REPORT OF INVESTIGATIONS/1994 

Evaiuation of Alternative Placement 

of Longwall Gob Gas Ventholes 

for Optimum Performance 
U.S. Department of the Interior 

Mission Statement 

As the Nation's principal conservation agency, the Department of 

the Interior has responsibility for most of our nationally-owned 

public lands and natural resources. This includes fostering 

sound use of our land and water resources; protecting our fish, 

wildlife, and biological diversity; 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 ensure that their development is in the best interests of 

all our people by encouraging stewardship and 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 9500 

Evaluation of Alternative Placement 

of Longwall Gob Gas Ventholes 

for Optimum Performance 

By William P. Diamond, Paul W. Jeran, 

and Michael A. Trevits 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Bruce Babbitt, Secretary 

BUREAU OF MINES 
International Standard Serial Number 

ISSN 1066-5552 
Page 

Abstract ........................................................................... Introduction ........................................................................ Acknowledgments .................................................................... Evaluation of production trends ......................................................... Experimental evaluation of near-margin gob gas ventholes ...................................... Evaluation of subsidence monitoring data ................................................ Field experiment of near-margin gob gas ventholes ......................................... Summary .......................................................................... References ......................................................................... 

1 . Mapofstudyarea ................................................................ 2 . Histograms of methane production from gob gas ventholes through 12 months and 9 months after 

completion of mining for panels 1 through 4. and panel 5. respectively ......................... 3 . Schematic diagram of drilling and completion details for gob gas ventholes 174 and 174.B. panel 1 ..... 4 . Schematic block diagram of speculated variable drainage patterns for normal and diminished mininginduced fracture permeability ....................................................... 5 . Schematic diagram of typical subsidence trough developed over extracted longwall panel ............ 6 . Schematic map view of distribution of horizontal strain above extracted longwall panel .............. 7 . Static surface subsidence curves at completion of mining on panel 3 ........................... 8 . Dynamic surface subsidence curves. panel 3 ............................................. 9 . Comparison of measured and predicted static surface subsidence .............................. 10 . Histogram of 7 months of methane production from gob gas ventholes on panels 6 and 7 ........... 11 . Cumulative methane production curves for gob gas ventholes 156-A and 156.B. panel 6. and horizontal 

gob gas ventholes. panel 7. relative to panel 7 face position ................................. 

TABLES 

1 . Mining and methane production data for panels 1 through 7 ................................. 2 . Cumulative methane production data for individual gob gas ventholes. panel 7 .................... 3 . Cumulative methane production data for individual gob gas ventholes. panel 6 .................... 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

cm centimeter m3/d cubic meter per day 

m meter mm/m millimeterpermeter 

m3 cubic meter Pet percent 
EVALUATION OF ALTERNATIVE PLACEMENT OF LONGWALL 

GOB GAS VENTHOLES FOR OPTIMUM PERFORMANCE 

By William P. ~iamond,' Paul W. ~eran,' and Michael A. ~revitr~ 

ABSTRACT 

In this U.S. Bureau of Mines study, production trends have been analyzed for 82 gob gas ventholes 

on 7 longwall panels in the Lower Kittanning Coalbed, Pennsylvania. Cumulative production from 

individual holes was found to vary considerably, ranging from 0.03 x 106 to 10.1 x 106 m3. Peak 

production rates were generally about 0.03 x 106 to 0.06 x 106 m3/d, but rates over 0.10 x 1@ m3/d were 

observed. The variability of gas production from individual holes can be attributed to several factors, 

including completion practices and problems, the extent of fracture development in the strata above the 

extracted panel, water production, and location on the panel. Of particular significance is hole location. 

Holes on the ends of the panels, where overburden strata are in tension because of the support of the 

surrounding pillars, were generally the highest and longest producers. This observation led to the 

experimental placement of gob gas ventholes in the zone of tension along the margin of a panel, instead 

of in the traditional centerline location, which is in compression. Analysis of 7 months of gas production 

data indicates that the experimental near-margin holes produced 77 pct more gas than did centerline 

holes on the same panel. 

'~eolo~ist. 

2~upervisory geologist. 

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

The U.S. Bureau of Mines (USBM) and BethEnergy 

Mines, Inc. began a cooperative research program in 1989 

to determine the factors influencing the production of gas 

from longwall gob gas ventholes. The USBM's initial 

involvement in this study resulted from a request for assistance in evaluating unexpected high methane emissions 

on the fust of a new series of longwall panels at the 

Cambria No. 33 Mine, near Ebensburg, PA. In the past, 

at this mine, it had been sufficient to drill three to four 

gob gas ventholes per panel to aid the ventilation system 

in controlling methane emissions underground. On the 

first longwall panel in the new development area, 12 gob 

gas ventholes were drilled. Even with this large number 

of ventholes, mining delays due to high methane emissions 

were still experienced. 

In the 3 years that this study has been in progress, 7 

longwall panels have been completed in the study area 

(fig. 1) and the production results from 82 gob gas ventholes have been evaluated. The analysis of the production 

trends has led to several recommendations for alternative 

methane control practices, including the repositioning of 

the gob gas ventholes from the traditional centerline location to locations closer to the margin of the panel. The 

analysis of the production data and the theory that led to 

this recommendation, along with the results from the fust 

application of the concept, are the focus of this report. 

This work was done as part of a USBM research program to prevent or reduce the impact of disaster-causing 

events, such as explosions caused by the ignition of methane gas. 
ACKNOWLEDGMENTS 

The substantial contributions of services and personnel by BethEnergy Mines, Inc., Ebensburg, PA, are gratefully 

acknowledged. 

