Bureau of Mines Report of Investigations/l975 

Rotary Drilling Holes in Coalbeds 

for Degasification 

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

Thomas S. Kleppe, Secretary 

Jack W. Carlson, Assistant Secretary-Energy and Minerals 

BUREAU OF MINES 

Thomas V. Falkie, Director 
This publication has been cataloged as follows: 

Cervik, Joseph 

Rotary drilling holes in coalbeds for degasification, by 

Joseph Cervik, H. 11. Fields, and G. N. Aul. [Washington] 

U.S. Bureau of Mines [I9751 

21 p. illus., tables. (U.S. Bureau of Mines. Report of investigations 8097) 

lncludes bibliography, 

1. Degassing of coal. 2. Boring. 3. Mine safety. I. U.S. Bureau of hlincs. 11. Fields, Herbert H., jt. auth. 111. Aul, G. N., 

jt. auth. IV. Title. (Series) 

TN23.U7 no. 8097 622.06173 

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

Page 

Abstract ................................................................ Introduction ............................................................ Acknowledgments ......................................................... Drilling hardware ....................................................... Bits ............................................................. Drill rod ................,......................................... 

Centralization of drill string ..................................... Control of hole trajectory .............................................. Hole surveying ..................................................... Drilling parameters ................................................. Drilling procedures ................................................ Human factors ...................................................... Azimuthal control .................................................. Methane control .................................................... Drilling example ................................................... Alternate drilling assembly ............................................. Drill and power unit .................................................... Conclusions ............................................................. References .............................................................. 

ILLUSTRATIONS 

1 . Drill bits ......................................................... 2 . Drill string with bit and drill casing ............................. 3 . Centralizer ........................................................ 4 . Drill string with one centralizer behind bit ....................... 5 . Drill string with one centralizer 10 feet behind bit ............... 6 . Drill string with two centralizers 10 feet apart ................... 7 . Drill string with two centralizers 20 feet apart ................... 8 . Drill string with two centralizers separated by a 20-foot-NW drill 

rod .............................................................. Drill rod ..............................*........................... 

Single-shot survey instrument ...................................... Protective case for survey instrument ............................ Survey instrument .................................................. Stuffing box for methane control ................................... BQ one-way check valve ............................................. Plot of hole trajectory ............................................. Drill unit ......................................................... Power unit .......... .............................................. 

TABLES 

1 . Bit penetration rates (Ohio strip pit) .............................. 2 . Casing specifications .............................................. 3 . Effect of thrust and bit rotational speed on hole trajectory ....... 4 . Summary of drilling parameters ... .................................. 5 . Penetration rates using a 3-112-inch bit ........................... 6 . Penetration rates using a 3-518-inch bit ........................... 
ROTARY DRILLING HOLES IN COALBEDS FOR DEGASIFICATION 

by 

Joseph Cervik,' H, HI Fields12 and G, N, Aul3 

ABSTRACT 

Coal is a soft and brittle material. Drilling rates in the Pittsburgh 

coalbed using a drag bit exceed 3 ft/min at 2,500-pound thrust. However, 

maintaining the bit on a horizontal trajectory or parallel to bedding planes 

to attain lengths of 1,000 feet is difficult. This Bureau of Mines report 

presents a drill string configuration that can be guided through the coalbed. 

The angle of the borehole is measured periodically and by proper combinations 

of bit thrust and rotational speed (r/min), bit trajectory can be lifted or 

dropped to keep the bit in the coalbed. An example of a hole drilled to 

2,126 feet is presented to show levels of thrust and rotational speed used to 

lift or drop the bit, the seemingly unexplainable events that occur, and the 

strategy used during drilling. Thrust levels during drilling range from 

600 to 2,500 pounds. Hydraulic motors powered by a 30-hp, 440-volt motor 

provide ample power for drilling horizontal holes 2,000 feet long. 

INTRODUCTION 

Experience has shmn that methane flow rates from horizontal holes 

drilled into coalbeds are directly proportional to length. For holes drilled 

from underground locations in a coal mine in the Pittsburgh coalbed (Fairview, 

W. Va., area), a characteristic flow rate is 25,000 ft3/d per 100 feet of 

hole. However, in areas of the coalbed remote from mining, such as in shafts 

located 1 mile or more from an underground mine, initial flow rates are much 

greater and approach 65,000 ft3/d per 100 feet of horizontal hole. The lower 

flow rate is the result of partial degasification of the coalbed along the 

periphery of the mine. 

