*Presently at the National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory. 

 

CHARACTERIZATION OF GAS FLOW IN LONGWALL GOBS:

PITTSBURGH COALBED, PA

W. P. Diamond, S. J. Schatzel, F. Garcia, J. C. LaScola, F. E. McCall, and

P. W. Jeran, National Institute for Occupational Safety and Health (NIOSH),

Office for Mine Safety and Health Research, Pittsburgh Research Laboratory;

and T. P. Mucho*

, Cyprus Emerald Resources Corporation

ABSTRACT

The National Institute for Occupational Safety and Health, Office for Mine Safety and Health Research, is

investigating the geologic and mining factors influencing the migration of longwall gob gas. One goal of these

studies is to optimize methane drainage strategies to reduce the volume of gob gas that reaches the underground

workplace. As part of this research effort, longwall gob gas flow paths in a mine operating in the Pittsburgh

Coalbed, PA, were investigated using SF6 tracer gas. Tracer gas released into the ventilation airflow underground

generally stayed in the bleeder system and was vented at the associated bleeder fan shafts. Tracer gas injected into

an inactive (intaking) gob gas venthole migrated to the two inby producing ventholes on the study panel, indicating

that all three ventholes were in communication. The tracer gas that was injected into the gob generally stayed in the

gob and only migrated to the ventilation system in response to the venthole on the study panel closest to the injection

hole going off production. Tracer gas flow velocities in the longwall gob reservoir and factors influencing the

production performance of gob gas ventholes in the study area are also discussed.

INTRODUCTION

As part of its Mine Safety and Health research program, the National Institute for Occupational Safety and

Health's Pittsburgh Research Laboratory is investigating methane emissions associated with underground coal

mining. The current focus of this research program is to develop a better understanding of the influence of geology,

mining and ventilation practices, and methane drainage on the release and migration of methane gas during longwall

mining [1-5]. The objective of the current phase of the study is to measure gas migration characteristics associated

with longwall gobs using a tracer gas to simulate methane flows. The knowledge gained from this research will be

used to optimize longwall methane control strategies to intercept a larger portion of the gob gas before it enters the

underground workplace. 

Research completed to date includes seven tracer gas releases into underground ventilation airways between

mined-out longwall panels. Additionally, one release, which is the focus of this paper, was made into an inactive

(intaking) gob gas venthole. This combination of release modes allowed for a complete evaluation of the interaction

(or lack thereof) between gob gas ventholes and the ventilation system and between individual gob gas ventholes on

the same longwall panel.

STUDY AREA

The study mine operates in the Pittsburgh Coalbed in Greene County, PA. Longwall panels in the study area

(Fig. 1) were generally 253 m (830 ft) wide initially (Panels A+ through E) and increased to 305 m (1,000 ft) starting

with F Panel. Panel length generally increased with each successive panel. G Panel at the southern end of the study

area where the SF6 borehole injection experiment was conducted had a mined length of approximately 32,713 m

(8,900 ft) (Fig. 2). Overburden depths ranged between 152 and 274 m (500 and 900 ft).

Methane control for the study area includes bleeder fan shafts and gob gas ventholes. Bleeder exhaust fans (BF2

and BF3), located at the top of 1.8-m (6-ft) -diameter air shafts, are designed to remove a portion of the gas liberated
2 CHARACTERIZATION OF GAS FLOW IN LONGWALL GOBS: PITTSBURGH COALBED, PA

during active mining on the current panel, as well as gas released in the large gob area formed by the previously

mined-out panels (Fig. 1). Commonly, three gob gas ventholes are located on the tailgate side of each panel in the

near-margin configuration developed in earlier research efforts [2]. The gob gas ventholes are equipped with

exhausters powered by the produced gas.

TEST PROCEDURES AND EQUIPMENT

General

The general concept of the gas flow characterization experiments is to release a defined volume of tracer gas into

the ventilation airflow or longwall gob and subsequently monitor (collect gas samples) all potential exit points for

the gas. The tracer gas selected for use in these studies was sulfur hexafluoride (SF6). SF6 is a nontoxic, colorless,

odorless, nonflammable gas commonly used in underground mine ventilation studies [6-8]. It is chemically and

thermally stable, but is heavier than air, requiring its dispersal and mixing to a low concentration (<100 ppm of SF6

in air) to simulate the movement of the host air stream.