EVALUATION OF PRODUCTION TRENDS 

The seven longwall panels in the study area (fig. 1) 

were mined at various times over the 3 years this study has 

been in progress. Therefore, there are varying lengths of 

gob gas venthole production histories for each panel. Cumulative methane production through 12 months after 

panel completion for panels 1 through 4 and through 9 

months for panel 5 is given in figure 2. The relative position of the holes on each panel is also shown in figure 2. 

The methane production data discussed in this paper 

were supplied by the cooperating mining company, and do 

not represent standard temperature and pressure (STP) 

corrected gas volumes. In a recent experiment at this site, 

a continuous-recording methane monitoring system was 

installed on a gob gas venthole to compare mine-supplied 

flow rates with those measured with more sophisticated 

instrumentation, and corrected to STP conditions. Preliminary evaluation of the experimental data indicates that the 

mine-supplied data are consistently 20 to 25 pct higher 

than the more accurately measured and STP-corrected 

values. Most of the discrepancy is due to the elevated 

temperature of the gas, which the mine measures after it 

has passed through the exhaust blower on the hole. Because the analysis of gob gas venthole methane production 

presented in this report is primarily based on comparisons 

of the relative levels of production between holes and 

panels, the absolute production values are not critical. 

The apparent consistency of the discrepancy between the 

mine-supplied data and the accurately monitored data 

supports the validity of the analysis to follow. 

The most obvious observation from figure 2 is that the 

gob gas ventholes in the study area have performed inconsistently. On the four panels for which 12 months of 

production data are available after panel completion, individual holes produced from 0.03 x 106 to 8.04 x 106 m3 

of methane. The highest producing gob gas venthole in 

the study area was 181-A on panel 3, which produced 

10.1 x 106 m3 of methane through 18 months after panel 

completion. 

Peak methane production rates were generally about 

0.03 x 106 to 0.06 x 106 m3/d, but rates over 0.10 x 

106 m3/d were observed for several holes (176,176-A, 182, 

and 185). Peak production usually occurred in the first 

week after a hole was intercepted by mining, and lasted 

only a few days. Gob gas ventholes generally produced for 

6 months or more after being intercepted by mining, 

though at declining rates. In fact, as can be seen in table 1, significant volumes of gas are still produced long 

after a panel has been completed. For the four panels for 

which 12 months of production time elapsed after panel 

completion, 40 to 55 pct of the total gas production occurred during that time. Two holes on panel 1,173-D and 

175-B, were still producing gas at rates of 0.003 x 106 to 

0.005 x 106 m3/d, 31 months after panel completion. 

The large variation in gas production, in particular low 

production volumes, can to some extent be explained by 

examination of the drilling, completion, and servicing 

records of the holes. Holes 174 and 174-C on panel 1, and 

176-C on panel 2, which had only 0.79 x 106, 0.03 x 106, 

and 0.48 x 106 m3 of methane production, respectively, 

were apparently adversely affected by invasion of cement 

into the strata around the wellbore. It had been common 

practice at the mine to not cement the well casing in place, 

or to only place a small amount of cement above cement 

baskets on holes drilled in advance of the longwall. 

Table 1 .--Mining and methane production data, lo6 cubic meters, for panels 1 through 7 

Panel 1 Panel 2 Panel 3 Panel 4 Panel 5 Panel 6 Panel 7 

Methane (12 holes, (12 holes, (9 holes, (1 3 holes, (1 5 holes, (7 holes, (1 4 holes, 

production 8.5 months 6.3 months 5.7 months 4.9 months 5.6 months 8.1 months 4.5 months 

mining time) mining time) mining time) mining time) mining time) mining time) mining time) 

To panel completion: 

Cumulative . . . . 18.55 20.98 20.16 20.27 22.65 4.73 9.34 

Per month . . . . . 2.18 3.33 3.54 4.14 4.05 .58 2.08 

Per hole . . . . . . . 1.55 1.75 2.24 1.56 1.51 -68 .68 

To 6 months after 

completion: 

Cumulative . . . . 23.73 27.01 27.78 26.79 26.36 5.69 

Per month . . . . . 1.64 2.20 2.37 2.46 1.76 .40 

NAP 

Per hole . . . . . . . 1.98 2.25 3.09 2.06 2.27 .8 1 

NAP 

To 12 months after NAp 

completion: 

Cumulative . . . . 26.3 29.42 31.21 29.93 NAP N Ap NAP 

NAp Not applicable. 
KEY 

I7 

Before mining V 

I 

. 

- After mining Q 

- Direction of mining 

9 

BOREHOLE DESIGNATION 

Total, 

26.3 ~10 m

- - 6.29 - 

- 

3-28 1 -0.03 Panel 1 - ,-, 4.08 - 

Total 

29.42 X lo6 rn3 

6 

3 

9 

6 

3 

* In j: g (1,265 mi e t ", ", 

BOREHOLE DESIGNATION 

2.63 

I 1.05 "I3 

I 

? m 0 c~) c~) 2:~ & pL a 4$XX <1,301 m $k $s cc ss 

BOREHOLE DESIGNATION 

1 - 

- 

.- 8.04 d 

- Panel 2 4.11 Z - 3.43 - 

0 J I 

? a 

6 '? g 1,311 m - - Q, 2 0 

E S m P PI$$ k ", 

Scale, m 

0- I 

Total 

31.21 X lo6 

- 

= 

- 

= 

- 

Tot01 

29.93 X lo6 m3 

9 .- 

9 d 

Figure 2-HIatograma of methane production from gob gaa venthoka through 12 montha and 

9 montha after comphtlon of mining for panda 1 through 4, and pad 5, respectively. 

= 

- 1.87 1-13 - 

p4~oqm q 0 ornu 

o o oAoA o n A &JXIXIN".*~~~ 

2 s E S 2SE2 

BOREHOLE DESIGNATION 

J 

3 6 4 

3, 

6 

3 

- 

- 

1.25 2.38 

I 

8.01 - 

Panel 3 6.26- 

4.02 - 1 3.68 

I I 

1.25 

02 

- Panel 4 5.18 - - 5.01 - 5.75 )I 

2027 2.0 7 2-27 

- 

1.42 

Total 

27.58 lo6 m3 

0 -. 

