Bureau of Mines Report of Investigations/l972 

Effect of a Surface Borehole 

on Longwall Gob Degasification 

(Pocahontas No. 3 Coalbed) 

s 

UNITED STATES DEPARTMENT OF THE INTERIOR 
Report of Investigations 7657 

Effect of a Surface Borehole 

on Longwall Gob Degasification 

(Pocahontas No. 3 Coalbed) 

By T. D. Moore, Jr., and M. G. Zabetakis 

Pittsburgh Mining and Safety Research Center, Pittsburgh, Pa. 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Rogers C. B. Morton, Secretary 

BUREAU OF MINES 

Elburt F. Osborn, Director 
CONTENTS 

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

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

Test site ................................................................ 2 

Instrumentation .......................................................... 5 

Experimental procedure ................................................... 6 

Results and discussion ................................................... 6 

Conclusions .............................................................. 9 

ILLUSTRATIONS 

1 . Cross section of borehole .......................................... 3 

2 . Map of study area .................................................... 4 

3 . Face advance during study ............................................ 5 

4 . Fluctuations in methane flow before and after borehole intersection 

during working and idle periods .................................... 7 

TABLES 

1 . Coal production and methane flow rates ............................... 7 

2 . Monthly and cumulative surface borehole production ................... 9 
EFFECT OF A SURFACE BOREHOLE ON LONGWALL GOB 

DEGASlFlCATlON (POCAHONTAS NO. 3 COALBED) 

by 

T. D. Moore, Jr.' and M. G. Zabetakis2 

ABSTRACT 

The use of a partially slotted 6.5-inch-inside-diameter vertical pipe 

to remove methane from a longwall gob area at a depth of 2,260 feet in the 

Pocahontas No. 3 coalbed ms investigated. The value of gas removed by such 

a pipe under free-flow conditions was found to equal the cost of the installation in this case. Approximately 25 percent of the available methane 

passed through the pipe without the use of an exhauster; an additional 

10 percent was removed when an exhauster was employed. 

The diversion of methane through a vertical slotted pipe to the surface 

permitted the use of increased mining rates and decreased the ventilation air 

requirements. 

INTRODUCTION 

Extensive studies have been conducted here and abroad on the control of 

methane in active coal mines. To date, the primary emphasis has been on the 

dilution of methane in the ventilating air. However, several years ago, Rice 

and others proposed the use of vertical boreholes to reduce the quantity of 

methane in a coalbed and in abandoned areas. Spindler and ~oundstone~ used 

2-, 5-112-, and 6-518-inch-diameter pipes to remove methane from the 

Pittsburgh coalbed about 470 feet below the surface, and 6-1/4-inch-outsidediameter pipe to remove gas from a sealed area; ide en our^ used 36- and 

lMining engineer. 

'~esearch supervisor. 

3~ice, George S. Safety in Coal Mining. BuMines Bull. 277, 1928, p. 42. 

Selden, R. F. The Occurrence of Gases in Coals. Banes Rept. of Inv. 3233, 

1934, p. 2. 

'~pindler, G. R., and W. N. Poundstone. Experimental Work in the Degasification of the Pittsburgh Coal Seam by Horizontal and Vertical Drilling. 

Trans. AIME, v. 220, 1961, pp. 37-46. 

5~idenour, D. C. Core Drilling of Air Shafts and Manway Portals. Min. Cong. 

J., v. 43, October 1957, pp. 99-101. 

Ridenour, D. C. Ventilation by Large-Diameter Bore Holes. Mechanization, 

March 1959, pp. 69-70. 
48-inch-diameter boreholes to ventilate remote gobs 580, 733, and 976 feet 

below the surface. More recently, ~lder~ and ~er~uson~ described an experiment in which a partially slotted 8-inch-diameter pipe was used to remove 

methane from longwall gob areas in the Lower Kittanning coal about 600 feet 

below the surface. The present work is an extension of this experiment to the 

Pocahontas No. 3 coalbed at a depth of 2,260 feet. It is a part of a longrange program designed to investigate the feasibility of using degasification 

holes drilled from the surface into operating mines prior to mining. 

TEST SITE 

The site chosen for the present study was approximately 2,260 feet below 

the surface in the Pocahontas No. 3 coalbed in Buchanan County, Va. The coalbed in this region is approximately 51 inches thick (fig. I), and has a 

1-518-inch shale parting approximately 15 inches below the top of the bed; 

its temperature is 70" to 7Z0 F; the horizon above the coal is sandy shale 

with sandstone streaks and partings. Below the coal is a 10-1/8-inch zone 

of quartzite, and below this is a massive micaceous sandstone. 

An 8-3/4-inch-diameter degasification hole was drilled to a level 4 feet 

below the coal in the No. 3 panel (fig. 2). A 7-inch-outside-diameter 

casing (6.5 inches inside diameter) was placed in this hole and pressuregrouted from the surface to within 139 feet of the coal (fig. 1). This left 

an annular space between the lower slotted section of casing and the hole. 

