By David C. Oyler and Paul B. Stubbs 

m 

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

Donald Paul Hodel, Secretary 

BUREAU OF MINES 

Robert C. Horton, Director 
Oylcr. l)a\id (' 

ilcusuring form;lrion ;>ri.ssirres an<{ tllc dcprcr of .<.I.; .lrnirr.~gc in n 

large coalhed g.1.; clrnirlngc fic:l<i. 

(Hureau oI \lines report of inv(.stig:~tion\ : 8980) 

13ibliography: p. 15. 

Supt. of Ilocs. no.: 1 28.2): 8986. 

1. Co.rlbrd methane drainage. 2. (.oalbcci mcthane-Alabama-Oak 

(;rove Region. I. Stubbs, I'aul 13. 11. Title. 111. Series: Kcport of inrestigarions (United States. flureau of \lines) ; 8986. 

TN23.b43 [TN844.6] 622s [622'.334] 85-600 172 
CONTENTS 

Page 

Abstract ...................................................................... Introduction .................................................................. Monitor hole drilling and completion .......................................... Drilling .................................................................... Completion .................................................................. Pressure sensor system ........................................................ Sensor unit ................................................................. Surface unit ................................................................ Pressure sensor installation and wellhead equipment ......................... Results ....................................................................... Gas content ................................................................. Pressure data ............................................................... Isotherm curve .............................................................. Conclusions ................................................................... References .................................................................... 

ILLUSTRATIONS 

1 . Map of monitor hole and production well locations ......................... 2. Typical bottom hole assembly of monitor holes ............................. 3 . Geophysical logs of monitor holes showing jet slotting cavities ........... 4 . Sentry unit being installed in monitor hole M3 ............................ 5 . Wellhead assembly ......................................................... 6 . Pattern area and estimated area of gas drainage ........................... 7 . Pressure decline curves for monitor holes M1. M2. and M3 .................. 8 . Isotherm curve derived from field gas content and pressure data ........... 

TABLES 

1 . Gas content determination for monitor holes ............................... 2 . Gas content values and formation pressures measured at time of gas 

content determination .................................................. 3 . Estimated coalbed gas content reduction at monitor holes .................. 4 . Error in direct method gas content measurement caused by change in 

fracture system gas saturation ........................................... 
atm 

cm3 

cm3 /g 

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

atmosphere lb pound (mass) 

cubic centimeter lb/f t pound per foot 

cubic centimeter 

per gram 

degree Fahrenheit 

foot 

cubic foot 

cubic foot per 

short ton 

gram 

gram per cubic 

centimeter 

lb/f t3 pound per cubic 

foot 

lbf /in2 (ga) pound (force) per 

square inch, gauge 

lbf/in2 per pound (force) per 

month square inch per 

month 

~~stdf t3 million standard 

cubic feet 

~~stdf t3 per million standard 

month cubic feet per 

month 

gallon Pet percent 

hour 

inch 

volt, alternating 

current 
MEASURING FORMATION PRESSURES AND THE DEGREE OF GAS DRAINAGE 

IN A LARGE COALBED GAS DRAINAGE FIELD 

By David C. Oylerl and Paul B. stubbs2 

ABSTRACT 

The Bureau of Mines and United States Steel Corp. are conducting a 

joint project to monitor formation pressures at a large (23-well) coalbed gas drainage field near Oak Grove, AL. Three monitor holes were 

drilled in late 1981, and pressure monitoring began in December 1981. 

The Bureau of Mines direct method was used to obtain gas content data 

from cores taken in the monitor holes. Comparison of the 1981 gas content data from the monitor holes with initial gas content values obtained from the production wells in 1977 indicates a 50-pct reduction in adsorbed gas content inside the pattern and a 29-pct reduction at one 

point 500 ft outside the pattern. The combination of pressure and gas 

content data has also allowed an in situ isotherm curve, relating the 

formation pressure to adsorbed gas content, to be developed for the area 

of the production wells. From the current rate of pressure decline 

(1 lbf/inz per month), the observed changes in gas content, and the isotherm curve, it appears that most of the early gas production came from 

within the pattern, but since 1981 (and possibly earlier) most of the 

gas produced has been migrating to the wells from outside the pattern 

area. 

'~echanical engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 

2~ssociate research consultant, Technical Center, United States Steel Corp., Monroeville, PA (now independent consultant, Unconventional Energy Group, Export, PA). 
INTRODUCTION 

