Mission: As the Nation's principal conservation 

agency, the Department of the Interior has responsibility for most of our nationally-owned public 

lands and natural and cultural resources. This 

includes fostering wise use of our land and water 

resources, protecting our fish and wildlife, preserving the environmental and cultural values of 

our national parks and historical places, and providing for the enjoyment of life through outdoor 

recreation. The Department assesses our energy 

and mineral resources and works to assure that 

their development is in the best interests of all 

our people. The Department also promotes the 

goals of the Take Pride in America campaign by 

encouraging stewardship and citizen responsibility for the public lands and promoting citizen participation in their care. The Department also has 

a major responsibility for American Indian reservation communities and for people who live in 

Island Territories under U.S. Administration. 
Report of Investigations 9430 

Determining the Source of Longwall Gob 

Gas: Lower Kittanning Coalbed, 

Cambria County, PA 

By William P. Diamond, James P. Ulery, 

and Stephen J. Kravits 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Manuel Lujan, Jr., Secretary 

BUREAU OF MINES 

T S Ary, Director 
Library of Congress Cataloging in Publication 'Data: 

Diamond, W. P. (William P.) 

Determining the source of longwall gob gas : Lower Kittanning Coalbed, Cambria 

County, PA / by William P. Diamond, James P. Ulery, and Stephen J. Kravits. 

p. cm. - (Report of investigations; 9430) 

Includes bibliographical references @. 13). 

1. FiredampPennsylvaniaAmbna County. 2. Coal mine waste-PennsylvaniaCambria County. 3. Longwall mining-PennsylvaniaAmbria County. 4. Mine 

ventilation. I. Ulery, J. P. (James P.) 11. Kravits, S. J. (Stephen J.) 111. Title. IV. 

Series: Report of investigations (United States. Bureau of Mines); 9430. 

TN23.U43 [TN305] 622 S--dc20 [622'.42] 92-13059 CIP 
CONTENTS 

Page 

Abstract ........................................................................... Introduction ........................................................................ Acknowledgments .................................................................... Methodology ....................................................................... Criteria for corehole site selection ...................................................... Gas content determination and sample selection ........................................... Results and discussion ................................................................ In-place gas content ................................................................ Volumetric calculations of overburden gas ................................................ Material balance calculations ......................................................... Recommendations ................................................................... Conclusions ........................................................................ References ......................................................................... Appendix...Gas content data. coreholes 9. 10. 11. and 13 ....................................... 

ILLUSTRATIONS 

Mine map of study area ........................................................... Comparison of daily methane production rates for good- and poor-producing gob gas vent holes. 

panel1 ...................................................................... Stratigraphic columns with results of coalbed gas content .................................. Proposed zone of gas emissions ..................................................... Variation of in-place gas volumes during mining cycle ..................................... Cumulative gas production from gob gas vent holes. panel 1 ................................ Mine map of study area with zone of gas emission and proposed methane drainage borehole systems . . Schematic of speculated pressure gradients in study area ................................... Cross-sectional view of proposed cross-measure gas drainage holes ........................... 

TABLES 

1 . Calculated in-place gas volumes for 600- by 4.15 0. by 275-ft zone of gas emissions. panel 1 ......... 2 . Gas volume reduction between coreholes .............................................. A.1 . Measured gas contents from corehole 9 ............................................... A.2 . Measured gas contents from corehole 10 .............................................. A.3 . Measured gas contents from corehole 11 .............................................. A.4 . Measured gas contents from corehole 13 .............................................. 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

OF degree Fahrenheit in inch 

ft foot in Hg inch of mercury (atmospheric 

pressure) 

ft3 cubic foot 

mD millidarcy 

ft3/ft3 cubic foot per cubic foot 

MMcf million cubic feet 

ft3/st cubic foot per short ton 

PC~ percent 

g/cm3 gram per cubic centimeter 
DETERMINING THE SOURCE OF LONGWALL GOB GAS: LOWER I 

KITTANNING COALBED, CAMBRIA COUNTY, PA 1 

By William P. ~iamond,' James P. ulery,' and Stephen J. ~ravits* 

ABSTRACT 

This report presents results of a cooperative research project between the U.S. Bureau of Mines and 

BethEnergy Mines, Inc. Four coreholes (two before mining and two after mining) were drilled at a 

longwall mine operating in the Lower Kittanning Coalbed to obtain coal and rock samples from overlying strata to determine their gas content at various times in the mining cycle. The results of those 

tests indicate that 91 pct of the gas removed from the overlying strata came from coalbeds. Material 

balance calculations were made to compare the volume of gas produced both from gob gas vent holes 

drilled on the panel and gas removed by the mine's ventilation system with the volume of gas removed 

from strata directly overlying the panel to a height of 275 ft. This analysis indicates that only 40 pct of 

the total gas produced from the panel actually came from the strata directly overlying the panel. The 

remaining volume of gas production probably migrated to the Iongwall gob from overlying, and perhaps 

underlying, strata immediately adjacent to the panel and perhaps from greater distances downdip 

because of the establishment of a long-term pressure gradient in the study area. 

'~eolo~ist. 

2~ining engineer (now with Resource Enterprises, Salt Lake City, UT). 

Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 
LEGEND 

Date of mlninq 6-30-68 

Carehole Gob vent hole Pallors ertrocled 

Before rnlnlng A 

After mlnlng A d D~rectimn d mmmg 0 

Figure 1 .--Mine map of study area. 
ACKNOWLEDGMENTS 

The day-to-day assistance of Donald E. Raab, project provided access to geologic and engineering data and to 

engineer, BethEnergy Mines, was noteworthy in accom- mine maps, and he managed the field logistics for obtainplishing the goals of this project. In particular, he ing coal and rock cores for gas content determination. 

METHODOLOGY 

CRITERIA FOR COREHOLE SITE SELECTION 

In addition to evaluating all the available engineering 

and geologic data for the area of interest, it was recommended that a series of coreholes be drilled to obtain rock 

and coal samples for reservoir characterization of the overlying strata. This characterization, which is the focus of 

this report, was primarily the determination of the gas content of the overlying strata, including both the coal and 

rock. 