By determining the tracer gas concentration (if any) in the periodic gas samples taken at the various monitoring

locations and measuring the associated airflow rate, the volume of tracer gas passing through each monitoring site

can be calculated. Thus, the relative distribution of tracer gas flow to the various outlet points (bleeder fan shafts,

producing gob gas ventholes, underground ventilation system) can be determined. The velocities associated with the

identified gas flow pathways can be calculated knowing the path length and travel times for tracer gas flow to each

monitoring location. These basic gob gas flow characterization data will be used to model gob gas reservoirs and

optimize methane control strategies.

Gas Sampling Procedures and Equipment

The periodic gas samples necessary to determine the presence (or absence) of the released tracer gas at the

monitoring stations were collected in 20-ml glass sample tubes. Gas samples were obtained in two ways during the

experiments. Primary sampling was accomplished with an automated gas sampling (AGS) system (Fig. 3). The

automated system was supplemented with manual sampling during critical time periods, primarily just after a tracer

gas release, to better identify the first arrival of the tracer gas at a monitoring site.

Operationally, the complete AGS system consisted of the automated gas sampling device sealed inside an airtight housing and a permissible, programmable air sampling pump external to the sealed container (Fig. 3). The

intake side of the pump was connected via tubing to the AGS housing, which in turn was connected to the mine

air/gas production point to be monitored, i.e., either the bleeder fan housing or the surface piping system of a gob gas

venthole. Thus, when the air sampling pump was on, the atmosphere inside the AGS housing was evacuated, pulling

the mine air/gob gas to be sampled into the container. SF6 concentration data were obtained by gas chromatography

(GC). 

UNDERGROUND RELEASES

General

The locations for the seven underground SF6 releases were chosen to assess a complete geographic distribution of

gas flows within the study area (Fig. 1). The releases were conducted at the completion (west) ends of the panels

because the ventilation airflow for this part of the mine was directed towards the back (east) ends of the panels

where the two bleeder fan shafts are located. Prior to the underground tracer gas releases, ventilation surveys were

conducted to determine the airflow volumes and velocities in the vicinity of the proposed release sites. These data

were used to design each individual release, including tracer gas volume and release method, i.e., fast versus slow. 

The ventilation surveys were also used to determine potential locations where tracer gas might flow outby the release

points to the main return airways, which would necessitate underground monitoring for the presence of tracer gas. 

Release Specifications

Two methods of tracer gas release (fast or slow) are generally used for underground airflow studies [6-8]. A fast

release would empty a standard 0.04-m3

 (1.3-ft3) lecture bottle in as little as 30 seconds, producing a high peak
 3

concentration in the air stream at the release point and, subsequently, a short-duration, high-concentration peak at the

monitoring locations. The results of the development work for this study determined that an air velocity of about

10.2 m/s (2,000 ft/min) was generally the minimum airflow acceptable for a successful fast release of tracer gas. 

Most of the releases in this series of experiments were fast.

The slower release method is generally used in low airflow conditions to ensure the mixing of the SF6 in the

airstream. A slow release is expected to produce a longer duration, lower concentration peak at the monitoring

locations than would be observed with a fast release. The slow release method empties a standard lecture bottle in

about 20 minutes. The slow release method was required for only one underground release (Test 2-0). 

Results

The SF6 recovery percentage for monitoring locations where tracer gas from each underground release was

detected is summarized in Table 1. Data relative to the SF6 concentration curves for each release, including first

arrival time, duration (time interval from first to last measurable SF6 concentration value), and the highest measured

concentration are summarized in Table 2. 

Most of tracer gas released into the underground ventilation airflow stayed in the ventilation system and the flow

paths were inby the release points, i.e., towards the back (start-up) end of the panels, where it was vented at one of

the bleeder fan shafts. On three occasions (Tests 2-0, 2-3 , and 2-5), tracer gas released into the underground

ventilation airflow was detected at a gob gas venthole. All tracer gas recoveries (percent of total release volume) at

gob gas ventholes were low, with the highest being only 0.7% at hole G2 during Test 2-5 (Fig. 2). The trace amount

recovered at gob gas venthole A+1 during Test 2-3 consists of only two samples with SF6 over the 0.12 ppb lowest

detection limit (LDL) for the GC, and the results from Test 2-0 (holes G2 and G3) consist of only a single sample

over the LDL. On two occasions (Tests 2-4 and 2-5, Fig. 1, Table 1), a small amount of tracer gas was diverted from

the release location back towards the main mine fan. 

