Bureau of Mines Information Circular/l982 

Site-Specif ic and Regional Geologic 

Considerations for Coal bed 

Gas Drainage 

By W. P. Diamond 

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

James G. Watt, Secretary 

BUREAU OF MINES 

Robert C. Horton, Director 
This publication has been cataloged as follows : 

Diamond, W. P. (William P.) 

Site-specific and regional geologic considerations for coalbed gas 

drainage. 

(Information circular ; 8898) 

Bibliography: p. 22-24. 

Supt. of llocs. no.: 1 28.27:8898. 

1. (:oal mines and mining-United States-Safety measures. 2 

Boring. 3. Coal -Geology -United States. 4. Methane. I. Title. 11. 

Series: lnformation circular (1Jnited States. Bureau of Mines) ; 8898. 

TN295.U4 622s [622',8] 82-600 254 
CONTENTS 

. 

Page 

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

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

Acknowledgments ................ee.............................................. 3 

The coalbed as a gas reservoir ................................................. 4 

Site-specific considerations ..............e.........................e.......... 5 

Direct method determination of the gas content of coal .................... 5 

Sampliw ............................................................. 5 

Test equipment ....................................................... 8 

Calculation of gas content ........................................... 9 

Auxiliary test procedures ............................................ 12 

Geologic considerations ...........................................e....... 13 

Variations in gas content ........................................... 13 

Coalbed discontinuities .......................e..........e........... 14 

Multiple coalbed reservoirs .........................ee......e........ 17 

Regional considerations .......e......e......................................... 19 

Calculation of an area's in-place gas volume ................ee............ 19 

Additional regional considerations.. ...............................e...... 22 

Conclusions .................................................................... 22 

References ...............e........................ee........................... 22 

ILLUSTRATIONS 

Coalfields of the United States ........................................... 3 

Comparison of the gas storage potential of coal and 10-pct-porosity nonreactive reservoir rock versus reservoir pressure.. ..................... 4 

Gas content of coal versus actual mine emissions.......................... 5 

Conventional and wire line coring equipment ............................... 6 

Coalbed correlation problems .............................................e 7 

Variable distribution of coalbeds in three wells. Trinidad. CO ............ 8 

Sample containers used for direct-method testing of coal samples .......... 9 

Equipment for direct-method testing of coal sample. ....................... 9 

Lost-gas graph ............................................................ 11 

Gas content versus depth for the Mary Lee Coalbed. Alabama.. .............. 13 

Map of rank distribution and depth distribution of the Mary Lee Coalbed. 

Alabama ................................e......e......e.................. 15 

Section view of ideal coalbed and effect of coalbed discontinuities on 

horizontal gas drainage boreholes ....................................... 15 

Section view of effect of coalbed discontinuities on vertical gas drainage boreholes ........................................................... 16 

Examples of multiple coalbed reservoir^............^..,.......^........... 18 

Isopach of the Mary Lee Coal Group superimposed on the overburden isopach . 20 

TABLES 

1 . Estimates of total in-place gas volumes for U.S. coalbeds ................. 2 

2 . Highest measured gas contents of U.S. coalbeds .......................... 2 

3 . States with highest measured gas emissions from coal mines ................ 3 

4 . Data for lost-gas graph ................................................... 10 

5 . Classification of coal by rank ............................................ 14 

6 . In-place gas volume for the Mary Lee Coal Group. Alabama .................. 20 

7 . In-place gas volumes of selected U.S. coal bed^..............^............. 21 
SITE-SPECIFIC AND REGIONAL GEOLOGIC CONSIDERATIONS 

FOR COALBED GAS DRAINAGE 

By W. P. Diamond ' 

ABSTRACT 

The Bureau of Mines has been involved in the drilling of vertical, 

horizontal, and directional coalbed gas drainage boreholes for mine 

safety since 1964. In that time, boreholes have been drilled in most of 

the major coal regions of the United States under a wide variety of geologic conditions. Many of the geologic conditions that occur in the 

coal measures are detrimental to gas drainage; others may be beneficial. 

Analytical techniques to determine the gas content of coal samples and 

evaluate regional trends of gas distribution have been developed. 

Drilling techniques that maximize the acquisition of coalbed gas data 

and geologic information have been determined. 

Although some of the geologic factors influencing the placement and 

potential success of coalbed gas drainage boreholes have been reported 

in papers on individual projects, a complete, systematic compilation has 

not previously been available. The objective of this paper is to provide information on specific geologic factors that should be considered 

prior to, during, and after the drilling of coalbed gas drainage boreholes. Many of the commonsense considerations that have been learned 

through many years of Bureau of Mines experience, but have generally 

not been reported formally, are included for those who may be considering coalbed gas drainage drilling for the first time, or who have 

not had the opportunity to encounter a substantial number of geologic 

situations. 

l~u~ervisor~ geologist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, 

PA 
INTRODUCTION 

The Bureau of Mines has been investigating the occurrence of gas in coal and 

techniques to remove the gas in advance 

of mining since 1964 (28).2 The goal of 

the Bureau's research program has primarily been to increase mine safety by reducing the explosion hazard of methaneair mixtures. Many of the coalbed gas 

observations and techniques developed 

have applications both for mine safety 

and for energy resource delineation and 

utilization. The evaluation procedures 

and geologic considerations for drilling 

sites discussed in this paper are relevant to both dne safety and gas utilization programs. 

It is estimated that coalbeds in the 

United States contain as much as 21.7 

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

the end of this report. 

trillion m3 (766 trillion ft3) of inplace gas (table 1). The gas is 

distributed in varying unit volumes 

throughout the extensive coal reserves of 

the United States (fig. 1). Gas contents 

ranging from essentially 0.0 cm3/g (0.0 

ft3/ton) to 21.6 cm3Ig (691 ft31ton) have 

been measured. Table 2 is a list of the 

highest measured gas contents of U.S. 

coalbeds. A list of 583 gas content 

tests on 125 coalbeds in 15 States can be 

found in Bureau of Mines RI 8515 (8). - 

TABLE 1. - Estimates of total in-place 

methane volumes for U.S. coalbeds 

TABLE 2. - Highest measured gas contents of U.S. coalbeds 

Source 

Bureau of Mines (6). . - National Energy 

Plan I1 (33)........ 

National ~Zroleum 

Council (24)........ - 

Trillion 

m3 

21.7 

1.4-19.8 

11.2 

Trillion 

ft3 

766 

50-700 

398 

Coalbed or formation 

Peach Mountain...... 