Four coreholes were drilled by BethEnergy at the locations indicated in figure l. The drilling locations were 

based on the data requirements of the project and selected 

to provide the maximum amount of data with the fewest 

number of holes. At the time when the drilling program 

was being finalized, the longwall on panel 1 was approximately 75 pct complete and located just beyond gob gas 

vent hole 175-A (fig. 1). 

As shown in figure 1, the spacing between gob gas vent 

holes was increasing toward the end of panel 1. This was 

a result of an increase in gas production from the gob 

holes, as shown in figure 2, that compares the methane 

flow rate for hole 173-A, drilled at the beginning of the 

panel, with hole 175-B, drilled at the end of panel 1. As 

the gob gas vent holes produced at a higher rate, a concurrent decrease in methane emission problems was observed 

in the mine. The first corehole (9) was drilled at a 

location estimated to be near where the next gob gas vent 

hole would be needed. Based on the trend toward higher 

gas production from the gob gas vent holes toward the end 

2.0 

1.5 

I .o 

.5 

0 

Mar. June Sept. 

1989 

Dec. Mar. June 

1990 

Sept. 

ll(1l[ll~1lllrlll 

Hole 175-8 

End of mining 

Start of min~ng 

Hole 173-A 

Figure 2.--Comparison of daily methane production rates for 

good- and poor-producing gob gas vent holes, panel 1. 

of the panel, it was anticipated that corehole 9 would be 

located near a gob gas vent hole that would produce at a 

higher flow rate than those at the beginning of panel 1. 

At the time samples were collected from corehole 9, mining was approximately 210 ft and 2 weeks away from its 

location. Results of gas content testing of rock and coal 

samples from this before-mining hole were compared with 

results from an after-mining corehole to be drilled on this 

same site at a later date. This comparison allowed a 

direct calculation of the volume of gas drained from the 

overlying strata as a result of the extraction of the Lower 

Kittanning Coalbed and subsequent caving and fracturing 

of the overlying strata. The gas content results were also 

compared with results from the other holes that were 

drilled with varying relationships to the mining cycle. 

The second corehole (10) was drilled prior to mining at 

the beginning of the second longwall panel and close to a 

gob gas vent hole that was already being drilled in anticipation of its eventual need. This corehole was expected 

to have the highest gas contents of the four coreholes since 

its location was the least affected by mining of the four 

coreholes drilled for this project. By cornparing the gas 

contents from corehole 10, which would presumably only 

be affected by the development mining around the second 

panel, with the results of corehole 9, it would be possible 

to determine what, if any, influence the approaching longwall face and gob formation would have on the gas contents in the overlying strata at corehole 9. Corehole 10 

would potentially be drilled in an area where gob gas vent 

hole production might be low, if the general low gas production rates experienced at the beginning of panel 1 were 

related to some geologic trend that might extend into the 

adjacent area of panel 2. 

The third corehole (11) was drilled approximately 

7.5 months after mining in the vicinity of poor-producing 

gob gas vent holes at the beginning of panel 1. The location of this corehole was chosen to provide a direct comparison of geologic conditions between good- and poorproducing gob gas vent holes. Gas content data from 

corehole 11 would represent the maximum interval of time 

after mining for comparisons of gas contents. It was anticipated that gas contents from this corehole would provide 

the minimum in-place gas content, or conversely would 
show the maximum drainage of gas from the overlying 

strata of the four coreholes. 

The last corehole (13) was drilled approximately 

2 months after mining at the same site as corehole 9, 

which was drilled in advance of mining. These two coreholes were drilled at the same site so that before- and 

after-mining gas contents could be compared on an as 

equal as possible geologic basis. Also, by comparing gas 

contents from the before-mining coreholes (9 and 10) with 

the results from coreholes 13 and 11 (drilled 2 and 

7.5 months, respectively, after mining), an assessment of 

the timing of gas flow from the overlying strata could be 

made. 

GAS CONTENT DETERMINATION 

AND SAMPLE SELECTION 

Coal and rock samples were collected from each corehole for gas content determination using the Bureau's 

modified direct method (MDM) test (I)? The MDM was 

originally developed by the Bureau to accurately measure 

the generally low gas contents of oil shale (2). The MDM 

differs from the original direct method test (3), in that the 

volume of gas desorbed or released from the sample is 

determined at periodic intervals by measuring pressure 

differentials in the sealed sample container and calculating 

gas volumes according to the ideal gas law. Gas samples 

are taken for compositional analysis each time the 

pressure differentials are measured. This permits the 

calculation of the actual hydrocarbon gas content of the 

sample, as well as providing data on other gases present. 

The original direct method test relied on bleeding gas 

from the sample container and measuring the volume 

directly by a water-displacement method. The MDM was 

used on this project because of the expected low gas 

contents of the noncoal rocks in the strata overlying the 

Lower Kittanning Coalbed. For data consistency, the 

MDM was also used on the coal samples. 

The total gas content of a sample is composed of three 

parts: lost, desorbed, and residual gas. The lost gas is 

that volume of gas that escapes from the sample between 

the time the sample is cored in the hole and finally 

retrieved on the surface and sealed in a desorption container. For coal samples where gas is stored by adsorption, the lost gas can be estimated by a graphical procedure based on the relationship that the volume of gas 

emitted from a fine coal particle is proportional to the 

square root of time (4-5). No such relationship is known 

to exist for rocks where the gas is free gas in the porosity 

of the sample and not stored by adsorption. Therefore, no 

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

preceding the appendix at the end of this report. 

lost gas volume is included in the gas content values 

reported in this study for noncoal rocks. For rocks that 

are highly permeable, such as some sandstones, a sidcant volume of the original gas in place could be lost to 

the atmosphere in the time required to recover the core 

from the ground and seal it into a container. 

The gas desorbed or released from the sample while it 

is sealed in the container was measured by the MDM as 

described above. 

