Bureau of Mines Report of Investigations/ 1973 

Degasification of Virgin Pittsburgh 

Coalbed Through a Large Borehole 

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

Rogers C. B. Morton, Secretary 

BUREAU OF MINES 

Elburt F. Osborn, Director 

The work upon which this report is based was done under a cooperative agreement between the Bureau of mines 

and the Eastern Associated Coal Corp. 
This publication has been cataloged as follows: 

Fields, Herbert H 

Degasification of virgin Pittsburgh coalbed through a large 

borehole, by H. H. Fields [and others. Washington] U.S. Bureau of Mines [ 19731 

27 p., illus., tables. (U.S, Bureau of Mines, Keport of investigations 7800) 

Bascd on work done in cooperation with the Eastern Associated 

Coal Corp, 

1, Degassing of coal, 2. Mine gases, 3, Mine ventilation. I, 

U,Su Bureau of Mines. 11. Eastern Associated Coal Corp, 111. 

Title, (Series) 

TN23.U7 no. 7800 622.06173 

lJ,S, L)ept, of the Int, Library 
CONTENTS 

Page 

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

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

Acknowledgments .......................................................... 2 

The multipurpose borehole ................................................ 2 

Vertical drilling operations ........................................ 4 

Placement and grouting of casings ................................... 8 

Ventilation and hoisting facilities .............................. 11 

Horizontal drilling operations ...................................... 13 

Dewatering and degasification ....................................... 16 

Results and discussions .................................................. 18 

Conclusions .............................................................. 25 

Appendix...Drilling equipment ............................................ 26 

ILLUSTRATIONS 

Location of multipurpose borehole in virgin Pittsburgh coalbed ...... 3 

Assembled drilling rig with mud pond ................................ 5 

Assembled drilling rig with clarification pond ...................... 6 

Core hole record at center of multipurpose borehole location ........ 7 

Arrangements of centralizers. gas pipes. power conduits. and 

steel plate for air duct on 48-inch casing ........................ 9 

Perspective view of completed multipurpose borehole ................. 10 

Details of enlarged bottom of borehole .............................. I1 

Surface plant layout of multipurpose borehole ....................... 12 

Photograph of the surface plant ..................................... 13 

Drilling assembly for holes in coalbed .............................. 14 

Mechanical packer assembly for holes in coalbed ..................... 15 

Connection of degasification holes through water traps and 

receiver tank on bottom ......................................... 16 

Location of degasification holes underground with respect to 

closest mine workings to projected main headings and to 

face cleat ........................................................ 18 

Calculated trend curve of initial gas emission rates from 

degasification holes .............................................. 18 

Initial and subsequent gas flow from each hole ...................... 21 

Smooth curve of total gas emission rates (all holes connected 

to collector) ..................................................... 22 

Initial in situ gas pressure (no gas flow) .......................... 23 

In situ gas pressure (after flow was established) ................... 24 

Average water flow rate per hole .................................... 25 

TABLES 

1 . Initial data on the seven degasification holes ...................... 19 

2 . Total gas and water flows and in situ gas pressures ................. 20 

3 . Analyses of two samples from total gas emission ..................... 22 
DEGASlFlCATlON OF VIRGIN PITTSBURGH COALBED 

THROUGH A LARGE BOREHOLE 

H. H. Fields,' Stephen Krickovic,2 Albert Sainato,3 

and M. G. Zabetakis4 

ABSTRACT 

The Bureau of Mines is in the process of degasifying a virgin area in 

northern West Virginia from the base of an 839-foot -deep vertical borehole. 

An in situ gas pressure hole 199 feet long, and seven degasification holes 

ranging in depth from 500 to 850 feet (average 618 feet) were drilled in an 

8-foot-thick virgin Pittsburgh coalbed from the enlarged (14-foot-diameter) 

bottom of this borehole. The in situ pressure at the back end of the hole 

and the range of gas and water flows from degasification holes as they were 

completed and closed off were 203 psi, 79,000 to 257,000 cfd (average 160,000 

cfd), and 5 to 8 gpm per hole (average 6.8 gpm), respectively. Twenty-four 

hours after the seven holes were connected through individual water traps to a 

common collector for natural venting of the gas through pipes to the atmosphere, the total gas and water flows and the in situ pressure dropped, respectively, to 971,000 cfd, 1.3 gpm, and 18 psi. Eighty days later the corresponding values dropped to 529,000 cfd, 0.50 gpm, and 13 psi. 

After 180 days of degasification, 91 million cubic feet of gas had been 

removed from the affected virgin area. This represents 60 to 70 percent of 

the gas calculated to be contained in the area of coal affected by the holes. 

