Effect of thermal cycling-dependent cracks on physical and mechanical properties of granite for enhanced geothermal system

Thermal cycling induced micro-cracks can change the physical and mechanical properties of geothermal energy reservoir, which may influence the heat and mass transfer performance as well as the stability of the reservoir. In this study, thermal cycling treatment was carried out on granite over the temperature range of 20–300 °C with 1–20 thermal cycles. The results show that thermal cycling promotes the initiation and propagation of intergranular and intragranular cracks, which are evenly distributed in all directions. With increase in the number of thermal cycles, the crack density (Pl) increases, resulting in increased permeability (K). The path of seepage passage is mainly between the mineral particles. Moreover, the critical crack propagation radius (rc) of rock decreases with increasing cracking degree, which leads to the decrease in rock fracture resistance. In particular, the fracture toughness (Keff) of granite decreases most when it is subjected to 1–5 thermal cycles. Water-cooling thermal cycling and cooling rate can significantly affect the micro-crack evolution, permeability and ability of granite to resist fracture. The changes in mechanical and physical properties observed in this work can provide basic theoretical reference for the rational selection of geothermal energy mining methods and process parameters, as well as the study of reservoir stability evaluation.