The final constituent of the total gas content is the 

residual gas. At some point in time, a sample will reach 

a low level of gas release, beyond which only a small increment of additional gas will be measured. At this point, 

additional gas may still be contained within the micropore 

structure of a coal sample. However, most of this gas will 

not be released from the solid sample under ambient conditions. This residual gas in coal samples can be measured 

by crushing the sample to a fine powder in a sealed vessel 

and measuring the volume of gas released. The current 

apparatus available for measuring the residual gas content 

of coal is not suitable for crushing other rock types to 

evaluate the presence of any residual gas. 

Measuring the volume of residual gas in coal samples 

is of general interest for many fundamental and theoretical 

studies of the occurrence of coalbed gas. Since residual 

gas is gas that cannot be produced from a small diameter 

core at ambient conditions on the surface, it represents a 

volume of gas that probably will never migrate to a mine 

opening or to a methane drainage borehole. For this reason, it is not absolutely necessary to determine residual gas 

contents of samples where the primary interest is to access 

or compare desorbable volumes of gas that would migrate 

to a mine opening or to methane drainage boreholes. The 

residual gas component for the coal and rock samples collected was not thought to contribute to mine emissions and 

therefore was not determined. 

The sampling strategy for the cores obtained for this 

study included the gas content testing of all coalbeds in the 

strata overlying the Lower Kittanning Coalbed to a height 

of approximately 275 ft. A height of 275 ft was selected 

to cover the generally reported height of approximately 

200 ft for the fractured zone above an extracted longwall 

panel (6-9). For thin coalbeds (generally less than 3 ft 

thick) the entire coalbed was tested as a single sample. 

Some of the thicker coalbeds were divided into two separate samples for gas content testing; however, the results 

were normalized and reported as a single value in this 

study. At least one representative noncoal rock sample 

was tested in each 204 core run within the 2754 

stratigraphic interval above the Lower Kittanning Coalbed 

investigated for this study. On occasion, more than one 

noncoal rock sample was tested in a single core run when 

a simcant lithologic change occurred in the 20-ft section. 
5/ Upper Kittanning 

........... Lower Kittanning - rider 

Lower K ittanning 

Hole 10 Hole 9 Hole II 

Hole l3 Mined out 

KEY 

Limestone 0 Sandy shale or siltstone 

0 Shale or siltstone Bone or bony shale 

a Sandstone Coal 

0 Shale or siltstone a Estimated value 

with interbedded 278 Gas content, ft3/st sandstone 

Figure 3.--Stratigraphic columns with results of coalbed gas content 
and bone in the Upper Freeport Coalbed. This can significantly influence the volume of gas contained in the unit, 

in that the coal can contain a much greater volume of gas 

than can bone or bony shale. In general, the lithologies of 

the noncoal units and individual samples varied to a greater extent from location to location than did the coal units 

and associated samples. For that reason, it is recognized 

that, in many cases, the rock samples collected at similar heights above the Lower Kittanning Coalbed are not 

directly equivalent between holes. 

As shown in figure 3, in almost every case the gas contents of stratigraphically equivalent coalbeds decrease with 

greater influence of mining, both laterally (left to right, 

figure 3) and vertically (top to bottom, figure 3), as would 

be expected. The only si&icant discrepancy in this trend 

is the Lower Freeport Coalbed in coreholes 10 and 9, 

where the corehole in the location least affected by mining 

(corehole 10) has a gas content 34 pct less than in corehole 9. It seems that the Lower Freeport results from 

both coreholes do not coincide with the general increase 

in gas contents expected and generally shown in figure 3 

at increasing distances above the coalbed mined. To fit 

the general trend, the Lower Freeport value from corehole 10 should be higher and the value from corehole 9 

should perhaps be less. No apparent reason can be 

deduced to explain this discrepancy, except that it is 

possible that an undetected leak occurred in the canister 

containing the Lower Freeport coal sample from corehole 

10, thereby producing a lower-than-actual gas content 

value. This of course does not explain the apparent 

higher-than-expected value for the Lower Freeport Coalbed in corehole 9. 

One particularly interesting set of gas content numbers 

shown in figure 3 are those for the Lower Kittanning 

Coalbed from the two before-mining coreholes (10 and 9). 

The gas content values of 18 and 7 ft3/st are extremely low 

when compared with an expected virgin gas content of 

approximately 300 ft3/st as measured for this coalbed at 

locations in nearby Indiana County, PA (2). While these 

two coreholes were drilled into areas where the Lower 

Kittanning had not yet been extracted, the longwall panels 

had in fact been outlined by the development entries in 

preparation for longwall mining. The development entries 

around panel 1 (fig. 1) to the northwest of corehole 9 were 

mined approximately 9 months prior to the drilling of the 

corehole, and those to the southeast were mined about 

3 months prior to drilling. The development entries surrounding corehole 10 on panel 2 were mined as long as 

16 months prior to the drilling of the corehole. It is likely 

that some of the gas in the Lower Kittanning had migrated 

out of the outlined panels to the surrounding entries prior 

to the drilling of the coreholes. The release of gas adsorbed in the internal structure of coal is initiated by the 

lowering of the reservoir pressure of the coalbed (10-11). 

Since mining of entries into the coalbed and the use of 

exhaust ventilation systems in the mine creates a very efficient pressure sink, a mechanism for initiating and sustaining gas flow from the outlined panels, as well as the virgin 

coal downdip, exists in the area of investigation. A second 

factor that has probably contributed to the formation of a 

pressure sink up dip of the new longwall panels is mining 

in the Upper Kittanning Coalbed (fig. 1). 

Looking beyond the area of the development entries 

immediately surrounding the longwall panels, it is seen 

that entries updip and less than 500 ft away to the 

northwest were mined out in the Lower Kittanning Coalbed as long as 22 years ago (fig. 1). Entries at the end of 

panel 1 closest to corehole 9 and less than 500 ft away 

were mined 14 years prior to drilling of the coreholes, and 

those at the end of panel 2 closest to corehole 10 and less 

than 500 ft away were mined 6.5 years prior to drilling. 

With such a large area of old workings surrounding three 

sides of the area of new panel development in the Lower 

Kittanning Coalbed, it is quite likely that gas has been 

migrating out of that coalbed to the large preexisting 

pressure sink created by the old workings. The most likely 

explanation then for the extremely low gas contents in the 

Lower Kittanning Coalbed in the study area is the longterm migration of gas to the old workings updip from the 

new panels and the more recent migration to the new development entries surrounding the panels. 