INTRODUCTION 

The Bureau of Mines has been responsible for the promotion of health and 

safety in mining since its establishment in 1910. Presently, in addition to 

other areas of research, it is engaged in methane control involving removal of 

methane from virgin coalbeds, from major panels being pillared, and from old 

gobs. Specifically, this study is an attempt to determine the effectiveness 

of long holes drilled in solid coal in degasifying a very gassy virgin area of 

the Pittsburgh coalbed. 

l~ining engineer. 

a Supervisory mining engineer. 

3~ining engineering technician. 

4~upervisory research chemist. 
Methane exists under pressure in micropores, joints, and fractures of 

gassy coalbeds. In many mines it also is present in adjacent strataatvarious 

distances above and below the coalbed. An in situ gas pressure of 275 psi has 

been measured in the Pittsburgh coalbed, and the permeability of this bed has 

been found to be high compared to that of other coal bed^.^ Further, in a number of mines where gas wells are located in this coalbed, significant bleed 

off of methane through the well barrier has been found.6 However, because of 

the high in situ gas pressure and permeability of the Pittsburgh coalbed, considerable degasification can be effected over a significant area if accomplished ahead of mining. The procedure considered in the present study 

involves the drilling of long holes in the coalbed from the enlarged bottom of 

a borehole; the gas then is bled through a piping system to the surface and 

vented (or utilized). 

Despite the use of methane detectors mounted on continuous miners for 

automatic disconnection of power at 2 percent methane concentration, ignitions 

do occur at the face and create an explosion hazard. Additionally, resultant 

stoppages reduce the productive time; this in turn reduces the rate of advance 

in mining with consequent lower production and higher costs. The handling of 

methane in the development of very gassy coalbeds with ventilation air alone 

in compliance with the 1969 Act is extremely difficult, if not totally impractical with existing technology. While the drilling of degasification holes in 

a coalbed from the outside headings in one or more sets of main headings being 

developed into virgin areas has been attempted in a number of mines over the 

past 12 to 15 years,7 apparently the technique described in this paper has not 

been undertaken in this country. 

ACKNOWLEDGMENTS 

The cooperation of the management of Eastern Associated Coal Corp., 

Pittsburgh, Pa., management of the Federal No. 2 mine, and contractors Fenix & 

Scisson and Loffland Bros., Tulsa, Okla., and Sprague and Henwood, Scranton, 

Pa. , is greatly appreciated. 

THE MULTIPURPOSE BOREHOLE 

The borehole utilized in this study is located in the barrier pillar 130 

feet east of the outside heading of a projected set of 10, 3 South Mains headings. Figure 1 shows the location to be approximately 1 mile from the existing faces of 3 South Mains. The borehole has been designated as being 

5~ervik, Joseph. An Investigation of the Behavior and Control of Methane Gas. 

Min. Cong. J., v. 53, No. 7, July 1967, pp. 52-57. 

'zabetakis, M. G., T. D. Moore, Jr., A. E. Nagel, and J. E. Carpetta. Methane 

Emission in Coal Mines: Effects of Oil and Gas Wells. BuMines RI 7658, 

1972, 9 pp. 

7~~~nd~tone, W. N. Private comunication, 1973. Available upon request from 

Stephen Krickovic, Bureau of Mines, Pittsburgh, Pa. 

Spindler, 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. 
FIGURE 1. - Location of multipurpose borehole in virgin Pittsburgh coalbed. 
multipurpose because after completion of degasification, or when 3 South Mains 

headings have been developed to the borehole, it may be used as an intake airway and later as a return airway. Still later, when the pillar line has been 

established, the borehole can be used to degasify the gob area nearby. While 

serving as an intake airway, the borehole may be used as an escapeway in an 

emergency, with power cables and comunication lines installed in the two 

degasification pipes located outside of the 48-inch-OD casing. 

Vertical Drilling Operations 

The multipurpose borehole was drilled with a specialized rig. All major 

components of the drilling assembly and supporting equipment are listed in the 

appendix. Views of the drilling rig and mud and clarification ponds are shown 

in figures 2 and 3. 

After preparation of an access road, a standard NX core hole was drilled 

at the center of the multipurpose borehole location to determine the character 

of the superjacent strata, the thickness of the coalbed, and the depth to the 

bottom of the coalbed. Figure 4 shows the core hole record which the driller 

used as a guide in determining the extra weight required on the bit at each 

change in competence of strata. Upon completion of the coring operation, a 

200- by 100-foot site was prepared and graded for drainage, 480-volt ac power 

was provided, and the drilling rig was erected to drill an 84-inch-diameter 

borehole. This diameter was drilled at approximately 10 rpm to firm rock 

(50-foot depth), utilizing reverse-mud air-assist circulation. Two 60-inch 

Do-nuts (23,400 lb)8 were used on the 7,500-lb drill bit and 16,400-lb, 

84-inch stabilizer to provide the necessary cutting pressure; a single shot 

deviation survey was made. 