In the late 1970's the use of hydraulically stimulated vertical wells to 

drain methane gas from coalbeds developed 

from an experimental technique to a technology which is now beginning to be used 

by both the coal mining and the natural gas industries. Work at the United 

States Steel (USS) Oak Grove Mine in 

Jefferson County, AL (1-2) -- ,3 Emerald Mine 

(3) in Greene County, PA, and at other 

locat ions, especially in the Western 

States (4), has shown that vertical boreholes caz be drilled to remove methane 

from coalbeds, both to decrease methane 

emissions in mines and to produce methane 

as a marketable natural gas. Several 

projects have recently been initiated to 

drill vertical hole patterns with both 

purposes in mind, 

Both reservoir and ventilation engineers need to determine the conditions 

existing in the coalbed reservoir, so 

that the progress of depletion of the 

reservoir can be monitored, and so that 

intelligent decisions concerning continuing gas production and mining can be 

made. Unfortunately, geophysical logging 

techniques and many of the physical Sampling methods developed in the oilfield 

to determine gas reserves are generally 

not applicable to coal. However, a technique has been developed that is applicable to coal and replaces techniques used 

in the oilfield, This method uses a measurement of the original in-place gas 

content obtained from the Bureau of Mines 

direct method test (5, - - 7) together with 

coal adsorption isotherm (8-10) -- data, 

either laboratory or field, to relate 

pressures in the coalbed fracture system 

to gas content. The method assumes that 

the pressure and gas content are at equilibrium; this is not completely true in a 

producing reservoir, but the error from 

this assumption is relatively small. 

Several methods are available to determine the fracture system pressure, which 

can then be related to the adsorption 

isotherms to give estimates of the absolute in-place gas content of the 

coalbed. 

In the late 19701s, the Bureau of Mines 

and USS entered into a cost-sharing 

agreement to test the technique of coalbed degasification from vertical boreholes using a large pattern of wells, In 

late 1977, 17 wells of a planned 25-well 

pattern at the Oak Grove Mine in Jefferson County, AL, were completed, By early 

1979 the average daily gas production for 

the pattern had exceeded 1 MMstdft3, demonstrating the effectiveness of the improved techniques developed during the 

project, Six more wells were drilled in 

the summer of 1980, bringing the total to 

23 wells. Wells 10 and 20 were never 

drilled. 

In late 1981, the Bureau and USS began 

a project to drill three holes in the 

vicinity of the Oak Grove pattern to monitor pressure changes in the coalbed 

caused by gas production, and to obtain 

cores for desorption, so that the cumulative effects of gas production could be 

determined. The holes were completed in 

the fall of 1981, and pressure monitoring 

began in December 1981. 

MONITOR HOLE DRILLING AND COMPLETION 

Figure 1 shows the location of the 

pressure monitor holes with respect to 

the production wells, Holes, M1 and M2 

are within the area of the degasification 

well pattern, and M3 is 500 ft due west 

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

the end of this report. 

of the northwesternmost well in the 

pattern, 

DRILLING 

The holes were first drilled at a 10- 

in diameter to depths between 50 and 60 

ft, where 7-in-OD surface casing was set 

and cemented in place. The holes were 

then rotary-air-drilled at a 6-114-in 
FIGURE 1. - Map of monitor hole and production well locations. 

diameter to a point just below the upper 

bench of the Mary Lee coalbed (figs. 2-3) 

at about 1,060 to 1,080 ft, where 4- 

1/2-in-OD, 10.5-lb/ft casing was placed 

in each hole. The casing was then cemented in place using class A cement with 

18 pct salt. 

After the holes were cemented, a coring 

rig was used to core out the float shoe 

and bottomhole cement plug and to core to 

approximately 30 ft below the coal seam. 

The coal core was logged, photographed, 

and placed in air-tight canisters, so 

that the Bureau of Mines direct method 

test could be performed on all of the 

coal recovered from each hole. 

COMPLETION 

A geophysical log suite consisting of a 

gamma-ray, density, and caliper log was 

run in each hole after coring. The logs 

were run from total depth to surface, although only the gamma-ray log is useful 

in the cased portion of the hole. Gyroscopic directional surveys were then run 

on each hole to determine the exact bottomhole locations in the coalbed. 

To ensure that the pressure instruments 

would see the true formation pressure and 

all changes in it, a notch was cut in the 

coalbed in each hole. The notch was designed to remove any coal that might have 
had its permeability reduced by cement or 

drilling fluids. The notch was cut by 

running a tool containing three small 

(about 3/16-in-diam) ports on the bottom 

of the tubing to the depth of the coalbed. A slurry of sand and water was then 

pumped down the tubing and through the 

ports, at pressures of 2,500 to 3,000 

lbf/in2(ga), to create a high-velocity 

jet which abraded away the first 6 to 12 

in of the coal from the wall of the hole. 

Since some sand had been used in the 

notching process, it was also necessary 

to flush out the holes with water after 

notching to remove the sand. After the 

notching had been completed, an additional gamma-ray caliper log was run in holes 

M1 and M3 to determine the size and location of the notches, and to ensure that 

the packers used later were not set in 

the notches. This log was not run in 

hole M2 until after the packer had been 

set in the notch. A reproduction of the 

geophysical logs (including the density 

log run earlier) is shown in figure 3.' 

Col~per logs Before ]el slott~n 

After jet slotl~nq 

1,070 

</ 7- - - I 

r -- / 

Gmma ray log 

1,075 41210864 2 0 

CALIPER, in 

HOLE M3 

-Cas~ng bottom 

+~- .. - -~ 

Jet slot cov~ty 

FIGURE 3. - Geophysical logs of monitor holes 

showing jet slotting cavities. 