The maximum in-place gas contents measured for 

coalbeds in the four coreholes drilled for this study are 

nearly 300 ft3/st in the Upper Freeport Coalbed and rider 

coal in corehole 10, the corehole least affected by mining. 

This value is in the range expected for these coalbeds at 

this depth as reported for virgin samples (3). The gas 

contents for the Upper Freeport Coalbed and rider coal in 

corehole 10 are assumed to be at or near virgin conditions. 

Comparing these gas contents with the gas contents from 

the other coreholes indicates that coalbeds as much as 

200 ft above a longwall can contribute gas to the gob and 

mine ventilation system. In addition to the gas drainage 

induced by previous mining in the Lower Kittanning Coalbed, mining in the Upper Kittanning Coalbed, 100 ft above 

the Lower Kittanning and approximately 1,200 ft updip 

from the new longwall panels (fig. I), probably contributed 

to the gas content reduction in the overlying strata. 
volume in the zone of gas emissions is subdivided into two 

categories: gas in coal, which includes bone and bony 

coal, and gas in rock, which includes the remaining noncoal strata. All gas contents and unit thicknesses as actually measured were used in the gas in-place calculations, 

with the exception of those from the Upper Freeport Coalbed. Because of the varying character of this coalbed, in 

particular the large amount of bone and small amount of 

coal in corehole 11, an average thickness based on the 

values from the other three cores was used in the calculations. It was thought that this would give a more reasonable evaluation for the panel as a whole. In several 

cases where the gas contents of thin coalbeds were not 

measured, values were assumed, based on representative 

values from other coalbeds. Since, in general, only one 

rock sample was tested for every 20-ft core run, the gas 

content of the closest similar lithology was assigned to any 

lithology not actually tested in a particular core run. 

The calculated total gas volumes and the coal and rock 

subdivided gas volumes based on the gas contents for each 

corehole are given in table 1 and plotted in graphical form 

in figure 5. The zone of emission was defined as a rectangular prism with its base equal to the size of the longwall panel (600 by 4,150 ft) and its height at 275 ft (fig. 4). 

The gas in-place values for each stratigraphic unit shown 

intervening rock . . . . . . . . . . 12.2 8.1 10.1 6.4 

Lower Kittanning rider . . . . . 21.7 13.5 5.6 24.5 

Intervening rock . . . . . . . . . . 2.3 3.7 4.0 

Lower Kittanning Coalbed . . 7.6 3.6 (3) X 

Total coal . . . . . . . . . . . . 375.0 200.0 110.0 75.0 

Total rock . . . . . . . . . . . . 45.0 35.0 35.0 15.0 

Cumulative total . . . . 420.0 235.0 145.0 90.0 

NAp Not applicable. 

'Based on normalized coal thickness. 

'~ased on estimated gas content. 

3~ined out. 

NOTE.-Data may not add to totals shown because of independent rounding. 

- 

2oo- lLi 

U Z 

2 loo. 

Mined out ' 

Figure 4.-Proposed zone of gas emissions. 

Freeport 

Freeport 

Kittann~ng 

Kittonn~ng 
Table 2.--Gas volume reduction between coreholes 

Proximity to mining Gas in Matrix of gas volume reduction, MMcf (pct) 

place, MMcf Virgin Hole 10 Hole 9 Hole 13 Hole 11 

wrginl .... Wrgin ................. 730 

Hole 10 ... Unmined .............. 420 

Hole 9 .... 0.5 month before mining .. 235 

Hole 13 ... 2.0 months after mining ... 1 40 

Hole 11 ... 7.5 months after mining ... 90 

'~ased on estimated 300 elst gas content for all coalbeds. 

By comparing the in-place gas volumes based on the 

actual measured gas contents from coreholes 10 and 11, 

330 MMcf of gas or 79 pct of the gas assumed to have 

been present at the start of mining has migrated out of the 

overlying strata within 7.5 months after extraction of 

panel 1. Of the 330 MMcf of gas drained from the overlying strata, 300 MMcf or 91 pct originated from the 

overlying coalbeds (table 1). As mentioned in the "Gas 

Content Determination and Sample Selection" section, the 

lost gas volume for rock samples was not determined. The 

results of laboratory core analysis of selected rock samples 

of various lithologies obtained from two of the four coreholes indicate a very low permeability (air) of less than 

0.005 mD and a low porosity (helium), generally less than 

1 pct, for tested samples. Only two sandstone samples had 

higher porosities at 1.8 and 3.7 pct. Water saturation values were also relatively high, most in the range of 75 to 

100 pct. Based on the laboratory core analysis data, it 

seems likely that a relatively small volume of gas was lost 

from most rock samples. It is also likely that the rock 

samples were monitored for sufficient time, generally more 

than 6 months, for most of the gas contained in the samples to have been emitted into the container and measured 

by the MDM test, even with the very low permeability of 

the samples. Even though most of the gas contained in 

most of the rock samples was probably accounted for, the 

rock values should be considered minimum values. 

If an assumed virgin gas content of 300 ft3/st was used 

for all coalbeds in corehole 10, an estimated total in-place 

gas volume of 730 MMcf would be obtained for the zone 

of gas emissions, representing the estimated total original 

gas in place prior to any mining in the area. Comparing 

this value to the in-place gas volume based on the actual 

gas contents of corehole 10 (420 MMcf) suggests that 

310 MMcf of gas, or 42 pct of the original gas in place, 

could have migrated from the zone of gas emissions prior 

to the start of mining on panel 1. 

Another interesting gas volume comparison to make is 

between coreholes 9 and 13 drilled at the same location 

before and after mining. The total zone of gas emissions 

in-place gas volume based on corehole 9, drilled 210 ft 

(2 weeks) in advance of the longwall face, was 235 MMcf. 