Pocahontas No. 3.. .. Tunnel.............. 

New Castle.......... 

Mary Lee............ 

Hartshorne.......... 

Mesaverde Fm........ 

Beckley... .......... 

Vermejo Fm .......... 

Pratt............... 

County and State 

Schuylkill, PA.. 

Buchanan, VA.. .. Schuylkill, PA.. 

Tuscaloosa, AL.. 

..... do......... 

Le Flore, OK.... 

Sublette, WY.... 

Raleigh, WV..... 

Las Animas, CO.. 

Tuscaloosa, AL.. 

Depth 

m 

209 

568 

185 

650 

666 

439 

1,065 

253 

547 

416 

Coal rank 

Anthracite. 

Low-volatile 

bituminous. 

Anthracite. 

Low-volatile. 

bituminous. 

Medium-volatile 

bituminous. 

High-volatile A 

bituminous. 

Medium-volatile 

bituminous. 

Do 

Gas content 

ft 

685 

1,864 

608 

2,132 

2,185 

1,439 

3,495 

830 

1,793 

1,365 

cmvg 

21.6 

21.5 

18.3 

17.5 

17.4 

17.1 

17.0 

15.3 

15.3 

15.1 

ft'/tonm 

691 

688 

586 

560 

557 

547 

544 

490 

490 

483 
FIGURE 1. - Coalfields of the United States. 
THE COALBED AS A GAS RESERVOIR 

The fundamental principle to accept 

when considering coalbeds as gas reservoirs is that they are not the same as 

as "traditional" gas reservoirs (such as 

sandstones) and do not behave in accordance with the same reservoir mechanics. 

In a traditional sandstone reservoir, the 

gas exists as free gas in the void spaces 

between sand grains, and transport of 

that gas through the reservoir is governed by pressure gradients as described 

by Darcy's law. 

In a virgin coalbed reservoir, only a 

small portion of the methane is found as 

"free" gas in the fractures (cleat), 

Most of the methane is adsorbed on the 

coal surface in the extensive micropore 

structure of the coal (5). The transport 

of the methane from the micropores 

through the "solid" coal is governed by 

concentration gradients as described by 

Fick's law of diffusion. Once the methane has reached the coalbed fracture system, the transport of the gas through the 

cleat to a well bore or mine opening is 

governed by Darcy's law. 

In a virgin coalbed reservoir, the 

pressure in the fracture system and the 

The attractiveness of coalbeds for commercial gas production is illustrated in 

figure 2, which compares the theoretical 

gas volumes that can be stored at various 

pressures in equal rock volumes of both 

traditional reservoirs of 10-pct porosity 

and representative coalbeds. Even at the 

relatively low reservoir pressures commonly found in coalbeds, the unit volume 

of coal can store several times the gas 

volume of the 10-pct porosity nonreactive 

traditional reservoir rock. 

An additional factor to be considered 

fzlr zozllb2d - Le~GLvwLLe -nv*rn~ wc is the influence 

of "boundaries" on the reservoir and gas 

flow rates, A boundary, either natural 

(such as a fault) or created (interference from other boreholes or mine openings), can reduce the time needed to 

lower the coalbed pressure and induce gas 

flows (3, 21)- The boundaries effectively iimit the size of the reservoir, 

and the pressure is more efficiently reduced in the resulting area, with an 

accompanying decrease in the time needed 

concentration of methane in the micropore Pocahontas coal, 30°C structure are in equilibrium (5). To induce a flow of methane from- the solid 

25 coal, the equilibrium must be disrupted m+ 

by lowering the pressure in the fracture Y4 

system. Coalbeds are generally saturated 

with water, which when removed, either by 

pumping from a vertical borehole or by 

"natural" drainage into a mine opening or 

horizontal borehole, disrupts the equilibrium. Methane can then desorb from 

the coal micropores and is made available 

for flow through the fracture system. 

Gas flows will normally continue as long 

as equilibrium conditions are disrupted 

but can quickly decline if equilibrium is 

- 

Pittsburgh coal, 30°C / 

/ 

/ - 

- 

- 10-pct porosity 

reestablished, The continued lowering of 1 

the coalbed reservoir pressure to ensure k I I 

gas flow is completely opposite tu the 400 800 1,200 1,600 2,000 2,400 

PRESSURE, psig situation with a traditional reservoir, 

where maintaining a high reservoir pres- FIGURE 2. - Comparison of the gas storage potential 

sure is required to provide the energy to of coal and 10-pct-porosity nonreactive reserflow large volumes of gas. voir rock versus reservoir pressure (3). 
to achieve gas production. Several provide mathematical descriptions of the 

publications (2, 5, 20-21) detail the reservoir mechanics. 

reservoir characteristics of coalbeds and 

SITE-SPECIFIC CONSIDERATIONS 

Site-specif ic considerations of the 

me thane potential of coalbeds include 

both determining the in-place gas content 

and identifying geologic factors that may 

affect the flow of gas from the coalbed 

reservoir to methane drainage systems or 

underground mines. 

Direct Method Determination of the Gas 

Content of Coal 

The Bureau of Mines originally became 

interested in determining the gas content 

of virgin coal as an aid in estimating 

the amount of gas that would be released 

An additional problem that is frequently encountered with the conventional 

coring of selected zones is missing the 

coalbed that is to be cored, or coring 

excessive lengths of section while 

searching for the coal. In the conventional coring technique, the hole is 

rotary-drilled (no core taken) to a point 

(depth) estimated to be near the top of 

the coalbed; then the drill pipe is 

pulled from the hole, the rotary drill 

bit is taken off, and the core barrel is 

installed. The core barrel is then run 

into the hole on the end of the drill 

in active underground mines. The initial 

research results were used to construct a 3.2 

graph (fig. 3) that related direct method 

test values to the actual measured methane emissions of nearby mines. 2.8- t 

\ 

Sampling "+ + 

8 2.4 Coal samples for determination of gas 

content are obtained either from continu- - o, 

ous wire line core holes or from rotary- cn" 

drilled boreholes by means of con- y 2.0- - ventional coring of selected zones. Z 

Schematic diagrams of wire line and con- s 

ventional types of coring equipment are cr 1.6 

LL shown in figure 4. In general, the con- z 

tinuous wire line technique is preferred 2 

V) for obtaining coal samples for gas con- 1.2 

I I I 1 I I I I 

- - 

Beatrice Mines - 

- - 

- - 

/ 

/ - / d~owe Mine - 

- 

Loveridge - Mine3 - 

Federal No.2 Mine - - 

- - 

4 - - 

tent determination. The time required to Z 

W 

/ 

remove the coal sample from the hole and - V) sealit intoa desorption containeris a ,B - important for good test results. (See '3 