Maintaining a bit on a horizontal trajectory or inclined to follow the 

dip of a coalbed is difficult. The natural tendency of the bit during 

horizontal drilling is to arc downward because of gravity. Changes in hardness of the coalbed or hard inclusions such as pyrite balls may deflect the 

bit in an unpredictable manner. 

lSupervisory geophysicist. 2 Mining engineer. 

~eologis t. 
A bit deviating from a given trajectory by lo will intercept the top or 

bottom of a 7-foot-thick coalbed in 200 feet, and in a 4-foot-thick coalbed, 

within 115 feet if started in the center of the coalbed. Hence, precise 

control must be maintained on the trajectory to keep the bit in the coalbed 

and to attain lengths of 1,000 feet or more. 

Starting in 1957, the British conducted experimental drilling studies 

to develop tools and equipment for drilling horizontal holes in coal to 

depths of 450 feet (1) .4 Procedures were developed for inducing the bit to 

rise or fall; no infzrmation was published on bit thrust and rotational speed. 

A state-of-the-art study on horizontal drilling in 1970 (5) concluded there 

are no reliable published data on horsepower, thrust, rotational speed, bit 

configuration, and the effects of these factors on hole trajectory. In the 

Pittsburgh coalbed (L), horizontal degasification holes have been drilled 

ranging in length from 500 to 850 feet. Hole trajectory was controlled by 

close-fitting stabilizers (packed hole) that ranged from 2 to 10 feet long. 

Torque, drilling speed, thrust, and rotational speed were not measured. At 

a demonstration inastrip pit (4), a horizontal hole was drilled to a depth 

of 1,034 feet. Drilling parameTers and a drill-string configuration were 

developed experimentally to control hole trajectory. 

Holes in excess of 1,000 feet expose a large area of the Pittsburgh 

coalbed to degasification and, therefore, fewer holes are required along 

the mine's periphery to reduce significantly methane flow into the mine. 

addition, four or five horizontal holes (1,000 feet long) produce about 

1 million ft3/d of methane, which can be piped to the surface through an 

underground system of pipelines. Reduction in methane flow into mine 

openings improves safety, results in a savings in ventilation costs, and 

improves the potential for increased coal production. 

The objectives of this Bureau of Mines report are to describe the 

demonstrated methods of maintaining a bit on a predetermined trajectory and 

to provide data on drilling parameters and equipment. This report provides 

guidelines and reviews concepts for (those who are contemplating) a horizontal drilling program. Drilling procedures and parameters have been 

developed for the Pittsburgh coalbed. Although procedures and parameters 

may require slight modifications in other coalbeds, the principles of 

horizontal drilling are unchanged. 

ACKNOWLEDGMENTS 

The cooperation of Eastern Associated Coal Corp., Pittsburgh, Pa., is 

greatly appreciated. The authors thank William Laird, vice president-- 

research and development, for his cooperation in conducting some of the work 

described in this report and for his continued interest and support. We 

acknowledge the work of Fenix and Scisson, Inc., Tulsa, Okla., on horizontal 

drilling, which was sponsored by the Bureau of Mines (contract No. H0111355). 

"underlined numbers in parentheses refer to items in the list of references at 

the end of this report. 
(1 (, 

am- 

.;(*A1 / , 1r1( tit s, 

FIGURE 1. - Drill bits. A, Three-blade drag bit; i?, three-cone roller bit; C, plug bit; 
Water at flow rates of 15 to 20 gal/min is effective in removing drill 

cuttings and preventing blockage at penetration rates of 3 ft/min. In some 

underground drilling locations, disposal of drill water is a problem and a 

recirculating system must be used. Air as a circulating medium is not permitted. An explosive airlmethane mixture will form, and sparks from drilling 

could set off an explosion. 

Bits 

There are a variety of bits that can be used to drill coal (2-5j. These 

include a three-blade drag bit, plug bit, and a three-cone roller bTt (fig. 1). 

Choice of the type of bit used will be governed primarily by cost and performance of the bit. For fixed levels of bit thrust and rotational speed, the 

drag bit by far has the highest penetration rate in coal and is the least 

costly. Table 1 shows comparative penetration rates of the various types of 

bits (4). The drag bit has one disadvantage. It will not penetrate a hard 

inclusion such as a pyrite ball. When this occurs, the drill pipe is pulled 

and a three-cone roller bit is used. If pyrite balls are prevalent in a coalbed, the three-cone roller bit is used throughout the hole-drilling program 

even through penetration rates are lower than the drag bit by factors of 3 to 

4. Use of the three-cone roller bit circumvents frequent pulling of drill 

pipe during drilling. Bit diameters used in the ~ureau's hole-drilling 

programs range from 3 to 3-518 inches. 

