1 

IMPLICATIONS OF RECENT NIOSH TRACER GAS STUDIES ON BLEEDER AND GOB GAS VENTILATION

DESIGN

T. P. Mucho, W. P. Diamond, and F. Garcia

Natl Inst for Occuptl Sfty and Health (NIOSH); Pittsburgh Rsrch Lab

Pittsburgh, PA, USA

J. D. Byars and S. L. Cario

RAG Emerald Resrcs Corp

Waynesburg, PA, USA

ABSTRACT

The National Institute for Occupational Safety and

Health (NIOSH) has been conducting research at a

Pittsburgh Coalbed longwall mine to evaluate and

optimize bleeder ventilation and gob gas venthole

longwall methane control systems. Gas flow into

these two methane control systems was investigated

using a combination of Sulphur Hexafluoride (SF6)

tracer gas studies and gob venthole pressure

monitoring experiments. The insights gained from

this research has facilitated the development of a

longwall subsided strata methane control system

conceptual model. This model provides the basis for

design suggestions to increase the efficiency of the

gob gas venthole system to permit the bleeder

ventilation system to become a "safety net" for

methane control.

INTRODUCTION

As part of its Mine Safety and Health research

program, the National Institute for Occupational

Safety and Health's (NIOSH) 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. Papers by Diamond and

Garcia (1999), Diamond, et al. (1992, 1994), McCall,

et al. (1993), and Schatzel, et al. (1992) document

previous research results. The objective of the

current phase of the research is to measure gas

migration characteristics associated with longwall

subsided strata (commonly referred to as gob) using

a tracer gas to simulate methane flows. Preliminary

results for the current phase of the research can be

found in Diamond, et al. (1999) and Schatzel, et al.

(1999). The knowledge gained from this research

will be used to optimize longwall methane control

strategies to intercept a larger portion of the gas

associated with the subsided strata over a mined-out

longwall panel before it enters the underground

workplace.

Research completed to date includes seven SF6

tracer gas releases into underground ventilation

airways between mined-out longwall panels and

seven releases into inactive (intaking) gob gas

ventholes. This combination of release modes

allowed for a comprehensive 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. Implications of the tracer gas study results on

the design of longwall methane control systems are

the focus of this paper.

The research results reported in this paper are

based on work conducted at a longwall mine

operating in the Pittsburgh Coalbed. Therefore,

some of the conclusions that follow may be specific

only to that mine site and perhaps to other mines

operating in the Pittsburgh Coalbed under similar

mining and geologic conditions. However, it is

reasonable to assume that a number of the

generalizations and concepts may have at least

some degree of application to other mines in other

coalbeds.
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BACKGROUND

Historically, the retreat mining process at many

mines produced high quantities of methane and,

therefore, posed a potential risk for a major mine

explosion; the most feared and devastating

workplace disaster faced by underground coal

miners. Bleeder ventilation and methane drainage

systems have been developed and refined to move

methane away from areas of the mine with high

potential for ignition sources through areas or

conduits with a very low potential for ignition

sources, and finally out of the mine. Bleeder

ventilation systems were originally designed to dilute

methane liberated from the subsided strata with

ventilating air at key points to maintain methane

concentrations below statutory limits. The main mine

ventilation system and the bleeder ventilation

systems had to mesh flawlessly for a mine to

operate safely from a methane control perspective.

Functionally, the bleeder systems rely on diluting the

gas sufficiently below its explosive range, while

methane drainage systems typically maintain the

gas at high percentages well above the explosive

range during transport.

Longwall methane control in the study area

includes the bleeder ventilation system and gob gas

ventholes. The bleeder ventilation system, as used

in this paper, includes the peripheral bleeder entries

surrounding the panel series, the former gateroads

between the mined-out panels, and the associated

bleeder fan shaft(s). Current state-of-the art bleeder

ventilation systems are a significant safety and

technological improvement over earlier bleeder

systems, such as the "wrap-around" systems. The

state-of-the-art longwall bleeder ventilation system

that has evolved at many mines today now relies on

dedicated high pressure bleeder fans to exhaust the

air and gases present in the bleeder system. This

partial separation of the bleeder system from the

main mine ventilation system apparently grew out of

difficulties associated with providing sufficient air

quantities and pressures with the main mine fans to

consistently dilute the methane liberated from the

subsided strata. The bleeder fans in the primary

study area (BF2 and BF3, Figure 1) are at the top of

1.8-m (6-ft) -diameter air shafts. The shaft for the

higher capacity bleeder fan in the new mining area

(BF4) is slightly larger [2.1 m (7 ft) diameter].

Beginning in the late 1960's, some coal mines in

the eastern United States began experimenting with

the use of small-diameter ventholes drilled from the

surface and equipped with high pressure centrifugal

fans or exhausters to help control the high volumes

of methane released from the subsided strata during

longwall mining (Diamond, 1994). Removing

methane directly from the subsided strata to the

surface before it could enter the bleeder system was

a major safety innovation. Commonly at this study

mine, three ventholes are placed on the tailgate side

of each panel in the near-margin configuration

developed in earlier research efforts by Diamond et

al. (1994), and the exhausters are powered by the

produced methane.

Figure 1. Map of primary study area.

The bleeder ventilation and gob gas venthole

methane control systems (if properly designed and

maintained) work well together and provide a much

higher measure of safety than in the past. While

these improved methane control systems represent

significant advances over previous practices,

analysis of the concepts and insights on the

behavior of longwall subsided strata gas resulting

from recent NIOSH investigations indicate that their

performance from both a safety and efficiency

standpoint can be improved even further.

While the study mine rarely experienced methane

control problems, some mines using bleeder

ventilation and gob gas venthole systems have, on

occasion, had problems meeting statutory methane

concentration limits at some point(s) underground

and/or at the bleeder fans. Assuming that these "outof-compliance" problems, even if rare, are indicative

of a lesser margin of safety, a methane control

system that minimizes the risk for such events would

undoubtedly lead to a safer underground workplace.