Two rolled and welded sections of 74-inch-ID casing (lower 30 feet with 

518-inch wall thickness and upper 20 feet with 112-inch wall thickness) were 

placed in the hole on firm rock. Type I1 cement, plus 2 percent calcium 

chloride mixed with 5.2 gallons of water per sack of cement, were pumped in 

the annulus through two strings of 2-inch tubing to a height of 5 feet both in 

the casing and the annulus. After waiting 8 hours for the cement to harden 

adequately, the second grouting stage was undertaken in the same manner as for 

the first stage, and completed to the top of the ground where a concrete pad 

had been poured prior to start of drilling. A total of 170 sacks of cement 

were used in the first stage and 380 sacks in the second. The upper 45 feet 

of casing had a safety factor of 2 against hydrostatic head, with an assumed 

mud weight of 8.8 lb per gallon. This 50 feet of 74-inch casing was installed 

to prevent entrance of surface water into the hole. 

Drilling of the 72-inch-diameter borehole was undertaken within the 

74-inch casing, and continued to the bottom of the Pittsburgh coalbed (789 

feet), again utilizing reverse-mud air-assist circulation with the mud weight 

maintained from 8.5 to 9.5 lb per gallon--average viscosity of 30 seconds flow. 

A geolograph was installed to.record depth, time, and rate of penetration. 

'use of trade names or makes and models of equipment is for information only 

and does not constitute endorsement by the Bureau of Mines. 
FIGURE 2. - Assembled drilling rig with mud pond. 
FIGURE 3. - Assembled drilling rig with clarification pond. 
FIGURE 4. - Core hole r
Single-shot directional surveys were run at each 30-foot depth drilled, with a 

Dowel1 sonar survey run at completion of drilling to obtain the exact direction and the amount of deviation from the vertical. The deviation was 19 

inches at the depth of 839 feet. 

In drilling the 72-inch hole, two separate 72-inch stabilizers (16,400 

and 8,400 lb); one 60-inch collar stem (19,500 lb); 10 Do-nuts (117,000 lb); 

one holddown clamp (800 lb); and the 72-inch drill bit (7,500 lb) were used. 

The rpm (approximately 10) and Do-nut weights were varied with competence of 

rock strata. Some difficulty was encountered in the upper strata of soft 

shales and fire clays from compaction of the soft material between the cutters. 

It was necessary to withdraw the bit and replace the cutters with a type more 

suitable for the conditions. Drilling operations were continued 24 hours per 

day, 7 days a week; the 72-inch hole was drilled in 54 calendar days. The 

drilling mud with the cuttings was discharged into the upper large pond 

(fig. 2) from which a ditch was dug approximately 20 feet from the lower end 

to decant the water and some mud to the clarification pond (fig. 3). 

Placement and Grouting of Casings 

The 48-inch-ID steel casing was fabricated in 50-foot lengths from AS'LM 

283-58, grade C steel in accordance with ASME Boiler Code Section VIII, division 1 specifications. The casing was designed to withstand full hydrostatic 

head, using the formula: 

P (psi) = - 

1 - h2 

where P = hydrostatic pressure, 

E = modulus of elasticity, 

h = ~oisson's ratio, 

t = wall thickness (inches), 

and D = outside diameter (inches). 

A safety factor of 2 was maintained, allowing for l/4-inch "out of roundness" 

and for presence of drilling mud within the casing. 

As the casing was lowered in the hole, arrangements were made to hold a 

section or sections at the top while the next section was welded. Welding 

was performed by the submerged arc fusion method; all field welds were X-ray 

inspected. Shop welds were inspected at random. Iridium 192, with a halflife of 75 days, was used for the radiographic inspections. 

Prior to placement of the casing in the hole, centralizers, two 4-inchdiameter gas pipes, two 3-inch conduits (one for power and communications and 

one for water pumping system), and 1/4-inch-thick steel plate to provide a 

segment for an air duct were welded onto the 48-inch casing (fig. 5). 
conduit 

546 in by 3-in 

steel plate 

Typical centralizer 

ELEVATION 

FIGURE 5. - Arrangements of centralizers, gas pipes, 

power conduits, and steel plate for air 

duct on 48-inch casing. 

hole bottom. A 

hemihead,  

with a 2-inch relief valve 

and a 1-inch connection for 

a pressure gage, was welded 

to the top section of the 

48-inch casing to prevent 

grout entering inside the 

casing which was filled with 

drilling mud. The predeter - 

mined excess safe pressure 

within the casing was computed to be 34 psi. 

The drilling mud was 

washed out from the annulus by pumping through three equally spaced 2-inch 

pipes followed by 40 barrels of "drill flush" to a height of 120 feet from the 

bottom of casing. This section was grouted immediately through the same 

2-inch pipes with cement slurry equal in specifications to that used in grouting 50 feet of the 74-inch casing at the top of the borehole. After the 

2-inch pipes had been raised from the bottom of the 48-inch casing and the 

grout allowed to set for 16 hours, drilling mud in another section of the 

annulus was removed and the section grouted as already explained. The grout 
FIGURE 6. - Perspective view of completed multipurpose borehole. 
Ventilation and Hoisting Facilities 

Figure 5 shows the segment of the 48-inch casing arranged for the ventilation duct; figures 6 and 7 show the total ventilation duct in perspective of 

the multipurpose borehole with adapters on top and bottom, and details of the 

enlarged bottom of borehole, respectively. Figures 8 and 9 show the locations 

of the ventilation, hoisting, and gas pumping facilities on the surface. 

