Report of Investigations 9238 

Rotary Drilling Techniques Used in the 

Beckley Coalbed 

By Tobias W. Goodman 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Manuel L. Lujan, Jr., Secretary 

BUREAU OF MINES 

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

Goodman, Tobias W. 

Rotary drilling techniques used in the Beckley coalbed. 

(Report of investigations; 9238) 

Bibliography: p. 11. 

Supt. of Docs. no.: I 28.23:9238. 

1. Coalbed methane drainage-West Virginia-Rotary drilling. I. Title. 11. Title: 

Beckley coalbed. 111. Series: Report of investigations (United States. Bureau of 

Mines); 9238. 

TN23.U43 [TN305] 622 s [622'.8] 88-600412 
CONTENTS 

Page 

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

Introduction ........................................................................ 2 

Acknowledgments .................................................................... 3 

Testprogram ....................................................................... 3 

Description of study area ............................................................ 3 

Drilling equipment and setup ......................................................... 4 

Drillingresults ...................................................................... 5 

Degasification results ................................................................. 9 

Gasanalysis ........................................................................ 10 

Summary and conclusions ............................................................... 11 

References ......................................................................... 11 

ILLUSTRATIONS 

Standard rotary drilling assembly with 18-ft collar ......................................... 2 

Degasification site ................................................................ 3 

Stratigraphic column .............................................................. 3 

Electrohydraulic drill .............................................................. 4 

Short-collared drilling assemblies ..................................................... 5 

Assembly type drag bits ............................................................ 5 

Location of holes map with entries .................................................... 6 

Vertical section. hole 1 ............................................................ 7 

Vertical sections. holes 2.3. and 4 .................................................... 8 

Gas flow rates. holes 1.2.3. and 4 .................................................... 10 

Totalgasflowrate ................................................................ 10 

TABLES 

1 . Drilling parameters. short-collared standard assembly ...................................... 7 

2 . Drilling parameters. bit plus centralizer assembly ......................................... 7 

3 . Drilling results of assemblies in holes 2. 3. and 4 ......................................... 9 

4 . Methane flow rates from holes ....................................................... 10 

5 . Beckley Coalbed studies compared .................................................... 10 

6 . Comparative gas analysis ........................................................... 11 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

cm centimeter L liter 

ft foot L/min liter per minute 

ft3 cubic foot lb pound 

ft3/d cubic foot per day m meter 

(ft3/d)/ft cubic foot per day per foot m3 cubic meter 

ft3lgal cubic foot per gallon m3/d cubic meter per day 

ft/min foot per minute (m3/d)/m cubic meter per day per meter 

gal gallon m3/~ cubic meter per liter 

gal/min gallon per minute m/min meter per minute 

hp horsepower r/min revolution per minute 

in inch rad radian 

in/min inch per minute W watt 

kg kilogram Y' year 

I 
ROTARY DRILLING TECHNIQUES USED IN THE 

BECKLEY COALBED 

By Tobias W. ~oodman' 

ABSTRACT 

The U.S. Bureau of Mines used a contact drilling strategy with short-collared assemblies in order to rotary drill 

long horizontal methane drainage holes in the Beckley Coalbed near Glen Daniel, WV. By decreasing the thrust, and 

increasing the rotation of the drill bit when in contact with the roof and floor rock, assemblies with 1-ft (0.3-m), 10-ft (3- 

m), and 144 (43-m) long collars we.re made tn de.flect and stay ir? the coalbed. Successful application of this proce-dure 

resulted in holes drilled full length in coal with the original assembly including bit. The test assemblies deflected away 

from the original bearings of the holes during drilling and arced. Four holes were drilled to depths ranging from 478 ft 

(145.7 m) before the strategy was developed, to 4,034 ft (1,230 m) when the strategy was applied. Total length of the 

holes in coal was 8,590 ft (2,618 m). After 577 days the total methane captured by the holes was 221,600,000 ft? (6,276,000 

m3). The methane flow rate peaked at 805,000 ft3/d (22,798 m3/d), then decreased to stabilize at 670,000 ft?/d (18,974 

m3/d) after 6 months. 

 i in in^ engineer, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 
drilling parameters that lift the trajectory of the bit in the 

coalbed. They also may deflect away from the desired 

bearing of the hole. Weight adds inertia and rigidity to the 

'1talic numbers in parentheses refer to items in the list of references 

at the end of this report. 