The total in-place gas volume based on corehole 13, drilled 

at the same location 2 months after mining, was 145 MMcf 

for a drained gas volume of 90 MMcf during the 2.5- 

month timespan, represented by coreholes 9 and 13. Since 

the gas content of corehole 9 is presumed to have already 

been reduced by drainage of gas to the approaching longwall, the in-place gas volumes based on corehole 10 

(420 MMcf) can be compared with those based on corehole l3 (145 MMcf) to better estimate the total volume of 

gas drained from the zone of gas emissions at the 2-month 

after-mining timespan. This comparison indicates that 

275 MMcf or 65 pct of the in-place gas was drained from 

the strata above panel 1 at this point in the mining cycle. 

By then comparing the volume of in-place gas drained as 

a result of mining panel 1 at the 2- and 7.5-month aftermining time intervals (figure 5 and table 2), it could be 

seen that most of the gas (275 MMcf) was drained from 

the overlying strata up to the 2-month after-mining point 

(corehole 10 versus corehole 13), with only an additional 

50

COREHOLE 

L 

In 

0 z r 

PROXIMITY TO MINING 

Figure 5.-Variation of in-place gas volumes durlng mining 

cycle. 
55 MMcf of gas drained in the next 5.5 months (corehole 13 versus corehole 11). As discussed previously, a 

larger volume of methane (310 MMcf) was potentially 

drained from the zone of gas emissions over an extended 

time period prior to mining of panel 1 (table 2). 

As can be seen from the data in table 1 and plotted in 

figure 5, there were generally minimal changes in the 

volume of gas drained from the rocks as opposed to the 

volume drained from the coalbeds in the presumed zone 

of gas emissions. By comparing the rock in-place gas 

values from the least mining influenced corehole (10) to 

the most influenced corehole (ll), it was determined that 

the gas volume drained was 30 MMcf, or 67 pct of the 

presumed original total. The biggest change in rock gas 

volumes was seen in the two after-mining coreholes (13 

versus ll), where 20 MMcf of gas, or 44 pct of the total 

rock in-place gas, was drained in the 2- to 7.5-month timespan after mining. 

MATERIAL BALANCE CALCULATIONS 

The final set of relationships to be considered are 

material balance calculations, based on the available data, 

that will help refine the evaluation of the source and 

migration patterns of longwall gob gas in the study area. 

Gob gas produced by longwall mining of the Lower 

Kittanning Coalbed at the study site in the Cambria No. 33 

Mine can be removed in two ways, by the mine's ventilation system or through gob gas vent holes. Mining on 

panel 1 began on March 6, 1989, and was completed on 

November 22,1989. This timespan encompasses 261 days, 

of which 152 days were actual coal production days. The 

nonproduction days included a strike, days the panel was 

shut down because of high gas emissions, and vacation and 

weekend days when production was not scheduled. Ventilation data supplied by the mine were used to calculate 

daily gas emission volumes for the various production 

and nonproduction days as described above. A total of 

175 MMcf of gas was calculated to have been removed by 

the ventilation system during the 261 days required to 

complete mining of longwall panel 1. During this same 

time period, the 12 gob gas vent holes produced 655 MMcf 

of gas from the panel (fig. 6). By adding the two gas 

production volumes calculated above, the total gas production from panel 1 is then 830 MMcf for the 261 days 

required to mine the panel. Comparing this value to the 

previously calculated value of 330 MMcf of gas drained 

from the zone of gas emissions (hole 10 versus 11) during 

the mining of panel 1 leaves 500 MMcf or 60 pct of the 

gas production unaccounted for. 

One way to account for the 500 MMcf of gas shortfall 

in the material balance calculations would be to simply 

enlarge the zone of gas emissions until the volume of gas 

drained from that zone equalled the 830 MMcf of gas 

calculated to have been vented from panel 1. This would 

seem to be a valid premise, since there is no reason to 

believe that gas could only be released from overlying 

strata confined by an area equal only to the size of the 

extracted panel itself. The mining of the coal and the 

subsequent fracturing of the overlying strata to form the 

gob certainly extends some distance laterally beyond the 

perimeter of the panel. It would also follow that a zone of 

reduced reservoir pressure would extend laterally beyond 

the perimeter of the extracted panel, and as discussed 

previously, the release of gas from coal, which has been 

shown in this study to be the primary source of gob gas, is 

initiated by lowering the reservoir pressure. It is also possible, if not probable, that the source of some of the unaccounted for 500 MMcf of gas production is coalbeds in the 

underlying strata. As discussed previously, it is not possible to estimate the volume of gas drained from the strata 

underlying the Lower Kittanning Coalbed with the available core data. 

By adding 350 ft on all sides of the original zone 

of emissions, whose base was defined as the size of the 

extracted panel, and keeping the height the same, the 

volume of drained gas would equal the volume of vented 

gas. The base of the new zone of gas emissions would be 

1,300 by 4,850 ft, as shown in figure 7. Increasing the size 

of the zone of gas emissions is probably the best way to 

reconcile the volume of gas drained from the overlying 

strata and the volume of gas vented from the panel. 

However, the shape and position of the base of the zone 

of gas emissions is probably not as shown in figure 7. As 

can be seen in figure 7, the extent of the larger zone of 

gas emissions overlaps onto areas mined out 7 to 16 years 

ago on three sides of the panel and approaches within 

about 150 ft of workings that are 21 years old. It is likely 

that gas released from the overlying strata in these areas 

migrated to those old workings both prior to and during 

the mining of panel 1. It seems more likely that the actual 

shape of the base of the zone of gas emissions extends 

further in the direction of panel 2 and less in the other 

three directions. 

2 V 

u Mar. June Sept. Dec. Mar. June Sept. 

1989 1990 

Figure 6.--Cumulative gas production from gob gas vent 

holes, panel 1. 
d Direction of ninlng 

Before mlnlng A 

After mlnlng A 0 

Proposed muhone dminaps systams 

. Vertiml well 

Horizonlal hole ~&&& cross-measure holes 

Figure 7.-Mine map of study area with zone of gas emission and proposed methane drainage borehole systems. 