A Buffalo type CB blower was installed for exhausting duty in a steel 

building, together with a 40-hp, permissible ac motor and control, and a V-belt 

drive to obtain a maximum air volume of 9,000 cfm at 12-inch water gage. 

All hoisting was accomplished by a single drum mine hoist (30-inch diameter by 24-inch face with 6-1/2-inch flange), model S-4000-300 manufactured by 

the Coeur dlAlene Co., Spokane, Wash. The hoist was powered by a permissible 

50-hp motor; with the power transmitted through a Sanstrand flow pump and a 

Houdaille piston-type hydraulic system. It has a capacity of 8,000 lb, and 

was equipped with 7/8-inch rope, overspeed, override, automatic braking facilities and simplex controls. Allhoistingequipmentwashousedina steel building. 

coal 

k-14 ft4 

FIGURE 7. - Details of enlarged bottom of borehole. 
ELEVATION 

FIGURE 8. - Surface plant layout of multipurpose borehole. 

A tripod headframe fabricated from Schedule 80 steel pipe, having two 

legs 6-5/8 inches and the third leg 8-518 inches in diameter, was installed 

over the borehole. The pulley was located 30 feet 9 inches above the top of 

the concrete pad and 40 feet from the hoist drum. The headframe was anchored 

to the concrete pad, 20 by 20 by 3 feet thick (figs. 8 and 9). All hoisting 

facilities were constructed to meet the requirements as set forth on page 36 

of the Mandatory Safety Standards, Underground Coal Mines, Title 30, Code of 

Federal Regulations, Part 75, March 30, 1970-December 31, 1972. 
in the 48-inch casing. Progressively, as these materials were removed, the 

borehole was enlarged on a 34" angle to a 14-foot diameter on top and bottom 

of the coalbed. All excavation was done with pavement breakers and chipping 

hammers for safety reasons. During this operation, water was usually pumped 

to the surface by a deep well pump and, at times, by bailing into the hoisting 

bucket. Also, the ventilation duct adapter was installed and reinforced flexible tubing attached to it for movement to each hole as drilling was started. 

The sloping rock section was bolted with 5/8-inch-diameter, 7-foot-long, steel 

bolts drilled on 4-foot centers on two circumferential lines (figs. 6 and 7), 

and coated with a 2-inch-thick layer of reinforced gunite. Because the coal 

surface area was wet continuously, it was not gunited. The ventilation facilities were adequate to maintain methane concentration under 1 percent during 

drilling operations. All work so far described below the 48-inch casing was 

performed in two 10-hour shifts 6 days a week for approximately 12 weeks. 

The degasification holes were drilled in the coalbed with a Sprague and 

Henwood model 40CL drilling machine powered by a 20-hp Ingersoll-Rand air 

motor through a standard four-speed gear box. The machine base was anchored 

to bolts drilled in the coalbed near the bottom in the direction of drilling. 

Control of torque, drilling speed, and thrust was not possible with this unit. 

approximately 400 feet, the 5-foot packed hole assembly was followed by a 

weighted 5-foot length of 2-1/2-inch drill rod and the hole advanced to a 

depth of 500 feet. The surveys at the 500-foot depth normally indicated a 

slight upward inclination. 

Surveying beyond 500 feet was not considered economical because of the 

time required to replace the rods and stabilizers. All holes, except one, 

were advanced beyond 500 feet, assuming only minor characteristic deviations 

would be realized by using the two basic stabilizer assemblies described above. 

Behavior studies of the 10-foot stabilizer, followed 10 feet behind with 

a 5-foot stabilizer, indicated the bit would drop between 1 and 2 feet in 

approximately 150 feet, and a single 5-foot stabilizer would cause a 1- to 

1.5-foot rise in approximately 100 feet. The holes were advanced in excess of 

600 feet using the two-unit stabilizer assembly (fig. lo), then changed to the 

single stabilizer assembly; one hole was drilled to a maximum of 850 feet. 

The bit stopped advancing at the 850-foot depth and drilling was stopped and 

no further advance was made due to high costs involved to advance beyond this 

point with the equipment used. No rock cuttings were evident, and if the 

response anticipated in the stabilizer action was correct, the hole had 

stopped in the upper third of the coalbed. It is possible that local concentrations of pyrite lenses could have been encountered in that zone. 

