2
(MSHA) along with CFR 75.1700 to manage gas well pil-
lar stability issues. Therefore, given the posed concerns and
the need for further scientific evidence in light of modern
mining technologies and practices, the National Institute
for Occupational Safety and Health (NIOSH) initiated
research to address these issues.
One of the first steps in quantifying the interaction
between the longwall mining and shale gas production is
to determine the change in ground permeability at depths
where longwall-induced deformations are of highest con-
cern. If there was a sequence of events that could produce a
casing breach, the changes in the ground permeability could
potentially increase the rate of shale gas infiltration into an
underground coal mine. Permeability field measurements
have been conducted at two mine sites with different depths
of cover. Boreholes drilled above the abutment pillar under
two different depths of cover and formation permeabilities
have been monitored during completion of the LW coal
seam. Under shallow cover (deepest at 127 m or 417 ft),
three monitoring boreholes were drilled to experimentally
measure borehole permeability through water head changes
over time. Watkins et al. [3] assessed these permeability
changes around boreholes above an abutment pillar under
shallow cover. In the same manner, two monitoring bore-
holes were drilled above an abutment pillar at a site under
deep cover conditions (up to 238 m or 1,047 ft). Harris et
al. [4] examined the initial permeability changes for a first
LW panel mine-by. This paper will present additional data
from the deep cover site and compare it with the shallow
cover data.
SITE DESCRIPTIONS
The most commonly sought-after shale gas reserve in the
southwestern Pennsylvania region is the Marcellus Shale
Reserve which can be located at a depth of about 2,100 m
(7,000 ft) to 2,750 m (9,000 ft). Commonly, several wells
can be positioned on a single drill pad to decrease drilling
costs and reduce the surface footprint of the well.
The Pittsburgh seam, a relatively flat deposit, is long-
wall mined in this region due to its consistent quality,
thickness, and pronounced lateral extent. The stratigraphic
zones monitored for possible changes in permeability due
to mining-induced deformation were the Sewickley coal
bed and the Uniontown coal bed, whose thicknesses vary
in the region. These coal beds typically have lower com-
pressive strengths and stiffnesses than the adjacent lime-
stone, shale, and siltstone/sandstone units [5, 6], leading to
further fracturing and horizontal displacement along their
bedding plane during the mining operation [7]. This stra-
tigraphy with a significant stiffness and strength contract
among overlying units is typical of Pittsburgh seam geol-
ogy at the study site with thick limestone units and thin
coal beds. Consequently, zones of high ground movement
and thus potential damage to the gas well casing would be
associated with the coal horizons due to mining-induced
deformation [8].
Based on the previous NIOSH research, cover condi-
tions were defined as three different segments: shallow (
152 m or 500 ft), medium (152 m or 500 ft – 274 m or
900 ft), and deep (274 m or 900 ft) [9]. This is similar
and corresponds relatively well with the cover depths listed
in the MSHA risk matrix [10]. Both studies at the shal-
low and deep cover sites were conducted at a coal mine
in southwestern Pennsylvania. The LW coal mine is part
of a three-entry gate road system with 460-m (1,500-ft)
wide LW panels. The panel lengths are about 3,700 m
(12,000 ft). Ventilation at the mine is by exhausting airflow
using bleeders and gob gas ventholes (GGVs) to extract gob
gas from the mined-out LW areas. The ventilation air quan-
tity at the active panel face is about 28 m3/s (60,000 cfm)
at the headgate.
Shallow Cover Site
At the shallow cover site, the location of the four monitor-
ing boreholes drilled for this study were in the overburden
directly above an abutment pillar between two planned LW
panels. The boreholes were centered above a 38-m (125-ft)
by 84-m (275-ft) abutment pillar that was part of a three-
entry gate road system (Figure 1). The distance from the
center of a borehole collar to the edge of the pillar adjacent
to an active panel varied between 12 m (40 ft) and 18 m
(60 ft). Using the guidance from the 1957 Joint Coal and
Gas Committee Gas Well Pillar Study, this distance may be
as small as 15 m (50 ft). The Pittsburgh seam mined at this
location was at a depth of 147 m (482 ft) from the surface.
Ventilation at the mine utilizing bleeders was controlled by
exhausting airflow and provided approximately 28.3 m3/s
(60,000 ft3/min) of airflow to the active panel face at the
headgate.