G3 BOREHOLE INJECTION

General

The final tracer gas release experiment conducted at the test site in 1997 was an injection into gob gas venthole

G3. Venthole G3 is located on the completion (west) end of G Panel, approximately 76 m (250 ft) off the centerline

[tailgate side of the 305 m (1,000 ft) wide panel], 381 m (1,250 ft) from the longwall recovery room and 2,332 m

(7,650 ft) from the start-up end (Fig. 2). Structurally, the Pittsburgh Coalbed at the injection hole (G3) was about 1.2

m (4 ft) and 21 m (69 ft) higher than at holes G2 and G1, respectively. It was anticipated that an injection of tracer

gas directly into the gob and subsequent monitoring of probable outlet points would provide significant insight into

the migration of gas through the gob. At the time of the release on July 21, 1997, it had been approximately 5

months since hole G3 had been mined through, and 3.6 months since G Panel had been completed. The adjoining H

panel longwall face was approximately 701 m (2,300 ft) east of hole G3 and 55 m (180 ft) west of hole G2 at the

time of the injection (Fig. 2). Hole G3 was inactive at the time of the injection, and had not produced gob gas for 2.3

months. The hole was shut in, but when opened, it would intake into the gob. These conditions were ideal for

dispersal of the tracer gas into the gob. 

All well-head equipment, except for the gate valve, was removed from gob gas venthole G3 prior to the injection

experiment. A new well-head assembly (Fig. 4) was installed on G3 to facilitate the tracer gas injection and

subsequent monitoring of the shut-in pressure. Depth to the Pittsburgh Coalbed at the G3 site was approximately

191 m (628 ft). The borehole (Fig. 5) was originally drilled to a depth of 179 m (588 ft), and 17.8-cm (7-in) casing

with 61 m (200 ft) of slotted pipe on the bottom was installed. The pipe above the slotted casing was cemented in

place. The slots were approximately 5.1 cm × 0.3 m (2 in × 1 ft), oriented vertically, with four rows (two slots/row)

for each 6 m (20 ft) length of pipe in the 61-m (200-ft) slotted interval. Approximately 12 m (40 ft) of strata were

present between the bottom of casing and the top of the Pittsburgh Coalbed prior to this location being intercepted by

mining and the subsequent collapse of the overburden into the mine void.

Release Specifications

The original concept for the borehole injection experiment was to inject the tracer gas into the borehole via

tubing at a point near the bottom of the borehole. However, an obstruction was encountered at 42 m (137 ft),

limiting the depth for the release. Two 0.3-cm (1/8-in) -ID polyethylene lines were installed in the hole, one to a
4 CHARACTERIZATION OF GAS FLOW IN LONGWALL GOBS: PITTSBURGH COALBED, PA

depth of 37 m (120 ft) to release the tracer gas and the other to a depth of 34 m (110 ft) for periodic monitoring of

the SF6 concentration after the release. G3 was open and intaking [at approximately 0.03 m3

/s (66 cfm)] for a short

time before and after the SF6 release to aid in dispersing the tracer gas into the gob. After the injection into G3 was

completed, the hole was shut in. A 0 to 6,089 kPa (0 to 1 psi) range pressure transducer was attached (at the surface)

to the polyethylene line installed to the 37-m (120-ft) depth to continuously monitor the shut-in pressure at the G3

location (Fig. 4). 

The tracer gas release into gob gas venthole G3 was the largest (by volume) of the study, with 0.27 m3

 (9.6 ft3)

(STP) of SF6 being released over a 25-min time interval. Underground tracer gas releases into areas of low airflow

velocity (as was expected at the release point in G3) had produced relatively low recovery rates. In an attempt to

enhance the recovery of the SF6 during the borehole injection experiment, the decision was made to significantly

increase the volume of the tracer gas released compared to that of the prior underground releases and to utilize the

slow release mode. 

Monitoring

The monitoring strategy for the borehole injection experiment included sampling all outlet points for the most

likely migration path(s) for gas released into the gob from the G3 gob gas venthole location. The probable migration

pathways were determined by evaluating a combination of factors, including ventilation surveys, methane drainage

system configuration, and results from the previous underground releases. The available data indicated that the most

likely direction of flow for the tracer gas released into G3 was towards the back (start-up) end of the panel, primarily

due to the influence of the two bleeder fan shafts and the two producing gob gas ventholes on the panel (Figs. 1 and

2). The two producing gob gas ventholes on G Panel (G2 and G1), as well as producing gob gas venthole F1 on the

adjoining F Panel, and the two bleeder fan shafts (BF2 and BF3) were equipped with AGS systems. In addition,

monitoring stations (FR and GR) were established underground (Fig. 2) in the event that any of the tracer gas was

diverted back towards the main mine fan. 