- Panel 5 

4.67 1 3.68: 

- 3.94 3.34 a: 

i 

- 3.60 

I - .23 

a 

= .57 a m 

3.60 

e * ?:' 0 * ma 

r- r-r- r-@ $ 2 CD ,A A ,Ahm'1,301m E E SE 2% 2 S E 2 222ES 

BOREHOLE DESIGNATION 

0 200 400 600 800 1,000 1,200 1,400 I 11 I I I I I J 

.17 1.22.~~ ( 

.54 

: 

1.53 1-13 1.53 

37 1 I 1 $0 
Hole 174 on panel 1 was initially drilled in advance of 

the longwall to within 3 m of the Lower Kittanning Coalbed. The hole was cased with 56 m of slotted casing at 

the bottom (fig. 3). Water inflow restricted gas flow after 

the hole was mined under by the longwall. In an attempt 

to stop the water inflow that was thought to be entering 

the hole above the slotted casing, the casing above this 

point was cemented in place. Cement baskets had been 

placed on the casing approximately 61 m above the mined 

coalbed to control the downward flow of the cement. 

After cementing was completed, water continued to be 

produced, and the hole was drilled deeper to below the 

depth of the base of the extracted coalbed. Even after 

penetrating the level of the mined-out coalbed, the accumulated water did not drain out of the hole. This suggests that the cement had bypassed the cement basket and 

invaded the gob zone and slotted casing below, damaging 

the mining-induced fracture permeability and cementing 

the slots in the casing. This may have been caused by 

either a failure of the basket or because the basket was 

adjacent to a mining-induced fracture. This apparently 

also happened on hole 176-C, panel 2, where after an 

attempt to cement the annulus to control water inflow, gas 

production was never reestablished. 

Hole 174-C was drilled into the gob after mining in an 

attempt to help alleviate the severe methane emission 

problems that prompted the initial investigation at this 

mine site. A similar cementing of mining-induced fractures probably occurred, as evidenced by the reported lack 

of cement returns to the surface during the cementing 

operation and the poor gas production history. 

Gob hole 

174 

Gob hole 

174-8 

Cement 159.4 m cement 194.8 m 

basket 171.3 m basket 200.9 m 

External 207.3 m - -. 

casing 

- 56.4 -m 

slotted 

pipe, 

20.32 cm ID 

TD 227.4 m 

U 

TD 234.1 m 

after mining 

TD 247.2 m I I, before mining 

Lower Kittanning Coalbed 

251.8 m 

TD= 251.8 m 

after mining 

Even without the added complication of cement invasion into the gob, water-bearing zones that are included 

in the gob or are connected to the gob by fractures can 

also hinder the flow of gas by blocking the mining-induced 

fractures. This is particularly important when the fracture 

permeability is abnormally low. In fact, abnormally low 

mining-induced fracture permeability is one of the speculated contributing factors to the high methane emissions 

experienced during mining of the first panel, and to the 

continuing need to drill large numbers of gob gas ventholes within the study area (I).' 

Hole 174-B on panel 1 was originally drilled to within 

3 m of the top of the Lower Kittanning Coalbed, prior to 

being mined under by the longwall (fig. 3). The casing 

was not cemented, and there was 40 m of open hole below 

the casing. A standing water level was measured in the 

hole prior to mining. Due to low methane flows after the 

longwall passed, the hole was probed again, and standing 

water was still present in the hole. Even though approximately 1.2 m of coal had been removed just 3 m below the 

bottom of the open hole, apparently insufficient caving and 

fracturing occurred to allow the water in the hole to drain 

into the mine void. It was only after the hole was dried 

deeper through the elevation of the base of the extracted 

coalbed that the water drained from the hole and gas 

production increased. 

As illustrated in figure 4 (holes C and D), if less 

fracturing occurs above the extracted longwall panel, the 

small surface area of the gob gas ventholes would connect 

with significantly fewer fractures. The drainage radius of 

the individual holes would be severely restricted, lowering 

gas production. More gas would, therefore, be diverted to 

the mine, which would have a significantly larger surface 

area in contact with the fracture system. 

The rate at which mining advances, while not necessarily influencing the ultimate gas production from individual holes, does increase the volume of gas liberated by 

mining in general, and gas drainage in particular, on a unit 

time basis. As can be seen in table 1, the time to mine 

the first four panels progressively decreased from 8.5 

months to 4.9 months, while the total methane volume 

produced by the gob gas ventholes remained relatively similar, especially on panels 2 through 4 where fewer methane 

emission problems were encountered than on panel 1. 

The methane production from the gob gas ventholes 

steadily increased from an average of 2.18 x 106 m3 per 

month on panel 1 to 4.14 x 106 m3 per month on panel 4; 

double the initial rate! Similar methane emission increases have been observed in other coal mining operations. In the Pocahontas No. 3 Coalbed, Virginia, it was 

reported that a 200- to 400-pct increase in longwall 

Figure 3.--Schematic diagram of drilling and completion dotail8 for gob gar venthoier 174 and 174-8, pnd 1. 

31talic numbers in parentheses refer to items in the list of references 

at the end of this report. 
induced fracture 

Mined out Drainage area 

Flgure 4.--Schematic block dlagram of speculated varlable dralnage patterns for normal (holes A and B) and 

diminished (holes C and 0) mlnlng-Induced fracture permeability. 

productivity was accompanied by a 200- to 300-pct increase 

in methane emissions (2). It is quite clear that mining 

companies must be prepared to deal with increased methane emissions on a unit time basis as mining productivity 

increases because of the use of advanced mining technology and larger longwall panels. 