Therefore, throughout this paper the concept is

advanced that methane flow from the subsided

strata into a bleeder system can be minimized by

optimizing the performance of the gob gas venthole

system. Thus, the bleeder system can function as a

"safety net" in the event of unforeseen or

unanticipated methane emission/control problems.
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Conceptually, the bleeder ventilation system "safety

net" would surround the gob gas venthole methane

control system which would function as the primary

system to control gas emissions from the subsided

strata.

Another safety issue behind some of the methane

control concepts in this paper is to not only have less

methane reaching and being exhausted by the

bleeder ventilation system, but also to minimize the

aerial extent of the interface zone between the

bleeder ventilation system (usually low methane

concentration, yet high oxygen), and the gob gas

venthole system (high methane). While these

interface zones are typically in locations which

generally have a low probability for ignition sources,

minimizing their aerial extent would obviously result

in a safer methane control system. The conceptual

model and design suggestions that follow, attempt to

advance an overall longwall methane control system

that takes into account the following concepts as

guidelines (1) the "bleeder ventilation system as a

safety net," and (2) "minimizing the aerial extent of

the interface between the two methane control

systems in the subsided strata" by limiting the

interactions between the two systems.

STUDY AREA

The study mine operates in the Pittsburgh Coalbed

in Greene County, PA. Longwall panels in the

primary study area (Figure 1) were initially 253 m

(830 ft) wide and were increased to 305 m (1,000 ft)

starting with F Panel. Overburden depths ranged

between 152 and 274 m (500 and 900 ft). A new

mining area at the mine where the latest gob gas

venthole injection experiment was conducted (3

North Panel) is located north-northeast of the area

shown in Figure 1. A generalized stratigraphic

section of strata above the Pittsburgh Coalbed is

shown in Figure 2. Several coalbeds with a

combined thickness of almost 2.9 m (10 ft) are

present in the 26 m (85 ft) of strata immediately

above the Pittsburgh Coalbed, and based on

previous research (Diamond, et al., 1992), are most

likely the primary source of subsided strata gas at

the study mine.

GAS ASSOCIATED WITH LONGWALL MINING

As will be further developed, the bleeder ventilation

and gob gas venthole systems are treated as

separate but related components of the overall

longwall methane control system. To understand the

development of the longwall subsided strata

methane control conceptual model in this paper, it is

important to review the source and pathways of gas

movement associated with longwall mining.

Figure 2. Generalized stratigraphic section for study area.

Gas Emissions from the Ribs Surrounding the

Bleeder Ventilation System

This methane source originates from the unmined

coalbed gas reservoir adjacent to the development

entries of the bleeder system and is emitted from the

solid coal ribs. Emissions from a specific rib area

generally decline with time after development

mining. However, the continual addition of freshly

mined coal surface area as new panels are

progressively developed makes the cumulative

effects of these emissions at some mines (including

the study mine), a significant contributor of gas to

the bleeder ventilation system. Other than utilizing

vertical methane drainage boreholes in advance of

mining or horizontal holes during mining, there is

little that can be done about this methane emission

source. Therefore, it is considered "residual" bleeder

system gas in the longwall methane control

conceptual model.

Gas Emissions from the Active Longwall Face

Mining of the longwall face is another obvious

potential source for methane entering the bleeder

ventilation system. Methane from the face area

includes both the gas in the mined coal itself as well

as methane being emitted from the fresh face on the

longwall (Diamond and Garcia, 1999). If, during the

early phases of mining on the longwall panel, the

face ventilation airflow is directed inby to the bleeder

fan (as the study mine does), then gas associated

with mining on the face will report to the bleeder

system until the face ventilation is switched to outby,

and is directed to the main mine ventilation system.

Other than using methane drainage in advance of

mining, there is little that can be done to reduce this

volume of methane reporting to the bleeder

ventilation system. The inby ventilation mode is

sometimes used due to the high ventilation
4 

resistance of today's long tailgate entries which is

often exacerbated by the high resistance of current

tailgate support configurations (Barczak, et al.,

1999).

Gas Emissions from Subsided Strata

The final source for methane associated with

longwall mining is that which originates or

accumulates in the subsided strata above the mined

out longwall panel (Figure 3). This gas includes (1)

methane released from the strata in the caved zone,

in particular coalbeds, boney coal, and coaly shales,

and (2) methane that migrates via mining induced

fractures from gas bearing strata adjacent to the

caved zone (both vertically and horizontally). Gas in

the strata adjacent to the caved zone will migrate to

one or the other longwall methane control system,

depending on the relative magnitude of the pressure

sinks associated with each system at a particular

time and location.

Figure 3. Schematic cross-section view of subsided strata zones and methane control system influence zones above mined-out

longwall panel (modified from Peng and Chiang, 1984 and Singh and Kendorski, 1981).

Methane Production Distribution Between Longwall

Methane Control Systems

All active gob gas ventholes in the primary study

area collectively produced an average of 3.94 x 106

m3 (139.0 MMcf) of methane per quarter during 1996

(the only extended time period with complete data

sets available) (Figure 4). By comparison, methane

production from the two bleeder fan shafts averaged

7.6 x 106

 m3/qtr (268.5 MMcf/qtr), or nearly double

that of the gob gas ventholes. There is some

evidence to suggest that methane reporting to the

bleeder ventilation system was influenced by

production variations in the gob gas venthole

system, i.e., an increase or decrease in methane

production from the gob gas ventholes was reflected

in a corresponding decrease or increase,

respectively, in methane production from the bleeder

fan shafts.

Analysis of Historic Gob Gas Venthole Production

Trends

One aspect of the gob gas venthole system at the

study mine that can be exploited to increase its

efficiency is understanding the areas of high and

sustained gas inflows into the longwall subsided

area. Inspection of the methane production curves

for individual gob gas ventholes on individual panels

in the study area suggests that ventholes on the

start-up ends of the panels generally performed the

best (Figure 5). 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 (Figure 6).