During the drilling of each hole, a flexible tubing attached to the 

adapter at the bottom of the main air duct (segment of 48-inch casing), was 

extended to the hole with approximately 9,000 cfm of air conducted to the work 

area. Methane concentration in the discharge of the exhauster never exceeded 

1 percent. Upon completion of each hole, a Bureau-designed mechanical packer 

(fig. 11) was inserted in the hole and the hole was shut in to prevent outflow 

of gas until all degasification holes were completed. The in situ pressure 

7 -in schedule 40 pipe 5ft long I Vspipe t hreod 7 

I !"$n schedule 40 pipe 7f t long 3-in hex nut 7 

\ '3-in rubber sleeve 

\ 3-irrod X 1 kin-id X !@-thick 

steel washer, 4 required 

11/2 pipe union / 

FIGURE 11. - Mechan ical packer assembly for holes in coalbed. 
hole was sealed as shown in figure 6, with provision for measuring the gas 

pressure at four equally spaced locations. 

6 

PLAN 

4-in pipes to 

methane fan 

-in valve 

n pipe and flexible 

TYPICAL SECTION 

Dewa ter ing and 

Degasification 

Approximately 

10 gpm of water 

was required to 

flush cuttings out 

of each hole 

drilled in the 

coalbed (fig. 10). 

During drilling 

operations, an air 

displacement pump 

was used to remove 

drill water and 

cuttings from a 

sump in the bottom 

of the large borehole to the surface. Subsequently, thebottom 

of the large borehole was dewatered 

by a 50-gpm, submersible pump in a 

sump powered by a 

15-hp, permissible 

ac motor. A 100- 

gpm horizontal 

pump powered by a 

40-hp, permissible 

ac motor was used 

for standby duty. 

A few inches of 

water always covered the bottom of 

the borehole. 

A connection 

was made from the 

mechanical packer 

in each degasification hole to a 

Bureau-designed 

water trap at each 

hole, and then to 

FIGURE 12. - Connect ion of degasification holes through water traps 24-inch-diameter 

and receiver tank on bottom. by 72 -inch-high 
receiver tank. An orifice plate was installed in the pipes between the water 

traps and the receiver tank for measuring pressure in and flow from each hole. 

The two 4-inch steel pipes grouted behind the 48-inch casing were connected to 

the tank (fig. 12) . 

Figures 8 and 9 show the surface plant layout, including the ventilation, 

hoisting, and degasification facilities. A Buffalo Forge Co. high-pressure, 

low-volume blower, model 45-1, type CB, operating at 3,500 rpm and capable of 

delivering 1,500 cfm of gas at 4 inches of mercury static pressure, was 

installed to assist in exhausting the gas from the holes in the coalbed. The 

fan is powered by a direct drive permissible 40-hp ac motor, with variable 

capacity obtained through a "butterfly" valve arrangement located at the discharge end of the fan. The gas is piped from the receiver tank on the bottom 

of the borehole through two 4-inch-diameter steel pipes extending to the surface, and connected by a 6-inch-diameter, 100-foot-long, steel pipe to a gas 

pump. The gas is exhausted through the pump and a 6-inch-diameter, 40-foot 

vertical pipe, with the gas flow being directed in the direction directly opposite the borehole and the ventilation fan setup. To date, the gas pump has 

not been used, but will be placed in operation when the total gas flow drops 

to approximately 350,000 cfd. Total gas flow, pressure, and time are measured 

by a Rockwell model T-16, 8-inch turbo-flowmeter located on the intake side of 

the gas pump. Due to the presence of moisture in the gas emitted from the 

degasification holes, it has been found necessary to insulate the surface 

piping handling the gas. 

All the surface plant ground has been leveled, cleared, graded for good 

drainage, and covered with a layer of crushed stone, and isolated with a 

cyclone-type steel fence and two gates. In addition to the ac powerline and 

facilities previously described, a diesel electric unit has been installed for 

use in case of the main ac power failure. This spare unit is necessary to 

prevent excessive flooding of the bottom of borehole in the event of electric 

power failure. 

Figure 13 shows the locations of the degasification holes, the lengths 

and angles made with respect to the face cleat, and the relationship of the 

holes to the multipurpose borehole and to the projected set of 10 main headings (3 South Mains). 

In situ gas pressure and volume measurements are made periodically (7 to 

10 days) at the bottom of the borehole. In addition, total gas flow, pressure, 

and time also are read with the turbo-flowmeter located on the surface. 
cu 0.4 

F B F B F 

1 I I I 1 1 I C I rC 

\ 

w I - 

E 

c .3- I5 

U 

X / 2 - 

- X X 

LL 6 

W X x7 

I 

Li - - 

= 0 

I I I I I I I - 

90 180 270 360 

DISPLACEMENT, 8, deg 

The initial gas flow 

rates (obtained during the 

first 24 hours) from the 

seven 3-inch-diameter holes 

drilled in the coalbed from 

the bottom of the multipurpose borehole are shown in 

figure 14 and listed in 

table 1. If we assume that 

holes 7, 8, and 1 are 

shielded from the wells by 

the other holes (particular ly 2 and 6) , then the 

initial gas flow rate (GFR) 

in the absence of the wells 

FIGURE 14. - Calculated trend curve of initial gas emis- would be given by the 

equation: sion rates from degasif ication holes. 

