Mechanisms and controlling factors of casing deformation in horizontal wells of coal-bearing shale gas in continental–marginal marine transition zones

This study investigates recurrent casing deformation in the transitional marine–terrestrial shale gas reservoirs of the Ordos Basin, where complex sedimentary structural features intensify integrity risks during multistage hydraulic fracturing. By integrating field deformation characteristics with geo-mechanical and structural data, a fully coupled 3D “casing–cement sheath–natural fracture–formation” model is established, enabling systematic thermo-mechanical simulations and parameter-sensitivity analyses. The mechanisms of casing failure are quantitatively identified as shear slip–induced yielding and thermo-mechanical extrusion. Results show that fracture or bedding slip with an inclination of 45° ± 10° and displacement ≥10 mm directly drives the shear deformation, while stress redistribution combined with low-temperature-fluid-induced thermal shock produces a von Mises peak of 311 MPa at the casing shoe, triggering extrusion failure. Sensitivity analysis reveals formation constitutive behavior as the dominant control on slip energy transfer, whereas conventional structural strengthening provides limited mitigation. Based on numerical insights and field verification, a hierarchical strategy—target optimization, adaptive wellbore design, and real-time fracturing regulation—reduces casing-damage incidence by 62% in pilot wells. This work advances the mechanistic understanding of casing deformation and offers a validated framework for wellbore-integrity design in deep shale gas development.