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.

Figure 4. Methane production from both bleeder fans and

active gob gas ventholes in primary study area.

In the sequence of panels at the study mine, the

ventholes on the start-up end of the panels are also

the ones adjacent to extensive virgin strata beyond

the subsided strata zone. However, ventholes on the
5 

completion end of the panels are across the mains

from previously mined panels (Figure 1), where

subsided strata gas has already been exhausted.

Typically, holes on the start-up end of the panels

produce enough methane to operate the methane

fueled engines running the exhausters for a long

time, even well after the panel has been completed.

After the methane concentration in the produced gas

becomes insufficient to operate the exhausters

efficiently (~45 to 55%), these same holes continue

to free flow methane for an even longer time, unlike

the outby holes on the same panels.

Figure 5. Methane production from G Panel gob gas

ventholes.

Figure 6. Average Cumulative gob gas venthole methane

production by location on longwall panel.

The high initial and sustained methane flow rates

for the first venthole on the start-up end of an active

panel are easy to understand, given their location

relative to the virgin (non subsided) strata gas

reservoir. The early caving action of the overburden

on a new longwall panel fractures into the strata

beyond the start-up end and the headgate side of

the panel. These mining induced fractures connect

to the naturally occurring permeability of the

surrounding virgin strata gas reservoirs. These strata

gas reservoirs should be at a higher pressure than

the strata immediately above and inby the face area

as it progressively moves away from the start-up end

of the panel, thereby providing a mechanism for

additional gas flow to the pressure sink of the first

gob gas venthole. Additional factors contributing to

the high level of methane production from holes on

the start-up end of the panels at the study mine

include (1) ventholes 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, and (2) a general flow

direction for gas in the gob and ventilation airflow

towards the startup 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 Diamond et

al. (1994) 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. 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.

TRACER GAS STUDY RESULTS

Underground Releases

To properly design and operate a longwall

methane control system, the action and interaction

between the two main components, the bleeder

ventilation system, and the gob gas venthole system

must be understood. The first step towards

characterizing this interaction was to conduct seven

underground SF6 tracer gas releases. The purpose

of these releases was to assess a complete

geographic distribution of the flow of air-gas mixtures

within the bleeder ventilation system, including

determining if any substantial portion of this airflow

interacts directly with the gob gas venthole system.

The tracer gas was released into the bleeder

ventilation system airflow at the completion end of

the panels (Figure 1), into or near the inlet

evaluation points (IEP's). Ventilation airflow for this

part of the mine was directed towards the back

(start-up) ends of the panels where the two bleeder

fan shafts are located.

The details of these releases have been reported

elsewhere (Schatzel et al., 1999), however, the key

findings from the underground tracer gas releases

were:

• -Paths of the tracer gas in the bleeder

ventilation system were along the peripheral

bleeder entries surrounding the panel series or
6 

apparently along the gateroads between

panels.

• -Tracer gas injected into the ventilation airflow

in the bleeder system generally stayed in the

bleeder system and reported to the associated

bleeder fan.

• -Only minor amounts of tracer gas were

detected at operating gob gas ventholes on

three occasions (maximum recovery was 0.7%

of released volume).

Venthole Injections

A series of seven tracer gas injections were

performed into selected gob gas ventholes on G, J

and 3 North Panels to further characterize the

interaction between the bleeder ventilation system

and the gob gas venthole system. Another goal of

the venthole injection tests was to characterize the

factors influencing the flow of gas within the

subsided strata area itself.

Gob gas ventholes at this mine site are generally

drilled to within 12 m (40 ft) of the top of the

Pittsburgh Coalbed and 17.8-cm (7-in) casing, with

61 m (200 ft) of slotted pipe on the bottom is

installed, as shown in Figure 7. For the venthole

injection tests, the SF6 tracer gas is released through

tubing extended some distance down the venthole.

The venthole, which must intake for the injection

test, is open to the atmosphere during the injection

to facilitate the mixing and dilution of the tracer gas

to a sufficiently low concentration to simulate the

flow of the host air stream (Timko et al., 1986). The

monitoring strategy for the venthole injection tests

included collecting periodic gas samples at all outlet

points for the most likely migration path(s) for the

tracer gas. The samples were analyzed with a gas

chromatograph for the presence and concentration

of SF6. The monitored outlet points generally

included the bleeder fans and producing gob gas

ventholes associated with each injection test.

Surface shut-in pressures and methane flow rates

for key ventholes associated with the injections were

also monitored to further evaluate the ventilation

mechanics of the subsided strata gas reservoir.

The details of the G Panel gob gas venthole

injection test have been published previously by

Diamond et al. (1999), and the details of the six

additional tests will be reported in a forthcoming

NIOSH publication. A summary of the venthole SF6

injection tests follows.

G Panel Venthole Injection, Test 3-1: The

injection venthole G3 is located on the tailgate side

of the panel near the completion end (Figure 8). At

the time of the test, venthole G3 was off production,

but the two inby ventholes G2 and G1 were online.

Figure 7. Schematic of gob gas venthole G3 configuration.

TRACER GAS RESULTS: Data relative to the

tracer gas first arrival and peak concentration at the

monitored locations, as well as the calculated

velocity between the injection venthole and the

recovery points, are shown in Table 1. Tracer gas

from the G3 injection was recovered sequentially

from the two inby producing ventholes on the panel

(Figure 9). Tracer gas was first detected at G2, 1.1

days (27 h) after the release [velocity of 0.008 m/s

(1.5 ft/min)], and was detected next at G1, 15.3 days

after the release [the velocity of 0.002 m/s (0.3

ft/min) was influenced by G1 being off production for

8 days prior to the SF6 arrival]. The only other

confirmed location to which tracer gas migrated from

the G3 venthole injection experiment was at the

associated bleeder fan shaft (Figure 1). The arrival

of tracer gas at BF2 on day 30 after the release is

probably directly related to gob gas venthole G2

going off production on day 25 of the test (Figure 9).