10-in centralizer, 

immediately after making a large change in vertical 

trajectory, the amount of experience with short-collared 

assemblies was less than desired. For example, nothing 

was known about the horizontal paths that they would 

follow, whether random because of deflection off 

inclusions, or to the right because of bit rotation (1-8). 

10- in centralizer, 

3i-n OD 7 

BQ drill rod, 1 

2 l%-in OD ' 18-11 NO drill collar. 

3 24 -in OD 

Bit, 32 I_] in 

Figure 1.-Standard rotary drilling assembly with 18-ft collar. 
Test area 

/ 

&Q .7 

Scale, miles 

KEY 

Clay 

Quartzite 

Shale 

Sandy shale 

Sandstone 

Coal 

Lower Raleigh Sandstone 1 Beckley Coalbed 

Figure 2.-Degasificatlon site. Figure 3.-Stratigraphic column. 
Figure 4.-Electrohydraulic drill. 

DRILLING EQUIPMENT AND SETUP 

A 20-ft- (6.1-m), 4-in- (10.2-cm) diam steel pipe is 

grouted into the coalbed to isolate the liberated gas and 

water. A full-port valve is connected to the pipe. Drilling proceeds through a gas-water separator, which separates gas from the drill cuttings and water. The gas produced from the hole is ducted by flexible tubing to a 6-in 

(15.2-cm) underground pipeline and subsequently vented 

through an 8-in- (20.3-cm) diam vertical pipe to the atmosphere at the surface. 

The holes were drilled with a 40-hp (298.4 x lo2-W) 

electrohydraulic drill (fig. 4), which provided bit thrust and 

rotation. The drilling assemblies were composed of 3-112- 

in (8.9-cm) drag bits, 1-ft- (0.3-m), or 14-ft- (4.3-m) long 

NQ drill collars, 10-in- (0.25-cm) long spiral centralizer(s), 

and 2-3/16-in (14.2 cm) BQ drill rods (fig. 5). The assembly type drag bit (fig. 6), which has stepped carbide blades, 

is capable of responding to driig parameters, and has a 

high penetration rate in coal (I). 
10-in centralizer, 

D 3 7/i6-in OD 7 10-ft NO drill collar, 

BQ drill rod- z3/,-~n OD, it, 3i/2 inFigure 5.-Shortcollared drilling assemblies. Figure 6.-Assembly type drag bits. A, New bit; B, worn bit. 

DRILLING RESULTS 

Initially, the plan was to use control procedures, trajectory of the assembly (fig. 5A) in the center of the 

excepting that the 14-ft (4.3-m) collar was substituted for coalbed led to three aborted short holes, A, B, and hole 1 

the 18-ft (5.5-m) collar. However, efforts at keeping the (fig. 7). 
Scale, ft 

Figure 7.-Location of holes map with entries. 

Figure 8 shows the trajectory of hole 1 in the vertical 

plane. The primary drilling assembly used in hole 1 was 

the short-collared standard assembly (fig. 5A), except the 

rear centralizer was removed so that the bit would tend to 

drill slightly upward in the soft coal. However, the assembly was still too heavy for the soft coalbed, and the trajectory nosed over towards the floor rock. It was replaced 

with the bit plus centralizer assembly (fig. 5B), which is 

configured to bring the trajectory of the bit upward. This 

assembly behaved as predicted, however, it failed to deflect 

from the roof at 214 ft (65.2 m) which it hit at about 4" 

(0.07 rad). Later, when used between 316 and 406 ft (96.3 

and 123.7 m) as a substitute for the primary assembly, it 

bounced off the floor shale which it hit at about 3" (0.05 

rad). The floor rock was then encountered using an 

assembly configured to lower the trajectory of bit (fig. 5D) 

at about 490 ft (149.4 m) after the floor was mistaken for 

roof. 