Results

Tracer gas was not detected at either of the underground monitoring locations or at gob gas venthole F1. The

first arrival of tracer gas after the release into gob gas venthole G3 was at gob gas venthole G2, the closest inby

producing hole on the same panel (Fig. 2). Tracer gas was first detected 1.1 days (27 h) after the release at

concentrations of 0.54 ppb (Fig. 6). The velocity (first arrival) of tracer gas flow from the release point in G3 to the

measurement point at the surface on hole G2 is 0.012 m/s (2.40 ft/min). The path length (for the velocity

calculations) includes the distances traveled vertically down the injection borehole, the horizontal distance (straight

line) through the gob, and the vertical distance up the monitored boreholes. Peak concentration was 1,308.59 ppb,

7.1 days after the release. As can be seen on Figure 6, hole G2 was taken off production on day 25 before tracer gas

had completely cleared this location. The hole was shut in due to insufficient methane concentrations to run the

methane fueled engine that powered the exhauster. At the time that hole G2 was taken off production, 23.9 days

after the SF6 tracer gas was first detected, 75.3% of the injected gas had been recovered at this location (Fig. 7, Table

1), and the SF6 concentration was still at 110.85 ppb (Fig. 6).

The second monitored location where SF6 tracer gas was detected was gob gas venthole G1, the next inby

producing hole on the same panel (Fig. 2). Tracer gas was first detected 15.3 days after the release at concentrations

of 0.43 ppb (Fig. 8). Peak SF6 concentration was 43.65 ppb, 25.1 days after the release. The significantly lower peak

concentration level as compared to that at hole G2, is an indication that the injected gas had become much more

widely dispersed in the gob atmosphere as it migrated inby past hole G2. Flow velocity (first arrival) to this location

was 0.002 m/s (0.36 ft/min), which is significantly slower than the 0.012 m/s (2.40 ft/min) calculated for flow to G2. 

It is important to note that the exhauster on hole G1 stopped running late on the first day of the release due to

mechanical problems and was not back on production until day 9. It seems likely that if gob gas venthole G1 had

been on production the entire time, tracer gas would have migrated to this location in a shorter time. Total SF6

recovery (287.8 days duration) from G1 was 3.2% (Fig. 7, Table 1).

The only other confirmed location to which tracer gas migrated from the G3 borehole injection experiment was

bleeder fan shaft 2 (Fig. 1). Tracer gas arrived at this location on day 30 after the release (Fig. 9, Table 2). Tracer

gas was detected at BF2 only 5 days after gob gas venthole G2 went off production on day 25 of the test. The tracer

gas probably migrated out of the gob into the mine’s ventilation system because of the disruption of the primary gob

gas flow path towards the diminishing low pressure sink of G2 after this hole went off production. Peak SF6
 5

concentration at BF2 was only 0.25 ppb on day 35, or about 2× the analytical lab’s detection limit. The low

concentration level at the bleeder fan shaft, compared to the gob gas ventholes, is due to the significantly larger

volume of ventilation air in which the tracer gas volume is diluted. At the time of the borehole injection experiment

in July 1997, the total airflow from BF2 ranged from about 60 to 75 m3

/s (120,000 to 150,000 cfm), compared to

only about 0.17 m3

/s (330 cfm) at G2 and G1. The duration of tracer gas at BF2 was 9.2 days, for a total recovery of

4.5% (Fig. 7, Table 2). 

Discussion of Borehole Injection Test Results

The primary findings from the borehole injection experiment were: (1) most of the tracer gas that was injected

into the gob stayed in the gob and was recovered from the ventholes inby the injection borehole, and (2) all three gob

gas ventholes on the test panel were in communication with each other. Eighty-three percent of the tracer gas

injected into gob gas venthole G3 was recovered (Fig. 7). Only 4.5% of the released tracer gas was recovered at a

location (BF2) outside the longwall panel. Tracer gas was first observed at BF2 approximately 5 days after gob gas

venthole G2 went off production. 

