Dynamic impacts on the survivability of shale gas wells piercing longwall panels

Gas wells traversing active longwall panels are susceptible to failure on account of various strata movements induced by mining activities. To reveal the dynamic impacts of longwall mining on the survivability of these shale gas wells, particularly during the mining cycle as the twin flanking panels successively approach and then pass beyond the wells, a three dimensional finite element model is applied to explore the evolution of potential distress to the wells by defining the magnitudes of various well deformations. The results demonstrate that, wellbores undergo accelerated or significant deformation (increase or decrease) when each advancing panel approaches within tens of meters and the deformation stabilizes as the panel passes the well by tens (deep mining, similar to 300 m) or up to one hundred plus (shallow mining, similar to 100 m) meters. Wells are more susceptible to instability when each panel, especially the second panel, advances past the well by approximately 50 m in the case of deep mining and more than one hundred meters for shallow mining with the accumulative deformation reaching the maximum. For mining at shallow depths (similar to 100 m), wells deform by axial compaction, longitudinal distortion, lateral compression and tension in the vicinity of the seam, and by shear in layers close to the surface. Conversely, for deep mining (similar to 300 m) the most severe deformations are mainly restricted to the seam and its shallow roof and floor. Locating the well slightly closer to the rib of the second advancing panel (e.g., 7.5 m out from the pillar-center for a 50 m wide three-entry longwall pillar) achieves improved well integrity. (C) 2015 Elsevier B.V. All rights reserved.