Comparison of Measured and Modeled Casing Deformations of a Test Well in a Longwall Abutment Pillar

Abstract

ABSTRACT: Shale gas wells in longwall abutment pillars are subjected to longwall-induced ground movements, and their stability has been a safety concern for both the mining and gas industries. To understand the impact of longwall mining on the stability of gas wells in longwall abutment pillars, a test well was drilled in a longwall pillar between two adjacent longwall panels to measure longwall-induced deformations in the casing as the two longwall panels on either side were retreated. The longwall panels extracted the Pittsburgh coal seam under a depth of 177 m and were developed with a 3-entry system with 52-m-wide chain pillars and 366-m-wide panels. Casing deformations induced by longwall mining were measured by 60-arm caliper logging and observed with a downhole camera. A FLAC3D model was set up to predict longwall-induced stresses and deformations in the test well casing. The model was validated with measured surface subsidence and longwall-induced pillar pressure. Both measurements and numerical modeling showed that the casing deformed because of relative displacements along distinct weak rock interfaces. Casing deformations at the weak claystone interfaces were found to be confined within the claystone layers. In this case, casing deformations occurred at the same locations with both first and second panel mining. Second panel mining caused additional deformations as the relative horizontal displacements along the interfaces shifted in the opposite direction. The FLAC3D model proved to be capable of predicting the locations and magnitudes of casing deformations with reasonable accuracy. 1. INTRODUCTION When gas wells penetrate through coal seams in the vicinity of longwall gob, they are subjected to longwall-induced subsurface movements, and casing deformations could occur at certain locations. Longwall-induced deformations in the casing could diminish casing integrity and allow high pressure gas to leak into the mine, potentially causing fires or explosions. Over the past decade, over 1,500 shale gas wells have been drilled in the coal reserves of the Pittsburgh coal seam. The influence of longwall mining on these shale gas wells will have to be evaluated as future longwall panels are mined. Therefore, it is important to understand how longwall-induced subsurface movements and stresses affect gas well casings and to develop engineering guidelines to safely mine-by these shale gas wells.