It is speculated that the presence of tracer gas in the ventilation system (BF2) is a direct result of G2 going off

production. In addition to the time coincidence factor noted above, the close proximity of the gob gas venthole

locations to the gate roads (ventilation system) on the tailgate side of the panel provides additional support for a

connection between the arrival of the tracer gas at BF2 shortly after the loss of active gas production at G2. 

Injection hole G3 is only about 76 m (250 ft) from the gate roads on the tailgate side of the panel, a considerably

shorter flow path than the 762 m (2,500 ft) between G3 and G2 (Fig. 2). If the pressure sink created by active gob

gas production (aided by the exhauster) at G2 had not been so dominating, it would seem likely that tracer gas would

have been able to flow the 76 m (250 ft) to the gate roads from G3 in a shorter time than the observed 30 days. 

Therefore, when G2 went off production, tracer gas flow from the vicinity of G3 was eventually influenced by the

pressure sink of the ventilation system. As the pressure sink in the gob diminished in the vicinity of G2, some of the

remaining tracer gas at the completion end of G panel was able to migrate to the ventilation system and was

eventually detected at BF2. No other occurrence of tracer gas was observed at any location outside the longwall

panel.

One important caution must be noted at this point relative to the observations of gas flow paths discussed above. 

At the time that the borehole injection experiment was conducted, G panel had been completed for 3.6 months, and

considerable gob gas had already been removed from the panel via the three gob gas ventholes. It does not

necessarily follow that similar results, i.e., minimal gas flow into the ventilation system from the gob would be

observed when the longwall was active. During active mining on the panel, some (but at this point unknown)

portion of the gob gas probably reaches the ventilation system, especially if the exhausters on the gob gas ventholes

are undersized. It is also possible that gob gas may enter the ventilation system on the headgate side of the panel

during longwall mining, because the gob gas ventholes are placed on the tailgate side. Gas may also reach the

mine’s ventilation system via mining-induced fractures that extend into the gas bearing overburden strata beyond the

footprint of the mined-out longwall panels. These additional pathways for gas flow are important unknowns that

will be addressed in subsequent investigations.

All three gob gas ventholes on G Panel were shown to be in communication with each other, as indicated by the

detection of the tracer gas injected into G3 at both of the inby ventholes. A second line of evidence that supports

communication between the three gob gas ventholes on the study panel is that variations in gas production at G1 and

G2 influenced the shut-in pressure at G3, and in some instances, production from each other. As can be seen in

Figure 10, at the start of the borehole injection experiment when G3 was shut in, both G1 and G2 were producing

gob gas. The shut-in pressure at G3 was decreasing, as would be expected with the extraction of gas from the gob

with the aid of an exhausters via holes G1 and G2. However, at 0.9 days of the experiment, G1 went off production. 

Concurrently, the shut-in pressure at G3 began to rise. G1 was off production for approximately 8 days, during

which time the shut-in pressure at G3 stabilized at about !0.1866 kPa (!7.5-in H2O), and gas production from G2

rose slightly. On day 9, production was restored on G1 for about 4 days, before going off production again for 2

days. The shut-in pressure at G3 declined again because of the extraction of gob gas from G1 during the 4 days of

production, and gob gas production from G2 declined. During the short time (2 days) that G1 was off production

again, the shut-in pressure at G3 increased, as did the production from G2. When G1 came back on production on

day 15, the shut-in pressure at G3 and gas production from G2 once again decreased. 

The SF6 migration time (first arrival) from the injection point in G3 to the next gob gas venthole inby the release

point was relatively quick (1.1 days) considering that it had been approximately 5 months and 6 months since G3
6 CHARACTERIZATION OF GAS FLOW IN LONGWALL GOBS: PITTSBURGH COALBED, PA

and G2, respectively, had been undermined. With this amount of time for compaction to have taken place in the gob

between G3 and G2, and the obstruction at 42 m (137 ft) in the injection borehole, it is somewhat surprising that a

velocity (first arrival) of 0.012 m/s (2.40 ft/min) was observed between these two gob gas ventholes. The maximum

migration path length [1,185 m (3,888 ft)] includes the 158 m (518 ft) from the injection point in G3 to the top of the

coalbed at that location, the horizontal distance between G3 and G2 [762 m (2,500 ft)], and the 265 m (870 ft) from

the bottom of G2 to the surface monitoring location.

As mentioned previously, gob gas venthole G1 was off production for approximately 8 days, beginning at 0.9

days after the tracer gas release. The calculated velocity (first arrival) for the 15.3 days of total elapsed travel time

for the tracer gas from the injection point in G3 to the surface monitoring location at G1 is 0.002 m/s (0.36 ft/min). 