G3 SHUT-IN PRESSURE MONITORING

RESULTS: As can be seen in the G3 shut-in

pressure monitoring data (Figure 10), variations in

gas production at G1 and/or G2 influenced the shutin pressure at G3. The venthole exhausters at this

mine typically operate at between 3.733 and 9.954

kPa (15 and 40 in w.g.), as measured at the top of

the venthole. It is evident from Figure 10 that gas

production variations at G1 were reflected in the gas

production rates at G2 and the shut-in pressure at

G3, i.e., when G1 was offline, the gas production at

G2 increased and the shut-in pressure at G3 rose

(became less negative). Similar influences are

evident when G2 was offline, i.e., a slight increase in

gas production at G1, and the shut-in pressure at G3

rose.
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Figure 8. Detailed map of F, G, and H longwall panel area at time of the G3 gob gas venthole tracer gas injection test.

 Table 1. Borehole to borehole tracer gas velocities (first arrival) and tracer gas peak data

The 0.622 kPa (2.5 in w.g.) shut-in pressure

differential observed at G3 when G1 was offline and

G2 stayed on production is an estimate of the

contribution the exhauster on G1 had on the -2.488

kPa (- 10.0 in w. g.) of shut-in pressure when both

G1 and G2 were producing gob gas (Figure 10). The

resulting pressure differential of 1.493 kPa (6.0 in

w.g.) when G2 is offline and G1 was operating is

then a measure of the contribution the exhauster on

G2 had on the -2.488 kPa (10.0 in w.g.) of shut-in

pressure at G3. The larger contribution of the

exhauster on hole G2, as compared to G1, to the
8 

observed shut-in pressure at G3 is due to G2 being

1,311 m (4,300 ft) closer to G3 than is G1 and the

higher total gas production from venthole G2 at the

time of the test. Venthole G2 was permanently off

production on day 26, resulting in a sustained rise in

shut-in pressure at G3 to an average of about -0.995

kPa (-4.0 in w.g.), which represents the combined

influence of the exhauster on G1 and the bleeder

fans. Since the influence of G1 was calculated to be

0.622 kPa (2.5 in w.g.), the contribution from the

bleeder fans is then only 0.373 kPa (1.5 in w.g.) of

the -0.995 kPa (-4.0 in w.g.) of shut-in pressure at

G3. BF2 and BF3 were operating at approximately

4.230 and 3.235 kPa (17 and 13 in w.g.),

respectively.

Figure 9. SF6 tracer gas cumulative recoveries from gob

gas venthole injection Test 3-1.

Figure 10. Influence of gob gas venthole production on G3

shut-in pressure.

KEY FINDINGS, G PANEL VENTHOLE

INJECTION TEST:

• -Most of the tracer gas injected into the gob

gas venthole system at G3 stayed in the

subside strata and was recovered at the inby

producing ventholes.

• -The recovery of tracer gas at the bleeder fan

is probably directly associated with venthole

G2 going off production 5 days before the

arrival of tracer gas at BF2.

• -All three ventholes on G panel were in

communication with each other as evidenced

by the tracer gas arriving sequentially at the

two inby producing ventholes. This was also

substantiated by fluctuations in the shut-in

pressure measurements at G3 in response to

production variations at the two inby ventholes.

• -The G3 shut-in pressure monitoring data

indicate that the producing gob gas ventholes

on G Panel had a greater influence on the

shut-in pressure than did the bleeder

ventilation system.

• -Based on venthole injection Test 3-1 it can be

concluded that the portion of the subsided

strata gas primarily influenced by the gob gas

venthole system tended to report to that

system, and not to the bleeder ventilation

system, as long as the gob gas ventholes that

were capable of producing methane at

concentrations sufficient to run the methane

fueled engines were operating.

Figure 11. Detailed map of I, J, and K longwall panel area at time of the J Panel gob gas venthole tracer gas injection tests.
9 

J Panel Venthole Injections: Five venthole tracer

gas injection tests were conducted on J Panel

(Figure 11) over a six month period. The placement

of the five gob gas ventholes on J Panel presented a

unique opportunity to investigate several aspects of

gas flow in longwall subsided strata. Three of the

holes were placed in the traditional configuration on

the tailgate side of the panel, and the other two

holes were placed on the headgate side of the

panel. The first two venthole injection tests on J

Panel were designed to test the ability for gas to

migrate across the centerline of this super critical

panel. A super critical panel, as used here, is one in

which the panel width is greater than the height of

the overburden, which results in a more complete

caving of the overburden strata into the mine void

(Figure 12). The remaining three venthole injection

tests on J Panel were designed to characterize other

potential gob gas flow paths, including those along

the tailgate and headgate sides of the panel.

Summary data for the J Panel venthole injection

tests is given in Table 1.

Figure 12. Schematic cross-section view of super critical,

critical, and sub critical longwall panels (modified from Jeran

and Barton, 1985).

TEST 4-1 AND 4-2 RESULTS: Many longwall

mines have adopted the near-margin gob gas

venthole placement concept reported by Diamond,

et al. (1994). However, as longwall panels have

steadily increased in size, the ability for gas to

efficiently flow from the headgate side of a panel to

gob gas ventholes placed on the tailgate side of a

super critical longwall panel came into question. Gob

gas venthole tracer gas injection Tests 4-1 and 4-2

were designed to investigate this concern. Tracer

gas was injected into venthole J2-A on the headgate

side of J Panel, with only two ventholes (J2 and J1)

on the tailgate side of the panel operating (Figure

11). Tracer gas was recovered sequentially at the

two inby producing gob gas ventholes on the tailgate

side of the panel during both Test 4-1 and Test 4-2,

conducted 41 days apart (Figures 13 and 14). The

migration time for tracer gas to reach the two inby

producing ventholes increased only moderately (by

1.2 hours to J2 and 10.8 hours to J1) from the first to

second test. These results suggest that there is little,

if any, impediment to gas flow across the super

critical panels at the study mine.

