Lessons Learned from Recent Longwall Mine-By Cases with Uncemented Casings

Abstract

ABSTRACT: Over 2,000 shale gas wells have been drilled in the longwall reserves of the Pittsburgh Coal Seam. When gas wells are in the vicinity of longwall mining, casing deformations are likely to occur due to longwall-induced subsurface movements and abutment stresses. One way to prevent casing deformation is to use a cementing alternative by leaving production casing uncemented. This paper presents three mine-by cases in which shale gas wells were in the chain pillars and the production casings were uncemented. These cases represent mine-by scenarios under shallow, medium, and deep cover. Casing deformations were measured with multi-finger caliper survey and predicted with numerical modeling. These mine-by case studies provide a general demonstration of how uncemented casing can uncouple production casing from longwall-induced stresses and deformations. Based on measurements of casing deformations and numerical modeling, the factors mentioned above are further analyzed to indicate in what conditions a cementing alternative may not eliminate production casing deformations. The knowledge gained from the mine-by case studies would help longwall operators and gas companies to optimize pillar design and casing design, thus minimizing the influence of longwall mining in the stability of shale gas wells in longwall pillars. 1. INTRODUCTION With the shale gas boom, about 2,000 shale gas wells have been drilled through the current and future Pittsburgh coal reserves over the past 20 years. These shale gas wells, if located in longwall pillars, will be influenced by longwall mining. Longwall-induced subsurface ground movements would induce deformations in gas well casings. The concern is that if casing deformation is excessive, a casing breach could occur, and high-pressure shale gas could leak into the longwall mine with serious results. Because of the safety concern of gas wells in longwall pillars, the National Institute for Occupational Safety and Health (NIOSH) has been conducting research since 2016 to evaluate the effects of longwall mining on casing deformations under various mining depths and geological conditions. NIOSH research has found that casing deformations are mainly caused by subsurface horizontal movements, especially under shallow and medium cover. NIOSH researchers have done several case studies with test wells in chain pillars under different overburden depths [Su (2016, 2017), Su et al. (2018, 2019), Zhang and Su (2021), and Zhang et al. (2022)]. These studies showed that longwall-induced relative horizontal displacements occur at weak-to-strong rock interfaces and are higher under shallow cover than under deep cover. The studies also showed that longwall-induced horizontal displacements are also affected by surface topography and strata dip.