However, if the time G1 was off production (and the velocity presumably reduced), is subtracted from the total test

time before the first arrival of SF6 at G1, then a velocity of 0.004 m/s (0.77 ft/min) is calculated as an estimate for

gas flow to gob gas venthole G1, if it had remained on production early in the experiment. The maximum migration

path length [2,449 m (8,036 ft)] includes the 158 m (518 ft) from the injection point in G3 to the top of the coalbed at

that location, the horizontal distance between G3 and G1 [2,073 m (6,800 ft)], and the 219 m (718 ft) from the

bottom of G1 to the surface monitoring location. 

G2 was off production for approximately 2 days starting on day 20 of the experiment, at which time the shut-in

pressure at G3 rose by about 0.1742 kPa (7.0-in H2O) (Fig. 10). This is a significantly greater influence on the shutin pressure at G3 compared to the pressure increases observed when G1 was not producing. This is due to G2 being

1,311 m (4,300 ft) closer to G3 than is G1. Finally, by day 26, G2 was off production for the rest of the experiment,

causing a permanent rise in shut-in pressure at G3 to an average of about !0.0995 kPa (!4.0-in H2O). There was

also a slight rise in average gas production from G1 after G2 was off production.

EVALUATION OF GOB GAS VENTHOLE PERFORMANCE

Because the ultimate goal of the gob gas characterization studies is to optimize methane control strategies for

longwall gobs, it was necessary to document the baseline performance of the current gob gas ventholes at the study

mine. These baseline data would then be one benchmark against which the success of any suggested optimization

strategies could be judged. The gob gas venthole production data necessary for this part of the study were supplied

by the mine operator. The time interval between data points for gob gas venthole production volumes was widely

spaced, typically reported every 2 weeks. These widely spaced data made it difficult to correlate the gob gas

production to specific mining events or coal production rates. 

Inspection of the methane production curves for individual gob gas ventholes in the study area suggests that

ventholes on the start-up ends of the panels generally performed the best, especially on the later panels (D through

G, Fig. 11). To quantify this observation, the average gob gas venthole cumulative methane production by location

on the panel (start-up end, center, and completion end) was plotted (Fig. 12). It is quite evident that the average

cumulative methane production from gob gas ventholes on the start-up end of the panels is by far the highest of the

three general hole locations. The average cumulative methane production of approximately 4.13 × 106

 m3 (146

MMcf) for the holes on the start-up end of the panels is nearly 3.5 times the production from the holes located near

the center of the panels and more than six times the production from the holes located at the completion end of the

panels.

A combination of factors probably contributes to the high level of methane production from holes on the start-up

end of the panels. Holes on the start-up end of the panels are the first holes to come on production; therefore, they

have a time advantage for longer term production, which generally results in higher cumulative gas production. 

Also, a general flow direction for gas in the gob and ventilation airflow towards the start-up end of the panels is

established early by the presence of the first gob gas venthole on this end of the panels as well as the bleeder fan

shafts located at the back end of the panels. 

In contrast to the gob gas production trends established at this study site, studies by the former U.S. Bureau of

Mines at the Cambria 33 Mine operating in the Lower Kittanning Coalbed, Cambria County, PA, showed that gob

gas ventholes on both ends of the panels produced at the highest rates [2]. The Cambria 33 Mine study demonstrated

that given enough time, cumulative gob gas production from the holes on the completion end of the panels

eventually caught up to that of holes on the start-up end of the panels. The primary difference between these two

mine sites is that the Cambria 33 Mine did not use bleeder fan shafts on the back ends of the panels, but relied on the

main mine fans for bleeder ventilation.

Gob gas ventholes in the Pittsburgh Coalbed study area produced 3.94 × 106

 m3/qtr (139.0 MMcf/qtr) of methane

during 1996. By comparison, methane production from the two bleeder fan shafts average 7.6 × 106

 m3/qtr (268.5
 7

MMcf/qtr), or nearly double that of the gob gas ventholes during the same time period (sufficient bleeder fan shaft

data were not available to extend this comparison into 1997). There is some evidence to suggest that methane

production from one of the methane control systems influenced the other, i.e., when there was an increase or

decrease in methane production from the gob gas ventholes, there was a corresponding decrease or increase in

methane production from the bleeder fan shafts.