J "Calculation of Gas Content" section.) a - 

The retrieval of samples by the wire line e4 coring technique is very fast since the 2 Mary Lee Noel Mine - 

inner barrel containing the coal is - 

brought through the drill pipe to the 

surface by the wire line without having 1 I I I, 

0 4 8 12 16 20 

to pull the entire string of drill pipe GAS CONTENT MEASURED 

from the hole, as with conventional cor- BY DIRECT METHOD, crn3/~ 

ing equipment. The difference in retrieval time can be several hours at FIGURE 3. - Gas content of coal versus actual 

depths greater than 305 m (1,000 ft). mine emissions. 
FIGURE 4. - rI, Conventional, and H, wire line 

coring equipment. 

pipe, and hopefully the position of the 

coalbed has been estimated with sufficient accuracy to have the entire coalbed 

in a single run of the barrel, which is 

typically 3.05 to 6.10 m (10 to 20 ft) 

in length. 

Figure 5 illustrates several of the 

problems that can be encountered in 

attempting to predict (correlate) where 

coalbeds should be encountered if sufficient geologic data are not available or 

if the geologic section is variable. 

Commonly a nearby data source, such as a 

previously drilled well or regional geologic trends, is used to determine the 

core points where the conventional core 

barrel will be installed to obtain samples. Usually core points are projected 

2. Assume that the intervals between 

coalbed A and coalbed B are the same in 

wells A and B, which would put the core 

point (11) in the correct position to 

obtain a sample. 

3. Assume that the intervals between 

the marker bed and coalbed B are the same 

in wells A and B, which would put the 

core point (111) below the actual position of coalbed B, and the coalbed would 

probably be missed for coring. 

4. Assume that the interval between 

coalbeds A and B in well B thins by the 

same amount as the interval between the 

marker bed and coalbed A in the same 

well, which would put the core point at a 

depth slightly below the protected core 

point (I), requiring unnecessary and 

expensive coring before encountering 

coalbed B. 
Well A Well B 

k Marker bed 

Coalbed A I 

Coalbed B 

-- Projected core point ( r ) --- --- 

---- --- 

I= Projected core poi nt 

------ (II) 

-- 

? ----- 

--- (III) ? --- 

Coalbed C and rider 

Fl GURE 5. - Coalbed correlation 

I Marker bed 

Coolbed B 

Coalbed C 

Coalbeds are frequently overlain by 

thin rider coals which can be used as 

marker beds for conventional core points. 

Coalbed C in well A has a rider coal 

above it; however, the rider is not present in well B. If the driller on well B 

had been told to stop when the rotary 

drilled into the first coalbed below 

coalbed B (supposedly coalbed C rider) 

and switch to the core barrel to core the 

main coalbed, part of coalbed C would 

have been penetrated and the coal lost. 

Many of the situations described above 

for picking core points for conventional 

coring would result in a loss of coal for 

gas content determinations and/or additional unprogrammed expense for excessive 

core drilling. The use of continuous 

wire line coring would eliminate or minimize the described problems. 

In unknown geologic areas where wire 

line coring cannot be used for reasons 

such as cost or lack of suitable drilling 

rigs, recovery of all coalbeds using conventional coring can be enhanced by 

"twinning." A rotary hole is first 

drilled through the entire section of 

interest, without taking any cores. The 

hole is then logged with geophysical 

equipment to precisely define the depth 

and thickness of the coalbeds. The 

drilling rig is then moved over a - few 

meters (feet), and a second ("twin") hole 

is rotary-drilled. Since the exact location of each coalbed is known from the 

first hole, the conventional core barrel 

can be used at core points precisely 

located above each coal. The variability 

in distribution of coalbeds over a small 

geographic area is illustrated with an 

example from a drilling project near 

Trinidad, CO (fig. 6), where the wells 

are approximately 150 m (500 ft) apart. 

Multiple testing, or preferably testing 

of the entire coalbed, is the preferred 

sampling strategy. Variations in gas 

content are commonly observed on multiple 

samples from the same coalbed in a core 

hole. A single test on a small portion 

of a coalbed may yield a falsely low or 

high value for the entire coalbed. 
Well 2 Well / Well 3 

BULK DENSITY, BULK DENSITY, BULK DENSITY, sdu sdu sdu 

6.000 0 6.000 0 6.000 0 I 

Zone 

C 

Zone 

6 

-Zone 

A 

LEGEND 

sdu Standard 

dens~ty un~t 

I COLORADO I 

Key map 

0 I 3000 

Scale, ft 

FIGURE 6. - Variable distribution of coalbeds in 

three wells, Trinidad, CO. (('orrcIm/ions 

coo7rr/ c .u!l of' ('o lormtlo il'c.olo!yicaol S?rr??c y .) 

Test Equipment 

Sample containers of several shapes and 

sizes that have been constructed for various testing purposes are shown in figure 7. The standard container (can A) 

used by the Bureau is made from a 0.3- 

(1-ft) piece of aluminum pipe, having an 

inside diameter of 10 cm (4 in). A 

top flange and bottom plate have been 

welded to the pipe section, and a removable lid that attaches to the top flange 

can be fitted with a gage and various 

types of valve assemblies. Valves with a 

quick-connect capability are preferred 

for convenience and time savings if a 

large number of samples are tested at the 

same time. 

Less expensive alternatives to the 

metal canisters are the various plastic 

water filter housings (cans B, C, and D) 

available from many plumbing supply outlets. These containers are sometimes 

awkward to use because of their rounded 

bottoms (cans C and D), or because of the 

difficulty of opening and/or sealing the 

large screw-type caps. Thus, standard 

metal containers are preferred because of 

their flat bottoms and durability, especially in long-term collection programs. T- ---#..,..- Ill stllcLQ1, any czntziner that 3 be 

easily sealed airtight, can contain about 

2 kg (4.4 lb) of sample, and can hold 

approximately 414 kPa (50 lb/in2 g) of 

internal pressure would be adequate for 

the test. 

