Hydration-induced weakening effect in deep coal rock and implication for hydraulic fracturing performance: insight from experimental and theoretical analysis

This study investigates hydration-induced weakening effect in deep coal rocks through integrated experimental and theoretical analyses. Nanoindentation, SEM, and XRD tests reveal that hydration reduces elastic modulus and hardness, increases plastic energy dissipation, and selectively dissolves calcite, weakening particle bonding and inducing microfracture propagation. The measured permeability of coal after hydration exhibits a strong inverse relationship with confining pressure, it increases approximately fivefold at 4 MPa but drops sharply to twofold at 24 MPa, confirming pronounced stress sensitivity. A time-dependent damage constitutive model based on the Weibull statistical damage theory and the Drucker-Prager criterion is established and embedded into a thermal-hydro-mechanical-damage (THMD) coupled framework. The model captures hydration-induced strength degradation and the brittle-to-plastic transition. Numerical simulations indicate that hydration strongly promotes fracturing fluid imbibition and enhances pore pressure diffusion. During the shut-in period, the formed damage area reach 3-5 times that of the hydraulic fracture, while simulated matrix permeability increases hundreds of times under coupled mechanical and hydration damage, significantly expanding the hydration-induced area. The high matrix permeability, abundant natural fractures, large hydration coefficient, and prolonged well shut-in time amplify this synergistic effect. This work provides improved mechanistic insight into hydration-induced damage in coal rocks and offers an experimental and theoretical framework for studying hydration in deep coals.