Based on the evaluation of gob gas venthole performance, it would appear that optimizing the performance of

gob gas ventholes outby those on the start-up end of the panels would probably have the greatest impact, potentially

reducing the number of ventholes to two per panel from the current three per panel. Additional data on gob gas flow

characteristics during active mining would be helpful for research efforts relative to improving gob gas venthole

performance.

SUMMARY

Tracer gas studies can be a valuable tool to study ventilation airflows associated with longwall gobs, bleeders,

and gob gas ventholes. The primary findings resulting from the analysis of the data obtained from the tracer gas

experiments and ancillary investigations at the study mine are as follows:

Underground Tracer Gas Releases

1. Tracer gas released into the ventilation airflow generally stayed in the bleeder system. A maximum of only 0.7%

of the tracer gas released from an underground location reached a gob gas venthole. 

2. Flow paths for tracer gas released at the completion ends of the panels were inby, i.e., towards the bleeder fan

shafts at the back (start-up) end of the panels.

Borehole Tracer Gas Injection

1. Tracer gas released into the gob generally stayed in the gob, as long as the associated gob gas ventholes were

operating. The flow of tracer gas (4.5% recovery) to BF2 is probably associated with gob gas venthole G2 going

off production 5 days prior to the SF6 arrival at BF2. 

2. The arrival of tracer gas at both G2 and G1 from the injection into G3 indicates that all three gob gas ventholes

on G panel are in communication.

3. Production variations at gob gas ventholes G2 and G1 influenced the shut-in pressure at injection hole G3, which

further confirmed that all three boreholes on G panel were in communication during the borehole injection

experiment.

4. The relatively quick arrival of tracer gas at G2 [1.1 days, 0.012 m/s (2.40 ft/min) velocity] indicates a relatively

high permeability flow path between these two points in the gob, even 5 and 6 months after G3 and G2,

respectively, were mined through.

Gob Gas Venthole Performance Evaluation

1. Gob gas ventholes located at the start-up end of the panels produced methane at higher rates for a longer time

than holes located in the central or completion end of the panels. 

2. The bleeder fan shafts generally produced more gas than the active gob gas ventholes during the same time

intervals.

ACKNOWLEDGMENTS
8 CHARACTERIZATION OF GAS FLOW IN LONGWALL GOBS: PITTSBURGH COALBED, PA

The interest and assistance of the management and engineering staff of the cooperating mine that provided access

and data for the successful completion of this study is gratefully acknowledged. It is through cooperative research

efforts of this kind that important mine safety and health innovations can be made readily available to the industry as

a whole.

REFERENCES

1. Diamond, W.P., J.P. Ulery, and S.J. Kravits. Determining the source of Longwall Gob Gas: Lower Kittanning

Coalbed, Cambria County, PA. USBM RI 9430, 1992, 15 p.

2. Diamond, W.P., P.W. Jeran, and M.A. Trevits. Evaluation of Alternative Placement of Longwall Gob Gas

Ventholes for Optimum Performance. USBM RI 9500, 1994, 14 p.

3. Diamond, W.P., and F. Garcia. Influence of Mine Design on Longwall Methane Emissions and Gas

Production from Methane-Drainage Boreholes. Paper in Proceedings of the 1997 International Coalbed

Methane Symposium (Tuscaloosa, AL, May 12-16, 1997) The University of Alabama, Tuscaloosa, AL, 1997,

p. 541-550.

4. McCall, F.E., F. Garcia, and M.A. Trevits. Methane Emissions During Longwall Mining. Paper in

Conference Papers, Longwall U.S.A. (Pittsburgh, PA, June 8-10, 1993) Maclean Hunter Mining and

Construction Group, 1993, p. 267-279.

5. Schatzel, S.J., F. Garcia, and F.E. McCall. Methane Sources and Emissions on Two Longwall Panels of a

Virginia Coal Mine. Paper in Proceedings of the Ninth Annual International Pittsburgh Coal Conference

(Pittsburgh, PA, Oct. 12-16, 1992) Univ. of Pittsburgh, Pittsburgh, PA, 1992, p. 991-998.

6. Vinson, R.P., F.N. Kissell, J.C. LaScola, and E.D. Thimons. Face Ventilation Measurement with Sulfur

Hexafluoride (SF6). USBM RI 8473, 1980, 16 p.

7. Timko, R.J., and E.D. Thimons. Sulfur Hexafluoride as a Mine Ventilation Research Tool - Recent Field

Applications. USBM RI 8735, 1982, 15 p.