It has been suggested that containers 

of greater length, perhaps even long 

enough to hold an entire core of an coalbed, should be used for testing. Although it would be preferable to test the 

entire core, several complications may 

arise in using large containers. Occasionally, a sample container will leak, 

invalidating the test. If six individual 

0.3-m (1-ft) sections of a 1.8- (6-ft) 

coalbed are tested separately, a leak in 

one can is of little consequence. But if 

the entire 1.8 m (6 ft) is placed in one 

can and it leaks, little usable data may 

be obtained. Furthermore, coal samples 

that are friable and very gassy will usually give off large volumes of gas early 

in the desorption procedure. If very 

large amounts of coal of this type are 

sealed into a large canister, then bleeding the large volume of gas into the measuring apparatus, which will be described 

later, can require an excessive amount of 

time (several minutes). Long measuring 

times may invalidate the calculation of 

the lost gas, which requires graphing of 

gas volumes at instantaneous points in 

time. 

The equipment (fig. 8) needed to measure the actual volume of gas desorbing 
from the coal sample consists of an in- and clamps to hold the graduated cylinder 

verted graduated cylinder sitting in a in place. The desorbed gas that collects 

pan filled with water and a ring stand in the canister is periodically bled into 

the graduated cylinder and measured as 

the volume of water displaced. This procedure is performed both at the drill 

30-PSI g site and subsequently in the laboratory. 

Calculation of Gas Content 

The gas content of a particular sample 

is composed of lost, desorbed, and residual gas, each of which is determined by 

slightly different techniques. A core 

sample actually begins to desorb gas 

before it is sealed in the sample 

container. The amount of this lost gas 

depends on the drilling medium and the 

time required to retrieve, measure, and 

describe the core, and seal the sample in 

FIGURE 8. - Equipment for direct-method test- the can. The shorter the time required 

ing of coal sample. to collect the sample and seal it into 
the can, the greater the confidence in Time core reached surface (C)-- 

the lost-gas calculation. As discussed 12:40 a.m. 

previously, because of its speed, wire 

line retrieval of the core is preferable Time core sealed in canister (D)-- 

to conventional coring. If air or mist 12:50 a.m. 

is used in drilling, it is assumed that 

the coal begins desorbing gas immediately Lost gas time: 

upon penetration by the core barrel. 

With water, desorption is assumed to be- (D-A) if air or mist is used 

gin when the core is halfway out of the 

hole; that is, when the gas pressure is C-B (0-C) + - 2 if water is used assumed to exceed that of the hydrostatic 

head. 

The lost gas can be calculated by 

a graphical method based on the relationsnip that for tne first r'ew hours 

of emission, the volume of gas given 

off is proportional to the square 

root of the desorption time. A plot 

of the cumulative emission after each 

reading against the square root of the 

time that the sample has been desorb- = 15 minutes. 

ing ideally would produce a straight 

line. The resulting graph is shown in figure 9. The intercept on the X axis is 

A sample of experimental data (ta- the square root of the elapsed time 

ble 4) and supplementary information (lost-gas time) in minutes from the time 

used to construct a lost-gas graph gas desorption begins and the sample is 

follows : sealed in the container. The estimated 

value of the lost gas is the point at 

Drilling medium--water. which the constructed line intercepts the 

the negative Y axis. 

Time coalbed encountered (A)-- 

12:Ol a.m. The desorbed gas is simply the total 

volume of gas drained from the sample and 

Time core started out of hole (B)-- measured in the graduated cylinder. The 

12:30 a.m. desorbing of a sample is generally 

TAbLE 4. - Data for lost-gas graph 

Reading No. 

l........... 

2........... 

3........... 

4........... 

5........... 

6........... 

7 ........... 

Time, 

a.m. 

12:50 

1:05 

1:20 

1:35 

1:50 

2:05 

2:20 

Time since 

placed in 

can, min 

0 

15 

30 

45 

60 

7 5 

90 

Time in 

can+l5, 

minl'2 

3.87 

5.48 

6.71 

7.75 

8.66 

9.49 

10.25 

Gas released 

cmd 

0 

92 

84 

55 

36 

40 

33 

Total gas 

10-3 f td 

0 

3.25 

2.97 

1.94 

1.30 

1.41 

1.17 

cmj 

0 

92 

176 

231 

267 

307 

340 

10'3 ftj 

0 

3.25 

6.22 

8.16 

9.46 

10.87 

12.04 
FIGURE 9. - Lost-gas graph, 

allowed to continue until a very low 

emission rate is obtained, generally an 

average of less than 10 cm3 (0.35 x 10-3 

ft3) of gas per day for 1 week. The time 

required to reach this low rate of 

emission will vary considerably and is 

affected by many things, including the 

size of the sample, the physical characteristics of the coal, and the amount of 

gas contained in the sample. 

When it is determined to discontinue 

the measurement of desorbed gas, the coal 

sample will usually still contain gas. 

To complete the gas determination procedure, the amount of residual gas must be -.-. uGasured. The prccedure recornended by 

the Bureau is to crush the coal in a 

sealed ball mill. The ball mill constructed for crushing coal was fabricated 

from a piece of 0.64-cm (114-in) wall, 

17.78-cm (7-in) diameter steel pipe. A 

steel plate was welded to the bottom, and 

a lid was fitted to the top. At the top, 

a short section of pipe with 2.54-cm 

(1-in) wall thickness was welded inside 

the 17.78-cm (7-in) pipe to provide sufficient surface area for machining a 

groove for an O-ring seal and for bolt 

holes to secure the lid. 

The ball mill is tumbled on a roller 

machine for approximately 1 hr to crush 

the coal. The mill is allowed to cool to 

room temperature, and the volume of gas 

released is then measured by the water 

displacement method. The crushed powder 

and any uncrushed lumps are weighed separately. The volume of gas released is 

attributed only to the crushed powder. A 

set of residual gas data and calculation 

procedure follows: 

Weight of crushed powder--735 g 

Weight of uncrushed lumps--45 g 

Volume of gas bleed off--1,082 cm3 

gas bleed off , cm3 Residual gas calculation = weight of sample crushed to powder, g 
Theoretically, it is possible to crush 

a coal sample in the ball mill at any 

point after collection and to obtain the 

total gas content (excluding lost gas) of 

the sample. This procedure is generally 

not considered appropriate if maximum information from the sample is desired. By 

crushing the sample before the desorption 

process is complete, it is impossible to 

obtain the relative amounts of desorbed 

and residual gas. This distinction is 

important because the actual residual 

gas, which will not desorb from the sample while sealed in the canister, probably represents gas that will not flow to 

a methane drainage borehole and possibly ->...".-.7--0-- ,.--A +I.-+ -*: 11 -..+ L.. --: ++..A LCpLCDCllLb 663 LLldL Wlll l1UL Ut: tZ1IllLLt:U 

into a mine atmosphere. It is true 

that during the process of mining coal, 

the coal is broken up into variously 

sized pieces; however, the majority of 

these pieces will not usually duplicate the very fine powder that the ball 

mill produces in the residual gas 

procedure. 

