Bureau of Mines Report of Investigations/l972 

The Methane Migration and Storage 

Characteristics of the Pittsburgh, 

Pocahontas No. 3, and OklahomaHartshorne Colalbeds 

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

Rogers C. B. Morton, Secretary 

BUREAU OF MINES 

Elburt F. Osborn, Director 
I. Methane. I. l'itle. (Series) 

TN23.U7 no. 7667 622.06173 

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

Page 

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

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

Acknowledgments .......................................................... 1 

Gas pressure and flow measurements ....................................... 2 

Analysis of pressure curves .............................................. 2 

The crushed zone ......................................................... 9 

Calculation of permeability .............................................. 9 

Increase in permeability with time ....................................... 10 

Possible reasons for permeability increase ............................... 13 

Permeabilityofdifferent coalbeds and effect of overburden ............... 14 

Anisotropy in permeability ............................................... 15 

Other variations in permeability ......................................... 17 

Conclusions .............................................................. 18 

References .............................................................. 20 

Appendix.--Applying the unsteady-state Darcy equation to experimental 

pressure gradient curves .............................................. 21 

ILLUSTRATIONS 

1 . Pressure curves from the Pittsburgh coalbed. Federal No . 2 mine. 

holes 1 and 2 ...................................................... 3 

2 . Pressure curves from the Pittsburgh coalbed. Federal No . 2 mine. 

holes 3. 4. and 5 .................................................. 3 

3 . Pressure curves from the Hartshorne coalbed. Howe No . 1 mine. 

holes 2 and 3 ...................................................... 4 

4 . Pressure curves from the Hartshorne coalbed. Howe No . 1 mine. 

holes 4 and 5 ...................................................... 4 

5 . Pressure curves from the Hartshorne coalbed. Howe No . 1 mine. 

holes 8 and 9 ...................................................... 5 

6 . Pressure curves from the Hartshorne coalbed. Howe No . 1 mine. 

holes 10 and 11 .................................................... 6 

7 . Pressure curves for the Pocahontas No . 3 coalbed. Kepler mine. 

holes 1 and 4 ...................................................... 7 

8 . Federal No . 2 mine. steady-state flow from 6-foot increments of 

open hole .......................................................... 13 

9 . A mine entry in a coalbed with pronounced face cleat and 

butt cleat development ............................................. 16 

TABLES 

1 . Coalbed permeability data ............................................ 8 

2 . Methane flow data .................................................... 12 
THE METHANE MIGRATION AND STORAGE CHARACTERISTICS 

OF THE PITTSBURGH, POCAHONTAS NO. 3, 

AND OKLAHOMA HARTSHORNE COALBEDS 

Fred N. Kissell 

ABSTRACT 

Methane flows and pressures were measured in mines in the Pittsburgh, 

Hartshorne, and Pocahontas No. 3 coalbeds. From the data, the permeability 

of each of these coalbeds was computed. It was found to depend strongly on 

the type of coal and on the amount of water in the coalbed, and it was found 

to increase with time. All of the coalbeds had a high-permeability zone 

adjacent to the face or rib, and the Pittsburgh bed exhibited directional 

anisotropy. Surprisingly, little or no overburden effect was observed. 

INTRODUCTION 

The Bureau of Mines has been engaged in a program of drilling deep 

horizontal holes into coalbeds from the working areas of coal mines (3-4).2 - - 

These holes are sealed with inflatable packers, and the pressure of the gas 

that seeps into the hole from the coal is measured. These pressures, in 

conjunction with other data from the mine and the laboratory, can provide 

information about the gas flow characteris tics of the coalbed (I). 

In this report, gas pressure and flow measurements from horizontal 

holes in the Pittsburgh, Hartshorne, and Pocahontas No. 3 coalbeds are used 

to calculate the coalbed permeability and methane sorption capacity. These 

measurements show that the permeability of a coalbed depends greatly on the 

nature of the coal, exhibits spatial variations and anisotropy, and exhibits 

a remarkable increase with time, possibly owing to loss of water from the 

coalbed. 

ACKNOWLEDGMENTS 

Grateful acknowledgment is made to these Bureau of Mines people who have 

contributed data and helpful advice: J. H. Perkins, T. C. Ruppel, Albert 

Sainato, and C. H. Elder. Also, G. R. Haworth of the Central Research Division, Continental Oil Co., has submitted helpful comments regarding permeability anisotropy and its effect on rib emission rates. 

