreholes into the gob 

the axis on the longwall. This is a de- is about the same for the two panels. 

Borehole parameters 

Angle, deg: 

Vertical............................ 

Horizontal.......................... 

kngth............................ft.. 

Terminal height.......... ........ .ft.. 

Spacing, ft: 

1st half of panel................... 

2d half of panel.................... 

parture from the European design, where (The penetration depth is the horizontal 

the holes must be drilled diagonally to projection of the hole parallel to the 

reach the gas ahead of the face in ad- line of the face.) All holes were 

vancing longwall systems. In European surveyed after completion of drilling tc 

retreating systems, the holes must be determine the actual path of the bit 

dismantled when the face reaches the through the roof strata. In general, the 

Panel A 

28 

45 

280 

130 

200 

300 

collar of the hole owing to caving of the surveys showed that the holes turned 

single entry. upward more steeply and to the right 

Table 1 summarizes the design param- because of clockwise rotation of the 

eters for the two panels. All boreholes bit. 

terminate about 130 ft (40 m) above the 

Panel B 

35 

90 

225 

130 

200 

300 

SURFACE GOB HOLES 

Surface gob holes have been used at the 

Cambria 33 Mine since 1968. A typical 

borehole plan is shown in figure 7. The 

first gob hole is located about 400 

ft (122 m) from the starting face, and 

others are spaced equidistant along the 

remainder of the panel. 

The surface gob hole fan is energized 

after the hole is undermined by the 

longwall miner. When the concentration 

LEGEND x Equidistant spacing @ Surface gob hole 

Centerl~ne 

.--$- q+ + -----A. 

x-i--x+x 

Direction of retreat - 

FIGURE 7. - Surface gob hole location plan. 

of the methane drops below 25 pct, the 

hole is shut in. Methane concentration 

and flow readings are made daily during 

mining of the longwall panel, using a 

methane detector and anemometer. 

For panel A, only one surface gob hole 

could be drilled because of a populated 

area on the surface (fig. 3). It was 

drilled about 1,300 ft (396 m) from the 

start of the panel. Even this borehole 

had to be displaced about 75 ft (23 m) 

from the centerline toward the return 

side of the longwall to avoid surface 

structures. Panel length is 2,700 ft 

(823 m). For panel B, two surface gob 

holes were drilled 650 and 1,350 ft (198 

and 411 m) from the start of the panel. 

Both surface gob holes were displaced 

from the centerline of the panel because 

of surface right-of-way problems. Panel 

B is 2,340 ft (713) long. 
DATA ANALYSIS 

PANEL A 70 

Panel A was worked three shifts per day 

but on a predominantly 3-d/wk schedule 

because of the low demand for coal (fig. 

8). The panel was completely mined in 69 

working days, which were spread over a 

240-day interval and included three idle 

periods of 73, 32, and 10 days. 

Cross-Measure System--Gas Production 

Most boreholes did not produce gas 

until a partial vacuum was applied and 

the longwall face passed 75 to 100 ft (23 

to 30 m) beyond the end of a borehole 

(but before the face reached its collar). 

Holes 5 and 10 were exceptions. A free 

flow of methane 160 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. 

The methane flow from the cross-measure 

system is shown in figure 9. Also shown 

are the times when each borehole started 

methane production, which is greatest 

when the borehole first goes on production. Methane flow from the crossmeasure system gradually increased as the 

number of boreholes undermined increased. 

That methane flow rates are dependent on 

whether coal is actively being mined is 

clearly demonstrated by the much lower 

flow rates during prolonged idle periods. 

During the first idle period (73 days), 

methane flow declined from 400 to 100 

ft3/min (0.19 to 0.05 m3/s); when mining 

resumed, the flow increased and returned 

to about 400 ft3/min (0.19 m3/s). These 

data indicate that about 75 pct of the 

methane in the gob comes from the newly 

fractured roof strata near the face. 

Figure 10 shows the variations in methane concentration in the gas flow from 

hole 2; they are typical of the methane 

concentration variations in gas flow from 

0 40 8 0 120 160 200 240 

TIME, days 

FIGURE 8. - Daily longwall coal production (panel A). 

