A Case Study of Shale Gas Well Casing Deformation in Longwall Chain Pillars Under Deep Cover

Abstract

ABSTRACT Shale gas wells in longwall chain pillars are subject to longwall-induced overburden movements. Longwall mining on either side of the chain pillars can induce deformations in gas well casings. The casing deformations induced by longwall mining has raised the safety concern that casing integrity might be diminished so that intrusive shale gas could leak into the longwall mine with serious consequences. This study deals with longwall-induced casing deformations of eight shale gas wells in the chain pillars between two adjacent longwall panels in the Pittsburgh coal seam under a cover depth of 314 m and a mining height of 2.1 m. The longwall panels were 457-m wide, and the chain pillars were 66-m wide. The longwall faces passed by the shale gas wells as each adjacent longwall panel was retreated. Gas well casing deformations were measured by a multi-finger caliper after each panel was mined. The first panel mining induced casing deformations less than 1.7 cm above the coal seam horizon. The second panel mining caused an increase of casing deformation up to 20%–30% on average. The casing deformations were also predicted by the FLAC3D modeling technique based on site-specific mining and geological conditions. The study demonstrates that the predicted casing deformations are generally in good agreement with the measurements. The study shows that the casing deformations first occur at the weak/strong rock interfaces after first panel mining and then increase by a small amount at the same locations after second panel mining. The study reveals that longwall-induced casing deformations under deep cover are smaller than those under shallow cover. The study also provides a quantitative method for using numerical modeling to assess the stability of shale gas wells in longwall chain pillars. INTRODUCTION Gas wells drilled in longwall pillars are influenced by longwall mining. Due to longwall-induced subsurface ground movements, the gas wells in the vicinity of longwall panels are subject to longwall-induced stresses and deformations. Excessive stresses and deformations induced in gas well casings could lead to a casing breach. More safety concerns have been focused on unconventional shale gas wells in the presumption that the casing breach could allow high-pressure gas to leak into the mine, potentially causing a fire and explosion. Over the past decade, more than 1,500 shale gas wells have been drilled in the current and future reserves of the Pittsburgh coal seam. The impact of longwall mining on these shale gas wells will have to be evaluated as future longwall panels are mined. Therefore, it is important to quantify longwall-induced subsurface movements and casing deformations and to develop reliable models to assess the stability of shale gas wells in longwall pillars.