The total gas content of a particular 

sample is the volume of lost gas and desorbed gas divided by the total sample 

weight plus the residual gas content. 

The calculation procedure and sample data 

set follow: 

Lost gas--240 cm3 

lost gas + desorbed gas Total gas = ------------------ total sample weight 

+ residual gas 

Auxi liary Test Prdcedures 

l'roximate, ultimate, and Htu analyses 

are obtained on the crushed powder from 

the residual gas test. These test results can be used to further evaluate the 

gas content results on a practical and 

tlleoret ical bas is. 

Because the gas content is presented on 

a volume-to-weight ratio, the presence of 

noncoal material, primarily shale and 

pyrite (which adds weight but not gas 

storage capacity), can produce seemingly 

erroneous data. Thus two samples from 

the same coalbed core may have gas contents varying by several cubic centimeters per gram if one sample contains 

appreciably higher noncoal material. The 

coal analysis will help determine if noncoal material is influencing the total 

gas content. 

Evaluation of the influence of depth of 

burial on the gas content is preferably An.-.- uurlc OG 21 ~122111 ZOS?, thiis rsmoving th2 

noncoal material variable from the evaluation. However, because coalbeds do contain noncoal material, the actual inplace methane in a particular volume of 

coal should be related to the as-received 

coal data. 

Theoretically, the gas content of coal 

is influenced by the rank of the coal, 

with higher ranks generally having higher 

gas contents. The coal analysis can be 

used to determine the apparent rank of 

the coal by ASTY Standard U388 (2) for - evaluation of the rank parameter. Coal 

petrography , specifically vitrinite ref lectance raeasurernents, can also provide 

a measurement of coal rank. Determining 

the microscopic constituents of the coal 

(macerals) may also be useful in investigations of the factors influencing the 

rnethane content of coalbeds. Adsorption 

isotherm tests (18) will give data on the 

theoretical storage capacity of a sample, 

and along with the other analytical 

tests, can be an important tool for evaluating the direct method test results. 

Gas samples should be obtained periodically during the desorption testing of 

coal samples. Gas cornposit ional analysis 

will provide information on the gas quality, including the presence of gases 

other than hydrocarbons. Ethane, propane, and butane are common hydrocarbons 

found in srnall amounts (geuerally less 

than 2 pct combined) in coalbed gas. 

Carbon dioxide (occasionally in amounts 

as high as 15 pct) is a common, putent ial ly undesirable, conponerlt of coalbed 
gas which has been found at the higher necessarily gassy. Many coalbeds in the 

levels primarily in a relatively small Eastern United States contain appreciable 

area of the Pittsburgh Coalbed in Penn- gas (>5 cm3/g [160 ft3/ton]) at depths of 

sylvania and West Virginia, and in sev- 305 m (1,000 ft) (8). However, many 

era1 western coals. Several publications coalbeds in the western United States at 

(17, --- 19, 29) discuss the origin and com- depths of 305 m (1,000 ft) contain little 

position of coalbed gas. gas (<2 cm3/g [64 f t3/ton] ). The reason 

for these variations in gas contents of 

Geologic Considerations coalbeds at similar depths are in many 

cases due to differences in the rank of 

Variations in Gas Content the coal. Coal rank (table 5) is a measure of the stage of coalification that a 

The gas content of individual coalbeds coal deposit has reached. The coalificahas been observed to increase as the tion process progressively transforms the 

depth of the coalbed increased (9-10, -- - 13- original plant material into higher ranks 

---- 14, 25, 27, 30). A coalbed contains more of coal, depending primarily on temperagas at greater depths primarily owing to ture and time and to a lesser extent on 

the increase in reservoir pressure, which 

allows the coal to "hold" more gas, if it 

is available. Figure 10 illustrates increasing gas content with increasing 

depth for the Mary Lee Coalbed in Alabama. It is important to note that this 

graph is only for the Mary Lee Coalbed 

and should not be used to estimate the 

gas content of any other coalbed or coal 

region. 

pressure (29). Methane is generated 

throughout xe coalification process in 

varying amounts, with an increased yield 

of methane associated with reduction of 

hydrogen, which begins at approximately 

29 pct volatile matter content in the 

medium-volatile bituminous coal range 

(29). Owing to the influence of the 

czlification process, it is theref ore 

possible to have deep coalbeds that have 

not gone through the stages that produce 

Even though the gas content of a coal- high volumes of methane and that do not 

bed increases with depth, this does have high gas contents. 

not mean that all deep coalbeds are 

0 500 1 ,000 1,500 2,000 2,500 

DEPTH, ft 

FIGURE 10. - Gas content versus depth for the Mary Lee Coalbed, Alabama. 
TABLE 5. - Classification of coal by rank (2) - 

Group 

Fixed carbon 

limits, pct 

(dry, mineralVolatile matter 

limits, pct 

(dry, mineralThe graph (fig. 10) that is a plot of 

direct method gas contents versus depth 

of samples from the Mary Lee Coal Group 

is also influenced by rank variations of 

the coalbeds. Figure 11 shows the distribution of coal rank and depth in the 

basin. In general, the rank increases 

with depth; however, this relationship is 

variable and does not precisely correlate 

throughout the area. Because of the high 

numbers of samples that would be needed 

for direct method testing to document the 

change in gas content with rank in a coal 

basin and the relative ease of mapping 

the changes in depth and obtaining Samples from a variety of depths for gas 

content determinations, the relationship 

of gas content to depth for coalbeds is 

most commonly presented. The Bureau of 

Mines is currently conducting research to 

Calorific value 

Btu/lb (moist, 

mineral-mat t ermatter-f ree basis) 

document and relate the influence of coal 

rank as well as depth on gas content. 

Equal to 

or greater 

than-- 

Lignite A.............. 

Lignite B.............. 

Coalbed Discontinuities 

matter-f ree bases) 

Less 

than-- 

CLASS 1.--ANTHRACITIC 

... 

... 

There are many geologic features that 

disrupt the continuity of a coalbed. 