Efforts to keep the trajectory of the bit near the center 

of the coalbed were abandoned at about 400 ft (122 m) 

depth in hole 2 after the short-collared standard assembly 

(fig. 5A) nosed over towards the floor rock. From that 

point in hole 2, and for holes 3 and 4, a strategy was followed which let the short-collared assemblies drill along 

the bedding plane contacts instead of being modified to 

keep away from the contacts as in control procedures. 

This strategy, which was adapted to the relatively thin, 

undulating coalbed, was to let the assemblies drill to the 

roof or floor rock and make them deflect or bounce along 

the rock contact in coal. Because of their configuration, 

which make them drill either up or down, the shortcollared assemblies drilled to the roof or floor contact as 
Sidetrack holes 

z 

a 

m Hole / n 

a 

HORIZONTAL DEPTH, ft 

Table 1 .-Drilling parameters, short-collared 

standard assembly (fig. 54) 

Hole depth, Bit thrust, Bit rotation, Drill . rate, Location . 

ft Ib r/min in/min of bit 

700 ..... 1.200 100 30 In coalbed. 

700 ..... 1 ;200 140 21-30 Deflecting 

off floor 

rock. 

1,500 . . . . 1,500 110 20 In coalbed. 

1,500 . ... 1,000 135 5-18 Deflecting 

off floor 

rock. 

Figure 8.-Vertical section, hole 1. 

Table 2.-Drilling parameters, bit plus 

centralizer assembly (fig. 5B) 

expected. The initial deflection was made by briefly substituting a short-collared assembly configured to drill away 

from the contact. For instance, the bit plus centralizer 

assembly (fig. 5B) was used to make the initial deflection 

off the floor rock in hole 2. The short-collared standard 

assembly (fig. 5A) was then replaced in the hole with the 

trajectory about parallel to the floor rock in the coalbed. 

After this maneuver was made, drilling became repetitious 

and simple. Deflecting was relatively easy because the 

short-collared assemblies drilled trajectories that stayed 

close to the contacts, essentially skimming the surface at 

10- to 20-ft (3- to 6.1-m) intervals. The procedure was to 

modify the drilling parameters by judiciously decreasing 

the thrust and increasing the rotation of the bit to ease the 

bit away from the rock contact. This procedure was invariable, making it unimportant to know which contact the bit 

was following. Thrust was kept high enough to keep the 

bit moving so that rotation speed did not become a factor 

by cutting a dent in the rock which the bit might try to 

follow. The drill cuttings varied frequently from black 

when the bit drilled in coal parallel to the bedding plane 

to stained brown when the bit was deflecting off the shale. 

This strategy allowed each hole to be completely drilled 

without removal of the assembly. 

Examples of the drilling parameters used to deflect the 

bit off floor or roof rock, compared to normal drilling 

parameters used to drill in the coalbed, are shown in tables 1 and 2. In table 1, which shows the parameters used 

with the short-collared standard assembly (fig. 5A), only 

the bit rotation was increased at 700-ft (213-m) depth to 

deflect the bit. The penetration rate, which had decreased 

to 21 in/min (0.53 m/min) because of resistance by the 

floor rock, gradually increased during the successful deflection back to the original 30 in/min (0.76 m/min). At 

1,500-ft (457-m) depth, it was required to both lower the 

thrust and increase the rotation rate to make the deflection. In table 2, which shows the parameters used with the 

bit plus centralizer assembly (fig. 5B), no adjustments were 

needed at 750 ft (229 m) to deflect the bit. It was a common experience during drilling that contacts with the roof 

rock required less adjustment and a shorter drilling interval to deflect than contacts with the floor rock. This was 

apparent because the bit plus centralizer assembly, which 

Hole depth, Bit thrust, Bit rotation, Drill rate, Location 

ft Ib r/min in/min of bit 

750 ..... 1,000 180 20 In coalbed. 

750 . . . . . 1;000 180 13-20 Deflecting 

of! !on? 

rock. 

1,800 . . . . 1,400 200 30 In coalbed. 

1,800 . . .. 1,000 250 10-15 Deflecting 

off roof 

rock. 

was much less constrained in the hole being drilled, deflected into alinement with the bedding planequicker than 

the short-collared standard assembly, which also had to - - 

overcome the force of gravity. 