TABLE 1. - Bit penetration rates, (Ohio strip pit) 

(3,000 -1b thrust , 200 r/min) 

Drill Rod 

Bit type 

Drag ........................... Three cone roller... ........... Plug ........................... 

Flush joint casing is used for drill rod in horizontal drilling programs 

conducted by the Bureau (2, 5). Specifications of casing are shown in table 2. 

Generally, for holes drilled500 or more feet, BQ flush joint casing is used. 

Ten-foot lengths are used for ease of handling in underground locations. 

Penetration rate, 

f t /min 

11 -12 

3- 4 

2- 5 

TABLE 2. - Casing specifications 

Casing 

Outside diameter ................. inch.. 

Inside diameter ................... do... 

Threads per inch. ...................... Weight .............p ounds per 10 feet.. 

EW 

1-13/16 

1-112 

4 

28 

BQ 

2-3/16 

1-13/16 

3 

40 
Centralization of Drill String 

The primary objective during drilling is to maintain the trajectory of 

the hole parallel to bedding planes of the coalbed. If the bit deviates from 

this course, some means of correcting its path must be applied to prevent the 

hole from intercepting the floor or roof. 

Experience has shown that the trajectory of a Lole being drilled with a 

bit and flush joint casing only (fig. 2) is unpredictable. Rotating this 

assembly at 500 r/min and low levels of thrust causes the bit to wear the 

bottom side of the hole and arc downward. However, as thrust levels are 

increased, the hole may arc upward or downward and the trajectory is 

unpredictable generally. 

If a short centralizer (fig. 3) (4-5) is placed directly behind the bit, 

the trajectory of the hole will follow an arc into the roof. The centralizer 

is about 8 to 10 inches long, and its diameter is 1/16 inch less than the 

diameter of the bit. This drill string is shown in figure 4. The weight of 

the drill string is borne by the centralizer. However, the drill casing, 

which lies on the bottom of the hole, tends to tilt the centralizer and bit 

slightly upwards, and consequently, the bit follows a trajectory into the roof. 

The spirals on the centralizer (clockwise or counterclockwise) do not affect 

the performance of a drill-string configuration (4). Clockwise spirals are 

used because of the tendency to assist in the removal of cuttings. 

Shifting the centralizer from a position directly behind the bit to a 

position one casing length (10 feet) from the bit causes the trajectory of 

the hole to follow a curved path downward (fig. 5). The flexibility of the 

casing and the weight of the bit bends the drill string downward slightly and, 

consequently, the trajectory of the hole will be downward. If two lengths of 

drill casing followed by a centralizer are used, the trajectory of the hole 

into the bottom is steeper. 

Based on the trajectories of the drill strings (figs. 4-5), one would 

assume logically that the use of two centralizers--one directly behind the bit, 

and another 10 feet behind the first centralizer (fig. 6)--would keep the bit 

on a horizontal trajectory. However, this is not so. Experience has shown 

that the trajectory of this drill string is a downward path. There is no 

satisfactory explanation for this behavior. Separating the two centralizers 

by two joints of drill casing (20 feet) does not solve the problem either. 

The joint between the two centralizers and the flexibility of the drill casing 

causes the drill striilg to bow upward between the two centralizers; 

consequently, the trajectory of the hole is upward (fig. 7). 

The drill string that gives the best overall results is the configuration 

shown in figure 8 (A). This configuration is similar to that in figure 7, 

except the BQ drill rod is replaced by a heavy NW drill rod (fig. 9). This 

rod is 2 -5/8-inch-OD, 1 -3/4 -inch-ID, and weighs about 205 pounds. The 

equivalent length of BQ drill rod weighs about 80 pounds in comparison. The 

heavy NW drill rod stiffens the drilling assembly and adds weight to the 
SCALE, inches 

FIGURE 3. - Centralizer. 

FIGURE 4. - Brill string with one centralizer behind bit. 
__. - - ---- 

FIGURE 7. - Drill string with two centralizers 20 feet apart. 

FIGURE 8. - Drill string with two centralizers separated by a 20-foot-NW drill rod. 
FIGURE 9, 

 Drill rod, A, BQ; R, NW 

bottom side of the bit. The drilling assembly (fig. 8) can be made to drill 

upward, downward, or horizontally. 