Increased methane emission problems were encountered on panel 5, which resulted in a slower mining rate. 

Gob gas venthole methane production on a unit time basis 

declined slightly from that on panel 4 (table 1). Cumulative methane production of 22.65 x 106 m3 through the end 

of mining on panel 5 was the highest of the first five 

panels evaluated. With 15 gob gas ventholes, panel 5 also 

had the highest number of holes in the study area, which 

may explain its higher total gas production. The average 

gas production per hole through the end of mining for 

panels 1 to 5 is given in table 1. The average methane 

production for panels 4 and 5 is almost identical on a per 

hole basis; therefore, panel 5 with more holes has a higher 

total production. 

In general, the average methane production results 

on a per hole basis are quite similar for each panel, except for panel 3, which had the fewest number of holes (9) 

and the highest average gob gas venthole production at 

2.24 x 106 m3 per hole. It appears that from a methane 

control perspective in the study area, mining conditions 

were optimum on panel 3. It should be noted that for a 

complete analysis of methane control conditions, the 

volume of methane removed from each panel by the 

underground ventilation system should also be taken into 

account. Unfortunately, this type of data is difficult to 

obtain, and was not available for this analysis. 

Perhaps the most significant observation relative to factors apparently influencing methane production from individual gob gas ventholes is position of the holes on the 

longwall panels. Examining the methane production data 

through 12 months after panel completion for panels 1 

through 4 (fig. 2), it is apparent that the holes at the ends 

of the panels were generally the highest producers. On 

panels 2 and 3, the holes at each end (176 and 178-D, 179 

and 181-A) were the highest producers, with combined 

production equaling 41 pct and 46 pct of each panel's total 

production, respectively. On panel 4, the highest producer 

was the second hole from the start (182-A), followed 

closely by the first hole (182). The hole at the completion 

end of panel 4 (184) was tied for fourth highest producer 

with hole 184-A. On panel 1, the highest producer was 

hole 173-D, the fourth hole from the start of the panel 

drilled into the gob after mining, followed by the two holes 

(173 and 175-B) at either end of the panel. 

In addition to the holes at the ends of the panels' generally producing more gas, they also tended to remain on 

production for a longer time. Many of the gob gas ventholes were no longer producing gas by the time a panel 

was completed, and most holes were finished within 6 

months after mining. However, of the eight holes remaining on production at 12 months after panel completion 

(panels 1 through 4), six were on the ends of the panels 

(175-B, 176, 178-D, 181-A, 182, and 184). This includes 

the holes at the completion ends of the four panels, and 

two at the startup ends. The remaining two holes on 

production after 12 months were 173-D, drilled into the 

gob on panel 1, and 180, the third highest producer on 

panel 3. 

The reason for the holes at the ends of the panels' 

generally producing more gas for a longer time is speculated to be related to behavior of the overburden as it 

subsides into the void left by the extracted coal. Figure 5 

is a schematic diagram of a typical subsidence trough that 

develops over an extracted longwall panel. In the zone of 
I Finish , 

Flgure 5.--Schematic diagram of typical rubridence trough developed over extracted longwall panel. Note: 

Subridence la exaggerated. 

maximum subsidence located in the central area of the /------- r-----‘ t. 

panel, the overburden is in compression. This compression diminishes the fracture permeability of the gob, which 

in turn adversely affects the production of gas from the 

gob gas ventholes. 

Conversely, the strata overlying the ends of a panel are 

draped into the maximum subsidence zone since they are 

supported on three sides by the surrounding pillars of the 

development entries. Because of this partial support, the 

overlying strata at the ends of a panel are in tension for 

some distance into the panel, which would be expected to 

enhance the fracture permeability and gas production in 

this area. 

As can be seen in figure 5, the overburden strata are 

also draped into the maximum subsidence zone along the 

side of a longwall panel. A tension zone in the overburden also exists along the margins of a panel, which should 

enhance fracture permeability and gas production in this 

area along both sides (fig. 6). It would, therefore, seem to 

follow that the least effective place to locate gob gas ventholes is along the centerline in the central area of the 

panel where maximum subsidence occurs. Potentially, in 

the portion of the pane', gob gas be &ahemadc map view of dlrtribuaon of horlzonal 

more effective if drilled closer to the panel margins. rtrain above extracted longwall panel. There are production data from panel 1 in the study 

I I 1 

area to support this theory. Hole 173-D, mentioned 

I 

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Extracted I 

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Tension- I 

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previously as one of the eight holes to produce a large 

volume of gas for at least 12 months, was drilled 40 m 

from the updip margin of the 192-m-wide panel (fig. 1). 

In fact, this hole was the highest producer on panel 1. 

Hole 174 was also drilled close to the margin of this panel. 

However, as discussed previously, it was one of the holes 

where cement apparently invaded the fractures in the gob, 

and there was no appreciable gas production. 

Geologic factors, in addition to water-bearing zones and 

variable fracture permeability, may also influence the production from gob gas ventholes. Any significant variability 

in the in-place gas volume contained within the overburden 

strata that is influenced by the panel extraction could 

influence the volume of gas available for flow to individual gob gas ventholes. Examples of this would include 

changes in the available and/or utilized gas storage capacity of surrounding coalbeds, which in previous studies 

at this mine site were identified as the primary source of 

longwall gob gas (I), or the presence or absence of other 

gas-bearing strata, such as sandstones. Localized geologic 

anomalies, such as structural faults, clay veins, sandstone 

channels, and lithologic and permeability variations, could 

influence the flow of gas to individual gob gas ventholes. 

No evidence of these additional geologic factors were 

found within the study area. 