GFR = (0.18 - 0.1 Cos 29) cfm/fp (1) 

where 0 is the angle (displacement) measured clockwise from the face cleat 

located between holes 2 and 4 (fig. 13). A plot of equation 1 is included in 
figure 14. Note in particular the effects of the directional permeability on 

the flow; in the absence of wells, the maximum flow is from holes drilled 

along the butt cleats (B) and nearly perpendicular to the face cleats (F). 

However, as boreholes that pass near wells may have flow rates above the predicted values, it is not surprising to find that the initial flow rates from 

holes 2 and 4 were approximately twice as high as those to be expected from 

equation 1. Further, comparing the flows from holes 4 and 7 (two diametrically opposed holes), we see that the flow from hole 4, which passes near a 

gas well, was about 2.5 times higher than that from hole 7, which does not 

pass near a well. This is in line with earlier findings on the effects of oil 

and gas wells on emission of methane in coal mines.g 

TABLE 1. - Initial data on the seven degasification holes 

 a ace cleat is appreciably more permeable than the much shorter butt cleat. 

No. of hole1.. .................... Length ........................ ft.. 

Gas emission .........( 1,000 cfd).. 

Gas flow per sq ft of coal surface 

in holes .................... cfd.. 

Angle of hole with respect to face 

cleat ....................... deg.. 

Distance from gas well ........ ft.. 

Average water discharge ......gp m.. 

In situ gas pressure at 199-ft 

depth ........................ si.. 

After the initial flow rates were measured, the seven bleeder holes were 

connected to a common collector. The flow from each hole is shown in figure 15. The total methane emission from these holes fell for approximately 

50 days and then began to increase (table 2). Interestingly, the emission 

rate fell by a factor of 2 (fig. 16) while the gas pressure in the coalbed 

fell by a factor of 20 (figs. 17 and 18). However, during this period, a fair 

amount of water was removed from the coal (the average initial water flow rate 

was 6.2 gpm per hole; in 50 days this figure fell to 0.5 gpm per hole (fig. 19) 

and the gas permeability of the coal increased. As a first approximation, the 

total emission rates (MER) are initially proportional to the increase of the 

square root of the elapsed time, t: 

1 

646 

201 

396 

83.5 - 

6.5 

- 

MER = 970 

J1 + t-10; 0 < t < 40 days 

l~igure 1. 

This simple equation yields values within about 5 percent of the smoothed 

(curve) data given in figure 16. 

gZabetakis, M. G., T. D. Moore, Jr., A. E. Nagel, and J. E. Carpetta. Methane 

Emission in Coal Mines: Effects of Oil and Gas Wells. BuMines RI 7658, 

1972, 9 pp. 
TABLE 2. - Total gas and water flows and in situ 

gas pressures 

Although 40 million cubic feet of gas was removed in 80 days from the 

virgin area, the subsequent periodic readings to March 13, 1973 (total of 

180 days), showed the volume to have increased to 91 million cubic feet. This 

is equivalent to an overall average of slightly over 500,000 cfd of gas with 

the water discharge per hole and in situ pressure dropping to 0.34 gpm and 

12.7 psi, respectively, on March 13, 1973. Based on the average depth of 

holes (618 ft), at least 9,600,000 cubic feet, or 384,000 tons of coal 

(8-ft coalbed thickness), are involved. Therefore, the total gas emitted 

per estimated ton of coal in place is 237 cubic feet. This significant 

volume is approximately 98 percent of the total estimated volume (250 cubic 

feet) of gas contained in a ton of virgin coal in this general area. Since 

the daily gas emission rate and in situ pressure were 538,000 cfd and 

12.7 psig, respectively, after 180 days of degasification, there is no doubt 

that gas liberation is occurring beyond the average length of holes. 

In situ 

gas 

pressure, 

psi 

203 

190 

18 

18 

17 

11 

10 

10 

13 

13 

13 

13 

13 

12.7 

12.7 

12.7 

12.7 

12.7 

12.7 

12.7 

Average water 

discharge 

per hole, 

~pm 6.2 

1.3 

1.2 

1.2 

.8 

9.5 

.5 

.5 

.5 

.5 

.5 

.5 

.5 

.45 

.43 

.45 

.44 

.34 

.34 

.34 

Gas 

emission, 

1,000 cfd 

Initial ................................. Twenty-four hours after all holes connected to collector ...................... Days after above 24 hours: 

10 ..................................... 20 ..................................... 30 ..................................... 40 ..................................... 50 ..................................... 60 ..................................... 70 ..................................... 80 ..................................... 90 ..................................... 100 ..................................... 110 ..................................... 120 ..................................... 130 ..................................... 140 ..................................... 150 ..................................... 160 ..................................... 170 ..................................... 180 ..................................... 

