Study on mechanical properties and damage constitutive model of deep coal rock under multi-field coupling

The mechanical failure of deep coal rock poses significant challenges for wellbore stability and resource extraction. However, its behavior under the coupled effects of temperature, confining pressure, and drilling fluid exposure remains insufficiently characterized. This study focuses on deep coal rock from the Benxi Formation in the Ordos Basin. Triaxial mechanical experiments were conducted under varying confining pressures, temperatures, and drilling fluid immersion durations to investigate the stress-strain behavior, mechanical properties, and failure characteristics of the deep coal rock. Based on the Lemaitre strain equivalence hypothesis, a segmented damage constitutive model was developed to simulate the coupled effects of confining pressure, temperature, and hydration. The model incorporates Weibull-distributed microelement strength, as well as damage variables related to hydration, temperature, and mechanical loading, and adopts the SMP criterion to account for intermediate principal stress. A parameter determination method for the proposed model is presented, and the evolution mechanisms of coal rock damage under different field conditions are analyzed. Finally, the reliability of the model is validated through comparison with experimental data and classical theoretical models. The results show that confining pressure and drilling fluid immersion time are the dominant factors controlling damage in deep coal rock, with temperature acting as a secondary factor. However, under high-temperature conditions, thermal shock significantly intensifies damage evolution, making temperature the primary driver. Both peak stress and crack closure stress exhibit an exponential increase with confining pressure and a decreasing trend with rising temperature and immersion duration; specifically, crack closure stress decreases linearly with immersion time, while peak stress declines exponentially. Correspondingly, failure modes evolve with loading conditions: single-shear failure dominates at 15 MPa, whereas elevated temperatures (120 °C) and prolonged fluid exposure induce tensile–shear conjugate failure and matrix detachment. Model validation under all test conditions yields RMSE values below 2.97 and R² values above 0.94, indicating high predictive accuracy. The proposed model effectively captures the complete stress-strain process and damage progression, offering concise parameterization and wide applicability for geo-mechanical analysis under high-stress, multi-field conditions.