They can be stratigraphic in origin and 

characterized by an interruption in sedimentation, either nondeposition or erosion, such as a sand channel; or they can 

be structural in origin and characterized 

by a surface separating two unrelated 

groups of rock, such as a fault (1). 

Discontinuities are an important consideration in evaluating the gas drainage 

potential of coalbeds. The presence of 

discontinuities can cause serious problems in the drilling and completion of 

both vertical and horizontal gas drainage 

Greater 

than-- 

free basis) 

... 

... 

Equal to 

or less 

than-- 

Equal to or 

greater 

than-- 

Meta-anthracite........ 

Anthracite............. 

Semianthracite......... 

... 

... ... 

... 

Less 

than-- 

2 

8 

14 

98 

92 

86 

CLASS 11.--BITUMINOUS 

6,300 

... 

... 

... 

... 

Low-volat ile 

bituminous coal....... 

Medium-volatile 

bituminous coal....... 

High-volatile A 

bituminous coal....... 

Hig h-volat ile B 

bituminous coal....... 

High-volatile C 

bituminous coal....... 

8,300 

6,300 

... 

... 

... 

... 

98 

92 

... 

2 

8 

78 

69 

... 

... 

... 

CLASS 111.--SUBBITUMINOUS 

86 

78 

69 

... 

... Subbi tuminous 

A coal................ 

Subbituminous 

B coal................ 

Subbituminous 

C coal................ 

14 

22 

3 1 

... 

... 

CLASS IV .--LIGNITIC 

... 

... 

... 

11,500 

10,500 

9,500 

... 

... 

... 

10,500 

9,500 

8,300 , 

... 

... 

... 

14,000 

13,000 

22 

3 1 

... 

... 

... 

... 

... 

... 

... 

... 

14,000 

13,000 

11,500 

... 

... 

... 
of gas (decreasing, or in some cases increasing, production as previously discussed) to the boreholes. 

An ideal coalbed from both a mining and 

a gas drainage standpoint would be of 

uniform thickness with no interruptions 

(fig. 12A). This is seldorn the case, as 

stratigraphic discontinuities as shown in 

figure 12B. A horizontal gas drainage 

borehole drilled into this coalbed would 

probably encounter great difficulty both 

in drilling and in staying in the coalbed. A vertical borehole (resource confirmation corehole or production hole) 

would also experience problem in sample 

Splits 

A Partings 

'~orizontal borehole 

FIGURE 12. - ;I, Section view of ideal coalbed, and H, effect of coolbcd discontinuities on horizontal gas drainage boreholes. 
recovery and gas flow if areas of thin or 

absent coal were encountered, as at the 

"roll" or in the area of the "splits." 

Boreholes drilled in an area of extensive partings (fig. 12~) could encounter 

gas flow problems. A horizontal hole 

drilled completely above or below an extensive parting that effectively separates a coalbed into separate reservoirs 

may drain methane only from that portion 

of the coalbed actually drilled. The 

other portion of the coalbed would remain 

undrained, and the gas would still be a 

hazard to future mining or would be unavailable for commercial production. A 

stimulation treatment in a vertical hole 

drilled into an area with an extensive 

parting may not completely stimulate both 

portions of the reservoir. If the treatment did not efficiently penetrate above 

and below the parting, the gas flows 

could be reduced with the same potential 

consequences as described for the horizontal holes. 

Impermeable discontinuities that completely disrupt a coalbed are particularly troublesome for gas drainage drilling activities. Figure 13 illustrates 

several geologic situations that can 

Borehole A 

adversely affect drilling. Borehole A 

has been drilled into a sand channel and 

completely missed the coalbed. If this 

was a resource confirmation core hole, no 

coal would have been obtained for direct 

method gas content testing. If borehole A was for gas drainage, it would 

probably be ineffective unless gas had 

migrated (or would migrate) from the coal 

to the sand channel and was trapped. 

Clay veins are generally smaller than the 

sand channels, however, if they are encountered, they can cause equally serious 

problems. 

Borehole B (fig. 13) has encountered a 

full section of the coalbed; however, it 

is bounded by a clay vein and a fault. 

This situation can be bad or good for gas 

production, depending on the size of the 

"cell" that borehole B has intercepted. 

If the bounded area is small, a limited 

amount of coalbed reservoir will be 

available to feed gas into the borehole, 

therefore, its production potential is 

low. If borehole B has penetrated a 

larger "cell" or is not completely 

bounded by discontinuities, the situation 

may enhance gas production. A bounded 

coalbed reservoir of this type will potentially have a faster pressure drawdown 

Borehole 6 Borehole C Borehole D 

Coal dk . .. . -- \.:; ,,..y .,'. a .., 1 I:. ..* .;.;:; y- --.--- - - - - - - -- - .o. . 0 ..:. . .. -== ==;--- := ==--.=I lG>\ I I 

Sand channel Clay vein U 

Abandoned 

mine 

FIGURE 13. - Section view of effect of coal bed disc~ntinuities on vertical gas drainage 

boreho les, 
when dewatering is initiated, and higher 

gas saturations and production rates 

should follow as has been observed at a 

vertical borehole methane drainage pattern in Alabama (3, - - 23). 

Borehole C (fig. 13) has completely 

missed the coalbed by intercepting a 

fault; therefore, there are no samples or 

gas production from the coal. Borehole D, which was to be a commercial 

well, has intercepted the coalbed; however, it is very near an abandoned mine. 

An abandoned mine is not a natural coalbed discontinuity, but it does interrupt 

the coalbed reservoir and can have seric=nseque.;.lces. I+, is nil+ tn lilr~ly 1""- ------ 

that in addition to a portion of the 

coalbed reservoir having been removed by 

mining, a significant amount of the gas 

originally in the remaining coal migrated 

to the mine openings and is no longer 

available for production from a borehole. 

Abandoned mines abuve the target coalbed 

must also be considered, since if a void 

is encountered, all of the drilling fluids and the hole may be lost. If the 

hole could be saved, expensive remedial 

actions such as casing through the mine 

opening might be required. 

All of the geologic situations described in figure 13 would also have 

serious effects on horizontal drilling 

activities (11). Sand channels are part icularly trzblesome because of their 

large size and slow rate of penetration 

with the horizontal drilling equipment. 

Intercepting an abandoned mine with a 

horizontal borehole could be hazardous if 

the mine was full of water (or gas) which 

flowed uncontrolled into the mine workings from which the hole was being 

drilled. 