IPhysical research scientist. 

2~nderlined numbers in parentheses refer to items in the list of references 

preceding the appendix. 
The Darcy equation is 

Here P is the gas pressure (atmospheres), x is the distance into the coalbed 

from the working face (centimeters), 7 is the viscosity of methane (centipoises), K is the permeability of the coal (darcy), S is the sorption capacity 

of the coal r volume adsorbed per atmosphere ] , and t is time (seconds). volume of coal 

3~11 the mines studied were being worked by the advancing room-and-pillar 

system of mining in the sections where measurements were made. 
3 

I 1 1 1 I 1 I 1 1 

- - 

- 

- - 

- Hole 2 

Extrapolation to 

obtain crushed - 

zone thickness 

- 

- 

/ 

- / A - 

Crushed 

7 ,One 1 ,/ - 

- 

I 

--- / -- I I I - 

I - 

0 10 20 30 40 50 60 70 80 90 100 

DEPTH OF HOLE, ft 

0 

FIGURE 2. - Pressure Curves From the Pittsburgh Coalbed, Federal No.2 Mine, Holes 3,4, and 5. 

- 

0 10 20 30 40 50 60 70 80 90 

DEPTH OF HOLE, ft 

FIGURE 1. - Pressure Curves From the Pittsburgh Coalbed, Federal No.2 Mine, Holes 1 and 2. 
0 

0 10 20 30 40 50 60 70 80 90 

DEPTH OF HOLE, ft 

FIGURE 3. - Pressure Curves From the Hartshorne Coa lbed, Howe No. 1 Mine, Holes 2 and 3. 

b - I I I I I I 9 

- Hole 5 - 

- 

Hole 4 

/ 

- / - 

b 

/ i 

I I I I I I 

0 10 20 30 40 50 60 70 

DEPTH OF HOLE, ft 

FIGURE 4. - Pressure Curves From the Hartshorne Coalbed, Howe No. 1 Mine, Holes 4 and 5. 
10 20 30 40 50 60 

DEPTH OF HOLE, ft 

FIGURE 5. - Pressure Curves From the Hartshorne Coalbed, Howe No. 1 Mine, Holes 8 and 9. 
DEPTH OF HOLE,ft 

FIGURE 6. - Pressure Curves From the Hartshorne Coalbed, Howe No. 1 Mine, 

Holes 10 and 11. 
U 5 10 15 20 25 30 35 40 45 50 55 60 

DEPTH OF HOLE ,ft 

130 

110 

90- ul 

a 

w' 

CK 

2 70- m 

m 

w cK a 

m 50- 

a 

(3 

30 

10 

FIGURE 7. - Pressure Curves for the Pocahontas No. 3 Coalbed, Kepler Mine, Holes 1 and 4. 

- 1 I I 1 1 1 1 1 I 1 I - 

Hole I, - - 

- 

- A-l - 

- -- - 

-Hole 4 

- - 

- - 

This equation has no exact solution, but an approximate solution is (6) - 

P2 (x,t) - p2 seam 1 = erfc - 

"atm - p2 s e am 2t: ' 

where erfc is the complementary error function and erfc = 1 minus the probability integral. Values of the probability integral may be obtained from 

tm PK standard tables. Here t, is equal to dimensionless time = - , P(x,t) is U 

nx2 s 

the pressure in the coalbed at point x and time t, Ps am is the gas pressure 

in the undisturbed coalbed, - Patm is atmospheric pressure, tm is the elapsed 

time since mining, and P is the average pressure at x. The ratio K/S or 

permeability ] is obtained by fitting the best theoretical error function sorption capacity 

curve to the experimentally measured pressure. Table 1 gives the K/S values 

obtained from each hole, together with K/s values calculated from a number of 

published pressure curves (2, - 2). 
Coal bed 

Pittsburgh ........ 

Hartshorne ........ 

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

Mine and locat

Federal No. 2, 

Fairview, W. 

Howe No. 1, 

Heavener , Okl

Beatrice, 

Keen Mountain

Va. 