After notching, the tubing was removed 

from the holes and rerun with two Lynes 

production-injection packers4 on bottom 

to straddle and isolate the coalbed. The 

use of the packers was primarily a 

4~eference to specific products does 

not imply endorsement by the Bureau of 

Mines. 
precaution, since the formations exposed 

in the borehole above and below the coalbed were impermeable shales and fire 

clays. The packers were set simultaneously by inflating them with water injected through the tubing. A hand pump 

was used to establish a pressure of between 1,100 and 1,500 lbf/inZ(ga) within 

the tubing and the packers, and a check 

valve in each packer prevented them from 

deflating when the tubing pressure was 

bled down. (The packers are deflated by 

rotating and pulling on the tubing.) 

After the tubing pressure had been relieved, a 1-in steel bar was run into the 

hole on wire rope (the workover rig sandline) to shear a pin in a Lynes model B 

circulating sleeve (fig. 2) between the 

packers. Shearing this pin opened ports 

in the sleeve and allowed communication 

between the coalbed and the tubing. 

The first packers installed (in M2) 

were inflated to 1,400 ~bf/in~(~a), at 

which time the lower packer ruptured. A 

caliper, gamma-ray log, run subsequently, 

indicated that the lower packer had been 

set in the notched cavity. The packers 

were removed from the hole, a replacement 

was obtained for the one that had ruptured, and they were then run back in the 

hole and successfully set. Had the lower 

packer been set in a portion of hole that 

was in-gauge or to a pressure below 

1,400 lbf/in2(ga), no failure would have 

taken place. The incident did indicate 

the toughness of the packers. When 

packers were to be set in holes M1 and 

M3, caliper gamma-ray logs were run to 

check the exact location of the notched 

cavity. 

As soon as the sleeve was opened, the 

water in the tubing began to enter the 

coalbed, and within 48 h the hydrostatic 

pressure in the tubing was within 5 

~bf/in~(~a) of the formation pressure. 

PRESSURE SENSOR SYSTEM 

SENSOR UNIT 

Pressure measurements were made using a 

Lynes Sentry pressure sensor installed in 

each hole. This tool uses a helicalshaped bourdon tube, connected by a shaft 

to a sapphire disc covered with a pattern 

of small transparent and opaque areas. 

On one side of the disk is a row of 

light-emitting diodes, and on the other 

is a row of photosensitive diodes. Rotation of the disk caused by a change in 

pressure produces a change in the pattern 

of signals seen in the photosensitive diodes. Each pattern represents a different position of the bourdon tube and a 

different pressure. Because the disk is 

divided into 500 individual patterns, the 

sensitivity of the tool is limited to 

11500, or 0.2 pct of the full-scale reading. For the 0-500 ~bf/in~(~a) sensors 

used in holes Ml-M3, this means that 

pressures can only be discriminated to 

within 41 lbfIin2. The pressure signal 

and a temperature reading are sent uphole through a 7132-in-diameter armored 

single-conductor cable, and a computer at 

the surface converts the signal to a 

pressure reading. Power for the tool is 

supplied by the surface unit through the 

signal cable. The Sentry tool is 1.5 in 

in OD by 37.75 in long (fig. 4) and 

weighs about 15 lb. 

SURFACE UNIT 

The surface unit is a Lynes model DSK-2 

PIT digital surface recorder (DSR). The 

unit can handle signals from up to 15 

holes. At Oak Grove, a single centrally 

located DSR is being used, with signals 

transmitted from each hole by coaxial 

cable. The pressure data are recorded 

every 8 h and printed on a paper tape. 

Each reading includes the well number, a 

pressure reading, a temperature reading 

(the last two printed twice), and a time 

interval. The time interval is printed 

in place of a time and date, and to convert it to an exact time and date both 

the time of the initial reading and the 

interval length must be known. The unit 

does not contain batteries and can only 

operate on 120-V-ac power. This lack of 

a battery backup can be a handicap in the 

event of a power failure. 
FIGURE 4. - Sentry unit being installed in monitor hole M3. 

To obtain true pressure values, a correction must be applied to the DSR readings, since the sensors are actually located several feet above the coalbed 

(fig. 2). The vertical distance from the 

sensor to the center of the coalbed (used 

as the reference depth for all pressure 

calculations) ranges from 10.2 to 12.4 

ft. The additional pressure added to the 

DSR readings is 5 lbf/in2 in holes M1 and 

M3 and 6 lbf/in2 in hole M2. 

PRESSURE SENSOR INSTALLATION AND 

WELLHEAD EQUIPMENT 

The sensors in the monitor holes were 

run through the tubing and suspended in 

FIGURE 5. - Well head assembly. 

the hole (fig. 2) from a small steel 

clamp (fig. 5) tightened around the 

cable. The cable was run in the holes by 

gravity. 

