Rock damage and fault reactivation mechanisms under initial perturbation in enhanced geothermal systems

This study elucidates injection-induced fault reactivation mechanisms in Enhanced Geothermal Systems through integrated thermo-hydro-mechanical-damage (THMD) numerical modeling of China's Huangshadong geothermal field. A coupled TOUGHREACT-FLAC3D framework incorporating Weibull-based stochastic damage constitutive relations was formulated to characterize fault slip behavior under hypothetical cold fluid injection scenarios. Numerical simulations reveal a three-stage temporal evolution of fault reactivation: initial quiescence (0–10 d), rapid acceleration (10–35 d), and quasi-linear stabilization (35–55 d), with a cumulative displacement of 17 mm. Progressive damage accumulation fundamentally reconfigures stress transfer pathways, manifesting in an amplification of the effective principal stress ratio (σ₁/σ₃) to a peak of 4.4—a 100% increase from initial conditions—which coincides with peak damage network percolation (D > 0.9). Comparative analysis demonstrates that THMD-coupled model predicts 40% more fault slip than conventional THM formulations, attributed to damage-induced stiffness degradation (40%–50% elastic modulus reduction) and permeability enhancement, which collectively diminish fault normal stress and expedite pore pressure diffusion. These findings show that conventional THM approaches systematically underestimate fault activation potential by neglecting damage-slip feedback mechanisms. This study provides quantitative insights into injection-induced seismicity and establishes a theoretical framework for risk assessment in deep geothermal reservoir exploitation.