CONTROL OF HOLE TRAJECTORY 

There are two drilling parameters that affect the trajectory of a drill 

string and are controlled by the driller. However, before changes are made 

by the driller, he must know beforehand the path the bit is following with 

respect to a programed trajectory. This information is obtained with 

instruments such as Sperry-Sun or Eastman Well Surveying Co. single-shot 

survey instrument. 5 

Ho 1 e Surveying 

Figures 10 and 11 show the single-shot survey instrument and its protective case, respectively. This instrument contains electrical components; 

therefore, it must be made permissible and approved for use in coal mines. 

Only the path of the bit in a vertical plane is measured and controlled. 

Control of the path of the bit in an azimuthal direction will be discussed in 

succeeding paragraphs. 

5~eference to specific equipment does not imply endorsement by the Bureau of 

Mines. 
FIGURE 12. - Survey instrument. A, Timer; li', batteries; C, inclination unit. 

The essential elements (fig. 12) of the single-shot surveying instrument 

are the inclination unit, which is in the form of an inverted plumb bob, 

batteries to furnish electrical power, and a timer, which controls the 

electrical circuits and illuminates the angle unit at a preset time to 

record the inclination on a film disk. Special loading and developing 

tanks permit handling of the photographic record disk in daylight or without 

turning off cap lamps underground. 

When the inclination of the bit is to be determined, the surveying 

instrument is placed in its protective casing, which is then inserted into 

the drill casing and pumped with water to the end of the hole. At a preset 

time, the film disk is exposed and, subsequently, the instrument is retrieved 

by wire-line, which is attached to the protective casing. The film disk is 

removed, developed, and read. The determination of the inclination of the 

bit takes about 1 hour for a hole at a depth of 1,000 feet. Initially, the 

hole is surveyed every 30 feet. If the bit maintains its programed trajectory, 
surveys are extended to 50-foot intervals, then to 100-foot intervals. When 

the bit begins to deviate, the surveys are made again at 30-foot intervals. 

Drilling Parameters 

Two drilling parameters that affect the path of the bit in a vertical 

plane are bit thrust and rotational speed. Table 1 shows that the penetration 

rate of the drag bit is 11 to 12 ft/min using 3,000-pound thrust and 200 r/min. 

At higher thrust levels, penetration rates of 19 ft/min are obtained, providing the fluid return rate is 25 gal/min or more. However, the high penetration rates potentially obtainable in coal are never attained during controlled 

drilling (fig. 8). Thrust levels normally used during drilling range between 

500 and 2,000 pounds. Bit rotational speed ranges from 200 to 900 r/min. 

The thrust applied determines not only the penetration rate but also the 

path of the bit. Table 3 summarizes the effect of thrust and bit rotational 

speed on bit trajectory (4). Levels of thrust and rotational speed shown in 

table 3 are meant as guidelines and may vary from one coalbed to another. 

TABLE 3. - Effect of thrust and bit rotational speed on hole trajectory 

Drilling Procedures 

Thrust, pounds 

800 .............................. 1,200 ............................ 2,000 or greater ................. 

Experience in drilling holes in the Pittsburgh coalbed shows that the 

bottom 3.5 feet is relatively soft and free of pyrite inclusions. However, 

the top 3.5 feet is much harder and contains numerous streaks and bands of 

pyrite. No difficulty was encountered in lifting or dropping the bit, when 

necessary, in the lower 3.5 feet. However, when the hole was in the upper 

3.5 feet, considerable difficulty was experienced in attempts to drop the bit 

because of the resistance to penetration due to hardness of the bands of 

pyrite in the coalbed. 

The procedures for dropping the bit are to reduce thrust to about 

800 pounds and increase bit rotational speed to about 800 r/min. At the end 

of each 10 feet of drilling, several slow reaming passes are made to wear the 

bottom of the hole. This procedure was repeated for each 10 feet of penetration for distances of 50 to 100 feet. In many cases, these procedures dropped 

the bit; however, in several cases, the bit did not drop fast enough and, 

consequently, the roof rock was encountered. The procedure then is to pull 

the drill pipe and remove the centralizer behind the bit. The bit is backed 

off the end of the hole 20 to 30 feet, and slotting runs are made for each 

10 feet of drilling. Removing the front centralizer shifts the weight of the 

heavy NW drill rod to the blades of the drag bit. The bottom of the hole is 

quickly and effectively worn and a new hole started slightly below the old one. 

The new hole is surveyed every 20 feet until the desired angle is obtained, 

and the drill pipe is pulled, the front centralizer replaced, and drilling is 

continued. 

