Assessing the contribution of mineral composition to meso-macro progressive failure in granite under thermo-hydro-mechanical (THM) conditions: Inspiration for hydraulic fracturing in Enhanced Geothermal Systems (EGS)

The Enhanced Geothermal System (EGS) is a breakthrough technology for harnessing geothermal energy, and a comprehensive evaluation for the mechanical performance of reservoir rocks under complex geological conditions is crucial for ensuring effective hydraulic fracturing and the safe operation of geothermal system. In this study, a refined THM simulation algorithm was operated to simulate the meso mechanical behaviors of different mineral grain interfaces. Nine numerical granite models with distinct mineralogical compositions were subjected to 20 THM scenarios under EGS design environments, and the progressive failure mechanisms governed by mineralogical topology spanning meso-interactions (cohesion degradation, interface damage, shear slip) to macro fracture mechanics were evaluated. Results demonstrated that temperature and pore pressure synergistically govern fluid flow pathways in granite, with pore aperture evolution, micro-defect connectivity, and interfacial crack propagation emerging as critical factors affecting seepage behavior. With the increased temperature and pore pressure, two mechanisms affecting rock failure process exist, including the competitive mechanism between thermal behaviors of quartz-related interfaces and biotite-related interface breakage on rock structure, as well as the inducement of thermal-on pore pressure-induced micro-cracks at quartz/feldspar-related crystal boundaries. Finally, a quantified analysis framework was developed to integrate mineralogical heterogeneity, temperature and pore pressure interactions with the meso-macro mechanical properties of granite. It revealed that the effect of quartz on the mechanical strength of granite is the most significant under high temperature and low pore pressure conditions, while its effect on the elastic modulus is obvious under high temperature and high pore pressure. Feldspar has the greatest impact on the elastic modulus of granite under high temperature and low pore pressure, with a relatively stable effect on the mechanical strength of granite under THM conditions. The proposed framework synergistically integrates elastic modulus-based brittleness indices with strength parameters, offering novel insights for evaluating the hydraulic fracability of granite and optimizing site selection in EGS applications.