Figure 13. SF6 tracer gas concentration curves at gob gas

venthole J2, Tests 4-1 and 4-2.

Figure 14. SF6 tracer gas concentration curves at gob gas

venthole J1, Tests 4-1 and 4-2.

The only other monitored location where tracer gas

was detected was at BF3. Tracer gas reached the

BF3 location on day 7.8 of Test 4-1, just 12 hours

after venthole J1 went off production for

approximately 1 day. As was the case with tracer

gas being detected at BF2 during the G Panel

venthole injection test, the presence of tracer gas in

the ventilation system during the first J Panel

venthole injection is believed to be directly related to

one of the producing gob gas ventholes (J1) going

off production.

One additional experiment of note conducted at the

end of Test 4-2 was to start venthole J1A, 1,097 m

(3,600 ft) inby the J2A injection hole on the

headgate side of J Panel to determine if tracer gas

was present at this location, even though the
10 

recovery points for the test indicated a general gas

flow direction across the panel to the tailgate side.

Tracer gas was detected immediately after venthole

J1A was started on day 22 of Test 4-2.

TEST 4-3 RESULTS: The next J2A venthole

injection (Test 4-3) was conducted 3.5 months after

the Test 4-2 SF6 injection. Only venthole J1 was

running at the start of Test 4-3. Venthole J2 had

gone off production permanently on day 19 of Test

4-2, and was never restarted. Tracer gas was

detected at venthole J1, 26. 1 hours after the

release, or 8.4 hours sooner than during Test 4-2,

when ventholes J2 and J1 were both running. The

velocity of 0.017 m/s (3.3 ft/min) falls between the

0.018 m/s (3.6 ft/min) and 0.0 13 m/s (2.5 ft/min)

values calculated for Tests 4-1 and 4-2, respectively

(Table 1). Potential explanations for the observed

faster migration time to J1 during Test 4-3 would

include (1) a more direct physical "connection"

between J2A and J1 than between J2A and J2, (2) a

more efficient (higher permeability) tracer gas flow

path along the headgate side of the panel, around

the startup end of the panel to J1 on the tailgate

side, (3) the operating vacuum pressure on J1 had

been sufficiently greater than that on J2 [9.954 kPa

vs 3.981 kPa (- 40 in w.g. vs. -16 in w.g.)] so that the

dominant gas flow path was towards J1, and when

J2 was running, it only exerted enough influence to

slow the velocity towards J1, and (4) some

combination of the first three explanations.

One additional experiment of note conducted

during Test 4-3 was the restarting of venthole J1A

on day 5.9 to determine if tracer gas was present in

that area on the headgate side of the panel. As was

seen in the earlier experiment at the end of Test 4-2,

tracer gas was detected in the produced gas

immediately after starting venthole J1A. These data

(and the headgate gas flow velocity for the

subsequent Test 4-4) would seem to support the

potential for a higher permeability gas flow path

along the headgate side of J Panel to the producing

venthole J1 on the tailgate. Alternatively, it could

indicate a very wide flow path through the gob.

TEST 4-4 RESULTS: Test 4-4 was designed to

determine the velocity for gob gas flow along the

headgate side of J Panel. This SF6 injection into

venthole J2A was conducted with only venthole J1A

operating. Tracer gas reached venthole J1A in 20.2

hours for a velocity of 0.0 15 m/s (3.0 ft/min). This

velocity for gas flow along the headgate of J Panel

was double the velocity along the tailgate of G Panel

[(.007 m/s (1.5 ft/min)].

TEST 4-5 RESULTS: This test was designed to

determine the velocity for gob gas flow along the

tailgate side of J Panel. For this test scenario, tracer

gas was injected into venthole J3 for the first time,

with only venthole J1 operating (Figure 11). The

tracer gas arrival at venthole J1 was at 2.7 days for

a velocity of 0.007 m/s (1.4 ft/min). This is a value

almost identical to that for the G Panel tailgate

velocity test [0.007 m/s (1.5 ft/min)].

KEY FINDINGS, J PANEL VENTHOLE

INJECTION TESTS: The key findings from the G

Panel venthole injection test were reconfirmed and

in some cases amplified.

• -Tracer gas injected into the subsided strata on

J Panel was produced by the J Panel gob gas

venthole system, and only migrated to the

bleeder ventilation system in response to a

producing gob gas venthole going offline.

• -Tracer gas injected into the subsided strata on

the headgate side of this super critical panel

migrated to producing gob gas ventholes on

the tailgate side of the panel.

• -Tracer gas flow velocity along the tailgate side

of J Panel was essentially the same as that of

G Panel. Tracer gas flow velocity along the

headgate side of J Panel was double that of

the tailgate side.

• -Tracer gas appeared to move freely through

the J Panel subsided strata to the most

negative pressure sinks, indicating an effective

mining induced fracture system with relatively

high permeability to gas flow.

3 North Panel Venthole Injection: The final gob

gas venthole injection experiment was conducted on

the first longwall panel in a new mining area at the

study mine (Figure 15). The new panel (3 North) is

approximately 6 km (3.7 mi) north-northeast of J

panel (Figure 1). Test 4-6 was prompted by reports

from the mine that the performance of the gob gas

ventholes on this new panel was substandard, and

the holes were not on production. One possible

explanation to be investigated was that with such a

small mining area to act on, the high capacity fan

[4.479 kPa (18 in w.g.)] installed on the bleeder fan

(BF4) was actually overwhelming the gob gas

venthole methane control system, and pulling gob

gas away from the ventholes. A production test on

the first venthole on the panel (2NE-4) yielded a

typical methane concentration approaching 75%

when the hole was started with propane. Based on

the production test results, it did not appear that the

bleeder fan was overwhelming the gob gas venthole

methane control system, as originally speculated,

and the reported production problems were

mechanical in nature. Evaluation of the conditions at

the second hole on the panel (2NE-1A) revealed that

it was capable of intaking when opened. A tracer

gas injection test into hole 2NE-lA was designed to

further evaluate the gas flow characteristics of this

first panel in the new mining area.
11 

 Figure 15. Detailed mine map of 3 North Panel at time of the 2NE-1A gob gas venthole tracer gas injection test.