8. Timko, R.P., F.N. Kissell, and E.D. Thimons. Evaluating Ventilation Parameters of Three Coal Mine Gobs. 

USBM IC 9109, 1986, 16 p.
 9

Table 1. - Recovery percentage of SF6 tracer gas at monitored locations

Test

No.

Bleeder fan Gob gas venthole Underground Total

BF2 BF3 A+1 D1 E1 F1 G1 G2 G3 CBR E1 E2 F1 F5 FR GR

2-01 D2 D ND2 D D NA2

2-1 66.0 ND ND 66.0

2-2 89.1 ND ND 89.1

2-3 57.4 ND Trace 57.4

2-4 85.2 ND ND ND ND 0.7 85.9

2-5 56.2 1.1 ND ND 0.7 ND ND 1.13 59.13

2-6 68.8 ND ND ND ND ND ND 68.8

3-1 4.5 ND ND 3.2 75.34 Injected ND ND 83.0

1

Insufficient monitoring data for this test to calculate recovery percentages. 2

ND = Not detected, D = Detected, NA = Not applicable.

3

Underground sampling ended prematurely, therefore, recovery percentage is underestimated. 4

Venthole taken off production while SF6 concentration still high.

Table 2. - SF6 gas recovery data

Test No. Recovery location First arrival Peak concentration Last detected Duration Recovery

Test time Test time Conc., ppb Test time Test time Percentage

Test No. 2-01

Test No. 2-1 BF2 0.75 h 1.0 h 485.25 14.5h 13.75 h 66.0

2.7 h 594.98

Test No. 2-2 BF2 5.6 h 10.2 h 31.41 3.0 d 2.8 d 89.1

Test No. 2-3 BF2 1.2 h 4.5 h 56.56 4.1 d 4.1 d 57.4

A+#1 6.9 h 6.9 h 0.90 7.2 h 0.3 h trace

Test No. 2-4 CBR

(underground)

37 min 42 min 149.20 77 min 40 min 0.7

BF2 7.0 h 1.4 d 10.36 8.1 d 7.8 d 85.2

Test No. 2-5 F5 (underground)2 1.1 h 1.4 h 94.98 2.1 h 1.0 h 1.1

BF3 9.6 h 12.5 h 1.35 28.0 h 18.4 h 1.1

BF2 14.2 h 24.5 h 9.54 6.1 d 5.5 d 56.2

G2 15.5 h 31.8 h 18.37 23.1 d 22.4 d 0.7

Test No. 2-6 BF2 8.1 h 28.5 h 18.95 6.9 d 6.5 d 68.8

Test No. 3-1 G23 1.1 d 7.1 d 1,308.59 25.0 d 23.9d 75.3

G1 15.3 d 25.1 d 43.65 303.1 d 287.8 d 3.2

BF2 30.0 d 35.0 d 0.25 39.2 d 9.2 d 4.5

1

Insufficient data for characterization.

2

Underground sampling ended prematurely; therefore, recovery percentage is underestimated. 3

Venthole taken off production while SF6 concentration still high.
10 CHARACTERIZATION OF GAS FLOW IN LONGWALL GOBS: PITTSBURGH COALBED, PA

Figure 1.-Map of study area.

Figure 2.-Detailed map of F, G, and H longwall panel area at time of borehole injection experiment, Test 3-1.
 11

Figure 3.-Schematic drawing of AGS system. Figure 4.-Schematic of G3 wellhead configuration for

tracer gas injection and shut-in pressure monitoring.

Figure 5.-Schematic of G3 borehole

configuration for gob gas production.

Figure 6.-Graph of SF6 tracer gas concentrations at G2

during borehole injection experiment.

Figure 7.-Graph of SF6 tracer gas cumulative recoveries for

borehole injection experiment.
12 CHARACTERIZATION OF GAS FLOW IN LONGWALL GOBS: PITTSBURGH COALBED, PA

Figure 8.-Graph of SF6 tracer gas concentrations at G1

during borehole injection experiment.

Figure 9.-Graph of SF6 tracer gas concentrations at BF2

during borehole injection experiment.

Figure 10.-Graph of influence of gob gas venthole production on G3 shut-in pressure.

Figure 11.- Graph of methane production from

G Panel gob gas ventholes.

Figure 12.-Graph of average cumulative

gob gas venthole methane production

by location on longwall panel.