By Tobias W. Goodman and Joseph Cervik 

UNITED STATES DEPARTMENT OF THE INTERIOR 
Report of Investigations 91 77 

Horizontal Trajectories 

of Rotary- Drilled Holes 

By Tobias W. Goodman and Joseph Cervik 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 

T S Ary, Director 
Library of Congress Cataloging in Publication Data: 

, 

Goodman, Tobias W. 

Horizontal trajectories of rotary-drilled holes. 

(Report of investigations; 9177) 

Bibliography: p.11 

Supt. of Docs. no. I 28.23: 9177. 

1. Boring. I. Cervik, Joseph. 11. Title. 111. Series: Report 

of investigations (United States. Bureau of Mines); 9177. 

TN23.U43 [TN279] 622s [622'.24] 88-600114 
CONTENTS 

Page 

Abstract ....................................................................... Introduction ................................................................... Drilling procedures ............................................................ Data analysis .................................................................. Summary and conclusions ........................................................ References ..................................................................... 

ILLUSTRATIONS 

Preferred assembly with 18-ft collar ...................................... Drill bits ................................................................ Modified assembly-up drilling ............................................ Modified assembly-- down drilling .......................................... Horizontal trajectory of hole. Pittsburgh 8 Coalbed ....................... Horizontal trajectory of hole. Pittsburgh Coalbed ......................... Vertical trajectory of hole. Beckley Coalbed .............................. Horizontal trajectory of hole. Beckley Coalbed ............................ Bit drilling assembly ..................................................... Horizontal trajectory of hole. Mary Lee Coalbed ........................... Vertical trajectory of hole. Mary Lee Coalbed ............................. Preferred assembly with 14-ft collar ...................................... Horizontal trajectory of hole. Beckley Coal Mine .......................... Lower Sunnyside Coalbed (Utah) study ...................................... 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

cm centimeter km kilometer 

0 degree L/min liter per minute 

ft foot lb pound 

gal/min gallon per minute lb/f t pound per foot 

in inch m meter 

kg/m kilogram per meter rad radian 
HORIZONTAL TRAJECTORIES OF ROTARY-DRILLED HOLES 

By Tobias W. ~oodman' and Joseph cervik2 

ABSTRACT 

This Bureau of Mines report investigates the horizontal trajectory of 

boreholes drilled in coalbeds using the rotary drilling technique. Test 

holes drilled in coalbeds showed that the path of a borehole in the horizontal plane depends on drilling assembly configuration, drill bit 

rotation, and coalbed geologic features. An assembly that contained an 

18-ft (5.53) long drill collar and two centralizers had the best horizontal stability evidenced by trajectories that deviated less than 16 ft 

(4.9 m) from a target at 1,000 ft (305 m). Removal of one of the centralizers resulted in a loss of control of hole trajectory in the horizontal plane. Generally, holes tended to turn to the right, presumably 

because of right-hand rotation of the bit. Deviations from the original 

bearing line were as much as 200 ft (61 m) or more at a depth of 1,000 

ft (305 m). 

- - - - - - 

l~inin~ engineer. 

2~upervisory geophysicist. 

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

The Bureau of Mines experimentation 

with long hole drilling for coalbed 

degasification began in 1970 (1).3 The 

Bureau's horizontal drilling technique is 

an adaption of the "packed hole" method 

used in vertical hole drilling by the 

petroleum industry (2). The primary 

objective in horizontal drilling is to 

maintain the vertical trajectory of the 

hole within the coalbed by varying the 

drill rotational speed and thrust parameters or by modifying the drilling assembly to prevent the bit from entering roof 

or floor rock, which could terminate the 

hole. 

Horizontal trajectory, the course of 

the hole projected to the horizontal 

plane, is essentially uncontrollable in 

horizontal rotary drilling. However, the 

degree of deflection from a given path 

can be limited somewhat by choice of 

drilling assembly. A complete downhole 

drilling assembly, consisting of a bit, 

centralizer, 18-f t (5.53) long drill 

collar, and a second centralizer (fig. 1) (e), is a rigid assembly that can be 

deflected off course but does not drill 

in an arc. Experience shows that this 

assembly is vertically controllable in 

the Pittsburgh Coalbed by adjustments to 

bit rotational speed and thrust. Because 

of these factors, the complete assembly 

3~nde rlined nurnbe rs in, parentheses refer to items in the list of references at 

the end of this report. 