TEST 4-6 RESULTS: This test was an SF6

injection into venthole 2NE-1A, with only the inby

venthole 2NE-4 operating. The first place tracer gas

was detected after the 2NE-1A injection was at BF4

(Figure 16). Tracer gas reached this location in only

48 minutes. The calculated velocity for gas to flow

the 82 m (270 ft) from the injection hole to the

bleeder ventilation system is then 0.032 m/sec (6.3

ft/min) (Table 1). This velocity is higher than that of

any other venthole injection test. This was also the

only test where tracer gas from a venthole injection

reached the bleeder system before being detected

at a producing venthole on the test panel.

Figure 16. SF6 tracer gas concentration curves, Test 4-6.

The tracer gas from Test 4-6 also reached the inby

producing venthole 2NE-4. Based on the arrival time

of 11.2 hours (Table 1), the velocity along the

tailgate side of this panel is 0.014 m/sec (2.8 ft/min),

almost double the velocities calculated for G and J

panel tailgates. The 0.014 m/sec (2.8 ft/min) velocity

for the tailgate on the 3 North Panel is similar to that

calculated for the J Panel headgate (Test 4-4, Table

1). It is postulated that the similar gas flow velocity

results from these two tests (Tests 4-4 and 4-6) are

due to the gas flow paths being in an area of

enhanced mining induced fracture permeability due

to their proximity to unmined adjacent areas (Figures

12 and 16), even though the tests were conducted

on different sides of their respective panels. As can

be seen on Figure 16, venthole 2NE-4 went off

production for 0.6 days due to mechanical reasons

at day 1 of the test. During this down time the SF6

concentration, which previously had been steadily

declining at BF4, rose slightly indicating an

interaction between the two methane control

systems.

As a result of the atypical findings from the

venthole injection test on 3 North Panel, a

production test was conducted on venthole 2 NE-1A

to determine its capability for producing methane.

Methane concentrations never reached 20%, in fact

they declined slightly over the time of the test. The

reason for this low methane concentration was an

inadvertent error in the drilling and completion of

venthole 2NE-1A to a depth approximately 14.6 m

(47.8 ft) below the bottom of the Pittsburgh Coalbed.

Once the section of slotted casing placed across the

mined coalbed interval had been removed,

approximately 43 m (140 ft) of slotted casing

remained in the overlying roof strata, including the

caved zone, as shown in Figure 3 (venthole B).

Therefore, the 2NE-1A venthole configuration

provided a direct link to the bleeder ventilation

system which resulted in the atypical test results

discussed above. Based on the caving height

formula of Peng (1984) and bulking factors for the

area provided by Chen (1999), a maximum mining

height of 2.1 m (7 ft) would yield a maximum caving

height (and estimate for the functional top of the

bleeder ventilation system gas production zone) of

10.7 m (35 ft). The 10.7 m (35 ft) estimated caving

height is a point below the typical bottom hole depth

of the gob gas ventholes at the study mine (venthole

A, Figure 3), thus supporting the conceptual model

that the "typical" ventholes are generally producing

gas from a subsided strata zone separate from the

bleeder ventilation system.
12 

KEY FINDINGS, 3 NORTH PANEL VENTHOLE

INJECTION TEST:

• -The venthole inadvertently drilled into the

mined coalbed produced methane at low

concentrations due to a reduced length of

slotted casing in the fractured zone, and a

greater influence of the bleeder system on the

gas in the caved zone adjacent to the lower

section of the slotted casing.

• -The venthole completed to a depth 12.2 m (40

ft) above the top of the mined coalbed

produced methane at high concentrations

indicating it was producing gas primarily from

the subsided strata zone influenced by the gob

gas venthole system.

• -The height of the caved zone (and the

interface between the bleeder ventilation

system and the gob gas venthole system) is

below the typical completion depth of

ventholes at the study mine site.

• -The two methane control systems are

operating and extracting gas at two different

levels in the subsided strata, with the bleeder

ventilation system operating at the level of the

mined out coalbed and the lower part of the

caved zone, and the gob gas venthole system

generally operating in the fractured zone and

at the top of the caved zone.

IMPLICATIONS OF TRACER GAS STUDIES

FOR BLEEDER AND GOB GAS VENTHOLE

SYSTEM DESIGN

The tracer gas study results presented in the

previous discussions have been used to develop a

conceptual longwall subsided strata methane control

model. The insights for gas flow inherent in the

conceptual model can be incorporated into the

current state-of-the-art design practices for longwall

methane control systems to produce a more efficient

optimized system which would provide an increased

margin of safety for the underground workforce.

Longwall Subsided Strata Methane Control

Conceptual Model

The conceptual longwall subsided strata methane

control model is as follows:

• -The bleeder ventilation and gob gas venthole

longwall methane control systems exhaust

methane from different subsided strata zones.

• -In addition to the "residual" methane

emissions from the ribs and active face area,

the bleeder ventilation system exhausts

methane primarily from the lower part of the

caved strata zone in direct contact with the

bleeder ventilation system around the

periphery of the mined out panel.

• -The gob gas venthole system exhausts

methane primarily from the upper caved and

fractured strata zone, due in part to the

layering effect of methane and the primary

source of long term methane flow being in and

adjacent to this zone.

• -An interface for methane/air mixtures exists

between the two longwall methane control

systems, but as long as both systems are

operating efficiently, they are essentially

operating independently.

• -Disruptions in methane production from the

gob gas venthole system may move the

interface between the two systems, i. e., gas

previously exhausted by the gob gas venthole

system will then flow to the bleeder ventilation

system.