HOLE PRODUCT ION STARTUP 

12 3 456 89 11 10,12 

1,000 II I Ill I1 I I 

KEY 

- 

0 40 80 120 160 200 240 

TIME, days 

FIGURE 9. - Methane flow from cross-measure 

system (panel A). 

0 40 80 120 160 200 240 

TIME, days 

FIGURE 10. - Methane concentrations in gas flow 

from hole 2 (panel A). 
other cross-measure boreholes. The methane concentration averaged about 90 pct 

but dropped to about 30 pct in 20 days 

during the first idle period. The distance between hole 2 and the longwall 

face was 835 ft (255 m) at this time. 

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 longwall face was 1,630 ft 

(497 m) from hole 2. During the second 

idle period (day 180 to 210), the methane 

concentration again declined, but it increased after mining resumed and reached 

90 pct. At this time, the distance between the longwall face and hole 2 was 

2,120 ft (646 m). These data clearly indicate the gob is quite permeable and 

that methane migrates easily through the 

gob for distances of at least 2,120 ft 

(646 m). 

The effectiveness of the cross-measure 

system in controlling methane in the return air from the longwall is shown in 

figure 11. The point in time when each 

borehole started to produce gas is shown 

along the top on the graph. Methane 

flows in the return air gradually decreased from 250 to 100 ft3/min (0.12 to 

0.05 m3/s) as the number of the boreholes 

producing gas increased. The effect of 

each borehole on methane flow in the return air is clearly demonstrated by the 

large decrease in flow in the return air 

immediately after each borehole started 

gob gas production. These peak flows 

ranged from about 200 to 450 ft3/min 

HOLE PRODUCTION STARTUP 

(0.09 to 0.21 m3/s), suggesting that the 

200-ft (61-m) spacing should be reduced. 

The first large roof fall after mining 

of the panel was started, 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 was on production at this time, 

and the quantity of the gob gas that 

can be drawn through a 2-in (51-mm) diameter borehole is small in comparison to 

the tremendous quantities generated in 

the gob. Consequently, gob gas began to 

spill into and overload the return air 

system. Additional 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 returns. 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. 

Surface Gob Hole--Gas 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 

1,000,000 ft3 (28,300 m3) of methane 

during its short productive life. Gas 

production started on day 156, and no indications exist that the gob hole affected methane flows in the return air, 

despite the fact that hole 7, which was 

undermined around day 160 and produced no 

gas, is in the same area as the surface 

gob hole. The large peak methane flow C - 12 3 456 89 11 10~2 around day 180 (fig. 11) was caused by a 5 600 , I 111 I I I I m .- KEY partially blocked pipeline; this may have Y- x Idle period 

2' 400 - masked the effect of the surface gob 0 

A hole. LL 

w 200 - Table 2 shows that the cross-measure z 

a 

I borehole system captured 70.7 pct of the F 

W methane produced by the mining operation. r 0 40 80 120 160 200 240 

TIME. days About 28.7 pct of the methane entered the , - mine ventilation system, and only 0.6 pct FIGURE 11. - Methane flow in return air (panel A). was captured by the surface gob hole. 
TABLE 2. - Panel A and B capture ratios 

=oss-measure boreholes 70.7 

Return air............. I / 28.7 

System 

Surface gob hole....... 

Tot a1 ............. I I 

PANEL B (DAYS 37-59) 

Cross-measure boreholes I 8 1 28.0 

PANEL A 

Methane, 

106 ft3 

Return air ............. 2 7.0 

Surface eob hole....... 1 19 1 65.0 

Capture 

ratio, 

PCt 

V 

Total.. ........... I 29 1 100.0 I 

PANEL B (DAYS 60-121) 

Surface gob holes...... 

Total............. 

Cross-measure boreholes 28- 62.0 

PANEL B 

Return air............. 

The panel was completely extracted in 

61 working days, which were spread over a 

121-day interval and included two idle 

periods of 11 and 25 days (fig. 12). 

