Longwall-Induced Deformations and Shale Gas Well Casing Stresses: Engineering Principles

Abstract

ABSTRACT This paper summarizes important findings from the ongoing National Institute for Occupational Safety and Health (NIOSH) gas well stability research. Over the past seven years, field instrumentation results from NIOSH case studies and active mine-by case studies indicate that, under shallower covers, the measured horizontal displacements within the longwall abutment pillar are one order of magnitude higher than those measured under deep cover. Longwall-induced horizontal displacements occur along weak-to-strong rock interfaces; the sharper the bending stiffness contrast across the interfaces, the larger the longwall-induced horizontal displacements. Longwall-induced vertical compressions under shallower covers are, on the other hand, one order of magnitude lower than those under deep covers. Subsurface strata dip may magnify or reduce longwall-induced horizontal displacements. Uncemented production casings serve to uncouple longwall-induced deformations and stresses from the production casing. A case study is presented to illustrate the importance of leaving the production casing uncemented. INTRODUCTION Since 2003, over 1,800 unconventional shale gas wells have been drilled through active and future Pittsburgh Seam coal reserves in Pennsylvania, West Virginia, and Ohio. These unconventional gas wells, whether tapped into the Marcellus or Utica formations, contain very high gas pressure and volume. Strata deformations associated with underground longwall coal mining could induce 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. To provide critical scientific data to the stakeholders, which includes the Mine Safety and Health Administration (MSHA), the Pennsylvania Department of Environmental Protection (PADEP), the West Virginia Department of Mine Safety (WVDMS), the Ohio Department of Natural Resources (OHDNR), coal operators, and gas operators, NIOSH initiated a research program in 2016 to evaluate the effects of longwall-induced deformations on shale gas well casing stability under deep as well as shallow covers. The effects of longwall-induced subsurface deformations on shale gas well casing stability under deep cover, under medium cover, and under shallow cover were published previously (Su et al., 2018a and 2018b; Su et al., 2019a and 2019b; Su et al., 2020; Zhang et al., 2020; Su and Zhang, 2021; Su et al., 2021). This paper focuses on a case study that illustrates the importance of leaving the production casing uncemented.