Figure 9 shows the locations of holes 2, 3, and 4 in the 

vertical plane. Vertical surveys, which are made with the 

sperry-sun3 downhole surveying instrument (I), are used 

in control drilling to monitor hole inclination so that bit 

trajectory can be maintained parallel to the bedding plane. 

In this test, surveys were routinely made to 1,675 ft (510 

m) in hole 2; 1,350 ft (411 m) in hole 3; and 700 ft (213 m) 

in hole 4, by which time the trajectory of each assembly 

was known to be bouncing along a roof or floor bedding 

contact. The elevation of the back portion of each hole 

was plotted from survey traverses made after drilling was 

completed. 

Figure 7 is a map of the test site that shows the horizontal trajectories drilled by the short-collared assemblies. 

Azimuth surveys made after each hole was drilled showed 

that these assemblies had drilled continuously arcing and 

not random trajectories. As observed in other studies (7), 

holes that are drilled in the coalbed, or against the floor 

rock, tend to arc to the right because of right-hand bit 

rotation. This study showed that left-hand trajectories 

were smaller than right-hand trajectories. Holes that arc 

to the left apparently only occur because of deflection of 

the bit off coalbed inclusions or the roof rock. Hence, because of bit rotation, left-hand trajectories are much less 

frequent, and usually less in magnitude than right-hand 

trajectories. 

3~eference to specific products does not imply endorsement by the 

Bureau of Mines. 
I 1 I I I 1 

,___-----

- 

- - 

- 

0 \sidetrack hole - 

1 1 1 1 1 

HORIZONTAL DEPTH, ft 

Figure 9.-Vertical sections, holes 2, 3, and 4. 
Table 3.-Drilling results of assemblies, in holes 2, 3, and 4 

Hole Assembly configuration Depth used, Results 

ft 

2 . . . . . . . bit, centralizer, 14-ft collar, and centralizer 3,980 Stayed on or near the bottom of the coalbed; the clockwise 

(fig. 5A). rotation gradually turned the hole in an arc to the right. 

3 . . . . . . . bit and centralizer (fig. 58) . . . . . . . . . . . 1,900 Stayed on or near the top of the walbed; the clockwise rotation 

of the bit against the roof gradually turned the hole in an 

arc to the left. 

4 . . . . . . . bit and 1-ft collar (fig. 5C) . . . . . . . . . . . 1,490 Reamed a new hole nicely but stayed against the floor rock, 

turning an arc to the right. 

4 . . . . . . . bit, 10-ft collar, and centralizer (fig. 50) . 460 Followed the floor rock; turned in an arc to the right; sub 

behind the assembly sheared off due to extreme turn off 

a roll. 

Table 3 summarizes the results obtained using the 

tested drilling assemblies. Hole 2 was drilled with the 

standard assembly (fig. I), except that a shorter, 14-ft (4.3- 

m) drill collar was substituted (fig. 5A). This short-collared assembly fell to the shale floor contact where it 

stayed for much of its length. Hole 2 was advanced to 

4,034 ft (1,230 m), but a (130°, 2.27 rad) wide arc to the 

right was experienced. Hole 2 was aborted because the 

hole was sufficiently long for degasification. There also 

was concern that the long drill rod string might fatigue and 

break. Hole 3 was drilled with a bit and one centralizer 

(fig. 5B), commonly used in standard drilling procedures 

to increase the bit inclination, by carefully bringing it to 

the roof, then allowing it to bounce along the shaley bedding plane. This assembly drilled by far the most linear 

horizontal trajectory, turning an arc of about 26" (0.45 rad) 

to the left. The left-hand arc resulted because bit rotation 

deflected the bit to the left when it was impinged against 

the roof during contact drilling procedures. Hole 3 was 

aborted because it was entering an area already being 

degassed by hole 2. 