The extension of the zone of gas emissions preferentially to the southeast in the direction of the development 

of future longwall panels may actually provide the explanation for the increased gas emissions experienced during the 

mining of panels 1 and 2. The presence of the old workings on three sides of the area developed for the new longwall panels, in particular those updip to the northwest of 

panel 1, which are present in both the Lower and Upper 

Kittanning, has probably induced a substantial downdip 

pressure reduction gradient. That pressure gradient, which 

on the basis of the material balance calculations could 

easily extend into and perhaps beyond panel 2, could have 

been inducing the desorption and flow of gas from the 

Lower Kittanning Coalbed as well as coalbeds in the overlying (and perhaps underlying) strata for as long as 

21 years (fig. &I). 

If, indeed, a flow of gas to the old workings in the 

Cambria No. 33 Mine had been established through the 

strata (primarily coalbeds) overlying the area of new panel 

r:'zz 60 

a3 

Eg 40 L~esorption Kg 20' pressure -Actual 

Panel 1 Panel 2 

a3 Desorption pressure 

Ponel 3 

Figure 8.--Schematic of speculated pressure gradients In 

study area. A, Prior to start of mining on panel 1; 6-C, progressive increase in pressure gradient if successive panels are 

mined faster than Initial pressure gradient can reestablish itself. 
development (fig. 7), then it is possible that the gas in 

transit to the old workings could be diverted to the closer, 

newly created pressure sink of the mine void and gob of 

panel 1. It is also possible that the free gas in transit 

through the strata overlying the panels would preferentially 

flow to the much larger area of the mine void instead of 

the small area represented by the twelve 4- to 6-in-diameter 

gob gas vent holes, thus resulting in the higher-thanexpected mine emissions and lower efficiency of the gob 

gas vent holes. 

The long-term flow of gas for relatively long distances 

through coalbeds and overlying strata to a created pressure 

sink has been documented previously by Bureau research 

(13). In that study, an area of reduced pressure gradients 

and gas flow from coalbeds was shown to extend at least 

3,400 ft and perhaps as much as 5,000 ft from a pattern of 

stimulated vertical coalbed gas drainage wells that had 

been producing from the Blue Creek Coalbed for 10 years. 

The study showed that of the 3.2 billion ft3 of gas produced by the 23 wells, only 1.14 billion ft3 came from the 

Blue Creek Coalbed within the perimeter of the pattern 

and the remaining 2.06 billion ft3 or 64 pct of the gas 

production migrated to the pattern both laterally and 

vertically from the surrounding strata. 

RECOMMENDATIONS 

If the flow of gas in transit to the old mine workings is 

indeed the cause of the higher-than-normal gas emissions, 

then several approaches to the problem are possible. The 

first would be to essentially do nothing. If a reduced 

pressure gradient and free gas flow has been established 

as speculated, then at some point beyond the currently 

mined panels virgin pressures must exist. If the mining of 

successive panels to the southeast can be achieved at a 

faster rate than the pressure gradient can decline to 

pressures as low as were adjacent to panel 1 when it was 

mined, then it should be possible to eventually mine coal 

that is higher on the pressure gradient (fig. 8B-C) where 

less gas would be in transit to be diverted to the current 

workings. A series of pressure monitoring holes could be 

drilled to the southeast at increasingly greater distances 

away from the panel being mined to evaluate the presence, 

magnitude, and changes in a gradient of reduced pressure 

with time. Data from such a system of monitoring holes 

could be used to forecast when methane emissions might 

return to normal. Should it become evident that sufficient 

progress is not being made in overcoming the gradient of 

reduced pressure, then it may be necessary to consider 

additional corrective measures. 

Corrective measures beyond simply mining faster than 

the pressure gradient declines would generally require 

another approach: the drilling of a system of methane 

drainage boreholes, downdip of current mining, to intercept the gas that would be flowing updip to the panel 

being extracted. There are basically three choices for 

methane drainage boreholes: stimulated vertical wells 

drilled from the surface, horizontal holes, and crossmeasure holes, the latter two drilled from inside the mine. 

The vertical wells would have the advantage of being 

drilled from the surface instead of the more restrictive 

underground environment, and they could be completed to 

drain gas from all the major coalbeds in advance of mining 

to provide the greatest relief. If the vertical wells were 

drilled in the proper locations, they could be used as gob 

gas vent holes on future panels (fig. 7), thereby defraying 

some of the costs. It is also possible that the gas produced 

from the wells could be sold to further reduce the cost to 

the company. 

Horizontal or cross-measure holes drilled underground 

would most likely be less costly initially than stimulated 

vertical wells and would probably be the option of choice 

to control immediate or short-term emission problems. 

Horizontal holes could perhaps be drilled into the roof 

strata from the opposite end of the panel from which mining is advancing (fig. 7). Drilling the holes in this manner 

would allow gas to be continually drained away from the 

advancing longwall face area and, depending on the length 

of the holes, could provide gas drainage for a substantial 

period of time. Using this system would probably require 

that the horizontal holes be manifolded to a gas gathering 

system that would transport the gas to the surface and not 

dump the gas underground. The gathering system would 

have to be kept under sufficient negative pressure by using 

exhausters on the surface so that gas could not backflow 

to the longwall face. Horizontal holes of this type could 

be drilled on the panel actually being mined to provide 

immediate relief and/or on the next panel to be mined 

downdip to intercept any migrating gas before it reaches 

the area of active mining. It might be necessary to drill 

several horizontal holes at different levels above the 

longwall panel to provide the greatest relief. 

The cross-measure holes, like the horizontal holes, 

could either be drilled over the panel currently being 

mined and/or over the next panel to intercept gas 

migrating updip. The cross-measure holes would be 

drilled from the development entries along the length of 

the panel (fig. 7), angled up into the roof strata above the 

longwall panel (fig. 9). A series of cross-measure holes 

has actually been drilled at this mine previously and were 

shown to be effective in reducing face emissions (14-15). 
Upper Freeport rider 

180 Upper -. ~reeport 

Lower Free port 1 

[Upper Kittanning 

4-to 5-pct dip J KEY 

Longwall panel 4 Coalbed 0 40 80 

lbIl Entry W 

Migrating gas Scale, ft 

/ Cross- measure hole 

Figure 9.--Cross-sectional view of proposed cross-measure gas drainage holes. 