The grout prevented the entry of water and gas from the upper horizons but 

permitted the passage of gas through the slots in the lower 120 feet of 

casing and through the end of the pipe. 

The degasification hole was completed in the spring of 1970; mining of 

the No. 3 panel was initiated approximately 3-112 months later, on August 13. 

No significant flow of methane was detected at the top of the casing until 

after the borehole was intersected on November 9 (fig. 3). 

The No. 3 panel was 370 feet wide and 3,920 feet long. It was located 

east of panels 1 and 2, which had been mined earlier. A bidirectional plow 

was used to mine the 370-foot face with the roof supported by self-advancing, 

double frame, heavy-duty, hydraulic units; each unit consisted of four doubleacting 140-ton jacks. The face equipment required a minimum space of 11 feet 

between the coal face and the gob. A single split of air was used to 

ventilate the longwall panel (figs. 2-3). The afrflow rate past the head 

end of the face ranged from 77,000 to 82,000 cfm during the study period; 

approximately 20 percent of this air reached the tail end of the face, with 

the remainder flowing past the roof support units into the gob. 

GElder, C. H. Use of Vertical Boreholes far Assisting Ventilation of Longwall 

Gob Areas. BuMines Tech. Frog. Rept. 13, 1969, 6 pp. 

 e erg us on, Pete. Methane Control by Boreholes. Coal Age, January 1972, 

pp. 76-77. 
LEGEND 

a Undefined 

m mle 

Sandy shale with& 

slOne streaks and 

partings 

Sondy shale 

Shaley sandstone 

Micoceaus sandstore 

Micoceous sondm 

with stale streaks 

m~andrtane md qwmife 

m auntlife 

WtziIe with quam 

pebbles 

Wrtz wnglomwote 

Gum wnglornemte with mol sneaks 

coal 

m~oney cool 

FIGURE 1. - Cross Section of Borehole. 

FIGURE 3. - Face Advance During Study. 

INSTRUMENTAT ION 

Both hand-held and recording units were used to measure air volumes and 

methane concentrations in the study area. Hand-held anemometers were used 

initially to determine the location of the average airspeeds at each of the 

monitoring sites 1-4 (fig. 2). Subsequent measurements were made by mounting 

Bureau of Mines sensors8 at each of these locations. These instruments were 

used to record methane levels and airspeeds during the study period. Similar 

measurements were made with commercial hand-held units at sites A-G; remote 

sensors could not be used at these locations because they interfered with the 

haulage equipment. Microbarographs were used to record the pressure changes 

on the surface and at sites 2-4. 

BLaScola, J. C., and Joseph Cervik. Development of Recording Methanometers 

and Recording Anemometers for Use in Underground Coal Mines. BuMines 

Tech. Prog. Rept. 15, 1969, pp. 9-13. 
EXPERIMENTAL PROCEDURE 

A preliminary survey of the study area was made by the senior author with 

representatives of Island Creek Coal Co. This survey was used to inspect the 

immediate and surrounding areas and to determine airflow patterns and methane 

levels. These data were then used to determine suitable monitoring sites for 

the study. 

The methane emission study was initiated on November 5, 1970, when the 

face was about 110 feet inby the borehole (fig. 3). Data were taken for 

14 consecutive days from 3:OO-11:15 p.m. .Airspeed and methane concentrations 

were determined at 15-minute intervals at sites A through G and recorded continuously at sites 1 through 4. In addition, periodic checks were made with 

hand-held instruments at each of the latter sites; gas samples were also 

taken periodically for gas chromatographic analysis to check the accuracy 

of the gas monitors. 

Records were made of the plow operating time and idle time, face advance, 

and coal production per shift. Coal production normally ceased about 

2:55 p.m. on the first shift and resumed about 4:20 p.m. on the second shift. 

The third shift was used primarily for maintenance, but coal was produced as 

time permitted. 

RESULTS AND DISCUSSION 

A summary of data obtained during this study is given in table 1. The 

methane flow rates from the study area and the borehole were obtained by 

assuming that the gas passing sites 2-4 and B, and that flowing from the borehole, consisted of gas liberated in the study area as well as that brought 

into the area through the passageways containing sites 1, A, and C-G. Accordingly, the values given here represent the average flow rates measured on the 

second shift at sites 2-4, and B, minus those measured at the seven inby 

sites 1, A, and C-G. The scatter per shift ranged from a low of 22.5 cfm 

(one standard deviation) after the borehole was intersected on November 9 

to a high of 119.0 cfm before it was intersected. Typical results obtained 

during working and idle days before and after borehole intersection are shown 

in figure 4. The effect of the borehole is quite evident here. Not only is 

there a decrease in the methane flow rate found underground, but the shortterm fluctuations in flow rate are diminished markedly. Presumably, much of 

the gas liberated by the overlying strata passes through the borehole and is 

thus prevented from being forced into the mine as immediate roof and adjacent 

strata collapse, eliminating much of the pumping action associated with the 

collapse of roof. Further, there was an associated marked decrease in the 

maximum measured methane concentration at sites 3 and 4. The maximum value 

recorded before borehole intersection was 1.93 volume-percent and the maximum 

value after intersection was 1.39 volume-percent in approximately 100,000 cfm 

of air; this represents a decrease of slightly over 25 percent, approximately 

the proportion of methane carried away by the borehole under free-flow 

(natural convection) conditions. However, an even larger portion of the gas 

was removed by the use of an exhaust fan (table 1, entries opposite 

November 17 and 18). 
TABLE 1. - Coal production and methane flow rates 

15 

16 46 

17 49 

18 20 

Extrapolated values. 