Kepler, 

Pineville, W. 
THE CRUSHED ZONE 

It has been shown (1) that no simple theoretical curve can be fitted to 

the experimental pressure curve unless the "crushed zone" is taken into 

account. The crushed zone is a region of high permeability in the coalbed 

adjacent to the face and rib and is probably similar to the relaxed zone 

around a longwall face. Presumably, it forms because coal in this region 

is unable to withstand the compressive load of the overburden. Partial 

crushing greatly increases the permeability of the region so that gas emerging 

from the intact coalbed behind the zone flows easily to the face or rib. The 

thickness of the crushed zone is calculated by fitting the best theoretical 

curve to the pressure gradient and extrapolating the theoretical curve 

to zero gage pressure (I). The distance from the face to this zero pressure 

point is the crushed zone thickness (fig. 1). 

The calculated crushed zone thickness for figures 1-7 are shown in 

table 1.4 A crushed-zone thickness from a previous report also is included 

(2). These zones ranged in thickness from 1.6 to 20 feet and averaged about 

10 feet. Hargraves (5) has published gas pressure data for a number of 

Australian mines. 1f the curves from this data are extrapolated to zero 

gage pressure most of them also yield a crushed zone thickness of about 

10 feet. 

It might be expected that a rough correlation exists between the thickness of the crushed region and the amountof overburden as shown in table 1, 

but this does not seem to be the case. In part, this lack of correlation is 

probably because shallow mines have pillarts and supporting members of smaller 

dimensions. Regardless, a high-permeability zone in the coal adjacent to the 

face and rib seems to be a feature common to all the mines even though the 

roof was not being caved. 

It should be mentioned that pressure curves do not provide the sole 

evidence for this high-permeability zone. Parsons (11) has measured dilations 

of pillars in the Pittsburgh coalbed and has found differential dilations up 

to a depth of 15 feet in pillars. 

Fitting the theoretical error function curve to the experimental pressure 

curve only yields the ratio K/S i permeability ] . Other methods must be 

sorption capacity 

used to obtain K or S independently. K can be estimated if both the pressure 

data from a hole and the methane flow from the face or rib region surrounding 

the hole are known (1). First, the pressure curve is fitted to the best 

theoretical curve, and the crushed zone thickness is obtained. Then, the 

theoretical curve is differentiated, and the slope at zero gage pressure is 

calculated. This slope and the face or rib flow are inserted into the steadystate Darcy equation (6-7), and K is obtained. - -- 

4Some of the pressure curves, such as Howe mine hole 11 in figure 6, do not 

have points close enough to the face to warrant extrapolation. 
coalbed at hole 1 is more than 10 times greater than at hole 2. The same 

effect is illustrated by hole 5 in the Federal mine; by holes 10 and 11 in 

the Howe mine, by holes 1, 2, and 3 in the Beatrice mine, and by holes 1 

and 4 in the Kepler mine. All three coalbeds show the effect. 

The reason for this increase in permeability with time is not entirely 

clear. Several possibilities are considered in the appendix. These include 

incorrect estimation of bed pressures, problems associated with the assumption that the coalbed at any given location begins to drain when that location 

is mined, possible incorrect pressures at long times owing to a delay in 

micropore desorption, and an assumption of constant sorption capacity. 

However, the variation in permeability appears too large to be accounted 

for by any of these. 

5Research chemist, Pittsburgh Energy Research Center. 
The borehole gas-flow data shown in table 2 provides evidence that this 

increase in permeability is real and not the result of the assumptions made in 

applying the unsteady-state Darcy equation. It is known that as methane seeps 

out of the coalbed into the mine, the gas pressure in the coalbed falls. If 

the coalbed permeability were constant, a decrease in gas pressure would lower 

the rate of emission from the horizontal test holes, but this never happened. 

For example, Federal hole 5, which was in an area 30 months old, had 4 psig 

at the back of the hole; whereas hole 4 in an area 2 months old had 21 psig. 

A constant permeability would indicate a much higher flow from hole 4, but as 

table 2 shows, the flow rates were about the same. Since the flow rate 

corrected for pressure and depth6 is roughly proportional to permeability, 

the permeability of the coalbed near hole 5 appears to be considerably larger 

than that of the coalbed near hole 4. Exactly the same effect is shown for 

holes 1 and 2 of the Beatrice mine, holes 1 and 4 of the Kepler mine, and it 

appears also to a much lesser degree at holes 10 and 11 of the Howe mine. 