The remainder of the wellhead assembly 

(fig. 5) consists of a 4-112-in tubing 

head with slips to hold 2-in tubing, a 

tee above the tubing bushed down for installation of a 318-in valve and a pressure gauge, a packing gland to clamp 

around the cable and seal off the tubing, 

and a clamp to hold the weight of the 

cable and sensor. The clamp rests upon 

the top of the packing gland, which carries the weight of the cable and the tool 

(about 120 lb). 
RESULTS 

GAS CONTENT 

Table 1 shows the gas content data for 

the cores obtained in monitor holes M1, 

M2, and M3. The core from each hole was 

divided into four samples, each of which 

was placed in a separate desorption canister. A gas content measurement was 

made on each sample using the standard 

Bureau of Mines direct method. Because 

the samples were split, with portions being sent for petrographic analysis, only 

about one-third to one-fourth of each 

sample was available for use in determining the residual gas content. The residual gas content is that portion of the 

gas which (at 1 atm pressure) is not 

given off until the coal is crushed to a 

fine powder. In this case, the residual 

gas content was determined for the smaller portion, and it was assumed that the 

value thus obtained was representative of 

the entire sample. To better ensure that 

this was the case, the original cores had 

been crushed to 118-in diameter after the 

desorption process was completed, and the 

TABLE 1. - Gas content determination for monitor holes 

Monitor 

hole 

and 

s ample 

M1: 

D...... 

C...... 

A...... 

B...... 

Total 

or av 

M2: 

B...... 

C...... 

A...... 

D...... 

Total 

or av 

M3 : 

A...... 

B...... 

C...... 

Dm..... 

Total 

or av 

NAp Not 

 he residual gas was measured for only this portion of the entire sample. 

 his gas came from crushing the part of the entire sample indicated in the column 

labeled "Residual gas portion." 

3~he sum of the lost and desorbed gas content and the residual gas content. 

4~omputed from totals. 

Sample 

interval, 

f t 

1,077.00-1,078.47 

1,078.47-1,079.94 

1,079.94-1,081.67 

1,081.67-1,082.60 

NAp .............. 

1,086.20-1,087.86 

1,087.86-1,089.74 

1,089.74-1,091.50 

1,091.50-1,093.20 

NAp .............. 

1,066.60-1,069.20 

1,069.20-1,070.11 

1,070.11-1,071.61 

1,071.61-1,071.50 

NAp.. ............ applicable. 

Es timated 

in 

situ 

gas 

content, 

ft3/st 

263 

256 

240 

211 

4244 

221 

208 

221 

205 

4211 

327 

356 

356 

378 

4350 

Laboratory 

Coal sample 

dry 

Entire 

sample 

1,204 

1,197 

1,309 

839 

4,549 

1,564 

1,753 

1,575 

1,493 

6,385 

1,312 

696 

1,038 

674 

3,720 

weight, g 

Residual 

gas 

portion' 

382 

462 

446 

378 

1,668 

561 

857 

395 

564 

2,377 

449 

312 

321 

292 

1,374 

gas content 

Gas volume, 

determination 

Lost 

and 

desorbed 

9,237 

9,137 

8,790 

4,934 

32,098 

10,167 

10,736 

9,530 

8,618 

39,051 

- 

13,136 

7,215 

10,796 

7,473 

38,620 

Gas 

Lost 

and 

desorbed 

7.7 

7.6 

6.7 

5.9 

47.0 

6.5 

6.1 

6.0 

5.8 

46.1 

10.0 

10.4 

10.4 

11.1 

410.4 

cm3 

Residua12 

185 

190 

360 

280 

1,015 

200 

350 

340 

350 

1,240 

-- 

105 

230 

220 

200 

755 

content, 

Residual 

0.5 

.4 

.8 

.7 

4.6 

.4 

.4 

.9 

.6 

4.5 

-2 

. 7 

.7 

.7 

.5 

cm3Ig 

Tota13 

8.2 

8.0 

7.5 

6.6 

47.6 

6.9 

6.5 

6.9 

6.4 

46.6 

10.2 

11.1 

11.1 

11.8 

410.9 
samples riffle-split to obtain the portion used for the residual gas determination. The residual gas content for each 

sample was computed on a cm3Ig basis by 

dividing the residual gas volume (cm3) by 

the weight of each residual gas sample 

(g). The lost-and-desorbed gas content 

was similarly determined for each sample 

by dividing the lost-and-desorbed gas 

volume (cm3) for each sample by the entire sample weight. The sum of the residual and lost-and-desorbed gas contents 

for a single sample (both in cm3Ig) gave 

the total gas content of that sample. 

To obtain the gas content of the entire 

coalbed in each hole, all four sample 

weights and gas volumes (both lostand-desorbed and residual) for each hole 

were summed to give the total weight of 

the entire sample in that hole, the total 

weight of the residual gas sample for the 

hole, the total 10s t-and-desorbed gas 

volume for the hole, and the total residual gas volume for the hole. The same 

methods used earlier in determining the 

gas contents of individual samples were 

then used to determine the lost-anddesorbed and residual gas contents for 

the entire coalbed at each hole (in 

cm3Ig). These two values were then added 

to give the total gas content of -the 

coalbed at each hole (table 2). The 

total gas contents of holes M1, M2, and 

M3 were, respectively, 7.6, 6.6, and 10.9 

cm3Ig (244, 211, and 350 ft3/st). 

