Progressive Evolution of Flow and Heat Transfer Channels in Hot Dry Rock Stimulated by Liquid Nitrogen Cold Shock

Hot dry rock (HDR) is a novel green, low-carbon energy. Its development requires the creation of fracture channels in deep thermal reservoirs. Traditional methods such as hydraulic fracturing have limited effectiveness in reservoir stimulation, so a method of liquid nitrogen cold shock was proposed. The ultralow temperature of liquid nitrogen induces a quenching effect on hot rock, thereby promoting the formation of a complex fracture network. In this study, hot rocks at different temperatures were subjected to cyclic liquid nitrogen cold shocks. The cascading evolution of the "temperature field-thermal stress-pore fracture development-three-dimensional seepage network" was analyzed. The main conclusions are as follows: As the cold-shock duration increased, the core temperature decreased exponentially, with a slower cooling rate at the center and a faster rate at the edges. Additionally, local temperature fluctuations existed at the edges. The maximum radial thermal stress at 200-600 °C ranged from 2.57 to 9.29 MPa. The initial cold shock and temperatures above 300 °C contributed the most to the reduction in wave velocity, with maximum decreases of 70.25 and 55%, respectively. The more developed the pores and fractures within the damaged core, the more pronounced the arrival time lag, frequency shift, and energy attenuation will be. Microscopic pore evolution followed two patterns. At 200-300 °C, the first peak increased significantly, with a few new micropores forming along grain boundaries. At 400-600 °C, the second peak grew and expanded to be dominant. Numerous mesopores formed both along grain boundaries and within grain interiors, interconnecting with the original pores. As the temperature shock effect intensified, the fracture increased and expanded. Branch fractures developed from the main fractures, ultimately forming a fracture network. The orientations of these fractures became more randomly distributed. What's more, the proportion of larger-sized and higher-connectivity pores increased. The flow velocity and flow rate increased significantly, with the maximum values in the representative elementary volume reaching 0.167 m/s and 1.75 × 10^-7 m³/s, respectively. When HDR underwent liquid nitrogen cold shock, the mineral grains contracted. Due to the various thermal expansion coefficients of different types of grains, local stress was concentrated. When the stress exceeded the tensile strength, tensile fractures were induced. The cyclic shock effect not only intensified the stress concentration but also lowered the damage threshold of the grains. The convergence of these two factors led to the continuous development of pore-fracture channels in the HDR.