Always, an increase in the age of an area caused an increase in permeabilityO7 

The result of this efgect is that holes drilled into older, low-pressure areas 

often seem to yield as much gas as holes drilled into freshly mined areas. 

Gas yield performance is of critical importance in coalbed degasification. 

A more critical test, especially with regard to degasification, would be 

to measure the flow from a single hole over a long time period rather than 

measure flows from holes in different age areas, as is done in table 2. The 

Bureau of Mines has recently drilled a 400-foot hole in a freshly mined area 

of the Federal No. 2 mine. The initial gas flow of 100,000 cubic feet per 

day continued virtually unabated for 2 months. 

At least one other investigator has observed a similar increase in 

permeability. Stewart (14), - in a study of Australian coalbeds, measured 

incremental flows from a borehole in a manner similar to that used to obtain 

the data of figure 8. He observed a peak in the flow, whereas in figure 8 

the flow increases smoothly with depth. In 30 days this peak moved inward 

from a depth of 6 feet to about 21 feet. Stewart attributed this change in 

permeability to "destressing," a reduction in stress with relaxation along 

cleat and bedding planes. This explanation might be valid for regionsclose 

to the face such as Stewart observed. (The crushed zone is probably such a 

destressed region.) However, hole 2 in the Beatrice mine (which exhibits a 

vastly increased permeability compared with hole 1) was 250 feet deep, and 

it seems unlikely that any stress removal would extend to this depth. It is 

significant that the crushed zone (which has been created by stress effects) 

is no thicker in older regions (table 1). 

flow 6~hown in table 2 as 

A 

[pressure x depth]' Or [B x depth]' 

?Since other factors affect permeability, th.ere appear to be some exceptions, 

which will be discussed later. 
Coalbed 

Pittsburgh ....... 

Hartshorne.. . . . . . 

Pocahontas No. 3. Pocahontas No. 3. 

Mine 

Federal No. 2

Howe No. 1.. . 

Beatrice .....

Kepler .......
FIGURE 8. - Federal No. 2 Mine, Steady-State Flow From 6-Foot Increments of Open Hole. 

5000 

> 

0 

u 

\ 

'% 

.e i 4000 

0 

-1 

LL 

3000 

W 

5 

Iv, 

, 2000- 

> 

0 

a 

W 

,000- 

0, 

POSSIBLE REASONS FOR PERMEABILITY INCREASE 

- - 

0 

- 

- - 0 

- - 

- - 

- - 

- 

- 

- - 

- 

- - 

I 1 I I I I I I 1 

0 10 20 30 40 50 60 70 80 90 100 

Present evidence suggests that the increase in permeability is due to a 

loss of water from the coalbed rather than a reduction in stress. Drill crews 

frequently have noticed that holes drilled into freshly mined faces and ribs 

emit much more water than those drilled in faces or ribs mined months earlier. 

From natural gas reservoir studies (k), it is known that the amount of water 

in the reservoir rock strongly affects the permeability to gas; more water 

results in less gas permeability. If a freshly mined area of a coalbed 

contains water, its permeability to gas is relatively low. However, as the 

water and gas drain into the mine, the water content of the coalbed is reduced, 

and the gas permeability increases. 

DEPTH OF HOLE, ft 

Aside from casual observation, the only firm evidence for this as yet is 

from borehole waterflows. Some borehole gas and waterflow data were obtained 

by the Bureau of Mines during a recent experiment at the Beatrice mine. In 

an attempt to drain methane from the coalbed ahead of the working face, 

several holes were drilled ahead of the face. One of these emitted 7,260cubic 

feet of gas per day and 3 gallons of water per hour (per 100 feet of hole) on 

the second, third, and fourth days after drilling. However, on the sixth day, 

the waterflow dropped to 2 gallons per hour and the gas flow increased to 

7,960 cubic feet per day. This was in a freshly mined area. Later, in the 

same mine, a 100-foot hole was drilled into an area mined 30 days earlier 

(shown as Beatrice hole 2 in tables 1 and 2). The waterflow was 0.2 gallon 
per hour, but the gas flow was about the same (7,500 cubic feet per day, 

table 2) as from the hole in the freshly mined area, even though the pressure 

was about half that of the freshly mined area. 