EXPERIMENTAL EVALUATION OF NEAR-MARGIN GOB GAS VENTHOLES 

A test of the near-margin gob gas venthole concept was 

implemented on panel 7, adjacent to the line of panels (1 

through 5) discussed previously (fig. 1). Subsidence monitoring data obtained previously by the mine within the 

study area were evaluated to determine a preliminary optimum distance from the panel margin for drilling the test 

holes. To test the concept, the holes needed to be within 

the zone of tension, but they could not be too close to the 

panel margin. If the holes were too close to the panel 

margin, they would probably pull in mine ventilation air 

and reduce the gas drainage efficiency. 

EVALUATION OF SUBSIDENCE 

MONITORING DATA 

Subsidence over a longwall panel is a dynamic process 

that is initiated with the collapse of the immediate roof as 

the face advances (3). As mining progresses and the size 

of the collapse zone increases, successively higher strata 

are fractured and sag into the mine void. Eventually the 

surface is affected. Monitoring of surface subsidence associated with longwall mining is typically conducted in two 

directions: along the centerline of the panel to obtain data 

on the dynamic subsidence process, and perpendicular to 

the centerline across the width of the panel to define static 

deformation. Regularly spaced monuments are installed 

along the lines to be monitored prior to surface movement. The monuments are periodically surveyed during 

the subsidence process to detect successive changes in 

their vertical and horizontal positions. 

During dynamic subsidence, which is directly influenced 

after subsidence has ended. Typically, the final result is a 

zone of compression over the centerline of the panel, 

flanked by a tensional zone in the vicinity of the rib lines 

(figs. 5-6). 

Subsidence monitoring data collected within the study 

area (figure 1, panels 3 and 4) were used to define the 

distribution of deformations induced by subsidence. The 

zone of tension, as defined by the inflection point on the 

strain curve in figure 7, was estimated to extend about 

45.7 m into panel 3 from the rib line. Fractures developed 

in this zone should remain open and provide effective permeability for gas flow. In the central area, compression 

would be expected to close fractures relatively quickly. 

Evaluation of the subsidence monitoring data (fig. 8) 

showed that the dynamic phase was completed after the 

face had advanced a distance equal to the thickness of the 

overburden. At this mine site, as is generally the case in 

the northern Appalachian Coal Basin (9, dynamic tension 

along the centerline was less than the static tension 

KEY 

--- Strain 

t - Subsidence 

by the position of the advance longwall face, the over- - -1 

burden initially undergoes tensional deformation parallel -4 . 1 b 

to the centerline of the panel, followed by compression, o 50 loo 150 200 250 

and finally ending in a neutral condition. Perpendicular to LOCATION, m 

the centerline across the width of the panel, static defor- Figure ‘1.4tatic rurface rubridence cuwer at completion of 

mations progress to maximum values that are retained mining on panel 3. 
developed over the ribs (figs. 7-8). Therefore, the intensity of tensional fracturing in the centerline area where 

most mining companies place gob gas ventholes should 

be diminished. It then follows that the centerline would 

generally be a less than optimum location for gob gas 

production. 

In the study area, one notable exception to the general 

behavior of subsidence is that only 70 pct of the final vertical movement occurred during the dynamic phase, versus 

the usual 90+ pct (fig. 8). A subsidence monitoring array 

at another location at this same mine site shows a typical 

static subsidence profile with about 0.85 m of surface 

movement (fig. 9B). In contrast, figure 9A shows the 

' d ens ion \ Strain 

0 - Subsidence 

Final subsidence - 60 "'L after mining of 

-18 

, , , panel , 4 , -20 

-50 0 50 100 150 200 250 300 

FACE POSITION, m 

Flgure 8.--Dynamic surface subsidence curves, panel 3. 

- 1 

0 50 100 150 200 250 0 50 100 150 200 250 

DISTANCE FROM CENTERLINE, m 

,' 

! rEdge of 

panel 

Figure 9.--Comparlwn of measured and predicted static surface subsidence. A, On panel 3 within study area; 8, at another 

locatlon on same mine site. 

profile for the array in the study area, which at 0.52 m of 

surface movement is far less than that at the other site as 

well as what would be predicted by a subsidence model 

(5). This lends additional support to the previous speculation (I) that for some still undefined reason, subsidence 

is not as great in this area, which in general could diminish 

the development of fracture permeability. 

FIELD EXPERIMENT OF NEAR-MARGIN 

GOB GAS VENTHOLES 

Mining on panel 7 began February 17, 1992. To evaluate the relative performance of the near-margin holes, 

several holes at the start of the panel were to be drilled 

along the traditional centerline location, then holes were 

drilled along the margin for comparison. Topographic, 

wetlands, and surface ownership considerations necessitated that the near-margin holes be drilled along the 

downdip headgate side of the panel. This positioning 

potentially had both positive and negative attributes. 

Previous research at this site (I) concluded that the 

methane emission problems may to some extent be related 

to a downdip pressure gradient that developed because of 

extensive old workings updip of the new mining area. This 

area of reduced reservoir pressure would induce the desorption of gas from the downdip coal reserves and, consequently, the flow updip toward the pressure sink of the 

old workings. Gob gas ventholes placed on the downdip 

margin of panel 7 would more effectively intercept this 

migrating gas before it crossed through the panel. One 

problem with having the holes on the downdip headgate 

side of the panel is that the holes would be working 

against the mine's ventilation system. The ventilation system is designed to bring intake air to the active face on 

the headgate side, and sweep across the face and through 

the gob to the bleeder entries on the updip tailgate side. 

Methane production by gob gas ventholes on the tailgate 

side would therefore be aided by the ventilation system. 