Three of the four monitoring boreholes were part of the
slug test portion of the previous research and were labeled
FEB Boreholes
Top View Panel 1
Panel 2 GCB Borehole
Figure 1. Three entry gate road layout
(MSHA) along with CFR 75.1700 to manage gas well pil-
lar stability issues. Therefore, given the posed concerns and
the need for further scientific evidence in light of modern
mining technologies and practices, the National Institute
for Occupational Safety and Health (NIOSH) initiated
research to address these issues.
One of the first steps in quantifying the interaction
between the longwall mining and shale gas production is
to determine the change in ground permeability at depths
where longwall-induced deformations are of highest con-
cern. If there was a sequence of events that could produce a
casing breach, the changes in the ground permeability could
potentially increase the rate of shale gas infiltration into an
underground coal mine. Permeability field measurements
have been conducted at two mine sites with different depths
of cover. Boreholes drilled above the abutment pillar under
two different depths of cover and formation permeabilities
have been monitored during completion of the LW coal
seam. Under shallow cover (deepest at 127 m or 417 ft),
three monitoring boreholes were drilled to experimentally
measure borehole permeability through water head changes
over time. Watkins et al. [3] assessed these permeability
changes around boreholes above an abutment pillar under
shallow cover. In the same manner, two monitoring bore-
holes were drilled above an abutment pillar at a site under
deep cover conditions (up to 238 m or 1,047 ft). Harris et
al. [4] examined the initial permeability changes for a first
LW panel mine-by. This paper will present additional data
from the deep cover site and compare it with the shallow
cover data.
SITE DESCRIPTIONS
The most commonly sought-after shale gas reserve in the
southwestern Pennsylvania region is the Marcellus Shale
Reserve which can be located at a depth of about 2,100 m
(7,000 ft) to 2,750 m (9,000 ft). Commonly, several wells
can be positioned on a single drill pad to decrease drilling
costs and reduce the surface footprint of the well.
The Pittsburgh seam, a relatively flat deposit, is long-
wall mined in this region due to its consistent quality,
thickness, and pronounced lateral extent. The stratigraphic
zones monitored for possible changes in permeability due
to mining-induced deformation were the Sewickley coal
bed and the Uniontown coal bed, whose thicknesses vary
in the region. These coal beds typically have lower com-
pressive strengths and stiffnesses than the adjacent lime-
stone, shale, and siltstone/sandstone units [5, 6], leading to
further fracturing and horizontal displacement along their
bedding plane during the mining operation [7]. This stra-
tigraphy with a significant stiffness and strength contract
among overlying units is typical of Pittsburgh seam geol-
ogy at the study site with thick limestone units and thin
coal beds. Consequently, zones of high ground movement
and thus potential damage to the gas well casing would be
associated with the coal horizons due to mining-induced
deformation [8].
Based on the previous NIOSH research, cover condi-
tions were defined as three different segments: shallow (
152 m or 500 ft), medium (152 m or 500 ft – 274 m or
900 ft), and deep (274 m or 900 ft) [9]. This is similar
and corresponds relatively well with the cover depths listed
in the MSHA risk matrix [10]. Both studies at the shal-
low and deep cover sites were conducted at a coal mine
in southwestern Pennsylvania. The LW coal mine is part
of a three-entry gate road system with 460-m (1,500-ft)
wide LW panels. The panel lengths are about 3,700 m
(12,000 ft). Ventilation at the mine is by exhausting airflow
using bleeders and gob gas ventholes (GGVs) to extract gob
gas from the mined-out LW areas. The ventilation air quan-
tity at the active panel face is about 28 m3/s (60,000 cfm)
at the headgate.
Shallow Cover Site
At the shallow cover site, the location of the four monitor-
ing boreholes drilled for this study were in the overburden
directly above an abutment pillar between two planned LW
panels. The boreholes were centered above a 38-m (125-ft)
by 84-m (275-ft) abutment pillar that was part of a three-
entry gate road system (Figure 1). The distance from the
center of a borehole collar to the edge of the pillar adjacent
to an active panel varied between 12 m (40 ft) and 18 m
(60 ft). Using the guidance from the 1957 Joint Coal and
Gas Committee Gas Well Pillar Study, this distance may be
as small as 15 m (50 ft). The Pittsburgh seam mined at this
location was at a depth of 147 m (482 ft) from the surface.
Ventilation at the mine utilizing bleeders was controlled by
exhausting airflow and provided approximately 28.3 m3/s
(60,000 ft3/min) of airflow to the active panel face at the
headgate.
Three of the four monitoring boreholes were part of the
slug test portion of the previous research and were labeled
FEB Boreholes
Top View Panel 1
Panel 2 GCB Borehole
Figure 1. Three entry gate road layout