Bit, r/min 

700-900 

400-600 

200-300 

Effect on bit trajectory 

Downward or dropping. 

Holds angle. 

Upward or lifting. 
The procedures for lifting the bit are easier and more effective, 

generally. Thrust is increased to 2,000 to 3,000 pounds and bit rotational 

speed is reduced to about 200 r/min. A hole deviating by 1" is brought back 

on course within 30 feet of drilling. Penetration rates are higher also 

because of the higher thrust levels. In some cases, such as a hole that is 

dropping 2" to 3O below its programed trajectory and approaching the bottom of 

the coalbed, removal of the back centralizer turns the hole more rapidly and 

may prevent the bit from entering the bottom. To do this, the drill pipe 

must be pulled twice. 

Human Factors 

There is presently no substitute for experience in long horizontal hole 

drilling. The driller must be familiar with the tools, equipment, and their 

response to drilling parameters. He must make frequent hole surveys, which 

detracts from actual drilling time. At depths approaching 1,000 feet, survey 

time requires about 1 hour. Therefore, the driller tends to drill for longer 

periods before surveying. Consequently, a hole may have diverted from its 

programed trajectory by more than 1" when finally surveyed. Considerable 

time may be lost redirecting the hole to its proper trajectory. No serious 

delay time will occur providing the bit is maintained within I" of its 

programed trajectory. 

The driller must react immediately to rod chatter sounds fron his drill, 

which indicate hard spots in the coalbed. These hard spots tend to deflect 

the bit in an unpredictable manner. The driller should reduce thrust until 

the zone has been drilled. Soft spots in the coalbed are a problem also. 

Penetration rates are extremely high (5 ft/min at 1,000-pound thrust and 

300 r/min), and thebit tends to arc downward rapidly. When soft spots are 

intercepted, the driller should reduce the rotational speed as low as 

possible and increase thrust to prevent the bit from wearing the bottom of 

the hole excessively and arcing downward. 

Azimuthal Control 

Thrust and bit rotational speed do not appear to affect the path of the 

b i t i n an azimuthal direction when drilling with the assembly (fig. 8). 

Surveys of holes drilled to depths of about 1,000 and 1,100 feet showed that 

the holes were 13 and 22 feet left, respectively, of the original bearing 

lines at depth (4). For degasification purposes, generally this distance is 

not critical, and no attempt is made to control the bit in a azimuthal 

direction. However, precautions are taken when pyrite balls are encountered 

because the bit canbedeflected both in the vertical and azimuthal directions. 

Methane Control 

Methane flow rates from horizontal holes drilled in virgin coal areas 

increase at a rate of 45 ft3/min per 100 feet of hole in the Pittsburgh 

coalbed. At horizontal distances of 1,000 feet, the flow rate is approaching 

450 ft3/min and sufficient air must be circulated to reduce concentrations of 

methane to levels below 1 pct at the drilling site. Therefore, at least 
45,000 ft3/min of air is required. The large quantities of air required are 

not available generally; therefore, other means of handling methane during 

drilling must be used. 

Figure 13 shows the method used in coping with large quantities of 

methane. Drilling is conducted through a Bureau designed and fabricated 

stuffing box attached to a 20-foot length of 6-inch-diam pipe grouted into 

the coalbed. Water and drill cuttings drop into the bottom of the box, and 

methane is drawn off the top of the box. Flexible tubing is used to connect 

the top of the stuffing box to a vertical 8-inch-diam borehole, which is 

maintained at a slightly negative pressure by a surface exhauster or natural 

draft. This method is a safe and efficient means of hacdling flows of methane 

during drilling. 

The stuffing box does not solve all methane problems. When drill pipe 

is being pulled or during hole surveys, gas pressure at the back of the 

horizontal hole clears the drill pipe of water and methane begins to flow 

into the mine opening. A one-way check valve placed in the drill string 

either directly behind the bit or behind the second centralizer eliminates 

this unsafe condition (fig. 14). 

Drilling Example 

Figure 15 shows a partial plot of the trajectory of a horizontal hole 

drilled with a 3-112-inch drag Sit and the drill string configuration (fig. 8). 

This example shows the response of the drill string to drilling parameters 

(thrust and rotational speed), the strategy used, and the seemingly unexplainable events that occur during drilling operations. The dip of the coalbed 

in the direction of drilling averages +0.25". The designation -I-0.25" 

indicates the dip of the coalbed is 0.25" above horizontal. The designation 

-0.8" indicates 0.8" below horizontal. 
FIGURE 14. - BQ one-way check valve. 
HORIZONTAL DEPTH, feet 

FIGURE 15. - Plot of hole trajectory. 