More data on waterflows fromboreholes are required to establish how much 

variation in permeability is caused by loss of water. It is likely that the 

total quantity of water involved is much less than that of a natural gas 

reservoir; the fracture porosity of coal has been estimated at less than 

1 percent. Most, if not all, of the water reaching the face evaporates as 

soon as it exudes from the coalbed; this may be why only one of the three 

mines investigated had puddles of seepage water on the mine floor. 

Another factor contributing to the permeability increase may be coal 

shrinkage. This was first suggested by Briggs (1). Moffat and Weale (10) - 

found that coal expands as methane is adsorbed and shrinks as it is desorbed. 

If the pressure is reduced from 40 atmospheres to zero, the bulk shrinkage is 

about 0.1 percent. Rarely does the pressure in coalbeds fall quite this 

much, but even with lower pressures such an effect may be great enough to 

open up cracks in the coalbed. 

It has been suggested (13) that the adjacent strata will have an effect 

on the pressure gradient; thenotion being that the gas from the coalbed 

may leak through the adjacent strata to the mine, thus short-circuiting the 

coalbed. The roof and floor in a mine will converge with time; this is likely 

to open cracks in the roof and floor strata. Given the way the analysis was 

done in this report, such an opening of cracks would be interpreted as a 

permeability increase. However, results from the horizontal boreholes 

indicate the permeability increase is real; there is no other way to explain 

the high flow rates from low-pressure holes in older areas. 

PERMEABILITY OF DIFFERENT COALBEDS AND EFFECT OF OVERBURDEN 

If the time effect is taken into account, it is apparent from table 1 

that the Pittsburgh coalbed is much more permeable than either the Hartshorze 

or Pocahontas No. 3 coalbeds. This difference in permeability also is 

reflected in the gas-emission rates from the horizontal holes (table 2). 

The gas-emission rate, corrected for differences in pressure and depth 

flow ] , is much higher for the Pittsburgh coalbed than for 

[pressure x depth 

the others. 

The permeability of coal measured in the laboratory greatly depends on 

the degree to which the sample is compressed (c), thus an inverse relationship might be expected between coalbed permeability and overburden depth. 

No such relationship is indicated in table 1. The deepest coalbed, Pocahontas 

No. 3, has a low permeability, which is expected. However, the permeability 

of the Hartshorne coalbed, which has the least cover, is just as low. Moreover, holes 10 and 11 in the Howe No. 1 mine were drilled in an area of the 

mine where the cover was 700 feet thick. No obvious variation in permeability 

exists between these holes and others in the same mine that were drilled where 

the overburden was only 285 feet. The only evidence for an overburden effect 
in table 1 is the 0.56 millidarcy permeability given for Kepler mine hole 1 

which may be compared with 0.32 for Beatrice mine hole 1. The difference in 

overburden is substantial (680 feet compared with 2,000 feet), but these 

represent relatively high overburden pressures. Patching (12) - has shown that 

by far the greatest permeability changes ti2ke place below compressive stresses 

of 500 psi. 

The nature of the coal may affect permeability more than overburden 

depth. Both the Pocahontas No. 3 and Hartshorne coals are friable, lowvolatile, highly fractured coals, whereas the Pittsburgh coal is hard and 

blocky with far fewer (but more prominent) fractures. Still another factor 

may be the amount of water in the coalbed. The only conclusion to be drawn 

is that permeability appears to be affected by the nature of the coal and 

other factors more than by overburden depth. 

ANISOTROPY IN PEliMEABILITY 

Permeability can be expected to have directional anisotropy in coalbeds 

that have pronounced face cleat and butt cleat characteristics. The permeability should be greatest in the direction of the surface of the face, or 

major, cleat. The face cleat is a continuous joint surface that crosses 

bedding planes in the coal and extends for many feet. On the other hand, 

the butt cleat is a short, discontinuous joint surface that often terminates 

against a face cleat. The surface of the butt cleat often is curved, and the 

butt cleat occurs less frequently than the face cleat. It follows that 

regions of the coalbed drained primarily by face cleats will be drained of 

gas more easily and will, therefore, exhibit lower pressures than those 

regions drained by butt cleats (given equal drainage times). The permeability values listed in table 1 are simply calculated using the hole pressure 

(the pressure curve); the lower the hole pressure, the higher the calculated 

permeability. 