1O;in centralizer, 

is preferred for use where possible in 

degasification drilling programs. Even 

minor modifications to this assembly, 

removal of a centralizer or use of a 

shorter collar, cause it to tend to drill 

in an arc. For instance, removal of the 

back centralizer, though useful in maneuvering the bit in the vertical plane, 

causes the back end of the drill collar 

to move off center and cock in the hole. 

Drill thrust tends to force the unsupported back end of the drill collar 

toward aside of the hole. The bit, 

then, begins to deflect away from the 

planned path in the horizontal plane. 

Continued drilling with this or other 

modified assembly causes deflection of 

the bit away from its original bearing 

line. 

During drilling, the hole is routinely 

surveyed to measure bit inclination (4- - 

- 5). Subsequently, changes are made in 

bit thrust and rotational speed to maintain bit trajectory parallel to the bedding plane. The objective during drilling is to maintain vertical control of 

the bit and prevent roof or floor interception, which slows drilling and 

increases drilling expense. The path of 

the bit in the horizontal plane is not 

measured during drilling because the horizontal trajectory of the hole cannot be 

controlled using the rotary drilling 

technique. In many cases where a hole is 

drilled to drain methane, horizontal 

deviation of the hole by 50 or more feet 

10- in centralizer, 7 

48-ft NP drill collar, 

1 

Ii Bit, 32 in 

FIGURE 1.-Preferred assembly with l&ft collar. 
aids in making changes in vertical angle 

easier. The cones on the roller bit are 

oriented ahead of the bit and do not provide as much side-cutting action as the 

drag bit. Water is the required drilling 

fluid since the use of air is hazardous 

when mixed with coalbed methane. Water 

rates of 15 gal/min (57 L/min) or more 

are needed to adequately lubricate the 

path of the bit and flush the hole clear 

of cuttings during drilling (4). 

The purpose of centralizers-is to keep 

the assembly centered in the drill hole. 

The centralizers are 10 in (25.4 cm) long 

and 3-7/16 in (8.7 cm) OD. Drill collars 

are used to provide weight and rigidity 

to the drill string. Fourteen- and 18-ft 

(4.3- and 5.531) long NQ rod drill collars were used in this study. NQ rod 

is 2-11/16-in (6.8-cm) OD, 1-13/16-in 

(4.6-cm) ID and weighs about 10.5 lb/ft 

(15 kg/m)- 

In standard practice, vertical attitude 

of the assembly is controlled by varying 

the drilling parameters and/or removing 

one of the centralizers if needed. When 

the vertical angle is less than 2" 

0 - 3 

Scale, in 

FIGURE 2.-Drill bits. A, Three-blade drag bit; B, tricone 

roller bid. 
18 oft NQ drill collar, 

10-in centralizer, 

FIGURE 3.-Modified assembly-up drilling. 

10-in centralizer, 

7 

BQdrill rod, 

1 I 18-ft NQ drill collar 

2 

FIGURE 4.-Modified assembly-down drilling. 

varying the drilling parameters with no 

alteration in the configuration of the 

preferred assembly. Higher thrust and 

lower bit rotational speed tend to raise 

the bit, and lower thrust and higher bit 

rotational speed tend to lower the bit. 

When the vertical angle is more than 2" 

(0.035 rad) out of alignment with the 

proposed trajectory, removal of the back 

centralizer cocks the drill collar, centralizer, and bit in the hole and turns 

the hole upward more rapidly than the use 

of drilling parameters (fig. 3). By 

removing the front centralizer (fig. 4) 

instead, the weight of the collar falls 

directly on the side-cutting blades of 

the bit, and the hole turns downward 

easily. These modified assemblies should 

be used only for short distances because 

of the tendency to lose control of the 

horizontal trajectory of the bit. 

