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No. 448, March 1995

Improving the Performance of Longwall
Gob Gas Ventholes

Objective

Provide longwall operators with an alternative strategy
for placement of gob gas ventholes to improve gas
production.

Background

Longwall productivity has been increased by improve-
ments in mining technology as well as by the mining of
larger panels, Unfortunately, with these productivity
improvements and the mining of deeper, gassier coalbeds,
methane gas emissions can become a problem. Increased
methane gas emissions are a safety concern and can also
reduce productivity when mining is halted to allow gas
concentrations to decline to statutory levels,

When ventilation alone is not sufficient to controi high
levels of gas emissions, methane drainage is often used as
a supplementary control measure. Gob gas ventholes are
the methane drainage method most often used on U.S.
longwalls. The completion configuration and spacing of
the holes on a longwall panel have generally been deter-
mined by trial-and-error experimentation. Once a comple-
tion configuration has becn established, changes are rarely
made.

The absence of fundamental scientific and engineering
principles in the design and placement of gob gas vent-
holes can cause difficulties for mine operators when gas
emissions increase because of a change in mine design or
because variable geologic conditions are encountered. In
an effort to improve on the trial-and-error method of de-
signing and locating gob gas ventholes, the U.S. Bureau of
Mines investigated the performance of such holes and how
and why they work.

Monitoring Venthole Production

‘Long-term gas production was monitored on 61 gob gas
ventholes on 5 longwall panels in a mine operating in the
Lower Kittanning Coalbed, Cambria County, PA. Analysis
of these data and associated drilling, completion, mining,
and geologic information revealed that scveral factors
influenced gas production rates and cumulative volume.

‘Low gas production was caused by drilling and comple-
tion practices such as cementing the casing after holes
were mined through and failing to isolate water-producing
zones from the gob gas-producing zones. Both of these
practices resulted in the blocking of mining-induced frac-
tures, which in turn impeded gas flow to the wellbore. On
the positive side, gob gas ventholes on the ends of the pan-
els generally produced the most gas, at the highest rates,
for the longest time.

‘The enhanced performance for holes at the ends of the
longwall panels is thought to be related to the behavior of
the overburden as it subsides into the mine void. The
central part of the panel experiences maximum subsidence
and is a zone of compression in the overburden (figure 1).
Mining-induced fracture permeability in the gob is reduced
under these conditions, resulting in generally lower pro-
duction rates for the gob gas ventholcs.

Conversely, the strata overlying the ends and sides of a
panel are draped into the maximum subsidence zone with
partial support provided by the surrounding pillars of the
development entries. Because of this support, overburden
strata at the ends and sides of the panel are in tension for
some distance into the panel, which results in enhanced
mining-induced fracture permeability and, hence, the high-
er production rates observed for the centerline gob gas
ventholes at the ends of the panel.

This document was prepared by the Bureau of Mines. Neither the United States Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the information contained in this document, or warrants that such use be free from

privately owned rights.



it appears that the current practice of placing gob gas
ventholes along the centerline, where maximum subsidence
occurs, is not the optimum for holes in the central area of
the panel. Locating gob gas ventholes closer to the panel
margin (figure 1), and in particular along the tailgate side
where the mine’s ventilation system would sweep gas to-
ward the holes, would be a more advantageous practice.

Improving Venthole Production

An experiment to evaluate improving production from
gob gas ventholes by placing the holes nearer the panel
margin was conducted adjacent to the longwall pancls
where the gas production analysis had been done. The
first five holes on the 216-meter-wide panel were drilled
near the centerline, as a control against which seven near-
margin gob gas ventholes could be compared. Owing to
surface restrictions on drilling locations, the near-margin
gob gas ventholes had to be drilled on the downdip
headgate side of the panel instead of the more advanta-
geous tailgate side of the panel. The seven near-margin
holes were located 16.8 to 59.4 meters from the panel
margin,

Over 7 months, the near-margin gob gas ventholes pro-
duced on average nearly 80 percent more gas than holes

-—Tension:

i
1] nate,
|

poocccsoeo

ongwall

pane! ——x

ra
ioe |
Neor-marsiny | | 7 |
Job ee | 1
gob gos rei Limit of
rey dy tf | surface
poy 1 || eefarmation
Pybood dd
rye bd
' foe t H
byt th
emroced | fe} 1 | compression
i i

Centerline
gob gos
venthote

Figure 1.—Relationship of centerline and near-margin gob
gas ventholes to zones of tension and compression asso-
ciated with an extracted longwall panel.

in the traditional centerline location on the panel (fig-
ure 2). In addition, three near-margin holes, positioned
over the central portion of the panel, produced as much or
more gas than did the centerline hole located at the start-
up end of the panel. Four (57 percent) of the near-margin
holes in the central portion of the panel actually remained
in production for 7 months, whereas only two (40 percent)
of the centerline holes produced for this length of time.
In contrast, on the previous five panels, centerline end
holes were predominantly the highest and longest gas
producers.

For More Information

More information on near-margin gab gas ventholes is
available in USBM Report of Investigations 9500, "Evalua-
tion of Alternative Placement of Longwall Gob Gas Vent-
holes for Optimum Performance.” Other applicable
methane drainage techniques are discussed in USBM
Information Circular 9395, "Methane Control for Under-
ground Coal Mines.” For a copy of these reports or addi-
tional information on this technology, contact William P.
Diamond, U.S. Bureau of Mines, Pittsburgh Research
Center, P.O. Box 18070, Pittsburgh, PA 15236, or tele-
phone (412) 892-6551.

108m? month
°

KEY
Near -margin

o2 —— Centeriine

° v z 3 4 5 6 7
MONTHS,

Figure 2.—Comparison of cumuilative methane production
for centerline and near-margin gob gas ventholes.

AVERAGE METHANE PRODUCTION PER HOLE,