3
the presence of a thick claystone layer at 23 feet above the
Pittsburgh seam. This thick claystone layer was not pres-
ent in the core hole 1,500 feet away from the study well.
As such, the FLAC3D model was reconstructed and rerun
with the overburden geology obtained from the on-site
gamma log. Specifically, many weak-to-strong rock inter-
faces were present and were simulated, which employed
over 400,000 zones. A hypo-elastic longwall gob model
with a maximum deformation of 25% was employed in the
FLAC3D models. The primary goal of the simulations was
to duplicate measured surface subsidence and measured
casing diameter changes by the 40-arm Caliper surveys.
The FLAC3D finite difference program was selected since
it has been calibrated and verified with field data from the
Pittsburgh seam. Figure 6 illustrates the longwall-induced
pillar pressure at the deep-cover test site, which shows an
Figure 2. Detailed locations of the five gas wells over the longwall gate-road pillars
Figure 3. Actual gas well location at the Pittsburgh seam horizon versus
the surface location due to well deviation