Impact of High-Temperature Cooling Cycles on the Physicomechanical and Seepage–Heat Transfer Properties of Granite

Geothermal energy, recognized as a renewable, environmentally friendly, low-carbon, and high-quality resource, possesses extensive reserves and promising application prospects. To enhance our understanding of heat extraction from hot dry rocks, it is crucial to investigate the deterioration behavior of high-temperature rocks subjected to cyclic cooling and the coupled seepage-heat transfer processes involving cold fluids. In this study, a series of high-temperature cooling cycle tests were performed on granite samples, ranging from room temperature to 600 °C, with an introduction of a low-temperature NaCl solution as the cooling medium. The variations in the physicomechanical properties of granite were systematically analyzed. Utilizing digital rock technology and mathematical morphology methods, the seepage pathways were extracted and characterized. Furthermore, finite element simulations were conducted to examine the seepage-heat transfer processes, considering four distinct fracture apertures and four levels of tortuosity. The key findings are as follows: (i) the deterioration effect induced by low-temperature NaCl cooling is substantially more pronounced than that caused by water cooling or natural cooling; (ii) the permeability coefficient and the proportion of large fracture apertures (190-266 μm) increase with the rising temperature and cooling rate. Specifically, the proportion of large fracture apertures reached 24.49% following low-temperature NaCl cooling at 600 °C. This phenomenon can be attributed to the higher thermal expansion coefficient of NaCl relative to granite; (iii) the tortuosity of the seepage pathways significantly enhances the seepage-heat transfer properties. When the tortuosity increases from 1.5 to 3, the total convective heat transfer coefficient increases by 80.23%. This study provides insights into the physicomechanical properties of high-temperature rocks post-cooling and offers novel perspectives for the calculation and numerical simulation of seepage-heat transfer processes.