These holes were drilled at an angle of about 30" for a 

total hole length of about 280 ft. Maximum height above 

the Lower Kittanning Coalbed generally ranged from 140 

to 170 ft, which would put the end of the gas drainage 

hole above the Lower Freeport Coalbed. Holes of similar 

length drilled at 45" would easily reach the Upper Freeport 

Coalbed if necessary (fig. 9). 

It would also be possible to use a combination 

of horizontal and cross-measure holes, perhaps with the 

cross-measure holes providing relief at the beginning of a 

panel and the horizontal holes being used at the completion end to whatever maximum length they could be 

drilled toward the advancing face (fig. 7). Assuming that 

the gas from these systems was manifolded and transported to the surface and that the gas was not contaminated with air to any great extent, it would be possible to 

sell this gas to help defray the cost of implementation. 

CONCLUSIONS 

The primary sources of longwall gob gas resulting from 

the mining of the Lower Kittanning Coalbed are the coalbeds in the overlying strata. As much as 91 pct of the 

longwall gob gas originated in the overlying coalbeds, with 

those as high as 200 ft above the mined coalbed contributing gas to the gob. Material balance calculations indicated 

that the volume of gas drained from the strata directly 

overlying the longwall panel could only account for 40 pct 

of the total volume of gas vented by the ventilation system 

and gob gas vent holes. It is concluded that most of the 

additional gas migrated from strata downdip from the new 

longwall panels. This migration of gas resulted from the 

presence of a large area of old mine workings that 

created a pressure sink, both in the Lower and Upper Kittanning Coalbeds, that facilitated the desorption and flow 

of gas updip to the old workings. It is possible that the 

flow of gas through the area of the new longwall panel 

development may be responsible for the higher- thanexpected methane emissions that led to this study. The 

high methane emission levels may decline if mining can 

advance downdip at a faster rate than the reduced 

reservoir pressure gradient can reestablish itself. The 

alternative is to employ one of the several available and 

effective methane drainage techniques to intercept the free 

gas in transit before it reaches the active longwall panel. 
REFERENCES 

1. Uiery, J. P., and D. M. Hyman. The Modified Direct Method of 

Gas Content Determination: Applications and Results. Paper 9163 in 

The 1991 Coalbed Methane Symposium Proceedins (Tuscaloosa, AL, 

May 13-17, 1991). Univ. AL, Tuscaloosa, AL, 1991, pp. 489-500. 

2. Schatzel, S. J., D. M. Hyman, A. Sainato, and J. C. LaScola. 

Methane Contents of Oil Shale From the Piceance Basin, CO. BuMines 

RI 9063, 1987, 32 pp. 

3. Diamond, W. P., J. C. LaScola, and D. M. Hyman. Results of 

the Direct-Method Determination of the Gas Content of U.S. Coalbeds. 

BuMines IC 9067, 1986, 95 pp. 

4. Kisseli, F. N., C. M. McCulloch, and C. H. Elder. The Direct 

Method of Determining Methane Content of Coalbeds for Ventilation 

Design. BuMines RI 7767, 1973, 17 pp. 

5. Sevenster, P. G. Diffusion of Gases Through Coal. Fuel, v. 38, 

1959, p. 403. 

6. Mazza, R L., and M. P. Molinar. Reducing Methane in Coal 

Mine Gob Areas With Vertical Boreholes (contract H0322851, Continental Oil Co.). BuMines OFR 142-77,1977,145 pp.; NTIS PB 272 768 AS. 

7. Davis, J. G., and S. Krickovic. Gob Degasification Research-A 

Case History. Paper in Methane Control in Eastern U.S. Coal Mines. 

BuMines IC 8621, 1973, pp. 66-72. 

8. Thakur, P. C., and J. G. Davis. How To Plan for Methane Control in Underground Coal Mines. Min. Eng., v. 29, No. 10, Oct. 1977, 

pp. 4145. 

9. Hasenfus, G. J., K. L. Johnson, and D. W. H. Su. A Hydrogeochemical Study of Overburden Aquifer Response to Longwall Mining. 

Paper in Ground Control in Mining (Proc. 7th Int. Conf., Morgantown, 

WV, Aug. 3-5, 1988). WV Univ., Morgantown, WV, 1988, pp. 149-162. 

10. Kissell, F. N., and J. C. Edwards. Two-Phase Flow in Coalbeds. 

BuMines RI 8066, 1975, 16 pp. 

11. Kuuskraa, V. A., and C. F. Brandenburg. Coalbed Methane 

Sparks a New Energy Industry. Oil and Gas J., v. 87, Oct. 9, 1989, 

pp. 49-56. 

12. Curl, S. J. Methane Prediction in Coal Mines. IEA Coal Research, London, England, Rep. ICI'IS/TR 04, Dec. 1978, 80 pp. 

13. Diamond, W. P., W. R Bodden 111, M. D. Zuber, and R A. 

Schraufnagel. Measuring the Extent of Coalbed Gas Drainage After 

10 Years of Production at the Oak Grove Pattern, AL. Paper 8961 in 

Proceedins of the 1989 Coalbed Methane Symposium (Tuscaloosa, AL, 

Apr. 17-20, 1989). Univ. AL, Tuscaloosa, AL, 1989, pp. 185-193. 

14. Garcia, F., and J. Cervik. Methane Control on Longwalls 

With Cross-Measure Boreholes (Lower Kittanning Coalbed). BuMines 

RI 8985,1985,17 pp. 

15. Campoli, A. A., J. Ce~k, and S. J. Schatzel. Control of Longwall Gob Gas With Cross-Measure Boreholes (Upper Kittanning Coalbed). BuMines RI 8841, 1983, 17 pp. 
APPENDIX.--GAS CONTENT DATA, COREHOLES 9,10,11, AND 13 

Table A-1.4easured gas contents from corehole 9 

Height above 

Sample Depth, top of Lower Gas content Specific 

ft Kittanning @/st @/@ rock gravity Lithology or walbed name 

Coalbed, ft 

1 ..... 677 301 0.27 0.02 2.52 Sandstone. 

2 ..... 692 286 1.7 -14 2.63 lnterbedded sandstone, siltstone. 

3 ..... 71 1 267 .4 .02 1.59 Coal (bony). 