During the drilling operation, a plot is maintained of the trajectory of 

the hole. The location of the bit relative to a horizontal plane is known 

at all times. If the coalbed undulates, another unknown is introduced. 

Table 4 summarizes the drilling parameters used during each interval drilled 

and the survey angles at the end of each drilling interval; the strategy 

used for the next drilling interval is provided under remarks. 
TABLE 4. - Summary of drilling parameters 

I Drilling I I 

Remarks 

Drilling 

interval, 

feet I pounds I speed, I deg 1 feet 1 

parameters 

Thrust,I~otational 

Survey 

Angle,I ~e~th, 

886-916 .... with upward parameters. 

Trajectory beginning to level 

out. Continue drilling with 

upward parameters. 

Continue drilling with upward 

1,306-1,356 

1,356-1,386 

parameters. 

Bit trajectory turned. Reduce 

thrust and increase 

rotational/speed. 

Reduce thrust to break upward 

path of bit during next 

interval of drilling. 

Bit angle is decreasing. 

Continue drilling with 

downward parameters. 

Increase thrust level slightly 

during next interval. 

Continue drilling with downward 

parameters for 10 feet to 

confirm previous survey. 

Continue drilling next interval 

with slightly dropping 

parameters. 

Continue drilling with slightly 

downward parameters for 

10 feet to confirm survey at 

1,286. 

Continue drilling with slightly 

1,900 

960 

600 

downward parameters for 

10 feet. 

Continue drilling with dropping 

 NO survey made. 

parameters. 

Angle decreasing. Continue 

drilling with dropping 

parameters. 

Pull drill pipe. Remove front 

centralizer. Restart hole at 

1,306 feet. 

r/min 

345 -0.8 916 Continue drilling next interval 
1,496-1,526 

1,526-1,556 

1,556-1,586 

1,586-1,686 

1,686-1,826 

1,826-1,856 

1,856-1,886 

1,886-1,896 

1,896-1,926 

1,926-1,966 

1,966-1,986 

1,986-2,006 

2,006-2,026 

2,026-2,046 

2,046-2,066 

2,066-2,126 

thrust to dropping parameters. 

Continue drillingwith slightly 

dropping parameters. 

Do. 

Do. 

Do. 

Hole near bottomof coalbed. 

Drill with lifting parameter 

to bring hole up. 

Continue drilling with lifting 

parameters. Increase thrust. 

Decrease thrust, but continue 

drilling with lifting 

parameters. 

Decrease thrust, but continue 

drilling with lifting 

parameters. 

Increase thrust level. 

Continue drilling. 

Do. 

Do. 

Do. 

DO. 

Do. 

Stop drilling. 

+.3 

+.2 

+.l 

0 

+.3 

0 

+.8 

+.9 

+.5 

+.7 

+.3 

0 

+.4 

+.2 

+.5 

- 

900 

1,080 

1,080 

1,080 

1,080 

1,440 

2,160 

1,740 

1,440 

2,160 

2,160 

2,500 

2,500 

2,160 

2,500 

1,440 

1,526 

1,556 

1,586 

1,686 

1,826 

1,856 

1,886 

1,896 

1,926 

1,966 

1,986 

2,006 

2,026 

2,046 

2,066 

- 

340 

340 

340 

340 

340 

'I. 0 0 

220 

225 

225 

225 

230 

225 

225 

225 

234 

310 
The interval of drilling between 1,246 and 1,296 feet provides an example 

of an unpredictable event. Drilling at 1,246 feet is progressing with holedropping parameters. However, a survey at the end of the drilling interval 

at 1,296 feet shows that the bit trajectory has reversed sharply. Drilling 

is continued with dropping parameters, but the bit trajectory angle continues 

to increase. Past drilling experience shows that if the bit deviates by more 

than +ZO from its programed path, pulling drill pipe, removing the front 

centralizer, and restarting a new hole are procedures to be used instead of 

attempting to turn the hole using 600-pound thrust and high rotational speed 

(>500 r/min). The only logical explanation for the hole angle reversal is 

that the bit encountered a hard spot at a glancing angle and was deflected 

upward. 

The interval between 1,926 and 2,126 feet is an example of drilling in 

soft coal. Penetration rates ranged from 30 to 40 in/min at thrust levels of 

2,16Oto2,500 pounds. Normally, the hole trajectory would turn upward at a 

sharper angle. Drilling was terminated at 2,126 feet because of a lack of 

drill pipe. The only mechanical problem with the drill was a worn hydraulic 

motor driving the spindle. Spindle rotational speed could not be increased 

beyond 350 r/min. Generally higher rotational speed is needed when hole 

trajectory is to be dropped. 

