Thermo-Mechano-Chemical-Time Damage Evolution of Granite for EGS

To address the challenge of multi-field coupled damage during acidizing stimulation in deep Enhanced Geothermal Systems (EGS) reservoirs, this study conducted gradient immersion and mechanical tests on granite under high-temperature, high-pressure, and acidic environments, and constructed a Thermo-Mechano-Chemical-time (TMCt) coupled damage constitutive model. The research reveals the nonlinear threshold characteristics of damage evolution: 170–180°C serves as the critical interval for the synergistic action of thermally induced micro-crack propagation and chemical dissolution. Crossing this threshold results in a precipitous drop in Young's modulus of approximately 15 GPa. After a 12-hour reaction, the peak strength and elastic modulus of the rock samples deteriorated by 37.1% and 43.6%, respectively, while the peak strain increased by 10.7%. Normalized regression analysis indicates that temperature significantly amplifies the corrosive efficacy of the acid via the Arrhenius mechanism, whereas confining pressures exceeding 6 MPa induce damage saturation through the crack closure effect. The constitutive model, constructed based on the Weibull distribution and chemical kinetics theory, demonstrates that the damage variable defined based on Young's modulus-compared to the secant modulus-effectively decouples elastic damage from plastic deformation. With a fitting accuracy exceeding 0.85 and clearly defined physical parameters, the model achieves a precise quantitative characterization of the mechanical behavior of granite under complex operating conditions, providing a theoretical basis for the fracturing design and stability evaluation of deep geothermal reservoirs.