Study on shear failure of surface wells in mining areas and wellbore optimization under extra‑thick coal seam mining

Under the condition of extra-thick coal seam mining, shear failure may inevitably occur in large-diameter surface wells in mining area, which significantly shortens the life cycle of surface wells. This study explored the casing failure law and wellbore structure optimization of large-diameter surface wells under the influence of extra-thick coal seam mining. Firstly, based on the movement process of broken rock strata in separation zone, this study expounded the mechanism of casing shear failure of surface well and established the mechanical model. On this basis, the discrimination index of casing shear failure was proposed to analyze the casing failure modes of surface wells. Secondly, with the Panel 854 as the prototype, a COMSOL numerical calculation model was established. Based on the calculation results and the afore-proposed discrimination index of casing shear failure, the risk zone of casing shear failure was predicted. Thirdly, considering that the risk zone height of shear failure in wells 854-2, 854-3 and 854-4 further increased, the technical schemes were proposed for installing placeholder pipes in the above three surface wells. With the installation of placeholder pipes, the gas drainage channel plugging induced by shear failure was avoided successfully in wells 854-2, 854-3 and 854-4, realizing the continuous and stable drainage of pressure-relief gas by surface wells.