To determine the reduction in gas content caused by degasification, initial 

gas content values for the Oak Grove pattern were required. The original drilling program in 1977 included coring and 

direct method gas content determinations 

for six of the wells. Table 2 shows the 

resulting gas content values and the initial formation pressures determined for 

those wells. The pressures were obtained 

from stabilized water levels measured in 

the wells in 1977 prior to the start of 

gas production. 

The arithmetic mean of the gas content 

values in the wells adjacent to each 

pressure monitor hole have been used to 

give estimates of 14.5, 14.9, and 15.3 

cm3Ig (465, 477, and 490 ft3/st) for the 

original gas contents at each of the monitor hole locations. The gas content 

values obtained from the monitor holes 

were 7.6, 6.6, and 10.9 crn3lg (244, 211, 

and 349 f t3/st) from holes Ml , M2, and 

M3, respectively. The data indicate that 

over a 4-year period of degasification 

the reductions in adsorbed gas content at 

the monitor hole sites were 48, 56, and 

29 pct (table 3), or roughly a 50-pct decrease in gas content within the pattern 

area and a 25- to 30-pct decrease as far 

as 500 ft away from the outside boundary 

of the pat tern. 

TABLE 2. - Gas content values and 

formation pressures measured at 

time of gas content deterwination 

I Gas content 

values 

Sample 

location I tory I mated 

I mea- lin situ, 

Formation 

pressure, 

lbf/in2 (ga) 

1 sured, I ft3/st I 

desorbed 

core l . . . . . I 0.0 1 

Completely 

cm3 /g 

244 

211 

350 

490 

439 

490 

465 

509 

490 

ressure. 

Ml......... 

M2......... 

M3......... 

Well 6..... 

Well 7..... 

Well B..... 

Well 14... . Well 15.... 

Well 25.... 

TABLE 3. - Estimated coalbed gas content reduction at monitor holes1 

7.6 

6.6 

10.9 

15.3 

13.7 

15.3 

14.5 

15.9 

15.3 

'~t 1 atm absolute p 

l~as production from 10177 through io/8i. 

The gas content reductions observed 

could be caused by uncertainty in the gas 

content measurements. There are two potential sources of uncertainty. The 

first is in the direct method itself, and 

M3 

15.3 

10.9 

4.4 

29 

Gas content, cm3/g: 

Initial 1977 estimate.. 

Measured in 1981....... 

Reduction (10177-10/81) 

Gas content reduction 

pct.. 

M1 

14.5 

7.6 

6.9 

48 

M2 

14.9 

6.6 

8.3 

56 
the second is possible variation in the 

actual gas content of the coalbed from 

point to point. To date no estimates 

have been published of the degree of error to be expected in a direct method measurement, so an estimate of the direct 

method uncertainty has not been attempted 

here. However, the 1977 and 1981 direct 

method measurements were made in the same 

manner and should have the same degree of 

uncertainty. Also, since the two sets of 

samples were taken from the same area of 

the same coalbed, the variations to be 

expected in gas content measurements due 

to actual variations in gas content 

should be similar. It is then anticipated that if a measure of the uncertainty of the direct method measurements 

could be found for either set of samples, 

it could be applied to both the 1981 and 

the 1977 gas contents. Fortunately, the 

1977 gas content values represent the 

coalbed at one condition, that is, before 

any degasification had taken place. This 

sample of six gas content values may then 

be used to obtain a statistical estimate 

of the variation of the Oak Grove gas 

content measurements. The mean gas content in the six holes is 15.0 cm3Ig with 

a standard deviation of 0.777 cm3Ig, indicating that the true original gas content of the coalbed at Oak Grove has a 

95-pct probability of being between 16.6 

and 13.4 cm3/g (within two standard deviations of 15.0 cm3Ig). If the same standard deviation is applied to hole M3, 

then the gas content at that location in 

1981 has a 95-pct probability of being 

between 12.5 and 9.3 cm3Ig. This analysis shows that there is a 95-pct probability that the gas content recorded in 

hole M3 represents a reduction in the gas 

content of the coalbed at that location 

of between 7 and 44 pct. Similar analyses can be made for holes M1 and M2. 

It has been assumed here that the adsorbed gas content values above represent 

the total gas content of the coalbed. 

However, this need not be the case. It 

is possible for gas to exist as free gas 

in the fracture system of the coalbed. 

Most of this gas would be lost during any 

coring operation and would not be measured, giving an erroneously low coalbed 

gas content figure. This problem exists 

both for the initial measurements and for 

those made in the monitor holes. However, it is potentially more serious in 

the case of the monitor holes, since it 

is possible that a larger fraction of the 

desorbed gas could be held in the fracture system, replacing the water originally present in the formation. 