Similar data have been obtained in the drilling of five additional holes 

in the Pittsburgh coalbed. The experience derived from those tests substantiates the data in table 4 and are not provided here because the purpose of this 

report is to describe a proven method for rotary drilling degas.ification holes 

in coalbeds. 

ALTERNATE DRILLING ASS EMBLY 

The preceding discussions were applicable to the drilling assembly 

(fig. 8). The diameter of the centralizers is 3-7/16 inches, which is 

1/16 inch less than the diameter of the bit (3-112 inches). Typical drilling 

parameters and associated penetration rates in the Pittsburgh coalbed are 

shown in table 5. 

TABLE 5. - Penetration rates using a 3-112-inch bit 

Increasing the thrust level to more than 1,200 pounds causes the bit 

trajectory to follow a curved path upward. Dropping a hole requires thrust 

to be reduced to the 800-pound level; accordingly, the penetration rate 

decreases to about 5 in/min. Certainly the full potential of the hydraulic 

drilling unit is not being utilized. 

Thrust, pounds 

800.. ........... 1,200 ........... 1,500 or greater 

Penetration rate, 

in/min 

4- 7 

10 

15 or greater 

Bit 

rotation, 

r/min 

700-900 

400-600 

200 -300 

Effect on bit 

trajectory 

Downward.. ... Holds angle.. 

Upward. ...... 
To improve the performance of the drill unit, the bit diameter of the 

drilling assembly (fig. 8) was increased to 3-518 inches. The centralizer 

diameter is 3116-inch less than bit diameter. Under this condition, the bit 

tends to wear the bottom side of the hole and, consequently, arcs toward the 

bottom more rapidly than the 3-112-inch bit. To overcome the tendency to drop, 

thrust levels were increased to about 1,700 pounds to maintain the bit on a 

horizontal trajectory. The drilling parameters and associated penetration 

rates are shown in table 6. 

TAELE 6. - Penetration rates using a 3-518-inch bit 

Increasing the bit diameter improves the performance of the drilling 

assembly. There is a general increase in penetration rate, changing bit 

trajectory especially downward is easier, and bit rotational speed is reduced, 

eliminating much of the drill rod vibration that occurs over 400 rlmin. The 

only disadvantage to the larger bit diameter assembly is bit wear. Normally, 

a 3-112-inch bit will become dull after drilling 1,000 to 1,200 feet. The 

points or corners of the blades become rounded and penetration drops slightly. 

The sides of the blades and corners of the 3-518-inch bit become rounded in 

about 800 feet of drilling. This is due to the weight of the heavy NW drill 

collar, which is shifted to the sides of the bit blades. However, any 

disadvantage due to bit wear is overshadowed by the improved performance of 

the drilling assembly. 

Thrust, pounds 

800 ............ 1,700 .......... 2,000 ........... 

Although experimentation with a larger bit diameter such as 3-314 inches 

was not attempted, results of the tests with the 3-518-inch bit indicate the 

performance of the drilling assembly can be improved further. The 3-314-inch 

bit will arc dom.ward more rapidly than the 3-518-inch bit. To overcome this 

tendency, thrust levels rnust be increased to about the 2,100-pound level to 

maintain the bit on a horizontal trajectory, Penetration rates will be 

increased in general. 

DRILL AND POWER UNIT 

Bit 

rotation, 

r Imin 

350 

250 

200 . 

Figures 16 and 17 show the drill and power units, respectively. The 

power unit consists of a 30-hp, 440-volt permissible motor and starter box, 

hydraulic pumps, and a 40-gal hydraulic fluid reservoir. A fire-resistant 

fluid (water in oil emulsion) is used in the hydraulic system. The drill unit 

consists of a 5-foot hydraulic cylinder mounted within its frame, a spindle 

mounted on a movable carriage, and valves, gages, and levers for controlling 

rotational speed, thrust, and movement of the carriage, 

The electric motor drives two hydraulic pumps. One pump (20 gal/min) 

pressurizes the 5-foot hydraulic cylinder on the drill unit, which provides 

Effect on bit 

trajectory 

Downward.. ... Holds angle.. 

Upward.. ..... 

' Penetration rate, 

inlmin 

7- 8 

17 -20 

22 or greater 
FIGURE 17. - Power unit. 

thrust on the bit. The second is a reversible, variable displacement pump 

(29 gallmin), which powers a hydraulic motor mounted on the spindle of the 

drill unit. 