4 ..... 715 263 .85 .07 2.58 Siltstone (shaley). 

5 ..... 729 249 .12 .O1 2.70 Do. 

6 ..... 747 23 1 .06 .O1 2.55 Do. 

7 ..... 76 1 21 7 0 0 2.73 lnterbedded sandstone, shale. 

8 ..... 773 205 .03 < .O1 2.74 Shale (sandy). 

9 ..... 786 192 263 11.16 1.36 Upper Freeport rider. 

11 .... 793 1 85 11 .80 2.32 Upper Freeport (bone). 

10 .... 794 1 84 1 83 7.60 1.33 Upper Freeport. 

12 .... 823 155 .26 .02 2.67 Limestone. 

13 .... 839 139 21 3 9.34 1 .# Lower Freeport. 

14 .... 848 130 .79 .06 2.62 Siltstone, sandstone. 

15 .... 864 114 4.13 .33 2.58 Siltstone (sandy). 

16 .... 874 104 91 4.3 1 1.52 Upper Kittanning. 

17 .... 907 71 .19 .02 2.61 Sandstone. 

18 .... 926 52 1.1 .09 2.66 Shale (black). 

19 .... 949 29 90 3.68 1.31 Lower Kittanning rider. 

20 .... 959 19 .98 .08 2.60 Siltstone. 

21 .... 968 10 .25 .02 2.65 lnterbedded sandstone, siltstone, shale. 

22 .... 973 5 .I0 .O 1 2.62 Sandstone. 

23 .... 978 NAP 7 .3 1 1.43 Lower Kittanning. 

NAp Not applicable. 

Table A-2.4easured gas contents from corehole 10 

~~~~~~ 

Height above 

Sample Depth, top of Lower Gas content Specific 

ft Kittanning @/st @/@ rock gravity Lithology or coalbed name 

Coalbed, ft 

1 ..... 416 262 0.57 0.05 2.7 1 Sandstone. 

2 ..... 449 229 .06 < .O1 2.67 Siltstone (shaley). 

3 ..... 466 21 2 .ll .O1 2.69 Sandstone, interbedded shale. 

4 ..... 477 201 1.50 .08 2.64 Siltstone, shale. 

5 ..... 479 199 278 11.68 1.35 Upper Freeport rider. 

6 ..... 49 1 187 359 17.41 1.55 Upper Freeport. 

7 ..... 493 185 21 9 1 1.07 1.61 Do. 

8 ..... 504 174 .94 .08 2.61 Sandstone. 

9 ..... 533 145 .39 .03 2.65 Siltstone (sandy). 

10 .... 542 136 140 6.24 1.43 Lower Freeport. 

11 .... 559 119 .19 .02 2.62 Siltstone, shale. 

14 .... 574 104 1.97 .16 2.64 Shale, siltstone. 

12 .... 575 103 256 11.40 1.43 Upper Kittanning. 

13 .... 577 101 152 6.34 1.34 Do. 

15 .... 595 83 .13 .O1 2.61 Siltstone. 

16 .... 606 72 320 12.90 1.29 Coal. 

17 .... 612 66 .65 .05 2.66 lnterbedded sandstone, shale. 

18 .... 627 51 1.56 .13 2.61 Shale (black). 

19 .... 643 35 .51 .04 2.35 Do. 

21 .... 657 21 -81 .07 2.67 Do. 

20 .... 660 18 117 4.62 1.26 Lower Kittanning rider. 

23 .... 673 5 .79 .06 2.60 Shale (black), interbedded sandstone. 

24 .... 676 2 .28 .02 2.66 Sandstone. 

22 .... 678 NAI> 18 .56 1 .# Lower Kittanning. 

NAp Not applicable. 
Table A-3.--Measured gas contents from corehole 11 

Height above 

Sample Depth, top of Lower Gas content Specific 

ft Kittanning elst VIP gravity Lithology or coalbed name 

Coalbed, ft 

Sandstone. 

lnterbedded siltstone, shale. 

Sandstone. 

Do. 

Sandstone, interbedded shale. 

Upper Freeport rider. 

Upper Freeport. 

Limestone, sandstone. 

Sandstone (shaley). 

Lower Freeport. 

Siltstone. 

Upper Kittanning. 

Sandstone. 

Shale (sandy). 

lnterbedded shale, siltstone. 

Shale (black). 

lnterbedded siltstone, shale. 

Sandstone. 

Table A-4.-Measured gas contents from corehole 13 

Height above 

Sample Depth, top of Lower Gas content Specific 

ft Kittanning *lst f3lft3 rock gravity Lithology or coalbed name 

Coalbed, ft 

1 .... 69 1 290 0.93 0.08 2.61 lnterbedded sandstone, shale. 

2 .... 71 8 263 .65 .05 2.57 Siltstone. 

3 .... 73 1 250 .21 .02 2.63 Do. 

4 .... 75 1 230 .19 .02 2.64 Do. 

5 .... 763 21 8 .04 <.01 2.67 Shale, interbedded sandstone. 

6 .... 774 207 .15 .O 1 2.67 Shale. 

7 .... 788 193 102 4.30 1.35 Upper Freeport rider. 

9 .... 797 184 30 2.26 2.42 Upper Freeport (bone). 

8 .... 798 1 83 118 4.95 1.34 Upper Freeport. 

10 ... 823 158 .18 .01 2.59 Limestone. 

11 ... 84 1 140 60 2.69 1.44 Lower Freeport. 

12 ... 852 1 29 .56 .05 2.59 Sandstone. 

13 ... 862 119 .60 .05 2.69 Siltstone. 

14 ... 877 104 6 1 2.58 1.36 Upper Kittanning. 

15 ... 904 77 .47 .04 2.60 Siltstone. 

16 ... 910 71 .17 .O 1 2.65 Sandstone. 

17 ... 929 52 2.21 .17 2.53 Shale (black). 

18 ... 952 29 39 1.57 1.29 Lower Kittanning rider. 

19 ... 964 17 .15 .O 1 2.61 Shale (silty). 

20 ... 972 9 .14 .O1 2.64 Shale, interbedded sandstone.