DATA ANALYSIS 

A drilling strategy is developed for is used. Then modifications are made to 

each site or coalbed. First, the pre- this assembly as needed. If the preferred assembly (fig. l), most readily ferred assembly can be used, drilling 

controllable using drilling parameters, proceeds by altering drilling parameters 
to correct the vertical angle with occasional removal of one of the centralizers 

when larger adjustments are advisable. 

If, however, the preferred assembly cannot control the vertical angle, then the 

back centralizer may be removed and a 

3-5/8-in (9.2-cm) bit substituted for the 

3-1/2-in (8.9-cm) bit. The tendency of 

the cocked drill collar to drill upward 

is compensated by the tendency of the 

larger bit to overbreak the coal and 

drill downward. This modified assembly 

can be controlled in the vertical plane 

by varying bit thrust and rotational 

speed and was successfully used in the 

Pittsburgh and Sunnyside Coalbeds. 

The longest interval drilled with the 

preferred assembly (fig. 1) occurred at a 

site in the Pittsburgh 8 Coalbed (3) that 

is relatively flat and blocky and Bimilar 

to the Pittsburgh coalbed. Figure 5 

shows the horizontal trajectory of the 

hole, which begins to bend abruptly to 

the left after the bit drilled into a 

band of pyrite at 350 ft (107 m). The 

bit never returned to its original bearing line though the total deviation was 

not extreme and the assembly was under 

control. At full depth of 1,034 ft (315 

m), the hole was only 15.5 ft (4.7 m) to 

the left of the original bearing line. 

Many coalbeds show a marked difference 

in the drilling characteristics in the 

upper and lower benches and may differ 

regionally as well. The upper portion 

may contain bands of pyrite compared with 

the lower half, which is relatively free 

of impurities and, hence, softer and 

easier to penetrate during drilling. As 

evidenced by penetration rates using comparable drilling parameters, the Pittsburgh Coalbed near Fairview, WV (A), was 

found to be much harder to drill than at 

Marianna, PA (7), located 30 miles (48 

km) north, apparently owing to the difference in overburden thickness. As a 

result, the preferred assembly (fig. I), 

which was very effective at the Fairview 

site, was not vertically controllable at 

the Marianna site. It repeatedly arced 

to the bottom. The preferred assembly 

was then modified by using a 3-5/8-in 

(9.2-cm) bit and removing the back centralizer (fig. 3). The penetration rate 

of the modified assembly was about twice 

that experienced using the preferred 

assembly. Figure 6 shows that the actual 

horizontal trajectory of the hole arced 

to the right. The deviation of the hole 

at 1,000 ft (305 m) from the original 

bearing line was 50 ft (15.2 m) but 

at 2,000 ft (610 m) measured along the 

200 400 600 800 1,000 1,200 

DISTANCE ALONG ORIGINAL BEARING LINE, ft 

FIGURE 5.-Horizontal trajectory of hole, Pittsburgh 8 Coalbed. 

- - 

KEY 

- X Point where bit hit pyrite - 

--- Original bearing line - 

- - 

- - 

- - 

- -, 

------------ d 

1 XI I I I 

f 
HORIZONTAL DEPTH,ft 

FIGURE 7.-Vertical trajectory of hole, Beckley Coalbed. 

riginal bearing line, the deviation was 

bout 160 ft (49 m). At the final depth 

f about 2,500 ft (762 m), the deviation 

from the original bearing line was about 

270 ft (82 m). Initially, the hole was 

drilled to about 340 ft (104 m) with the 

preferred assembly fig 1 Figure 6 

shows that there was very little horizontal deviation of the hole during this 

drilling interval. Deviation began 

shortly after the preferred assembly, 

which was unable to control vertical 

angle, was replaced by the modified 

assembly in the drill string. 

Drilling difficulties were experienced 

in the Beckley Coalbed because of the 

soft, friable nature of the coalbed, 

particularly in the lower portion, which 

drilled differently than the top portion, 

and severe undulations of the coalbed, 

which were unknown and made location of 

the bit difficult to interpret. Even 

when properly interpreted, the hole 

trajectory could not be altered rapidly 

enough to follow the coalbed. Figure 7 

shows the vertical trajectory of a hole 

drilled from the bottom of a ventilation 

shaft at Maple Meadow Mine (8) and why 
difficulties in interpretation occurred. 