The thrust potential of the drill unit is 13,000 pounds. The 5-foot 

hydraulic cylinder is connected by steel cable to the carriage through a 

system of pulleys. The carriage travels about 10 feet as the piston extends 

to 5 feet. 

Figure 13 shows a view of the control panel of the drill unit. The 

driller is able to monitor and control drill rotational speed, thrust, drill 

pipe water pressure, and direction of carriage travel from one location. 

Drill pipe water pressure is monitored to insure water circulation at the bit 

at all times. Monitoring the return water does not insure that bit cuttings 

are being properly flushed. A blockage can occur within the drill pipe or bit 

and the return waterflow will not be affected for several minutes. Therefore, 
several feet of coal will have been drilled with no cutting removal and, 

consequently, the drill string may become lodged. However, monitoring drill 

pipe water pressure would indicate the blockage by a sudden rise in pressure. 

A drop in drill pipe water pressure indicates a loss of water at the drill 

unit. Both require that drilling be stopped until the problem is solved. 

During drilling operations underground, the power unit is kept in fresh 

air entries. Hydraulic lines are run between power and drill units, which 

may be 500 feet apart. At the drilling site, the drill unit is cribbed so 

that the horizontal hole is started in about the center of the coalbed. In 

addition, the drill is anchored to the coalbed to prevent backward movement 

of the drill unit when high thrust levels are being applied to the bit. 

CONCLUSIONS 

A proven technique for horizontal drilling in coal has been presented. 

Drilling parameters have been derived for the Pittsburgh coalbed, which may 

or may not be applicable to other coalbeds. However, these parameters are a 

starting point, and, with experience, the levels for dropping, holding, or 

lifting hole trajectory may require slight modification for drilling in other 

coalbeds. This can be accomplished in about the first 300 to 500 feet of 

drilling. 

Surveying to obtain bit angle is important and necessary. The drilling 

example presented shows that bit angle can change radically in 30 feet. Best 

results are obtained when bit angle is maintained '1.0" of programed 

trajectory. 

When hole direction is being changed from an upward to a downward 

trajectory or vice versa, the surveying interval should not exceed 20 feet. 

The turn should be made gently to avoid angles of the order of +ZO from 

programed trajectory. 

Success in long hole horizontal drilling depends to some degree upon the 

driller. There is no substitute for experience at this stage of development. 

The driller must respond immediately to changes in hardness of the coalbed, 

which require thrust and rotational speed levels to be altered. 

The drill string configuration (fig. 8) must be used. This assembly can 

be turned upward or downward using either a 3-112- or 3-518-inch bit. Drag 

bits are recommended because of their high penetration rates and low cost. 

The drill and power units used in this study have ample reserve power, 

and at no time was the equipment taxed. The drill unit is about 18 feet long 

to acconnnodate the 10-foot feed and could be made shorter for ease in handling; 

however, the 10-foot feed would be sacrificed. The drill unit was equipped 

with gages for monitoring thrust, spindle rotation speed, and water pressure 

during drilling, a necessity for controlled horizontal drilling. 
REFERENCES 

1. Baxter, J. S. Drilling of Long Boreholes in Coal. Colliery Eng., 

v. 36, December 1959, pp. 520-525. 

2. Fields, H. H., S. Krickovic, A. Sainato, and M. G. Zabetakis. 

Degasification of Virgin Pittsburgh Coalbed Through a Large Borehole. 

BuMines RI 7800, 1973, 27 pp. 

3. adde en, J. D., and J. Cervik. Design and Development of Drill Equipment. 

BuMines TPR 11, May 1959, 11 pp. 

4. Rornmel, R. A., and L. A. Rives. Advanced Techniques for Drilling 

1,000-Foot Small Diameter Horizontal Holes in a Coal Seam. BuMines 

contract No. H0111355, v. 1, Fenix and Scisson, Inc., Tulsa, Okla., 

March 1973; available for consultation at U.S. Bureau of Mines, 

4800 Forbes Avenue, Pittsburgh, Pa., Building D152. 

5. Williamson, T. N. Evaluation of Horizontal Drilling Techniques in 

Coalbeds. BuMines Open File Report 34-72, 1970, 131 pp.; available 

for consultation at BuMines libraries in Minneapolis, Minn., Denver, 

Colo., Spokane, Wash., Pittsburgh, Pa., and Central Library, 

U.S. Department of the Interior, Washington, D.C.