Cross-Measure and Surface 

Gob Holes--Gas Production 

3 

All boreholes produced gas continuously 

after they went on production, except 

hole 10, which produced for only 3 weeks. 

Gas flows began after mining moved 75 to 

100 ft (23 to 30 m) past the boreholes, 

which are oriented parallel to the longwall face. For boreholes oriented at 45" 

(0.79 rad) with the longwall axis, gas 

production started after the longwall 

face passed 75 to 100 ft (23 to 30 m) beyond the end of the borehole but before 

the face reached its collar. 

Figure 13 shows that the partial vacuum 

of the surface pump ranged from 8 to 10 

in Kg (27 to 34 kPa) during the study. 

The partial vacuum, however, on individual boreholes generally decreases with 

time. During the early life of the panel, the capacity of the surface pump is 

distributed over a few holes, and the 

partial vacuum is high. Addition of 

boreholes to the system causes the partial vacuum on the older boreholes to 

decrease. 

7.0 

0 20 40 60 80 100 120 140 

TIME, days 

FIGURE 12. - Daily longwa I1 coal production (panel B). 

HOLE PRODUCTION STARTUP 

3 SGH-I +-x--4 

-I r 

- a - 8 KEY 

k I x ldle period I LL 

2 6A ;o 40 6'0 ;o Ib0 Id0 IlO 

TIME, days 

FIGURE 13. - Partial vacuum measured at surface 

pump (panel B). 

HOLE PRODUCTION STARTUP 

I SGH - 2 

x ldle period \ 

0 20 40 60 80 100 120 140 

TIME, days 

FIGURE 14. - Partial vacuum measured at hole 1 

(panel B). 

Figure 14 shows the partial vacuum 

curve for hole 1, which is typical for 

other boreholes. The partial vacuum decreased from about 4 in Hg (14 k~a) when 

only 2 boreholes were producing to about 
1 in Hg (3.4 kPa) when 13 boreholes 

were producing. The increase in partial vacuum on hole 1 around day 37 

was caused by the start of gas production from surface gob hole 1 (SGH-1) , which caused the surf ace exhauster 

to work harder to maintain flow 

through the cross-measure system. SGH-2, 

which was intercepted on day 92, is 

too far from hole 1 to have had any 

effect. 

Generally, gas flow (methane plus air) 

and methane flow (fig. 9) from the crossmeasure system on panel A increased as 

the number of boreholes undermined increased. On panel B, gas and methane 

flows increased when holes 1 to 3 were 

undermined (fig. 15). The differences 

between the two curves represent the 

amount of air drawn into the crossmeasure system. When SGH-1 was undermined, both gas and methane flows decreased. Gas flow recovered more quickly 

than methane flow and exceeded the gas 

flow that existed before SHGl was intercepted; the methane flow did not fully 

recover. These data indicate that the 

cross-measure system was drawing more 

mine air into the system to make up for 

the loss of methane to SGH-1. Methane 

flow was about 1,800 ft3/min (0.85 m3/s) 

when SGH-1 started production, and declined over a 7-day period to about 500 

f t3/min (0.24 m3/s) (fig. 16). The high 

methane flow from SGH-1 caused the declines in both gas and methane flows from 

the cross-measure system. Because the 

flow from SGH-1 declined rapidly, both 

methane and gas flows from the crossmeasure system increased again. For the 

next 23 days, both methane and gas flows 

remained constant even though holes 4 to 

8 were undermined. After day 80, gas and 

methane flows began to increase again 

as holes 9 to 13 were undermined. These 

data indicate that the zone of influence 

of SGH-1 extended about 700 ft (211 m). 

Beyond that distance, flows from the 

cross-measure system increased as additional boreholes were undermined. SGH2 caused a slight decrease in flow from 

the cross-measure system, but its effect was not as great as that of SGH-1 

(fig. 15). 

A Methane flow 

HOLE PRODUCTION STARTUP 

12 34567 8 9 10 11 12 13 \/ / \ / 1,000 - / II / 1 / \// 1111 / I / Ill I1 

I I I I x ldle period 

0 20 40 60 80 100 120 140 

TIME, days 

FIGURE 15. - Gas and methane flows from crossmeasure boreholes (panel B). 