To obtain an estimate of the error in 

the gas content measurement that could be 

caused by replacement of formation water 

by gas, assume that all of the water in 

the fracture porosity of a particular 

volume of coal is displaced by gas in the 

course of water and gas production. Also 

assume a fracture porosity of 5 pct (a 

value chosen to be as high as or higher 

than common estimates of coal fracture 

porosities) and a coalbed bulk density of 

1.32 g/cm3, a figure obtained from density logs run in the monitor holes. In 

table 4, the maximum fracture system gas 

storage has been calculated for the vicinities of holes M1, M2, and M3. The 

calculations indicate that the total error from this cause would be no higher 

than 8 to 11 pct. The error is sensitive 

to changes in the formation pressure and 

total current gas content, since the 

higher the pressure or the lower the gas 

content, the larger the proportion of the 

total gas content that could potentially 

be held in the fracture system. The calculations in table 4 assume a reasonable, 

but fairly high, porosity and probably 

an unreasonably high change in water saturation (from 100 to 0 pct). Taking 

these factors into account, it is reasonable to state that fracture system gas 

storage will not cause serious errors in 

gas content estimates. 

The gas content information from the 

pattern wells and the monitor holes was 

used to calculate the quantity of gas 

removed from the pattern through November 

1981. This calculated value was then 

compared to the actual gas production 

figures. Because the available data are 

so limited, a set of rather rough assumptions have been made: 

1. The original in-place gas content 

was 15 crn3lg (480 ft3/st). 

2. The average gas content within the 

area of the pattern (fig. 6) in November 

1981 was 7.1 cm3Ig (227 ft3/ton), based 
LEGEND 

Pressure monltor hole MI 

Product~on well w~th no 

/6 desorption data 

@ Product~on well with 

7 desorption data 

------ Pattern boundary 

Assumed outside boundary 

of gas dramage area 

I I Estimated av gas content I 1 7.1 cm3 /g, through 10/81 L----J 

Eshmated av gas content 

I1 cm3/g, through 10/81 

0 

u 

1,000 

Scale, ft 

FIGURE 6. - Pattern area and estimated area of gas drainage. 
upon the average of the gas content values at holes M1 and M2. 

3. The average gas content in a block 

of coal from the outside edge of the pattern to 500 ft away from the pattern was 

11 crn3lg (352 ft3/st), based upon the gas 

content at hole M3. 

4. No desorption of gas took place 

outside the two previously mentioned 

blocks of coal (fig. 6). 

5. The coal density is 1.32 g/cm3. 

6. The average coal thickness is 66 

in. 

Based upon these assumptions, the calculated reduction in gas content was 980 

MMstdft3, of which 860 MMstdft3 came from 

within the pattern and 120 MMstdft3 came 

from the area outside of the pattern. 

The actual gas production through October 1981 was 1,110 MMstdf t3. The calculated gas production is about 12 pct lower than the actual gas production. 

PRESSURE DATA 

The pressure decline curves for holes 

M1-M3 have been plotted in figure 7 for 

the period December 7, 1981, through 

June 30, 1984 (a total of 937 days). 

These pressures have been corrected for 

the distance from the center of the coalbed to the pressure sensors. This portion of the hole is water filled, so the 

correction is just the pressure of the 

water column. The initial stabilized 

pressures recorded in holes bl, M2, and 

M3 were 122, 112, and 247 ~bf/in~(~a) respectively. The initial pressure declines were about 4.5 to 7.5 lbf/in2 per 

month in hole M3 and 2.1 to 2.9 lbf/in2 

per month in holes M1 and M2. By late 

1982, the rates had been reduced to about 

1 lbf/in2 per month in holes M1 and M2, 

and to 2 lbf/in2 per month in hole M3. 

The probable reason for this is that gas 

and water production from the pattern, 

which had been deliberately curtailed in 

mid-1981, were abruptly increased in late 

1981 to begin gas sales. Gas production 

sharply increased in Novem7)er 198 1 and 

peaked in May 1982. Monthly water production, which hit a 3-year low of 

119,000 gal in October 1981, increased to 

- 

0 120 I I I I I -9' I I I 1 

C .- Sensor removed from hole (day 142) HOLE M2 f2 loo 1 \ Sensor repositioned 

(day 305) W 

cn 

cn Hole shut in W 

TIME, days 

FIGURE 7. - Pressure decline curves for monitor holes MI, M2, and M3. 

345,000 gal in November 1981 and peaked 

at 346,000 gal in December 1981. These 

large production changes caused a large 

transient change in formation pressure. 

Although it was not possible to install 

pressure-monitoring equipment soon enough 

to see the beginning of this transient, 

it appears that the rate of change in 

formation pressure was affected well into 

1982 by the large production changes. It 

can be assumed that the rates of pressure 

decline observed in the monitor holes in 

1983 probably more closely represent the 

normal pressure decline at the Oak Grove 

site for stable gas and water production 

rates. 