Since coalbed discontinuities can seriously affect the successful completion of 

resource confirmation core holes as well 

as vertical and horizontal methane drainage boreholes, they should be evaluated 

as part of the feasibility studies for a 

specific project area or drill site. 

While it is not possible to precisely 

locate all discontinuities (especially 

the smaller ones) before drilling a 

particular site, basic geologic mapping 

techniques can project probable areas of 

occurrence if sufficient data are available. It is also possible to estimate 

the probability of encountering coalbed 

discontinuities by statistically evaluating data from mines in adjoining areas 

(12). Impermeable coalbed discontinuities are also important from a mine ventilation standpoint since they can isolate large volumes of gas that can be 

liberated suddenly in high volumes when 

penetrated by a mine entry. 

Multiple Coalbed Reservoirs 

Multiple cnaLbed reservoirs can be 

attractive for a resource recovery 

program using vertical boreholes; however, their thickness and distribution 

must be amenable to efficient well completion practices. Completions of multiple coalbed reservoirs may also have 

applications in mining when more than onc 

coalbed is to be mined or where gas from 

surrounding coalbeds may migrate to the 

workings in the coalbed being mined. 

In general, it is preferable to have 

the coalbed completely exposed (open-hole 

completion) to the wellbore for the most 

efficient gas production (23). The completion of multiple coalbeds, if they are 

distributed over a large interval in the 

well, can necessitate the installation of 

casing through the upper coalbeds, which 

is less desirable. In well A (fig. 14) 

two thick coalbeds have been encountered 

at the bottom of the hole. Assuming that 

both coalbeds have sufficient gas to warrant completion, both could probably be 

completed open hole with the casing set 

above the upper coalbed. Depending on 

the actual distance between the two coalbeds and the competence and condition of 

the intervening rock unit, each coalbed 

could be stimulated separately or at the 

same time. Separate treatments would be 

desirable to increase the probability cf 

getting a good stimulation treatment in 

each coalbed. If both coalbeds were 

treated at the same time, there would be 

a chance that the treatment would only 

enter one of the coalbeds, in spite 

of treatment designs to avert that 
Well A Well B ............................ I.. ................. .I 

I... ........................ I 

I.... ................... I 

LEGEND 

Limestone, sandy shale Sandy shale 

FIGURE 14. - Examples of multiple coalbed reservoirs. 

situation. A good stimulation treatment 

is critical for efficient production of 

methane frorn a coalbed (23). - The additional cost for separate treatments must 

be balanced against the potential for 

incomplete stimulation of coalbeds in a 

zone treatment. 

Well B (fig. 14) represents an undesirable situation involving multiple 

coalbeds. Instead of one or more thick 

coalbeds being encountered, all of the 

coalbeds are thin, and they are spread 

over a large interval in the well. Even 

though collectively all of the coalbeds 

in well R may contain a volume of gas 

equal to or greater than that of a single 

thick gassy coalbed, the completion cost 

will probably be high and the production 

potential low. If the interval containing the coalbeds is large and the rock 

between the coals is subject to deterioration and sloughing, the upper coalbeds 

would have to be cased to prevent the 

hole from filling in. 
When casing is installed through a 

coalbed that is to be completed for production, communication between the coalbed reservoir and the wellbore must be 

established either using conventional 

perf orations or preferably by slotting 

(6, 28). The perforations or slots must 

b7 precisely located at the coalbed interval to have a reasonable chance of a 

successful completion. The thinner the 

coalbed, as in well B (fig. 14), the 

greater the chance of missing the coalbed 

when an attempt is made to perforate or 

slot. 

The coalbeds in well B (fig. 14) would 

probably have to be completed with three 

separate stimulation treatments. The 

upper three coalbeds would be grouped as 

a zone, and the bottom two coalbeds would 

be stimulated individually. The production potential of the coalbeds in well B 

would probably not justify the high cost 

of this completion program, or even the 

cost of the well itself. 

REGIONAL CONSIDERATIONS 

Regional considerations for coalbed gas 

potential are essentially an expansion 

and correlation of information gained 

from site-specific evaluation procedures. 

The regional considerations, like the 

site-specific considerations, have both 

mine safety and resource production potential applications (7). - 

Calculation of an Area's In-Place 

Gas Volume 

A calculation of the in-place gas volume in an area and mapping of the gas 

distribution are primary regional considerations. To calculate the in-place 

gas volume, the following are needed: A 

means of estimating the gas content that 

is related to mappable parameters (direct 

method gas contents versus depth, 

fig. lo), and maps of the parameters 

(depth of the coalbed in the area, and 

coal thickness, fig. 15). 

Once the appropriate maps have been 

constructed and gas contents from coal 

samples versus depth have been graphed, 

the actual calculation of the in-place 

gas volume is quite simple. The data 

from the Mary Lee Coal Group in the 

Warrior Basin of Alabama will be used as 

an example. The overburden map (fig. 15) 

has been drawn with a 152-m (500-ft) contour (depth) interval. The coal isopach 

(thickness) map has been superimposed on 

the overburden map so that the volume of 

coal in each 152-m (500-ft) depth interval can be calculated. 

For estimation purposes, the gas content of the median depth of each 152-m 

(500-ft) overburden interval (fig. 10) is 

used in the calculation as the average 

gas content of the interval. The gas 

content of the median depth of each interval is multiplied by the volume of 

coal in the interval to obtain the inplace gas volume. As an example, the 

gas content of the median depth of the 

305- to 457-m (1,000- to 1,500-ft) Mary 

Lee overburden interval is 14.0 

(448 ft3/ton), and the coal volume is 

1,421 trillion kg (1,566 billion tons), 

which when multiplied yields 19.9 billion 

m3 (702 billion ft3) of gas for the interval. Similar calculations are made 

for each interval (table 6), and the 

total in-place volume (52.3 billion m3 

[1.8 trillion ft3] for the Mary Lee 

Group) can be determined. 
TABLE 6. - In-place gas volume for the Mary Lee Coal Group, Alabama 

FIGURE 15. - lsopach of the Mary Lee Coal Group superimposed on the overburden 

isopach. 