800 

HOLE PRODUCTION STARTUP 

1 2 3 4567 8 9 10 11 12 13 / / 2,000 1 // I / I I I 

I 1 I 

I- Xcl 

- rSGH-I -x* sGH7 - 

a 

I 

Iw ' 5 400 "1 KEY 

x ldle period 

II~l~t~I~-

0 20 40 60 8 0 100 120 

TIME, days 

FIGURE 16. - Methane flow from surface gob holes 

(panel B). 

HOLE PRODUCTION STARTUP 

20 1 I I I I I 1 

0 20 40 60 80 100 120 140 

TIME, days 

FIGURE 17. - Methane concentration in gas flow 

from the cross-measure system (panel B). 
The concentration of methane in gas 

flow from the cross-measure system was 

close to 100 pct before SGH-1 was int ercepted (fig. 17). The concentration 

dropped to about 30 pct for a few days 

after SGH-1 was intercepted and recovered to about 55 pct. These measurements reinforce the gas and methane flow 

(fig. 15) data, which demonstrate the 

tremendous influence of SGH-1 on the 

cross-measure system. Except for the 25- 

day idle period when the concentration 

dropped to 30 pct and then increased 

again when mining resumed, the concentration changed only a few points for the 

remainder of the study. The effect of 

SGH-2 on methane concentration in the gas 

flow from the cross-measure system was 

negligible. 

Methane flow in the return air from the 

longwall showed characteristics similar 

to those noted in the panel A study (fig. 

11). It decreased dramatically after 

hole 2 was intercepted, began to build up 

again during mining, and then decreased 

again when hole 3 was intercepted (fig. 

18). Shortly after hole 3 went on production, SGH-1 was intercepted and the 

methane flow in the return dropped to 50 

f t3 /min (0.023 m3/s) or less for the remainder of the study. The peak methane 

flows that occurred during the early life 

of the panel suggest that the boreholes 

were spaced too far apart. A closer 

spacing of holes would eliminate the peak 

flows and lower the general level of 

methane in the returns. 

Capture Ratios 

Figure 16 shows the combined methane 

flow from SGH-1 and SGH-2. Methane flow 

from SGH-1 declined from 1,800 to 200 

ft3/min (0.85 to 0.09 m3/s) in about 22 

days (day 37 to 59); during this time interval, the hole captured 65 pct of the 

methane generated by the mining operation 

(table 2). The cross-measure system captured 28 pct, and the other 7 pct entered the mine ventilation system. However, for the remainder of the panel 

(days 60 to 121), when the combined flow 

from SGH-1 and SGH-2 averaged 200 ft3/min 

(0.09 m3/s) , the cross-measure boreholes 

were the dominant system controlling 

HOLE PRODUCTION STARTUP 

12 3 4567 8 9 1011 1213 

700 \\ \ \I// , \ \ ,,I 

TIME, days 

FIGURE 18. - Methane flow in return air a an el B). 

600 

500 

400 

300 

200 

100 

gz 0 

-200 0 200 400 600 800 1,000 1,200 1,400 1,600 

TIME, mln 

FIGURE 19. - Effectiveness of cross-measure system and surface gob holes (panel B). 

0 20 40 60 8 0 100 120 

SGH - I - KEY 

methane. Table 2 shows that the crossmeasure system captured 62 pct of the 

methane, compared with 28 pct for the two 

surface gob holes during this period. 

The capture ratios for the two systems 

might have been much higher than indicated in table 2 had only the surface gob 

holes or the cross-measure system been in 

operation on panel B. The surface gob 

holes did affect the performance of the 

cross-measure system, and undoubtedly the 

cross-measure system affected the performance of the surface gob holes. On 

panel A, the methane concentration in the 

gas flow from the cross-measure system 

averaged 80 pct or greater during mining. 

