Instability mechanism and wellbore stability of hard brittle mudstone in the Mahu Sag: a hydration-fracture coupling perspective

To address severe wellbore collapse in the hard, brittle shale of the Wusan Member, Permian Urho Formation (Mahu Sag), this study establishes a chemo-mechanical coupled stability model using mineral analysis, micro-CT, and triaxial hydration tests. Based on linear elastic porous media, the model integrates a time-dependent damage variable and modified Jaeger’s weak plane theory. Results show that despite low macroscopic swelling (<5%), filtrate invasion along primary weak planes induces local wedging stress and capillary effects, dominating micro-fracture initiation (hydration-fracture coupling). Hydration causes exponential mechanical degradation, reducing cohesion by 92.0% (±2.5%) after 10 days. Fracture orientation and seepage jointly control stability. Instability risk peaks when bedding dip is perpendicular to the maximum horizontal principal stress, and seepage increases the collapse pressure equivalent density to 1.96 ± 0.03 g/cm3. Consequently, we recommend optimizing wellbore trajectories along the maximum horizontal stress. For open-hole exposures exceeding 10 days, drilling fluid density should be maintained at 1.97–2.02 g/cm3, supplemented by effective plugging strategies.