Microstructural Characteristics of Damaged Hot Dry Rock Flow Network Stimulated by Cryogenic Liquid Nitrogen Shock

Hot dry rock (HDR) geothermal development faces challenges due to the difficulty of stimulating fluid flow and heat-exchange fracture channels within deep, low-porosity, and low-permeability reservoirs. A liquid nitrogen cyclic cold shock method was proposed, using liquid nitrogen as a fracturing fluid. The large temperature difference between the liquid nitrogen and the hot rock induces thermal stress, forming a complex pore-fracture network. In this study, HDR samples at various temperatures were subjected to 5 cycles of liquid nitrogen cold shock. Pore-fracture structures of the damaged cores were tested by low-field nuclear magnetic resonance and X-ray microscopy. Results showed temperature-dependent changes in porosity: at 200 and 300 °C, small-size pores rose, with maximum porosities of 3.13% and 3.37%; at 400-500 °C, large-size pores rose, with porosities reaching 4.59% and 12.76%. Different pore types exhibited distinct responses: mesopores are the most sensitive to temperature shock, micropores responded to the early damage stage, and macropores only responded under high-temperature differences and multiple cycles. Multifractal analysis revealed increased heterogeneity and concentration in pore distribution with damage escalation. The slice porosity, fractal box dimension, and probability entropy demonstrated exponential growth. Results indicate that temperature difference is the main controlling factor of pore damage in HDR, and cyclic shock can also contribute to continuous pore development. There is a high correlation between pore-fracture parameters and multifractal parameters, allowing the multifractal to reflect a more complex pore-fracture structure quantitatively.