On panel B, the concentration dropped 

from 100 to about 60 pct after SGH-1 was 

intercepted (fig. 17). Tests conducted 

on panel B during the second idle period 

(day 62) indicate that no change occurred 

- 

+x4 - 

in the methane flow in the return air 

over a 20-h period when the cross-measure 

system was closed and the surface gob 

hole was operating. The test showed that 

x Idle per~od 

-x- 

- 

- 

- 
the surface gob hole alone could control 

methane flow in the return air. However, 

in a later test on day 109, methane flow 

in the return air increased from 50 to 

220 ft3/min (0.02 to 0.10 m3/s) over a 

22-h period when the cross-measure system was not operating and the two surface gob holes were producing methane (fig. 19). When the situation was 

reversed--that is, the surface gob holes 

were shut in and the cross-measure system was in operation--the methane flow 

in the return air decreased rapidly 

from 220 to 50 ft3/min (0.10 to 0.02 

m3/s) in about 3.5 h. During the period 

when the cross-measure system was not in 

operation, over 50 pct of the methane 

produced by the mining operation entered 

the mine ventilation system. The test 

was terminated to avoid exceeding the 

permissible methane level in the return 

air. 

Table 2 shows that about 93 pct of the 

methane generated by the longwall operation on panel B was captured by the combined effects of the cross-measure system 

and the two surface gob holes. On panel 

A, where only the cross-measure system 

was in operation, about 71 pct of the 

methane was captured. 

Gas flows from the boreholes drilled 

parallel to the face on panel B were 

comparable to flows from boreholes 

drilled at a 45' angle to the axis of 

the longwall on panel A. No discernible 

difference was observed. Both SGH-1 and 

SGH-2 affected the concentration of methane in the gas flow from the crossmeasure system and prevented further 

comparisons. 

GOB GAS MIGRATION INTO A BOREHOLE 

Gas pressure differentials associated 

with flow of gas from a cross-measure 

borehole were calculated for three cases 

and then compared to observed values 

for the boreholes along panel B. In the 

first case, gob gas is assumed to flow 

into the borehole near the pillar line 

(fig. 20A) with the remainder of the 

borehole in the gob nonproductive. In 

the second case, gob gas is assumed to 

flow into the borehole uniformly along 

the length of the borehole in the gob 

(fig. 20B). In the third case, gob gas 

is assumed to flow into the borehole over 

the pillar with the remainder of the 

borehole nonproductive (fig. 20C). The 

following equation was used to compute 

the gas pressure differentials for the 

three cases (7): - 

where p = pressure differential (in 

Hg) , 

$ = 1 for case 1 (fig. 20A) and 

113 for cases 2 (fig. 20B) 

and 3 (fig. 20C), 

X = coefficient of resistance 

(dimensionless), 

L = pipe or hole length (ft), 

d = pipe or hole diameter (ft), 

3 

p = gas density (lb/ft ), 

- 

V = average gas velocity (ft/s), 

and g = accelerations of gravity 

(f t/s2). 

In most instances, the calculated gas 

pressure differentials agree with observed values for the case where gas 

enters the boreholes close to the pillar 

line (fig. 20A). In a few instances, the 

case where the gas flowed into the borehole over the pillar (fig. 20C) fit observed values more closely. Figure 21 
KEY 

d Case I 

-4 Case 2 .-.a Case 3 

---0 Observed 

B Gas flow . \LL 

Gas flow 

u. b**.4.w&.- .-.-. I ,+.*.-.&.-.-* .-. &J 

50 70 90 110 130 

TIME, days 

FIGURE 21. - Comparison of observed and calculated partial vacuums on hole 3 (panel B). 

shows the calculated values for the three 

cases compared to the observed values for 

hole 3, which are typical for most of the 

boreholes. Good agreement exists for the 

case where gas enters the borehole close 

to the pillar line. Comparisons between 

calculated and observed values for boreholes along panel A yielded similar results (5). Thus, boreholes drilled at a 

45' angie with respect to the axis of the 

longwall behave in a manner similar to 

boreholes drilled parallel to the face. 

These calculations also indicate that the 

boreholes can be shortened considerably, 

which will make the cross-measure system 

more cost effective. Cross-measure borehole length is 280 ft (85 m) on panel A 

and 225 ft (69) on panel B. These calculations show that borehole length can 

be reduced to about 140 ft (43 m) without 

affecting the performance of the system. 