1,121 

971 

720 

511 

464 

444 

460 

490 

438 

529 

500 

543 

480 

4 98 

515 

4 94 

5 18 

509 

529 

538 
KEY 

Hole No. " I 

x 2 

04 

v 5 

m6 

.7 

0 8 

TIME, days 

FIGURE 15. - Initial and subsequent gas flow from each hole. 

Table 3 shows that 87.7 percent of the gas is methane. When the daily emission rate drops to 350,000 cfd, the gas pump on the surface will be activated 

to supplement ~he in situ gas pressure. 
ELAPSED TIME, days 

FIGURE 16. - Smooth curve of total gas emission rates (all holes connected to collector). 

TABLE 3. - Analyses of two samples from total gas emission 

At least 2 years will be required before the 3 South Mains are developed 

to the multipurpose borehole location. Methane emission will be monitored 

periodically during development of the mains to determine the actual area of 

effectiveness of this degasification technique. It is expected that upon 

maximum practical degasification of the area involved, coal can be mined at 

normally maximum rate with no hazards from ignitions and explosions, provided 

normal and properly controlled ventilation is utilized. It is further 

expected that this situation will prevail some distance from the borehole. 

Ga s 

Ethane (C, & ) .................... ppm. . Carbon dioxide (CO,) .............p ct.. 

Oxygen (0,) ....................... ct.. 

Ni trogen (N, ) .................... pc t .. Methane (CH4) ..................... ct.. 

Regarding the cost effectiveness of the degasification technique used 

in conjunction with the experimental multipurpose borehole, it should be 

recognized that the expense of this study was quite high. For the depths of 

the borehole and the degasification holes used here, the total cost was 

$848,000. Assuming a minimum of 40 minutes' loss of production time per shift 

in development within virgin coal (this loss has been measured in Feder'~1 

Sample 1 

300 

9.5 

0.6 

2.0 

87.9 

Sample 2 

321 

9.0 

1.2 

2.4 

87.4 
TIME, doys 

FIGURE 17. - Initial in si tu gas pressure (no gas flow). 

No. 2 mine because of methane) and an average production rate of 4 tons per 

minute, 160 tons of coal would be lost per shift. Assuming further that 

480 tons of raw coal are produced per shift in the degasified development area, 

and 320 tons if the area is not degasified, on a clean coal basis, the tonnages would become 410 and 275, respectively. The face costs per shift in 

both cases would be $450 for a continuous miner operator, a loading machine 

operator, a ventilation man, two shuttle car operators, mechanic, general 

serviceman, boom operator, and foreman (all fringe benefits included). The 

face costs per ton of clean coal would be $1.10 for the degasified area and 

$1.62 for the nondegasified area, or a difference of $0.52 per ton. 

Using 336,000 tons of raw coal (estimated with 7-ft mineable coal thickness) available in the virgin area degasified through the holes drilled in the 

coalbed (286,000 tons of clean coal), the saving by degasification would be 

$148,000. It is obvious that the multipurpose borehole is too costly, even if 

the other progressive uses of the hole are considered. 

A much more practical approach is for coal companies operating in very 

gassy coalbeds to consider planning more air shafts with smaller crosssectional areas than is the increasing practice today. For example, a 

16-foot-diameter, concrete-lined shaft 840 feet deep (same depth as the 

multipurpose borehole) sunk by mechanized conventional method would cost 

$650,000 or less. Yearly cost of borrowing this sum at 6 percent interest 

would be $39,000. Thus, the savings due to this degasification would pay for 

nearly 4 years' interest. Actually, it would be adequate to sink the shaft 

3 years ahead of need and degasify the virgin coal by long holes as accomplished in conjunction with the multipurpose borehole. 
in the total cost. 

Other benefits would 

accrue as a lesser number of 

headings should be required 

between air shafts (these 

should be maintained in an 

open condition to the fulles t extent practicable) and 

from the possibility of 

isolating major panels of 

coal for at least 1 year and 

preferably 2 years for gas 

bleed off prior to mining 

within the panels. It is 

not intended to imply that 

air shafts should be sunk on 

approximately 2,000- to 

3,000-foot centers, but 

rather to carefully plan the 

layout of sets of main headings in the whole mine and 

determine the locations of 

both intake and return air 

shafts required for ventilation purposes and additional air shafts with the 

intent of degasifying the 

coalbed from the bottom of 

each. It may be found 

advisable to sink some 

shafts 14 feet in diameter. 

Two compartment shafts, in 

the opinion of the authors, 

are questionable from the 

standpoints of safety and 

cost, except possibly in 

some very unusual cases. 