The final useful variation of the standard assembly was 

used in hole 4. It consisted of a bit followed by a 3-in- 

(7.6-cm) diam centralizer (i.e., a 1-ft, 0.3-m drill collar) 

(fig. 5C), which followed the shaley floor rock intercept 

with coal similarly to the assembly used in hole 2. Another assembly variation, consisting of a bit followed by a 10- 

ft (3.0-m) drill collar and centralizer (fig. 5D), commonly 

used to lower the angle of holes, was used to drill a long 

segment along the floor contact in hole 4, but was ultimately found to be too stiff for the contours resulting in 

breaking a drill rod at the threads. Hole 4 was aborted 

to prevent the arcing trajectory from entering a mine 

opening. The total arc turned by the two assemblies was 

about 80" (1.40 rad) to the right. 

The best performing assembly in this test was the bit 

plus centralizer assembly (fig. 5B). Besides drilling the 

more linear hole, it deflected easier during the contact 

drilling procedure than did the short-collared standard 

assembly (fig. 54). 

The features of the assembly-type drag bit (fig. 6) that 

are recommended for drilling coal because of its penetration rate are also useful in contact drilling. The steps, 

which are 45" (0.79 rad) with the face of the bit, tend to 

deflect the bit off shale, or even clay floor rock if the bit 

hits the plane of the rock at a low angle (I). In contact 

drilling, a worn stepped drag bit is recommended because 

it has lost some of its ability to penetrate floor or roof 

rock and is still able to drill coal. Figure 6 shows a drag 

bit which was used to drill 6,073 ft (1,854 m) in coal. It 

became dull, but never entered the shaley roof or floor. 

The worn bit was still penetrating at 2-112 ft/min (0.76 

m/min) when it was removed from service. The same 

success could be achieved by dulling a drag bit in the shop 

before putting it into service. 

DEGASIFICATION RESULTS 

Gas flows from the individual holes are shown in figure 10. Average gas flows ranged from 50,000 ft3/d (1,416 

m3/d) from hole 1 to 440,000 ft3/d (12,460 m3/d) from 

hole 2 (table 4). Note the productiveness of hole 2 due to 

its length. Holes 1, 3, and 4 were less productive because 

they were shorter, nearer the mine (fig. 7), and their 

drainage patterns were shielded by the arced trajectory of 

hole 2. 

However, if hole 3 had been continued to 4,000 ft 

instead of hole 2, it would have captured more methane. 

A 4,000-ft- (1,219-m) long hole drilled with the bit plus 

centralizer assembly (fig. 5B) would have penetrated the 

coal-gas reservoir about 3,850 ft (1,173 m). By contrast, 

hole 2 penetrated virgin coal 2,900 ft (884 m) because of 

its chance location at the corner of a section (fig. 7). 

Gas flow rates were enhanced because of the low 

coalbed water in the test area. Average water flows were 

very low, from 0.2 gal/min (0.76 L/min) from hole 2 to 

only a trace from hole 1. Hence, it was unnecessary to 

dewater the coalbed before degasification could occur. 
7 Hole / 

Hole 2 

400 

500 1 

TIME, days 

Figure 1 1 .-Total gas flow rate. 

27 (ft3/d)/ft (2.5 (m3/d)/m) at the previous Beckley Coalbed site. Table 4 shows the flow rate from each hole at 

the current site. The 110 (ft3/d)/ft (10.2 (m3/d)/m) rate 

for hole 2 was constant for the final 350 days of the study. 

Table 5 compares results obtained in the two Beckley 

Coalbed studies. After about the same degasification period, the current study had captured 10 times the volume 

of gas due to the length of the four holes, compared with 

eight holes in the earlier study, and the dryness of the 

coalbed. The presence of coalbed water was a problem at 

the first site. In the first study, the ratio of gas to water 

from the holes was 3.3 ft?lgal (0.025 m3/~) and, in the 

current study, this ratio was 848 ft3lgal (6.3 m3/~). If a 

successful drilling strategy had been applied at the first 

Beckley Coalbed site, longer holes would have dewatered 

the coalbed, followed by degasification. 

Table 4.-Methane flow rates from holes 

Hole Length, Average gas Methane flow, 

ft production, ft3/d (ft3/d)/ft 

1 ........ 478 501000 105 

2 ........ 4,034 440,000 110 

3 ........ 2,039 156,000 76 

4 ........ 2,039 105,000 5 1 

TIME, days Table 5.-Beckley Coalbed studies compared 

Figure 10.-Gas flow rates, holes 1, 2, 3, and 4. 

