3
as FEB 1, 2, and 3 in Figure 1. The fourth borehole, shown
as GCB in Figure 1, was to monitor ground movement in
the horizontal direction. For the three boreholes specified
for the slug tests, FEB 1 had a bottom hole depth of 127 m
(417 ft), FEB 2 had a depth of 76 m (250 ft), and FEB
3 had a depth of 41 m (135 ft). The FEB boreholes were
cased and cemented, except for the test intervals, which had
screen lengths of 8 m (26 ft), 4.6 m (15 ft), and 2 m (7 ft)
respectively. An isometric view of the three permeability
monitoring boreholes is shown in Figure 2.
The stratigraphic zones monitored for possible changes
in hydraulic conductivity, or permeability, due to mining-
induced deformation were the Sewickley coal bed, the
Uniontown coal bed, whose thicknesses vary in this region
(Figure 2). These coal beds often have lower compressive
strength than the associated limestone, shale, and siltstone/
sandstone units. Therefore, it was anticipated that the coal
horizons would be the main zones of potential ground
movement and where higher permeabilities would be pres-
ent compared to the surrounding rock units.
The overburden depth of the Pittsburgh coal seam at
this location was about 147 m (482 ft) to the top of the
mined seam, which is commonly at 305 m (1,000 ft) since
shallower occurrences have often been previously mined
and the area of the mine studied was located near a major
stream valley. The coal zones in relation to the three bore-
holes in this study are displayed in Figure 3. The most com-
monly sought-after shale gas reserve in the region is the
Marcellus shale which can be at a depth of about 2,100 m
(7,000 ft) to 2,750 m (9,000 ft). The Utica shale with
variable overburden depths, typically at depths between
2,100 m (7,000 ft) and 3,660 m (12,000 ft) in the region,
can also be a drilling target. Commonly, up to 30 wells can
be positioned on a single drill pad to decrease drilling costs
and to reduce the surface footprint of the wells.
Ground movement and subsidence are the primary fac-
tors creating the gas transport network associated with LW
gobs. From prior ground control studies [8], the primary
component of movement that potentially deforms well
casings in LW pillars is subsidence (vertical movement) or
horizontal movement. Both vertical and horizontal move-
ment were monitored during the study with displacement
arrays positioned in the GCB borehole, which was in close
proximity to the FEB boreholes (Figure 4). The caliper data
showed that there was an insignificant amount of vertical
movement in the borehole. However, horizontal movement
measured by the monitoring array indicated significant hor-
izontal displacement, as shown in Figure 4. This data shows
that the primary zones of movement were the Uniontown
and Sewickley coal seams. The Sewickley horizon measured
over 16 cm of lateral movement in response to LW mining
passing the pillar. Therefore, these horizons were chosen as
the zones to monitor for permeability changes.
Deep Cover Site
Monitoring boreholes used to examine longwall-induced
permeability changes under deeper cover were drilled in the
overburden directly above an abutment pillar between two
planned LW panels. The deep cover field test site is located
on the top of a steep hill and has an overburden depth to
the Pittsburgh seam of about 353 m (1,160 ft). The moni-
toring boreholes are labeled as VEP-S and VEP-U to moni-
tor the Sewickley and Uniontown formations respectively.
VEP-S was drilled to a depth of 328 m (1,076 ft) ending
at the Sewickley formation and VEP-U was drilled to a
depth of 291 m (955 ft) ending at the Uniontown forma-
tion equivalent.
Figure 2. Borehole description and location in relation to pillar layout at the shallow cover site [11]
as FEB 1, 2, and 3 in Figure 1. The fourth borehole, shown
as GCB in Figure 1, was to monitor ground movement in
the horizontal direction. For the three boreholes specified
for the slug tests, FEB 1 had a bottom hole depth of 127 m
(417 ft), FEB 2 had a depth of 76 m (250 ft), and FEB
3 had a depth of 41 m (135 ft). The FEB boreholes were
cased and cemented, except for the test intervals, which had
screen lengths of 8 m (26 ft), 4.6 m (15 ft), and 2 m (7 ft)
respectively. An isometric view of the three permeability
monitoring boreholes is shown in Figure 2.
The stratigraphic zones monitored for possible changes
in hydraulic conductivity, or permeability, due to mining-
induced deformation were the Sewickley coal bed, the
Uniontown coal bed, whose thicknesses vary in this region
(Figure 2). These coal beds often have lower compressive
strength than the associated limestone, shale, and siltstone/
sandstone units. Therefore, it was anticipated that the coal
horizons would be the main zones of potential ground
movement and where higher permeabilities would be pres-
ent compared to the surrounding rock units.
The overburden depth of the Pittsburgh coal seam at
this location was about 147 m (482 ft) to the top of the
mined seam, which is commonly at 305 m (1,000 ft) since
shallower occurrences have often been previously mined
and the area of the mine studied was located near a major
stream valley. The coal zones in relation to the three bore-
holes in this study are displayed in Figure 3. The most com-
monly sought-after shale gas reserve in the region is the
Marcellus shale which can be at a depth of about 2,100 m
(7,000 ft) to 2,750 m (9,000 ft). The Utica shale with
variable overburden depths, typically at depths between
2,100 m (7,000 ft) and 3,660 m (12,000 ft) in the region,
can also be a drilling target. Commonly, up to 30 wells can
be positioned on a single drill pad to decrease drilling costs
and to reduce the surface footprint of the wells.
Ground movement and subsidence are the primary fac-
tors creating the gas transport network associated with LW
gobs. From prior ground control studies [8], the primary
component of movement that potentially deforms well
casings in LW pillars is subsidence (vertical movement) or
horizontal movement. Both vertical and horizontal move-
ment were monitored during the study with displacement
arrays positioned in the GCB borehole, which was in close
proximity to the FEB boreholes (Figure 4). The caliper data
showed that there was an insignificant amount of vertical
movement in the borehole. However, horizontal movement
measured by the monitoring array indicated significant hor-
izontal displacement, as shown in Figure 4. This data shows
that the primary zones of movement were the Uniontown
and Sewickley coal seams. The Sewickley horizon measured
over 16 cm of lateral movement in response to LW mining
passing the pillar. Therefore, these horizons were chosen as
the zones to monitor for permeability changes.
Deep Cover Site
Monitoring boreholes used to examine longwall-induced
permeability changes under deeper cover were drilled in the
overburden directly above an abutment pillar between two
planned LW panels. The deep cover field test site is located
on the top of a steep hill and has an overburden depth to
the Pittsburgh seam of about 353 m (1,160 ft). The moni-
toring boreholes are labeled as VEP-S and VEP-U to moni-
tor the Sewickley and Uniontown formations respectively.
VEP-S was drilled to a depth of 328 m (1,076 ft) ending
at the Sewickley formation and VEP-U was drilled to a
depth of 291 m (955 ft) ending at the Uniontown forma-
tion equivalent.
Figure 2. Borehole description and location in relation to pillar layout at the shallow cover site [11]