Meso-damage mechanism and physico-mechanical properties of Zhangzhou granite subjected to water-cooling treatment after elevated temperatures

In order to exploit deep geothermal resources, it is critical to comprehensively understand the meso‑damage mechanism and physico-mechanical properties of high-temperature rocks after water-cooling treatment. In this paper, water-cooling experiments were carried out on granite at varying temperatures (20 °C, 150 °C, 300 °C, 450 °C, 600 °C and 750 °C), and the corresponding damage degree of microstructure was quantitatively assessed by computed tomography, optical electron microscopy, and nuclear magnetic resonance The effects of thermal shock on physical and mechanical parameters such as elastic wave velocity, permeability, uniaxial compressive strength (UCS) and elastic modulus of granite were analyzed. The results showed that the number and size of cracks in granite increase gradually with the increase in heating temperature; in particular, when the temperature rises to 600 °C and 750 °C, the proportion of macropores significantly increase, indicating the development and penetration of pores and cracks, which leads to the aggravation of the meso‑damage degree of granite. The fractal dimension of pores decreases with the increase in temperature, which shows that high temperature refines the pore structure. However, the fractal dimension of mesopores and macropores is insensitive to temperature variations. The meso‑damage of granite directly affects its physical and mechanical properties. The P-wave velocity, uniaxial compressive strength and elastic modulus of granite gradually decrease with temperature increase, whereas the peak strain gradually increases. The permeability of granite also exhibits a trend of slow increase first and then rapid rise, similar to the porosity variation trend. The results provide valuable theoretical support for evaluating the thermal energy utilization efficiency of hot dry rock and the stability analysis of reservoir reconstruction projects.