The hole was started and drilled to 80 ft 

(24 m) with the modified assembly (f ig. 

3), which is cocked upward in the 

hole about 1" (0.02 rad). The smaller 

3-112-in (8.9-cm) bit was chosen over the 

3-518-in (9.2-cm) bit because of the extreme softness and friable nature of the 

coalbed. The assembly did not overbreak 

as expected, perhaps because of the small 

bit, and about a 2" (0.035 rad) upward 

vertical angle quickly developed that 

could not be decreased using drilling 

parameters. The back centralizer was 

reinstalled so that drilling parameters 

would be more effective in controlling 

vertical trajectory. The preferred 

assembly (fig. 1) leveled the hole near 

the roof, which it grazed occasionally 

between 240 and 370 ft (73 and 113 m) before hitting pyrite balls at about 390 ft 

(119 m). Drilling was continued until 

the bit hit the clay floor at 685 ft (209 

m). The existence of the undulation beyond 400 ft (122 m) was unknown. Figure 

8 shows that the hole deviated about 2 ft 

(0.6 m) at the 300-ft (91.5-m) depth. An 

abrupt change in deviation occurred after 

after the roof rock was intercepted by 

the bit at around that depth. Consequently the hole turned 18 ft (5.5 m) in 

the next 385 ft (118 m) of drilling. 

Thus, the total deviation of the hole was 

about 20 ft (6.1 m) at a depth of 685 ft 

(209 m). This example illustrates the 

impact of hard inclusions on hole trajectory. Vertical angle in this case was 

not affected. Figure 7 shows that the 

vertical angle remained horizontal after 

the bit intercepted the roof rock. 

In the Mary Lee Coalbed in Alabama, 

drilling with the preferred assembly 

(fig. 1) was abandoned because the holes 

squeezed and collapsed, which locked the 

assembly in place. An alternative assembly consisting of a 3-in (7.6-cm) bit and 

BQ rod was used in this coalbed (fig. 9) 

(9) because, in many cases, this assembly 

could be rotated and backed out of 

the hole when collapse occurred. Figure 

10 shows that this assembly arced to 

the right, and at a depth of 1,000 ft 

(305 m), the hole deviated over 200 ft 

(61 m) from the original bearing line. 

The first 120 ft (37 m) was drilled with 

the preferred assembly (fig. l), and figure 10 shows that very little horizontal 

deviation occurred during the drilling 

interval. Attempts to control the trajectory in the vertical plane with the 

modified assembly by adjusting bit thrust 

and rotational speed were unsuccessful. 

I I I I 1 1 1 I I 

- 

- - 

- - 

KEY 

- 

X Point where bit hit pyrite 

- --- Original bearing line 

- 

0 100 200 300 400 500 600 700 800 900 1,000 

DISTANCE ALONG ORIGINAL BEARING LINE, ft 

FIGURE 8.-Horizontal trajectory of hole, Beckley Coalbed. 
Bit, 3 in 1 

FIGURE 9.-Bit drilling assembly. 

KEY ---- -Original bearing line 

DISTANCE ALONG ORIGINAL BEARING LINE, ft 

FIGURE 10.-Horizontal trajectory of hole, Mary Lee Coalbed. 

1 

//////////////,,, 

I 

Coalbed - 

- 

/////////"///'/'/////: - 

- I I I I I I I I I 

0 100 200 300 400 500 600 700 800 900 1,000 

HORIZONTAL DEPTH, ft 

FIGURE 11.-Vertical trajectory of hole, Mary Lee Coalbed. 
FIGURE 12.-Preferred assembly with 14-lt collar. 

C I I I 

b--- 

- - 

- - 

- KEY - 

----- Original bearing line 

- - 

- - 

DISTANCE ALONG ORIGINAL BEARING LINE, ft 

FIGURE 13.-Horizontal trajectory of hole, Beckley Coal Mine. 
Hole 4 

Oft 

FIGURE 14.-Lower Sunnyside Coalbed (Utah) study. 

