This paper summarizes the results of a unique study conducted by the National Institute for Occupational Safety and Health (NIOSH) over the past four years to evaluate the effects of longwall-induced subsurface deformations on shale gas well casing integrity and underground miner safety and health. Results from the NIOSH instrumentation sites are compared to the results from a previous study by the coal and gas industries, which clearly indicate that under shallow and medium covers, the measured horizontal displacements within the abutment pillar are one order of magnitude higher than those measured under deep cover. Effects of casing couplers are evaluated and indicate that the couplers may aggravate longwall-induced casing deformations if the longwall-induced deformations coincide with the coupler locations. Cementing alternatives are found to have significant impact on longwall-induced casing stresses and deformations. Detailed evaluations of the 1957 PADEP Gas Well Pillar Guidelines indicate that the regulation can be applied to longwall mining if the depth of cover is less than 228 meters. Preliminary engineering guidelines are proposed on longwall-induced deformations, casing and cementing alternatives, and gas well setback distances under chain pillar and barrier pillar scenarios.
Due to a recent shale gas boom, approximately 1,500 unconventional shale gas wells have been drilled through active and future coal reserves in Pennsylvania, West Virginia, and Ohio over the past 17 years. These shale gas wells have penetrated both current and future Pittsburgh Seam coal reserves. These unconventional gas wells, whether tapped into the Marcellus or Utica formations, contain very high gas pressure. Strata deformations associated with underground longwall coal mining could induce high stresses and deformations in the shale gas well casings, which in certain situations could compromise the mechanical integrity of the production, intermediate, and coal protection casings. Damaged well casings could potentially introduce high-pressure, high-volume explosive gas into underground mine workings to jeopardize underground miners’ safety and health.