2~ith exhaust fan mounted on the borehole. - 

2.200, I I I I I I I I I I I I I I I I I 

Coal production, Methane flow rate, cfm 

tons Study area 1 Borehole 1 Total 

Date, 

November 

- 

- Sun6oy. Nov 8.1970 

- 

KEY 

0 

PIDW cpemung time 

ldle ?(me 

- 

- 

I I I I I 

Face advance, 

feet 

1.200 

"'i2 pm. 3 pm pm p.m. pm p.m. p.m p.m. pm. CLOCK ~.mp.m. TIME, 3k2 h pm p.m em, p.m. p.m. p.m. pm pm p.m. 

FIGURE 4. - Fluctuotions in Methane Flow Before and After Borehole Intersection During 

Working and ldle Periods. 
The methane concentration of the gas discharged by the borehole during 

the study period ranged from 77 to 100 percent after pure methane started to 

flow on November 10, about 24 hours after borehole intersection. At this 

time, the trailing edge of the support units was 27 feet outby the borehole. 

Because the concentration tended to decrease when the exhaust fan (exhauster) 

was turned on, it was used only intermittently to keep the methane content 

above about 30 percent. Because of the excellent flow behavior of the gas 

passing through the borehole casing, there is some question whether the use 

of an exhauster was particularly advantageous here unless designed to minimize 

its effect on the mine ventilation air. Although the exhauster increased the 

methane flow rate through the borehole from about 25 to 35 percent, it also 

entrained some air. 

Because the borehole diverts methane from the mine returns, the mining 

rate can be increased with a particular ventilation system without exceeding 

the legal methane limit. Higher mining rates were actually used here after 

borehole intersection occurred; the largest production figures encountered 

during the study period were those obtained on November 16 and 17 (table 1). 

Although the cost of a degasification borehole is difficult to assess 

because of the effect of such a 6ole on productivity and safety, nevertheless, 

a few conrments are in order. First, the borehole described here initially 

removed over 800,000 cubic feet of high-purity methane per day under free-flow 

conditions; over 179 million cubic feet of gas was removed during the period 

November 1970 through January 1972 (table 2). If we assme a finished cost 

of approximately $20 per foot for this borehole, this is sufficient gas to 

pay for such an installation at current w'ellhead natural gas prices. Further, 

the savings in ventilation air (initially 40 million cubic feet per day if we 

assume it is being discharged at maximum contamination level) and the 

increases in production rates and safety would be additional bonuses. For 

example, using the equation9 air hp = 2, where H is the pressure drop in 

6,350 

inches water gage, and q is the airflow rate in cubic feet per minute, the 

power required to ventilate the study area and adjacent gobs (fig. 2) is 

as follows: 

A decrease of 25 percent in airflow rate causes the pressure drop to decrease 

by about 44 percent. The corresponding decrease in power is 58 percent, so 

that 37.2 horsepower is saved; at a modest rate of $0.01 per kilowatt-hour, 

this represents a savings in power costs alone of approximately $200 per 

month. 

gKingery, D. S. Introduction to Mine Ventilation Principles and Practices. 

BuMines Bull. 589, 1960, p. 27. 
TABLE 2. - Monthly and cumulative surface borehole production 

January. 

February ................. March.................... 

April .................... May. ..................... June..................... 

July ..................... August ................... September ................ October.................. 

November............ ..... December................. 

Month 

- 1970 

November................. 

December. ................ 

Methane, Mcf 1 

CONCLUSIONS 

Days 

produced 

20 

31 

Monthly 

16,403 

27,967 

- 1972 

January. ................. 

A slotted pipe placed in a vertical surface borehole is usable in longwall gobs at depths in excess of 2,200 feet in the Pocahontas No. 3 coalbed. 

The gas removed by such an installation may be adequate to offset the cost 

of the borehole and placement of the pipe. In addition, further savings 

may be realized in the use of higher mining rates and decreased ventilation 

requirements. 

Cumulative 

16,403 

44,370 

. U.S. QWi 1.73-7106 17/a2 

Methane, percent 

lUnder free-flow conditions. 

'~hirt~ to 100 percent with exhaust fan in use. 

25 

Average 

98 

100 

Range 

'77-100 

100-100 

55.3 37- 97