As circumstances evolved, neither of these factors probably had any significant influence on the holes. Instead, 

production of gas from all of the holes was adversely affected by high water inflow, with the near-margin holes 

perhaps being most severely impacted because of their 

downdip location. Water inflow had sporadically been a 

problem for gob gas ventholes throughout the study area, 

as discussed previously, but virtually every hole on panel 7 

experienced some degree of water problems. The general 

lower level of gob gas venthole production on this panel 

(table 1) increased the level of methane emissions in the 

mine and adversely affected coal production. Water problems were so severe that the mine attempted to use downhole pumps to dewater several holes (161-C, 161-F, 162-A, 

162-C, and 163-A), and drilled several supplemental gob 

gas ventholes, after mining, into the gob near the start of 

the panel. 
The severe water problems encountered on this panel 

made it a less than ideal experimental site to test the 

concept of near-margin gob gas ventholes. In spite of the 

problems, analysis of 7 months of gas production data is 

encouraging. Figure 10 presents 7 months of cumulative 

methane production for individual gob gas ventholes on 

panels 6 and 7. Panel 6 was chosen as a general comparison for the performance of gob gas ventholes on the experimental panel because of its proximity. Also, it appears 

that, for some unknown reason, the gob gas holes in this 

part of the study area produce simcantly less gas than 

those on the five adjacent panels. Gob gas ventholes on 

panels 6 and 7 produced only 4.73 x 106 and 9.34 x 106 m3, 

respectively; whereas panels 1 to 5 produced between 

18.55 x 106 and 22.65 x 106 m3 through the completion of 

mining (table 1). 

There are several possible reasons why the gas production on panel 7 was double that of panel 6. Panel 6 

was completed approximately 1.5 years before mining on 

panel 7 began. This delay may have induced a flow of 

additional gas from the downdip coal reserves into the 

area of panel 7, similar to that speculated for panel 1 (I). 

Another possibility is that more gas was produced simply 

because 14 holes were drilled on panel 7, compared with 

KEY 

1.98 40 

Before : After I mlnlng mining 

Near-margin hole - Direction of mining 

BOREHOLE DESIGNATION 

3, 9 

- - 

2 - Panel 6 

1.68 1 - Total 

0 200 400 600 800 1,000 

Scale, rn 

- 

1 - 

W 

Figure 10.--Histogram of 7 months of methane production from gob gas ventholes on pnds 6 and 7. 

6 3 

.71 .Is. 5.32X 10 m 

41 - 

L 

1,204m e 

(0 

43 v; COY) 

P 

In m m 

i - z 5s - !c !E 

2 BOREHOLE DESIGNATION 2 t; 

=, 

J0 3 - 

=,g ZcL 2 - Panel 7 

3 1.821.98 - 

0 - 1.51 Total 

1 - .94 .93 .77 - .25 11.53 x lo6 rn3 

-37 .33 .32 'P.24 - 

I 13 . , 01 Y 
7 holes on panel 6. In fact, the average production to 

panel completion for both panels was identical at 0.68 x 

lo6 m3 per hole (table 1). A third possibility is that mining 

time may have been a factor. Panel 6 took 8.1 months to 

complete, primarily because of equipment and ground control problems. Panel 7, even with the methane control 

problems, only took 4.5 months to complete. It is possible 

that with the additional time, a higher proportion of the 

total gas released during mining of panel 6 was removed 

by the ventilation system. 

The comparison of relative effectiveness of the nearmargin holes versus centerline holes on this panel will be 

made on an actual production time basis for each hole. 

The comparison of gob gas production between panels 7 

and 6 will also be on this basis. This is in contrast to the 

previous discussion of panels 1 to 5, where the results were 

evaluated on a time basis relative to completion of each 

panel. 

Table 2 presents the cumulative production through 

7 months for each gob gas venthole on panel 7. The frrst 

five holes on the 216-m-wide panel were drilled near the 

centerline; the remaining nine holes were drilled from 16.8 

to 59.4 m from the downdip panel margin. The two nearmargin holes at the completion end of the panel (163-A 

and 163-B) are not included in the following analysis. As 

shown on figure 1, there were four experimental horizontal 

methane drainage holes drilled into the strata above the 

Lower Kittanning Coalbed from the completion end of this 

panel. These holes began producing gas approximately 

1 month prior to the interception of gob gas venthole 

163-A, and continued through the completion of the panel 

(fig. 11). It is probable that the 0.43 x 106 m3 of methane 

produced from these horizontal holes and their general 

diversion of gob gas away from the downdip margin of the 

panel adversely affected the gob gas production of holes 

163-A and 163-B. 

Because of the methane emission problems being experienced on panel 7, the mine reactivated several abandoned gob gas ventholes on the adjoining panel 6 (fig. I), 

in an effort to remove or divert gas from the active mining 

area. Because of the location relative to the near-margin 

holes at the completion end of panel 7, gas production 

from two of these holes on panel 6, 156-A and 156-B, 

probably contributed to the low production from holes 

163-A and 163-B, as did the horizontal gob gas ventholes. 

The cumulative methane production curves for holes 

156-A and 156-B, plus a total cumulative production curve 

for these holes and the horizontal gob gas ventholes is 

shown in figure 11. The total cumulative methane production from these additional methane drainage holes increased substantially from the time near-margin holes 

163-A and 163-B on panel 7 were intercepted by mining, 

reaching nearly 1.4 x 106 m3, at the completion of the 

panel. In view of this additional gas production in the 

vicinity of the near-margin holes 163-A and 163-B at the 

completion end of panel 7, the exclusion of these holes 

from the evaluation of the effectiveness of the near-margin 

holes is substantiated. 