Additional experimentation is necessary 

FIGURE 20. - Migration of gas into a borehole. to verify these calculations. 

SUMMARY AND CONCLUSIONS 

The cross-measure system is a viable 

method of controlling gob gas on retreating longwall faces, provided access to the main pipeline and individual 

boreholes can be maintained during the 

life of the panel. On panel A, where 

only the cross-measure system was operating, about 71 pct of the methane produced by the mining operation was captured. On panel B, where both the crossmeasure and surface gob holes were in 

operation, 93 pct of the methane was 

captured. 

Methane flows in the return air on 

both panels 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 crossmeasure 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. 

During prolonged idle periods, methane in the gob was depleted by the 
cross-measure system; consequently, the 

methane concentration in the gas flow 

from the cross-measure system dropped 

from 80 pct or more to 30 pct. When mining resumed, the concentration increased 

again to 80 pct or more even in boreholes 

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

face. These data indicate that the gob 

is permeable and that methane generated 

by the breakup of roof strata near the 

face will migrate through the gob for 

distances of over 2,000 ft (610 m). 

About 75 yct of the methane in the gob 

during mining emanates from newly fractured roof strata near the face. 

Gas flows from boreholes drilled parallel to the face on panel B were comparable to flows from boreholes drilled 45" 

to the axis of the longwall on panel A. 

No discernible difference was observed. 

Both sets of holes terminated at the same 

height above the mined coalbed and penetrated about the same distance into the 

gob. Penetration into the gob is defined 

as the horizontal projection of the 

borehole parallel to the face. Boreholes 

drilled parallel to the face are recommended because they are shorter and require less drilling. 

SGH-1 was very effective in controlling methane in the gob. Its influence 

extended about 700 ft (213 m). Beyond 

that point, the cross-measure system controlled gob gas. SGH-2 was not as effective as SGH-1. Both surface gob 

holes were offset from the centerline 

of the panel because of right-of-way 

problems. This may have affected their 

performance. 

Comparisons between measured and calculated gas pressure differentials in 

boreholes on both panels indicate that 

gob gas entered the boreholes close to 

the pillar line, which was about 140 ft 

(43 m) from the collar of the borehole. 

The remainder of the borehole was nonproductive. Reducing borehole length to 140 

ft (43 m) would lower drilling costs 

and make the cross-measure system more 

cost effective. Additional experimentation is necessary to verify the 

calculations . REFERENCES 

1. Browmilow, J. G., and J. H. Jones. 

Drainage and Utilization of Fire Damp. 

Colliery Eng. (London), v. 32, No. 6, 

June 1955, pp. 222-232. 

2. Cervik, J. Methane Control on 

Longwall--European and U.S. Practices. 

Ch. in Longwall-Shortwall Mining, Stateof-the-Art, ed. by R. V. Ramani. Soc. 

Min. Eng. AIME, 1981, pp. 75-80. 

3. Schatzel, S. J., G. L. Finfinger, 

and J. Cervik. Underground Gob Gas 

Drainage During Longwall Mining. BuMines 

RI 8644, 1982, 14 pp. 

4. Campoli, A. Am, J. Cervik, and 

S. J. Schatzel. Control of Longwall Gob 

Gas With Cross-Measure Boreholes (Upper 

Kittanning Coalbed). BuMines RI 8841, 

1983, 17 pp. 

5. Garcia, F., and J. Cervik. Methane 

Control on Longwalls With Cross-Measure 

Boreholes (Lower Kittanning Coalbed). 

BuMines RI 8985, 1985, 17 pp. 

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

Self-Actuating Vacuum ~as/~iquid Separator. U.S. Pat. 4,500,329, Feb. 19, 

1985. 

7. Boxho, J., P. Stassen, G. Mucke, K. 

Noack, C. Jeger, L. Lesher, Em J. Browning, R. Dunmore, and I. Monis. Firedamp 

Drainage. Verlag Gluckauf GmbH, Essen, 

1980, pp. 130-135. 

U.S GOVERNMENT PRINTING OFFICE. 1986-605-017140.015