Two problems discovered in late 1982 

make use of the pressure data difficult. 

The first is that the sensors in holes M1 

and M2 (but not hole M3) were originally 

set directly upon the tops of the packers, causing a metal-to-metal contact 

which may have sealed off the tubing from 

the coalbed and prevented the transmission of pressure changes to the sensors. 

These seals were not complete since some 

pressure changes were observed, but a 

partial seal may have been sufficient to 
cause small errors or a lag in the pressure readings. The magnitudes of these 

errors are not known for certain, and the 

errors may not have remained constantly 

at the same magnitude. For example, the 

sensor in hole M2 was removed on April27, 1982 (day 142) and reinstalled on 

May 1 (day 146) with no pressure change, 

but when the sensor was permanently 

raised on October 7 (day 305) to ensure 

that the tubing was not sealed, the pressure dropped by 16 l b f / i n 2 . The pressure 

drop in hole M1 on October 7 was only 4 

lbf /in2. 

The second problem was a small (essentially unmeasurable) gas flow which 

caused reductions in the measured formation pressures. This was corrected on 

November 12, 1982 (day 341) by shutting 

in and packing off the holes to prevent 

further gas flow. No pressure change was 

observed in hole M1, but the pressure in 

hole M2 increased by 13 lbf/in2 and a 3 

lbf/in2 increase was seen in hole M3. 

Although it is impossible to determine 

the exact magnitude of these errors in 

the pressure measurements, it appears 

that the total error from both causes was 

small in holes M1 and M3; it was at most 

less than 5 lbf/in2 and may have averaged 

less since the two causes had opposite 

effects, which to some extent tended to 

cancel out. Also, because the pressures 

measured in December 1981 were determined 

from a water column which was falling 

rapidly to equalize the wellbore and 

formation pressures, there was probably 

no gas flow and no sealing off of the 

tubing at the time, so the early pressure 

readings are probably accurate. None of 

the pressure data since November 1982 

have been affected by these problems. 

The most difficult data to interpret 

are those from hole M2 during the period 

October 7 to November 11, 1982 (days 305 

to 340). After the sensor was raised, 

the pressure dropped drastically (by 16 

lbf/in2). The pressures were maintained 

in this range until the hole was shut in. 

When the hole was shut in, the pressure 

immediately increased, apparently back to 

its initial range of October. It is 

tempting to assume that the pressures 

during that period are anomalous and that 

both the earlier (before October 7) and 

later (after November 11) pressures are 

correct. However, it is more likely that 

the pressures recorded in holes M1 and M2 

from May through October 1982 were in error due to two opposite effects. The 

raising of the sensors in October eliminated the source of an artifically high 

pressure reading, and the subsequent shut 

in of the wells eliminated a source of 

reduction of the recorded pressures. 

This means that most of the data from 

hole M2 from May to November 12, 1982, 

are in error by some unknown amount, and 

that similar, but fortunately smaller, 

errors are present in the data from holes 

M1 and M3 during the same period. Analysis of trends in the pressure decline before November 1982 is probably risky, although the trend of high initial pressure 

changes, with a gradual reduction of the 

rate of change, appears to be real. 

ISOTHERM CURVE 

An additional use can be made of the 

gas content and pressure data of table 2. 

Plotting the pressures versus the gas 

content values creates a graph which is 

the equivalent of an isotherm curve. 

This has been done in figure 8. In this 

case, the curve is based upon 10 data 

points: 1 each for the 3 monitor holes; 

6 closely spaced points representing initial or virgin coalbed conditions, obtained from production well data; and a 0 

gas content point at 1 atm, or 0 gauge 

pressure. A power curve regression was 

run to obtain the relationship shown in 

figure 8 between gas content and pressure. The curve thus obtained averages 

the properties of a large volume of coal, 

essentially representing the northern 

portion of the Oak Grove pattern, and because it is from in situ data, it in effect averages over such factors as coalbed moisture and differences in rank and 

volatile matter which can affect the 

ability of the coal to adsorb gas. 

The following assumptions have been 

made in developing this pseudo-isotherm 

curve. First, it has been assumed that 

the initial formation pressures and adsorbed gas contents obtained from the 
FORMATION PRESSURE ( P), I bf /in2(ga) 

FIGURE 8. - Isotherm curve derived from field gas content and pressure data. 

production wells are in equilibrium as 

specified by the isotherm for the coalbed. It is possible that in the coalbed 

the initial fluid pressures measured are 

higher than required to adsorb the gas 

actually present. If this is the case, 

then the curve gives a pessimistic estimate of the ability of the coalbed to adsorb gas at high pressure. The curve 

need not necessarily be inaccurate for 

the low-pressure range, however. It has 

also been assumed that the gas content 

and pressure readings are currently in 

equilibrium. However, for free gas to be 

present and for gas to flow, the system 

must be slightly out of equilibrium. The 

effect should be small and should not 

significantly affect gas content measurements made from pressure data (table 4). 