Coalbed dip appears to be an important the maximum dip of the coalbed (fig. 14). 

factor affecting the horizontal trajec- All holes were drilled with the modified 

tory of holes in coalbeds. Studies in assembly (figs. 3-4). Hole 1, which was 

the lower Sunnyside Coalbed where the dip drilled along the strike, turned to the 

is 5' (0.09 rad) showed that the horizon- right as would be expected when using the 

tal trajectory of the holes turned toward modified assembly. 

SUMMARY AND CONCLUSIONS 

Analysis of the horizontal trajectory 

of holes drilled in coalbeds showed that 

the standard assembly (fig. I), which includes an 18-ft (5.5-m) drill collar and 

two centralizers, had the best horizontal 

stability and missed a target at 1,000 ft 

(305 m) by only 15.5 ft (4.7 m). 

In coalbeds where the standard assembly 

(fig. 1) is modified by removal of the 

rear centralizer to maintain control of 

the bit in the vertical plane, horizontal 

control of the hole trajectory is lost 

and the bit tends to arc to the right. 

At a depth of 1,000 ft (305 m), the deviation was as much as 50 ft (15.2 m) and 

was 160 ft (49 m) at the 2,000-f t (610-m) 

depth. Thus, once a hole begins to arc, 

the deviation becomes more severe with 

depth. 

Short drill collars, though easier to 

handle in an underground environment, did 

not maintain control of hole trajectory 

in the horizontal plane. Deviations as 

much as 350 ft (107 m) occurred when hole 

depth measured along the original bearing 

line was 1,000 ft (305 m). At 1,600 ft 

(488 m) the trajectory of the hole in the 

horizontal plane was at a right angle to 

the original bearing line. 

Holes drilled without a drill collar 

deviated over 200 ft (61 m) from the 

original bearing line at a depth of about 
1,000 ft (305 m). In general, most holes in any direction. In dipping coalbeds, 

tended to drill to the right, presumably the horizontal trajectory of holes 

because of the right-hand rotation of the appears to turn toward the direction of 

bit. Hard inclusions in the coalbed, maximum dip. 

such as pyrite balls, can deflect a bit 

REFERENCES 

1. Williamson, T. N. (Jacobs Associates). Evaluation of Horizontal Drilling Techniques in Coalbeds. BuMines OFR 

34-72, 1970, 131 pp.; NTIS PB 212 746. 

2. Wilson, G. E. How To Drill a 

Usable Hole, Part 2. World Oil, Sept. 

1976, pp. 47-51. 

3. Rommel, R. R., and L. A. Rives. 

Advanced Techniques for Drilling 1,000-Ft 

Small Diameter Horizontal Holes in a Coal 

Seam (contract H0111355, Fenix & Scisson, 

Inc. ). BuMines OFR 17(1)-76, 1973, 

101 pp.; NTIS PB 240 713. 

4. Cervik, J., H. H. Fields, and 

G. N. Aul. Rotary Drilling Holes in 

Coalbeds for Degasification. BuMi nes 

RI 8097, 1975, 21 pp. 

5. NL Sperry-Sun (Oklahoma City, OK). 

Magnetic Directional Single Shot Operating Instructions. 1972, 12 pp. 

6. Kravits, S. J., A Sainato, and 

G. L. Finfinger. Comparison of Rotary 

and In-Hole Motor Techniques for Drilling 

Horizontal Boreholes in Coal. BuMines 

RI 8933, 1985, 31 pp. 

7. Prosser, L. J., Jr., G. L. Finfinger, and J. Cervik. Methane Drainage 

Study Using an Underground Pipeline, 

Marianna Mine 58. BuMines RI 8577, 1981, 

29 PP* 

8. Goodman, T. W., J. Cervik, and 

G. N. Aul. Degasification Study From an 

Air Shaft in the Beckley Coalbed. BuMines RI 8675, 1982, 19 pp. 

9. Perry, J. H., L. J. Prosser, Jr., 

and J. Cervik. Degasification of the 

Mary Lee Coalbed, Brookwood, AL. BuMines 

RI 8669, 1982, 13 pp. 

U.S. GOVERNMENT PRINTING OFFICE: 1988 - 547-000180,041