A subsidence-to-shear geomechanical framework for natural gas wellbore integrity in gas–coal co-exploitation areas

Natural gas wells in gas-coal co-exploitation areas commonly penetrate thick unconsolidated aeolian sand, underlying bedrock, and longwall-mining-disturbed coal-bearing strata. Mining-induced subsurface deformation can transfer along the wellbore, threatening integrity at mechanical discontinuities. This study integrates theoretical analysis, 3D numerical modeling, and field-measured subsidence data to investigate how incompatible deformation and shear slip near the sand-bedrock interface translate into localized transverse shear loading. Results show that differential sand-bedrock movement induces active–passive earth pressure asymmetry, concentrating an additional transverse shear force of up to 71.6 kN near the critical interface, accompanied by a positive-to-negative reversal of lateral bending moment with peak values of 26.4 and − 24.8 kN m. Under sustained lateral loading, the cement sheath reaches its shear limit before the steel casing, and local cracking may compromise zonal isolation and increase gas leakage risk. Based on an upper-bound shear-capacity model considering in-situ strength reduction, the ultimate cross-sectional shear capacity of the cement sheath was estimated at 107 kN. Numerical simulations calibrated a linear relationship between surface subsidence and transverse shear force, with a transmission coefficient of 1.087 kN/mm and an R-squared value of 0.982, yielding a critical surface subsidence threshold of 98.4 mm. Under conservative limit-state assumptions with an empirical safety margin, combined with field-measured dip-direction subsidence functions, the recommended risk-control offset from the subsidence-basin centerline was 404 m. This risk zoning framework supports wellbore integrity assessment, gas leakage risk control, and mining-boundary optimization.