Table 2.--Cumulative methane production data for indlvldual gob gas ventholes, panel 7 

Hole1 1 month 2 months 3 months 4 months 5 months 6 months 7 months 

Cumulative pr~duction,~ lo6 m3: 

1613 ................................... 1.07 

161 -A~ .................................. .24 

161-c3 .................................. .25 

161 -B~ .................................. .42 

161-F.. ................................. .23 

161-9 .................................. .17 

161-D .................................. .85 

162 .................................... .72 

1624 .................................. .73 

162-A ................................... .69 

162-8 .................................. .26 

163 .................................... .41 

16%~~ .................................. .19 

163-Et .................................. -22 

1.19 

NAP 

w 

.59 

N Ap 

NAp 

1.64 

1.40 

NAP 

1.14 

NAP 

.71 

NAP 

.31 

1.26 

NAp 

N Ap 

-63 

NAp 

NAP 

i .n 

1.58 

NAP 

1.23 

N Ap 

.8 1 

NAP 

.34 

1.32 

N Ap 

N Ap 

-66 

NAp 

NAp 

1.83 

1.63 

NAP 

1.33 

NAp 

.84 

NAp 

.37 

1.51 

NAp 

NAp 

.n 

NAp 

NAp 

1.98 

1.82 

NAP 

1.51 

NAP 

.94 

N Ap 

NAp 

Average production,' lo6 m3: 

All producing holes ........................ .50 .n 1.11 1.21 1.27 1.34 1.42 

Centerline holes ........................... .43 .58 .89 .95 .99 1.06 1.14 

Near-margin holes ......................... .56 .90 1.22 1.35 1.41 1.48 1.56 

Percent difference, near-margin versus centerline 

holes5 ................................. +30 +55 +37 + 42 +42 +40 +37 

NAp Not applicable. 

'Listed in order of position panel. 

2~hrough the final month of production for each individual hole. 

3~oles drilled near the centerline. 

4~oles not included in analysis because of probable interference of horizontal gob gas ventholes and gob gas ventholes 156-A and 

156-8, panel 6. 

'calculated for holes on production during a particular time period. 
rn 

b Relative position of 

5 gob gas ventholes , panel 6 

J. 

KEY 

- Total 

----- 156-A 

-.- Horizontal gob gas holes 

-- 156-B 

4 

3 Position of hori zonta I 

2 gob gas ventholes, panel 7 

1 

90 + 

Completion * rc) m 

? 

CU Position of gob gas Startup 

panel 7 'L) 2 5. 5. f ventholes, panel 7 J panel 7 1 

FACE POSITION, date 

955 900 800 700 600 500 400 300 200 100 0 

DISTANCE FROM STARTUP END OF PANEL 7, m 

flgure 11.--Cumulative methane production curves for gob gas venthole8 156-A and 156-8, panel 6, and horizontal gob gas 

ventholer, panel 7, rdatlve to panel 7 face posttion. 

A si@cant volume of methane (0.4 x 106 m3) had 

been diverted away from the downdip margin of panel 7 

by the time near-margin hole 163-A was intercepted by 

mining and began producing gas. In fact, reactivated gas 

production from gob gas venthole 156-A on panel 6 began 

about 2.5 months prior to the start of gas production from 

hole 163-A on panel 7. It is even possible that this early 

production from hole 156-A as well as that from hole 

156-B and the horizontal gob gas ventholes reduced the 

gas production from earlier near-margin holes, in particular holes 163 and 162-B (fig. 11). However, these holes 

were included in the evaluation of the relative production 

levels of centerline and near-margin gob gas ventholes. 

All but two holes on panel 7 produced gas for at least 

1 full month (table 2). Holes 161-A and 161-F produced 

for only 9 and 20 days, respectively. The average methane 
production for 1 month or less for the 12 holes being 

evaluated was 0.5 x 106 m3. As on most of the other 

panels in the study area, a centerline hole on the end of 

the panel (161) was the highest producer (1.07 x 106 m3). 

At 1 month of production, the average methane production from the near-margin holes (0.56 x 106 m3 per hole) 

was 30 pct greater than that of the centerline holes 

(0.43 x 106 m3 per hole). 

Six holes on panel 7 produced gas for 7 months (table 2), including two centerline holes and four near-margin 

holes. The four near-margin holes produced 37 pct more 

methane (1.56 x lo6 m3 per hole) than did the two holes 

located near the centerline (1.14 x 106 m3 per hole). If the 

average cumulative methane production for all the centerline and near-margin holes (except 163-A and 163-B) is 

compiled over the entire 7 months, then the seven nearmargin holes produced an average of 1.10 x 106 m3 per 

hole, or 77 pct more than the 0.62 x lo6 m3 per hole for 

five centerline holes. 

Of further interest is the observation that the average 

cumulative methane production (1.56 x lo6 m3) for the 

four near-margin holes that produced for 7 months was 

higher than the cumulative production (1.51 x 106 m3) 

from the best producing centerline hole (161), located at 

the startup end of the panel. In fact, two near-margin 

holes (161-D and 162) produced 31 and 21 pct more 

methane, respectively, than did hole 161. In addition, 

near-margin hole 162-A produced as much methane as 

hole 161. This is in marked contrast to the results from 

the fust five panels, where end holes were predominantly 

the highest and longest producers. 

The methane production results from gob gas ventholes 

on panel 7 can also be compared with those from panel 

6 (table 3). The holes on panel 6 were drilled slightly 

off the centerline toward the updip tailgate side of the 

panel (fig. 1). The average methane production for the 

seven holes on panel 6 after 1 month of production was 

0.46 x 106 m3 per hole. This is slightly more than the 

0.43 x 106 m3 per hole average for near-centerline holes 

on panel 7. However, the near-margin holes on panel 7 

vented 22 pct more methane through the fust month of 

production than the average hole on panel 6. 

Three holes on panel 6 produced for at least 5 months, 

for an average cumulative methane production of 0.83 x 

106 m3 per hole. The two centerline holes that produced 

for at least 5 months on panel 7 averaged 0.99 x 106 m3 

per hole, or 19 pct more than those on panel 6. The four 

near-margin holes on panel 7 that vented methane for 

5 months averaged 1.41 x 106 m3 per hole, or 70 pct more 

than the three centerline holes on panel 6. 