Second, it has been assumed that the 

pressures in the monitor holes in early 

December 1981 were the same as those 

present in the holes in mid-November 1981 

when the holes were cored. The rates of 

change of pressures in December were sufficiently low, about 2 to 5 lbf/in2 per 

month, that the errors here are small. 

Finally, the variability in measurements using the direct method and in the 

gas content of the coalbed must be considered. The 1977 gas content data from 

the production wells have been used to 

give an estimate of the 95-pct (twostandard-deviation) confidence limit of 

direct method gas content measurements 

made in the Oak Grove area. This value 

is the gas content value 51.55 cm3/g. 

This confidence interval applies only to 

direct method measurements made from the 

Lower Bench of the Mary Lee Coalbed in 

the Oak Grove area, since it includes 

local variations in the gas content of 

the coalbed whose magnitudes are unknown 

and cannot be independently estimated 

from the available information. Additional data points would help reduce the 
size of this confidence interval, as 

would improvements in the accuracy of the 

direct method. Some methods that have 

been suggested by other researchers for 

improving the direct method are described 

in reference 11. The use of additional 

data points is, of course, expensive 

since it is necessary to drill or core a 

hole and make a formation pressure determination to obtain each point, a process 

which can cost $30,000 per point. 

The material balance computation made 

earlier in this report in which the total 

desorbed gas volume through October 1981 

was estimated to be 980 MMstdft3, compared to an actual 1,110 MMstdft3, indicates that the isotherm curve of figure 8 

is reasonably accurate (within 12 pct), 

despite the limited data from which it is 

derived. 

The pseudo-isotherm curve was used to 

estimate the rate of gas desorption in 

April 1983 from within the area of drainage shown in figure 6; this includes the 

shaded area within 500 ft of the outside 

of the pattern boundary and the pattern 

area. At that time the rate of pressure 

decline was about 1 lbf/in2 per month 

within the pattern (1.2 lbf/in2 per month 

in hole M1 and 0.9 lbf/in2 per month in 

hole M2) and 1.8 lbf/in2 per month in 

hole M3. The pressures in holes M1 and 

M2 were 108 and 76 lbf /in2 (ga) , respectively, and the pressure in hole M3 was 

214 ~bf/in~(~a). The average pressure 

inside the pattern was assumed to be 92 

~bf/in~(~a), the mean of the pressures in 

holes M1 and M2. The average pressure 

used for the shaded area of figure 6 was 

the mean of the pressure in hole M3 and 

the 92 lb~/inZ(~a) value, or 153 

lbf /in2 (ga) . The calculated changes in 

gas contents for the two areas per month 

were 1.3 and 2.0 ft3/st. The pattern 

area has been assumed to contain 3.4 x 

lo6 st of coal and the shaded area 2.0 x 

lo6 st of coal. Using these figures, the 

3,875- by 3,875-ft pattern area should 

have produced 4.4 MMstdft3 of gas, and 

the shaded area within 500 ft of the pattern boundary should have produced 4.0 

MMstdft3, or a total of 8.4 ~~stdft'. 

The actual gas production from the pattern wells in April 1983 was 32.8 

~~stdft~. This large disparity indicates 

that, at the present time, most of the 

gas being produced by the Oak Grove pattern is migrating to the wells from far 

outside the pattern boundaries, although 

it appears that before 1981 most of the 

gas produced had come from within the 

pattern. 

CONCLUSIONS 

The core data from the Oak Grove pattern show that the initial 4 years of degasification (1978-81) caused a reduction 

of approximately 50 pct in the adsorbed 

gas content of the coalbed within the 

pattern area and a reduction of approximately 25 to 30 pct at a distance of 500 

ft from the outside boundary of the 

pattern. 

Simple calculations using core and 

pressure data indicate that, through 

1981, most of the gas produced from the 

pattern, perhaps 70 to 90 pct, had come 

from within the area of the pattern itself. However, calculations using a 

derived isothermlike curve indicate that 

since 1981 most of the gas produced 

is migrating from areas outside the 

pattern. 

The project data demonstrate that the 

adsorbed gas content of the coalbed has 

decreased as the formation pressure decreased. An isothermlike curve was developed from desorption data to allow 

computation of the adsorbed gas content 

given the formation pressure. This 

curve, although developed from limited 

data, appears to agree with the observed 

gas production data. 

Difficulties experienced in measuring 

formation pressures in the monitor holes 

indicate that, once desorption has begun 

to take place and free gas becomes present in the coalbed, water level measurements alone cannot be relied upon to give 

accurate formation pressure readings in 

coalbeds. It is necessary to shut in 

monitor holes to prevent the flow of free 
and C. H. Elder. The Direct Method of 

Determining Methane Content of Coalbeds 

for Ventilation Design. BuMines KI 7767, 

1973, 17 pp. 

y

g

and Application (U.S. Dep. Energy contract DE-AC21-78MC08089). U.S. Dep. Energy, Morgantown Energy Technology Center, Morgantown, WV, i981, 44 pp. 

*US. GPO: 1985-505019/20,113