Of the three coalbeds listed in tables 1 and 2, the Pittsburgh bed is 

the only one with pronounced face cleat anti butt cleat development. The 

face, or major, cleat direction is roughly east-west, whereas the butt cleats 

trend8 north-south. Hole 3 was the only hole drilled parallel to the face 

cleat direction into a region drained by the face cleats, and should give 

the greatest permeability (fig. 9 and table 1). The effect is obscured 

somewhat by the apparent increase of permeability with time, but it is 

clear that the relatively high value of 82.5 millidarcys listed for hole 3 

is not the result of drilling into an old region. 

The gas flow data from Federal mine holes 3 and 4 give additional 

evidence for a permeability anisotropy. Hole 4 was drilled parallel to the 

butt cleat direction into a region drained by butt cleats; therefore, the 

permeability calculated from the pressure curve (33 millidarcys) is lower 

B~arsons (2) has measured the lateral compressive stresses in strata adjacent 

to the Pittsburgh coalbed and has found that the major lateral stress is 

in an east-west direction. Thus the but:t cleats are squeezed more 

tightly, and the permeability difference is even more enhanced. 
FIGURE 9. - A Mine Entry in a Coalbed With Pronounced Face Cleat and Butt Cleat 

Development. 

ENTRY 

than that of hole 3 (82.5 millidarcys)(fig. 9). Despite this lower permeability, the flow of gas corrected for differences in pressure and depth, 

flow rate 

that is [pressure x depth 1, is over twice that of hole 3 (table 2). The 

COALBED I 
reason for this unusual effect is that the hiole pressures are controlled by 

the permeability parallel to the hole, and therefore the 33 millidarcys from 

hole 4 is a butt cleat permeability. However, the factor governing the 

borehole flow rate is the permeability of th~e cracks the hole intersects, and 

not the permeability parallel to the hole. Hole 4 intersects the face cleats, 

whereas hole 3 intersects butt cleats. It follows that hole 4 intersects more 

permeable cracks than does hole 3, and that the flow from hole 4 should be 

greater than from hole 3 over and above what: would be expected from differences 

in pressure and depth. Table 2 confirms this. 

If holes give different flow rates for different directions, the same 

might be expected for the mine entries. The Bureau of Mines recently did a 

careful methane survey in conjunction with water-infusion experiment in the 

No. 2 west mains of the Federal No. 2 mine. It was found that in this location the methane emission from the solid rib was far greater than the emission 

from the working face (the section was idle at the time, so there was no contribution from the advance of the mining machine (L)). This difference in 

emission rates was not surprising, for the rib intersected the face cleats 

(as in hole 4), and the face was draining the less permeable butt cleats. 

The same effect also took place elsewhere in the mine, with face cleats 

yielding more gas than the butt cleats (other things being equal). It has 

been implicitly assumed that the rib lines are perfectly parallel with the 

cleats. This is true in the Federal No. 2 mine. However in many other 

mines in the Pittsburgh coalbed the rib line and cleat vary by a few degrees. 

OTHER VARIATIONS IN ITRMEABILITY 

It has been suggested (g-2) that in lorigwall faces a tight abutment zone 

of abnormally low permeability exists between the crushed zone of high permeability and the intact coalbed. If such an abutment zone existed in mines 

worked by the advancing room-and-pillar method, a region of the pressure 

curves shown in figures 1-7 would be steeper than that expected from theory. 

No distinct evidence of such a steep region can be seen in most of these 

figures, nor in those publications cited previously (2, 1). There is a 

possibility that this zone is very narrow and easy to miss; however, at least 

one hole (Federal mine hole 4, fig. 2) was tested with the pressure points 

close together so that variations over a small region would not be missed. 

Flow measurements were taken for each pressure point in the same hole 

(fig. 8). The flow curve closely parallels the pressure curve shown in 

figure 2, which indicates that no sharp variations in permeability with 

distance exist beyond the crushed zone in this case. 