In case of a catastrophe, 

destruction of the curtain 

wall at the bottom would 

render the shaft useless; 

approximately 300 feet apart 

and if two shafts were sunk 

210 I 1 I 1 I I 1 1 I 

200 

TIME, days with the two open crosssectional areas equaling 

FIGURE 18. - In situ gas pressure (after flow was those in the two compartestab l ished). ments, there would be a 

relatively small difference 

190 

180 

170 

160 

150 

140 

cn 130- . - 

m 

Q 120 

w 5 110- 

cn 

V, 

w 100- E 

Q, 

3 90- 

I80- 

z 

- 70- 

60 

50 

40 

30 

- 

- 

- 

- 

-I$ 

- 

--- A A A a - 

- 

- 

- 

- 

2 holes connected 

- 

- 

i I I I I i I I 

- 

- 

- 

- 

- 

0 20 40 60 80 loo 

3 holes connected - 

- 

- 

- 

4 holes connected - 

- 

- 

"6 holes connected - 

- 

All 7 holes connected - 
0 10 20 30 40 50 60 70 80 

TIME, days 

FIGURE 19. - Average water flow rate per hole. 

CONCLUSIONS 

Based on the results obtained in 180 days, it can be concluded that the 

multipurpose borehole is a useful degasification technique. It is very probable that the final degasification will exceed appreciably 70 percent of the 

estimated volume of gas contained in 1 ton of virgin Pittsburgh coalbed. However, the multipurpose borehole is quite expensive; therefore, it appears 

advisable to consider using the technique of drilling degasification holes in 

the coalbed from the bottom of planned air shafts (sunk by conventional mechanized methods) in virgin coal areas approximately 3 years ahead of closest 

mine workings. 
APPENDIX. - -DRILLING EQUIPMENT 

The shaft drilling equipment used here was Loffland Brothers rig No. 142. 

This is a specialized drilling rig designed exclusively for shaft drilling 

operations. The major rig components are listed below: 

Derrick: Lee C. Moore; mast 133 feet; 750,000-lb capacity. 

Subs truc tures : Lee C. Moore; 12 feet 4 inches; 450,000-lb casing capacity; 

300,000-lb setback capacity. 

Draw works : Na t ional 75; maximum single line pull 27,330 lb; maximum 

hook load, 10 lines, 576,000 lb. 

Auxiliary brake: Parkersburg 40-inch double hydromatic. 

Engines : Four Czllrinins N855-1'250; 440 int. kW at 1,750 rpm and 

240 coni:. kW at 1,750 rpm driving four Reliance 640 pm dc 

genera tors. 

Desander : San Angel0 12-cone (three units of four cones each). 

Water storage: To comply wi th bid specifications. 

Generators: Two, 37.5 kV-A ac each. 

Rotary table: National 27-1/2-inch. 

Traveling block: National 5 sheave, 300-ton. 

Crown block: Lee C. Moore. 

Hook: Byron Jackson Model 4200, 200-ton. 

Swive 1 : High B. Williams, 2.50-ton, 12-inch ID. 

Special tools: All necessary shaft drilling equipment including: 

1. Drill pipe, 13-3/8-inch OD. 

2. Drill collar and stabilizers, 40-inch. 

3. Flat bottom bits, 84- and 72-inch. 

4. Casing running tools. 

Supporting equipment: 

1. Input draw works, 1,000-hp, 6-speed. 

2. AC powerhouse. 

One ac distribution panel. 

One 4 ac air compressor. 

TWO 671 CMC 125-hp. 

One generator, 50-kW ac. 
Draw work motors. 

Tv~o 400-hp Reliance dc motors. 

Oil rotary table, 27-1/2-inch, with 3.85 to 1 reduction. 

DC powerhouse. 

Four NH 250 Cummins 250-hp diesel drive engines with four 4,000-hp 

Reliance dc generators. 

Engine 1, used only for draw works. 

Engine 2, used for draw works, mud pumps, or sandline for survey. 

Engine 3, used only for mud pumps. 

Engine 4, used only for rig air compressor. 

Air compressor. 

Two 750 Joy air compressors driven by two 12-V 71 250-hp General 

Motors Corp. motors. 

Mud pumps. 

Two 14- by 16-inch Allen Sherman rotary pumps driven by two 400-hp 

Reliance motors. 

DC switch gear. 

Electrical controls for standby operations. 

Main electrical controls for normal operations. 

Ground operator's control panel. 

Rig air compressor. 

One 600-cfm I.R. air compressor driven by one 400-hp Reliance motor. 

Water pump. 

One 2-foot by 3-inch B.J. pump driven by one 20-hp ac motor. 

Sand1 ine . One 15,000-ft 5/16-inch wire line driven by one 400-hp Reliance dc 

motor. 

Drill line spool. 

One spool with 7,500 feet of 1-3/8-inch wire rope. 

Fuel tank. 

One 5,000-gal fuel tank with 2-inch pump. 

Power pole. 

One power pole with three 440-V three-phase ac output 100-kW 

trans formers .