The supposition that greater fracture permeability 

exists for a longer time near the panel margins, thus 

allowing for a longer gas production life, in addition to 

higher production rates, for near-margin gob gas ventholes 

can also be evaluated with the available data. Only one 

gob gas venthole (156-A) out of seven on panel 6 remained on production for at least 7 months (table 3). In 

contrast, 6 gob gas ventholes (out of 14), including 4 nearmargin holes (table 2) were still producing methane after 

7 months on panel 7. 

Table 3.--Cumulative methane production data, lo6 cubic meters, for individual 

gob gas ventholes, panel 6 

- - 

Hole1 1 month 2 months 3 months 4 months 5 months 6 months 7 months 

Cumulative prod~ction:~ 

155 ..................... 0.70 0.74 0.76 NAP NAp 

1 55-A 

NAP NAP .................... .39 .4 1 N Ap N Ap NAp NAp NAp 

155-5 ................... -53 .71 NAp NAP N Ap NAp NAp 

155-C ................... .4 1 .50 .5 1 N Ap NAp NAp N Ap 

156 ..................... -26 .40 .46 .47 0.51 0.55 NAp 

156-8 ................... .32 .57 .67 -69 .70 NAp N Ap 

1 56-A .................... .63 .86 1 .O1 1.14 1.27 1.48 1.68 

Average production3 ........... .46 .60 .68 .77 .83 1.02 N& 

NAp Not applicable. 

'~isted in order of position on panel. 

2~hrough the final month of production for each individual hole. 

'calculated for holes on production during a particular time period. 
The evaluation of gob gas venthole methane production at the Cambria No. 33 Mine has revealed several 

interesting observations, and has led to a test of a change 

in hole placement that may improve performance. Perhaps the most obvious observation relative to gob gas 

venthole production is the large degree of variability. 

Cumulative production for the 82 holes in the 7-panel 

study area has ranged from 0.03 x 106 to 10.1 x 106 m3. 

Low production rates can in some cases be attributed to 

drilling and completion problems, such as inadvertently 

cementing the casing slots or the mining-induced fractures 

in the gob. In several instances, the presence of water in 

the overlying strata impeded the flow of gob gas. 

Mining rates were found to have a general influence on 

gob gas venthole production rates. As the time to mine a 

panel decreased, the average gob gas venthole production 

rate on a unit time basis increased. This is because a 

relatively finite volume of methane is contained in the 

strata influenced by the longwall mining of the coalbed. 

Therefore, if the panel is extracted in a shorter time, the 

finite volume of gas will be encountered at an accelerated 

rate. This is of particular concern since the industry trend 

in coal mining is toward the use of more efficient mining 

equipment and larger dimension longwalls to increase 

productivity. It is quite clear that with increased coal 

production, it is highly likely that a corresponding increase 

in methane emissions will be encountered on a unit time 

basis. 

The analysis of production volumes within the study 

area also revealed that gob gas ventholes on the ends of 

the panels tended to have the highest cumulative methane production and produced for a longer time. This is 

probably related to the characteristics of overburden response after the longwall panel is extracted. A zone of 

tension and more effective fracture permeability exists 

close to the ends of the panels because of the support of 

the overburden on three sides by the surrounding coal 

pillars. A similar zone of tension also exists along the 

margins of the panel. This has led to the supposition that 

gob gas ventholes placed in this area, instead of in the 

more traditional centerline location, might produce gob 

gas at a higher rate and for a longer time. 

A test of this concept was conducted in the study area. 

However, the presence of high volumes of water that 

frequently interfered with gob gas venthole production 

made this a less than optimum test site. In spite of the 

problems on the test panel, the test results are encouraging. The four near-margin holes that produced for 

7 months on the test panel had a 37-pct higher average 

production than the two centerline holes that produced for 

the same length of time. The average cumulative methane 

production for the seven near-margin holes on the panel 

was 77 pct higher than for the five centerline holes. The 

near-margin holes produced more gas and were on production for a longer time than the centerline holes. Further testing of the near-margin gob gas venthole concept 

is needed to substantiate the results of this study. However, this novel concept of placing gob gas ventholes along 

the margin of a longwall panel to take advantage of the 

increased fracture permeability in that area may provide a 

very cost-effective method of increasing mine safety. 

REFERENCES 

1. Diamond, W. P., J. P. Ulery, and S. J. Kravits. Evaluation of the 

Source and Migration of Longwall Gob Gas: Lower Kittanning Coalbed, 

Cambria County, PA. BuMines RI 9430, 1992, 15 pp. 

2. Aul, G. N., and R Ray. Optimizing Methane Drainage Systems 

To Reduce Mine Ventilation Requirements. Paper in Proceedings of 

the 5th U.S. Mine Ventilation Symposium (Morgantown, WV). WV 

Univ., 1991, pp. 638456. 

3. Jeran, P. W., and V. Adamek. Subsidence Over the End of a 

Longwall Panel. BuMines RI 9338, 1991, 8 pp. 

4. Adamek, V., P. W. Jeran, and M. A. Trevits. Static and Dynamic 

Subsidence Prediction in the Northern Appalachian Coal Region Based 

on the Use of a Variable Subsidence Coefficient. Paper in Proceedings 

of the Third Workshop on Surface Subsidence Due to Underground 

Mining (Morgantown, WV). WV Univ., 1992, pp. 10-21. 

5. Jeran, P. W., V. Adamek, and M. A. Trevits. A Subsidence 

Prediction Model for Longwall Mine Design. Proceedings of the 

Longwall USA Conference (Pittsburgh, PA). MacLean Hunter, 1986, 

pp. 101-112.