Damage evolution and wellbore stability model of coal seams under the coupled effects of fluctuating pressure and hydration

Deep coalbed methane horizontal wells frequently undergo cyclic fluctuating pressures induced by drill string/casing movement during hole conditioning and casing running. Under the combined effects of long-term drilling-fluid soaking-induced weakening and pressure disturbances, the borehole wall is prone to progressive damage evolution and instability, yet existing models struggle to quantitatively capture this synergistic behavior. To address this gap, this study systematically investigates the fatigue-damage characteristics of coal under varying confining pressures, loading rates, and loading gradients using a true-triaxial cyclic loading-unloading test system. The results show that cyclic disturbances significantly alter the elastic modulus, peak strength, and failure mode of coal; higher loading rates and steeper loading gradients accelerate damage accumulation, whereas increased confining pressure enhances the load-bearing capacity of coal. Meanwhile, drilling-fluid soaking causes the elastic modulus and compressive strength to continuously decrease with time, and the two types of damage effects can be uniformly characterized using a damage-variable framework. Furthermore, an analytical model is developed for steady-state fluctuating pressure comprising three components-static shear force, viscous drag, and inertial force-and is coupled with the damage variable to establish a total damage model for coal under the combined action of drilling-fluid weakening and fluctuating pressure. By incorporating the Mohr-Coulomb criterion and a single-weak-plane criterion, a wellbore stability analysis method is formulated. Model validation indicates that, as the damage variable increases, the collapse-density and fracture-density windows shrink markedly, leading to a significant deterioration in wellbore stability. A case study of Well X in the western Ordos Basin demonstrates that the proposed model can accurately explain the pump pressure build-up and wellbore pack-off/bridging encountered during casing running at a depth of 4218 m, and reveals that an excessively high casing running speed increases instantaneous fluctuating pressure and triggers the propagation of borehole-wall damage. Finally, recommendations are proposed for optimizing the drilling-fluid density range and controlling casing running speed, providing theoretical support and engineering guidance for safe drilling, managed-pressure cementing, and casing deployment in deep coalbed methane wells.