• -It may be possible to optimize gas production

from the gob gas venthole system sufficiently

by minimizing the interaction between the two

methane control systems. Thus, the aerial

extent of the interface between the two

methane control systems is reduced and the

bleeder ventilation system can then junction as

a "safety net" for the longwall methane control

system.

Design Considerations for Optimized Longwall

Methane Control Systems

Bleeder System: The design of the bleeder

ventilation system envisioned for the longwall

subsided strata methane control model is fairly

straightforward using standard ventilation design

principals and methods. Key aspects of the

conceptual model are briefly summarized here, but

the overall design of the bleeder ventilation system

will be discussed in a forthcoming NIOSH

publication. An important result of the tracer gas

studies on bleeder system design is that for all

panels in the bleeder system series, the bleeder fans

must be capable of generating sufficient ventilating

pressures at the front of the panels to induce flow to

the back of the panels, as was the case at the study

mine.

The studies also indicate that given the minimal

interaction of the bleeder ventilation and gob gas

venthole systems at the study mine, ventilation

quantities along old gateroads can diminish, over

time and with panel age, without deleterious effects.

This assumes that the gob gas venthole system is

adequate to handle the gas inflows into the upper

subsided strata zone. It also assumes that the

bleeder ventilation system would be capable of
13 

delivering the required air flow quantity and,

therefore, ventilating pressures, at the desired points

in the system where most needed, e.g. near or at

the active panel where higher levels of emissions

are occurring (Figure 17). Obviously, under sizing of

the bleeder ventilation system from a ventilation air

flow quantity and pressure standpoint can negatively

impact compliance with statutory methane

concentration levels in the bleeder entries or at the

bleeder fan.

Figure 17. Conceptualized methane production curves for

gob gas ventholes and bleeder fan associated with active

longwall panel.

Gob Gas Ventholes: Design of gob gas venthole

systems are not as straightforward as bleeder

ventilation systems, which has provided the basic

impetus for the present direction of NIOSH longwall

methane control research. Most of the current gob

gas venthole systems at individual mines have

resulted from a trial-and-error design practice. Until

the reservoir mechanics of longwall subsided strata

and the interaction of the bleeder ventilation system

and the gob gas venthole system are more

completely understood, the trial-and-error approach

will probably continue, aided by the type of research

described in this paper.

One example of gob gas venthole design practice

highlighted in the work at the study mine (3 North

Panel venthole injection test) is that the completion

depth of the gob gas ventholes should be at or

above the height of the caving zone. It was

determined that the typical completion depth for gob

gas ventholes approximately 40 ft above the

Pittsburgh coalbed accomplished two goals, (1) it

protected the slotted casing from damage, and (2) it

was high enough in the subsided strata to minimize

the interaction between the two longwall methane

control systems.

Changes in other design practices at the study

mine might increase the overall efficiency of the gob

gas venthole system. Typically at the study mine,

gob gas ventholes at the start-up end of the panel

initially run at a high volume with a fairly low head,

and gradually shift to a lower volume at a relatively

high head. Since a reasonably consistent flow

velocity (permeability) in the gob gas ventholeinfluenced subsided strata zone on the tailgate side

of longwall panels is indicated by the current study, it

is assumed that the change in production

characteristics with time is due to changes in the

volume of methane available in the subsided strata.

Coordinating gob gas venthole system production

capacity with the volume of available gas is critical to

minimize the interaction between the longwall

methane control systems. Designing the gob gas

venthole system for the expected range of

production capacities is then the objective to be met

to make the system operate more efficiently.

It is generally understood that if the gob gas

venthole system capacity is operating at a

production volume level higher than the methane

inflow rate, then mine ventilation air is pulled into the

gob gas venthole system, thereby reducing the

methane concentration at the venthole. The

methane concentration then becomes a gauge of

this interaction, as was seen during Test 4-6.

Conversely, if the volume of methane available in

the subsided strata zone influenced by the gob gas

venthole system is greater than the gob gas

venthole system capacity, then more of this gas will

enter the bleeder ventilation system. A number of

the gob gas venthole tracer gas injection tests at this

mine have verified these kinds of interactions. This

rationale then provides additional guidelines for the

design of gob gas venthole systems (1) the gob gas

venthole system capacity must nearly equal or be

adjustable to the volume of gas inflows into the

system, and (2) sufficient monitoring and

maintenance of the producing ventholes is critical to

ensure that production of subsided strata methane is

uninterrupted.

Figure 17 shows conceptualized methane

production curves for a bleeder system and

associated active panel gob gas venthole system

based on data from the current study and previous

work (Diamond, 1994, Diamond et al., 1994, and

Diamond and Garcia, 1999). The major longwall

methane control system design concerns are the

rather sudden and large increase in methane

production at the bleeder fan early in the life of the

panel, and the sustained high level of methane

inflow from the subsided strata during most of the life

of the active panel. The initial high level of methane

production from the bleeder ventilation system

comes primarily from two sources (1) longwall face
14 

ventilation airflow reporting to the bleeder fan, and

(2) newly subsided strata gas prior to intercept of the

first gob gas venthole. Additional volumes of

methane may report to the bleeder ventilation

system after the first gob gas venthole is intercepted

due to subsided strata gas inflows above the

capacity of the gob gas venthole system. Optimizing

the gob gas venthole system should reduce the

volume of methane reporting to the bleeder

ventilation system, i.e., an overall lowering of the

bleeder fan methane production curve shown in

Figure 17.

At the study mine, the longwall face air usually

reports to the bleeder fan shaft for a relatively short

period of time before the face ventilation is switched

to outby, generally accounting for a relatively small

portion of the total panel methane volume at the

bleeder fan shaft. The subsided strata gas that

reports to the bleeder fan shaft prior to interception

of the first venthole is a significant source of gas.

Typically at the study mine the first venthole is

located 305 to 366 m (1,000 to 1,200 ft) from the

start-up end of the panel. One way of decreasing the

volume of subsided strata methane that reports to

the bleeder fan shaft is to move the location of the

first venthole closer to the start-up end of the panel.