Figure 11 shows the total gas flow for all holes. The peak 

gas flow was 805,000 ft3/d (22,798 m3/d). Cumulative gas 

and water roduction after 510 days was 186,900,000 ft3 P (529,300 m ) and 220,320 gal (833,911 L), respectively. 

Because of the low water rate, less than 112 gal/min (1.9 

L/min) for the four holes, the average methane flow was 

82 (ft3/d)/ft (7.6 (m3/d)/m) of hole drilled compared with 

Hole length Gas drained Water Ratio: 

Site in coal, after 51 0 drained gaslwater, 

ft days, ft3 after 51 0 ft3/gal 

days, gal 

No. 2 air 

shaft. 3,486 18,700,000 5,705,000 3.3 

(8 holes) 

2 NE 

mains. 8,590 186,900,000 220,320 848.0 

(4 holes) 

GAS ANALYSIS 

Tests conducted in both the Pittsburgh (2-3) and Beck- that obtained at the previous sites. The methane samples 

ley Coalbeds (6) have shown that coalbed gas can be sub- taken from the 2 NE mains site showed excellent quality, 

stituted for natural gas. Table 6 compares the average easily convertible to commercial use. 

composition of the coalbed gas at the Beckley Mine with 
Table 6.-Comparative gas analysis, percent 

Coalbed Ethane Carbon dioxide Oxygen Nitrogen Methane 

Pittsburgh, multipurpose borehole ........ 0.16 9.06 0.34 1.24 89.10 

Pittsburgh, 1Sftdiam shaft ............. .05 10.07 .12 0 88.6 

Beckley, No. 2 air shaft ................ 0 .O1 -16 3.12 96.71 

Becklev. 2 NE mains .................. 0 .41 .13 .24 99.2 

SUMMARY AND CONCLUSIONS 

Long methane drainage holes were drilled in the Beckley Coalbed by substituting short-collared assemblies for 

the long-collared assembly normally used to rotary drill 

horizontal holes. These assemblies drilled widely arcing 

holes along the roof or floor rock, depending on their configuration, but never left the coalbed. Simple adjustments 

to the drilling parameters made deflections of the bit that 

followed the contacts easier. Drilling along the contacts 

was abetted by the presence of shaley roof and floor rock, 

the friable nature of the coalbed, and by using a dulled 

drag bit. This strategy might have application to other soft 

coalbeds with similar geology. The bit plus centralizer 

assembly (fig. 5B) was the most suitable assembly tested 

because of its relatively linear trajectory. 

Large volumes of commercial quality methane gas were 

produced from the long horizontal holes drilled in this 

study though the area had degasified for several years. 

After 510 days, 186,900,000 ft3 (529,312 m3) of methane 

had been drained from the coalbed. The methane captured per increment of drill hole was about 110 (ft3/d)/ft 

(10.2 (m3/d)/m) from hole 2 (fig. 7). 

REFERENCES 

1. Cervik, J., H. H. Fields, and G. N. Aul. Rotary Drilling Holes 5. Perry, J. H., G. N. Aul, and J. Cervik. Methane Drainage Study 

in Coalbeds for Degasification. BuMines RI 8097, 1975, 21 pp. in the Sunnyside Coalbed, Utah. BuMines RI 8323, 1978, 11 pp. 

2. Fields, H. H., J. H. Perry, and M. Deul. Commercial-Quality 6. Goodman, T. W., J. Cervik, and G. N. Aul. Degasification 

Gas From a Multipurpose Borehole Located in the Pittsburgh Coalbed. Study From an Air Shaft in the Beckley Coalbed. BuMines RI 8675, 

BuMines RI 8025, 1975, 14 pp. 1982, 19 pp. 

3. Fields, H. H., J. Cervik, and T. W. Goodman. Degasification 7. Goodman, T. W., and J. Cervik. Horizontal Trajectories of 

and Production of Natural Gas From an Air Shaft in the Pittsburgh Rotary-Drilled Holes. BuMines RI 9177, 1988, 11 pp. 

Coalbed. BuMines RI 8173, 1976, 23 pp. 

4. 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. 

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