The possibility of an abutment zone in room-and-pillar workings cannot 

be ruled out completely. Theory based on constant permeability beyond the 

crushed zone would indicate that for inactive areas, the pressure curve should 

not level off in the first 100 feet. For instance, in Federal mine holes 4 

and 5 (fig. 2) the pressure is still climbing at the last measured point. 

However, in hole 1 (fig. 1) the pressure appears to approach an asymptote, 

which might be caused by a region of low permeability between 20 and 40 feet. 

Hole 10 in figure 6 shows the same effect. 
The in-situ permeability of three coalbeds has been calculated from field 

measurements. The findings are as follows : 

1. The nature of the coal seems to have a more important effect than 

overburden pressure on permeability. 

2. Where face and butt cleats are prominent, the coalbed is more 

permeable in the direction of the surface of the face cleat. 

3. A high-permeability crushed zone along ribs and faces appears to be 

a common feature of all mines. 

9This pressure was measured in conjunction with the BuMines program for 

coalbed degasification by vertical wells. 
4. Beyond the crushed zone there is no evidence for an abutment zone 

of low permeability. 

5. The effect of in-situ water on the permeability to gas is very great. 

Older regions of a mine from which the water. has drained have vastly increased 

permeabilities. The practical effect of this is to maintain a high borehole 

flow. 

6. The permeabilities given in table 1 are for coalbeds that have 

already been affected by the mining process. Virgin coalbed permeabilities 

are probably much lower. 

The analysis presented here is preliminary. Its objective is to 

delineate the major factors that control coalbed gas emission. A better 

theoretical treatment must await much more c:omprehensive field data. 
Patching, T. H. Variations in Permeability of Coal. Proc. 3d Rock 

Mech. Symp., Univ. of Toronto, Jan. 15-16, 1965, Mines Branch, 

Ottawa, Canada, 1965, 204 pp. 

Selden, R. F. The Occurrence of Gases in Coals. BuMines Rept. of 

Inv. 3233, June 1934, 64 pp. 

Stewart, Ian McC. Diffusional Analysis of Seam Gas Emission in 

Coal Mines. Canadian Min. and Met. Bull., April 1971, pp. 62-70. 
of pressure. It appears then that there is more gas in the Pittsburgh coal 

than the pressure in the fractures would indicate. This would not affect the 

computed coalbed permeability because permeability is defined in terms of the 

pressure curve and the observed emission rate. However, it would cause the 

sorption capacity to be lower than that expected from laboratory sorption 

experiments, which are done only under equilibrium conditions. 

Another way of stating the same thing would be to say the Pocahontas 

No. 3 coalbed has a good internal equilibrium; that is, deep in the coalbed 

. the amountofadsorbed gas is in equilibrium with the gas pressure in the 

cracks. It follows that the computed sorption capacity is close to values 

obtained from equilibrium laboratory measurements. On the other hand, the 

Pittsburgh coalbed does not have good internal equilibrium. Some gas is 

retained in the coal as the fracture pressure falls, and the result is a 

lowered sorption capacity. 
3. Another assumption is that the sorption capacity; that is, the 

[ per is constant. A constant sorption capacity volume of coal I 

would mean the equilibrium adsorption isotherm is linear, when in fact it is 

known the methane-coal isotherm is not linear, but follows the Langrnuir 

equation. Dry Pocahontas No. 3 coal adsorbs 10 cubic centimeters per gram of 

methane at 10 atmospheres pressure. (All volumes are given at standard 

temperature and pressure.) If a coal density of 1.3 grams per cubic centimeter is assumed, this corresponds to a sorption capacity of 1.3. At 

20 atmospheres, 13 cubic centimeters per gram is adsorbed, and this corresponds to a sorption capacity of 0.84. The Hartshorne coal is similar. 

Just a few percent moisture in coal considerably decreases the amount 

adsorbed, therefore it is not surprising that the in-situ sorption capacity 

for Pocahontas No. 3 coal should be 0.56. In any case, an increase in the 

sorption capacity, S, as the pressure falls is not likely to explain why the 

quantity K/S rises with time. 

4. A more exact solution to the Darcy equation for unsteady-state gas 

flow is available (6, p. 421), and, if it is used the calculated permeabilities are somewhat Tower. However, the major conclusions about the crushed 

zone, directional anisotropy, permeability increase, and so forth, are all 

unchanged.