However, there is a practical limit as to how close to

start-up the first gob gas venthole can be placed

without being detrimental, i.e., it can't be so close to

the start-up end that it is within the zone of influence

of the bleeder ventilation system. Some mines

operating in the Pocahontas No. 3 Coalbed in

Virginia place ventholes as close as 30 to 46 m (100

to 150 ft) to the start-up end of the panel, however,

they do experience high O2 concentrations (R.E.

Ray, Jr., Pers. Commun., 1999). The Mine Safety

and Health Administration (1996) reports that the

first gob gas venthole on a panel is generally drilled

within 152 m (500 ft) of the start-up end. Some

mines operating in the Pittsburgh Coalbed have

successfully drilled their first ventholes as close as

76 m (250 ft) to panel start-up, so placing the first

gob gas venthole closer to the start-up end of the

panel is probably a viable option for the study mine.

Reducing the volume of methane entering the

bleeder ventilation system due to an under capacity

of the gob gas venthole system can probably be

successfully addressed. A key design factor to

achieve optimized performance from the ventholes is

to allow for the variability in methane volumes

available in the subsided strata as a function of time,

and to some extent, position on the panel, as

discussed above. A consideration for the study mine

to evaluate is to increase the production capacity of

the first hole on the panel by increasing the size

and/or capacity of the gob gas venthole exhausters.

The exhausters at the study mine are four stage

centrifugal fans powered by diesel engines

converted to operate on methane/propane mixtures.

Quantities and pressures generated by the

exhausters are quite variable and are a function of

engine rpm, air/methane mixture percentages, air

temperature and humidity, elevation above sea level

(particularly at the higher elevations of some mines

in the western United States), and system

resistance. System resistance is mainly a function of

the external plumbing, venthole diameter and depth,

condition of the casing and surface piping, fracture

permeability, and volume of gas inflow from the

subsided strata. The current 17.8 cm (7 in) borehole

diameter casing used at the study mine contributes

little head losses to the system, however, this may

not be true for deeper mines and/or mines producing

higher volumes of gas.

A potential option for increasing the gas production

capacity from the first venthole on the panel is to run

two exhausters of the current design in parallel

during the early stages of longwall panel coal

production when gas inflows are generally higher.

This configuration of the exhausters should give

ample capacity for any conceivable level of gas

inflows at the study mine site, and has been

successfully implemented at some mines operating

in the Pocahontas No. 3 Coalbed in Virginia (R.E.

Ray, Jr., Pers. Commun., 1999).

A companion suggestion for optimizing the

performance of the gob gas venthole system at the

study mine is to consider eliminating the third

(completion end) venthole on panels, since they

have historically been poor producers (Figure 6).

Based on the results of the tracer gas venthole

injection tests and associated shut-in pressure

monitoring experiments, eliminating the third

venthole may be possible. Along with the elimination

of the ventholes on the completion end of panels,

the two exhausters on the first venthole may have to

be reconfigured from the parallel arrangement into a

series alignment (easily accomplished with minor

surface plumbing changes). This would be

implemented later in the panel's life to provide a

greater pressure sink in the gob gas venthole

system as the face progressively moves towards the

completion end of the panel and the level of

subsided strata gas inflow decreases.

The final consideration for optimizing the

performance of the gob gas venthole system is

related to monitoring and maintenance of the

system. Earlier it was noted that the gob gas

venthole and bleeder ventilation systems tended to

not interact as long as the gob gas venthole system

was operational, i.e., that all ventholes capable of

producing methane at concentrations sufficient to
15 

run the methane fueled engines powering the

exhausters were online.

One operational issue is associated with the use of

the produced methane to run the methane fueled

engines. In addition to the variability of the fuel

source and associated carburetor problems that

must be dealt with, problems with other mechanical

components such as self-oilers, provide ample

opportunity for these engines to cease operation. An

associated problem is that when an exhauster is

found offline, the question of whether the methane

fueled engine stopped due to a mechanical

malfunction or methane depletion must be

addressed. This distinction may be difficult to

determine without considerable effort. While

electricity would be a more reliable power source to

run the exhausters, providing electricity to the

remote locations at this site and at other mine sites

can be prohibitively expensive. Also, due to the often

remote locations for the ventholes and the frequency

of visits by maintenance personnel, it may be

several days before it is discovered that a venthole

is not online. The worst case scenario is that an

under performance of the gob gas venthole system

is discovered because of increased methane

concentrations in the bleeder ventilation system.

Recent advancements in monitoring technology

may help address these operational problems,

thereby minimizing the interactions between the gob

gas venthole and bleeder ventilation systems.

Remote data transmission systems that use solar

powered cellular phone technology are currently

available. It should be possible to combine this

remote data transmission capability with the data

acquisition and storage system developed by NIOSH

for use at the study site. Integration of the two

systems will then permit the transmission of real time

performance data directly to the mine office. System

operational problems can thus be identified and

corrected before they lead to additional methane

flow into the bleeder ventilation system. An

integrated system of this type is scheduled for

demonstration at the study mine in the near future.

SUMMARY

Tracer gas studies can be a valuable tool to study

ventilation airflows associated with longwall gobs,

bleeders, and gob gas ventholes. The analysis of

data obtained from the tracer gas experiments and

ancillary investigations at the study mine have been

used to develop a conceptual longwall subsided

strata methane control model. The gas flow insights

inherent in the conceptual model can be

incorporated into the current state-of-the-art design

practices for longwall methane control systems to

produce a more efficient optimized system which

would provide an increased margin of safety for the

underground workforce.

ACKNOWLEDGMENTS

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. The following NIOSH personnel are

acknowledged for their contributions to the

successful completion of this research effort: C. S.

Compton, D. L Dwyer, P. W. Jeran, J. C. LaScola, J.

K. Marshall, F. E. McCall, G. F. Persetic, S. J.

Schatzel, and J. P. Ulery.

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