Overburden 

m 

0-152 

152-305 

305-457 

457-610 

610-762 

Average gas content 

f t 

0- 500 

500-1,000 

1,000-1,500 

1,500-2,000 

2,000-2,500 

cm3/g 

0.5 

9.2 

14.0 

15.5 

15.7 

Gas in place 

ft3/ton 

16 

294 

448 

496 

502 

Billion m3 

0.8 

15.0 

19.9 

11.9 

4.7 

52.3 

Billion ft3 

28 

530 

702 

4 19 

165 

1.8 million 
The gas content information can be used The general regional estimates of inin conjunction with the regional over- place gas volumes, as calculated by the 

burden map to delineate areas of high in- procedure described above, can be a valuplace gas volumes (potentially bad for able indicator of areas to be seriously 

mining, good for commercial production) considered for commercial gas production. 

and low in-place gas volumes (potentially Previous studies (4, 9-10, 14, 16, 25, -- - - - 

good for mining, bad for commercial pro- -- 

- 

30-31) have estimatFd the in-place gas -- 

duction). Since the gas content of coal volumes for several of the gassiest coalincreases with depth, the deeper parts of 

the Warrior Basin, as delineated on the 

overburden map, have the highest potential for large volumes of in-place gas. 

At a depth of 610 m (2,000 ft), every 

2.6 km2 (square mile) of Mary Lee coal, 

1.8 m (6 ft) thick, would contain approximately 85 million m3 (3 billion ft3) of 

in-place gas, This volume of gas would 

probably be attractive for its resource 

production potential, but it would be a 

tremendous volume of gas to encounter in 

a mining operation. The gas content at a 

depth of 610 m (2,000 ft) from figure 10 

is approximately 15.7 cm3/g (502 ft3/- 

ton), which when plotted on the graph of 

expected mine emissions (fig. 3) yields 

an estimate of over 96 m3 (3,400 ft3) of 

gas emissions from all sources (roof, 

floor, ribs, and pillars in addition to 

that actually contained in the volume of 

coal mined at the face) for each 907 kg 

(ton) of coal production. If possible, 

it would be preferable from the standpoint of the potential methane hazard 

(which of course is not the only consideration for locating a mining operation) to locate in the areas of lower inplace gas volumes. 

beds and coal-bearing formations in the 

United States (table 7), and several estimates of the in-place coalbed gas volumes for the entire United States have 

been made (table 1). These estimates are 

only valuable if used with an understanding of their true meaning and significance. It is very important to realize 

that these values are for in-place gas 

volumes and do not represent the volume 

of gas that can physically and/or economically be recovered from coalbed gas 

drainage systems . 

The percentage of in-place gas that can 

physically be removed from a coalbed is 

presently unknown and will probably be 

different for each coalbed and even for 

different areas of the same coalbed. It 

is probable that the volume of gas that 

is residual gas in the direct method test 

will probably not flow to a wellbore and 

perhaps will not flow into a mining operation. Economically, it is unlikely that 

gas that is at low concentrations, as in 

the shallow portions of coal basins and 

in the low-rank coals, will ever be captured for utilization. 

TABLE 7. - In-place gas volumes of selected U.S. coalbeds 

Coalbed or formation, and 

State 

Mesaverde Fm. (32), (Southern 

Piceance ~asinr Colorado.. . Mesaverde Fm. (4), (Sandwash - Basin) Colorado............. 

Mary Lee (lo), Alabama....... 

Vermej o ~m7(31), Colorado.. . Pittsburgh (9x - Pennsylvania 

and West Virginia.. ......... Fruitland Fm. (16), Colorado. 

Lower ~artshornh(l4), - 

Oklahoma.................... 

Upper F'reeport (30), - 

Pennsylvania................ 

Beckley (25), - West Virginia.. 

Area 

km2 

4,079 

1,072 

2,161 

464 

3,367 

715 

1,554 

1,295 

518 

In-place gas volume 

mi 2 

1,57 5 

414 

835 

179 

1,300 

276 

600 

500 

200 

Billion m3 

887 .O 

396.7 

52.3 

44.2 

42.5 

39.6-283.2 

31.1-42.5 

5.7-11.3 

2.8 

Trillion ft3 

31.3 

14.0 

1.8 

1.56 

1.5 

1.4-10.0 

1.1-1.5 

0.2-0.4 

.1 
Additional Regional Considerations 

The other regional considerations for 

coalbed gas drainage activities are primarily related to geologic factors for 

selection of areas within a region for 

site-specific evaluation of proposed commercial gas recovery projects. The 

thickness of coalbeds can vary on a regional basis as well as locally, as discussed previously. The thicker the coalbed, the larger the reservoir for gas 

storage. Also, various coalbeds can 

appear and disappear independently of 

each other throughout a region. This is 

important if multiple zone completions of 

vertical wells are anticipated, For commercial ventures it is necessary to pick 

an area for potential development that 

has the optimum balance of gas content, 

coal thickness, and number and distribution (vertical thickness of producing 

zone and distance between individual 

coalbeds) of producible coalbeds if multiple completions are planned. 

Regional trends of water-bearing sands 

(water sands) should also be considered 

to determine where such sands may be in 

close association with coalbeds from 

which gas is to be extracted by vertical 

wells. If water sands are present, high 

volumes of extraneaous water may be produced, perhaps indefinitely, without 

dewatering the coalbed, and with little 

if any gas production. The type and cost 

of completion procedures for vertical 

wells can be seriously affected by the 

presence of water sands in a prospective 

producing zone. 

CONCLUSIONS 

Coalbeds can contain appreciable quantities of methane, the removal of which 

may be desirable for mine safety and/or 

energy resource utilization purposes. 

The direct method test can be used to 

determine the gas contents of coalbeds at 

specific sites of future mining operations or potential resource recovery and 

utilization systems. Geologic evaluation, including a determination of the 

potential of encountering coalbed discontinuities, is an important consideration 

when locating methane drainage drilling 

sites. Maximum information can be obtained from a resource confirmation exploratory hole if a continuous wire line 

core is obtained. If wire line coring is 

not possible, a twinned-hole approach is 

the second choice. 

Regional considerations influencing the 

gas potential of coalbeds are primarily 

related to the calculation of the total 

in-place gas for a coalbed (or group of 

coalbeds) in an area. Caution must be 

used not to confuse the in-place gas volumes with recoverable volumes. The distribution of the gas volumes in a region 

can be used to delineate areas of high 

in-place gas volumes where mining may be 

adversely affected but resource recovery 

and utilization may be enhanced. Regional mapping of geologic trends such 

as coal thickness, number of coalbeds, 

and water sands can aid in delineating 

areas with the highest potential for 

commercial production of methane from 

coal beds. 

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Glossary of Geology. Falls Church, Va., 

1974, p. 201. 

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Part 26, Gaseous Fuels; Coal and Coke; 

Atmosphere Analysis. Philadelphia, Pa., 

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