Experimental Study on the Elastic-Plastic Deformation and Damage Failure Mechanism of Hot Dry Rock after Alternating Temperature Loading (Unconfined Pressure Condition) Pretreatment

Summary Hydraulic fracturing technology is key to achieving efficient heat extraction from hot dry rock (HDR). During the fracturing process, the injection of large volumes of cold fluids induces thermal stresses, significantly affecting the physical properties of the rock, as well as its deformation and failure mechanisms. In this study, based on the principle of “thermal expansion and contraction,” we simulate the alternating thermal load environment induced by high-temperature steam and low-temperature liquid nitrogen injection. Our aim is to enhance internal damage in HDR through cyclic thermal stresses, thereby increasing the complexity of fracture networks. However, research on the elastoplastic deformation and failure mechanisms of HDR under alternating thermal loading remains insufficient. In this study, we use a high-temperature, high-pressure triaxial compression system combined with permeability and porosity measurements, along with advanced techniques such as ultrasonic velocity monitoring, computed tomography (CT) scanning, and nuclear magnetic resonance (NMR) imaging, to systematically investigate the elastoplastic deformation behavior and damage mechanisms of deep reservoir rocks under alternating thermal loading. By comparing these results with those from single thermal treatment experiments, we comprehensively analyze the changes in permeability, porosity, and rock mechanical parameters under alternating thermal loading. The experimental results show that the temperature differences induced by alternating thermal loading promote the formation of numerous thermally induced cracks in HDR, thereby weakening its mechanical strength and stiffness. According to the Mohr-Coulomb criterion, alternating thermal loading reduces the cohesion of the rock while increasing the internal friction angle, which enhances its elastoplastic deformation characteristics. By comparing rock failure patterns, CT scan images, and incorporating the Hoek-Brown and Griffith criteria, we reveal the nonlinear elastoplastic deformation and failure characteristics of rocks under alternating thermal loading. Under alternating thermal loading, the fracture network becomes more complex, with shear-type plastic failure being predominant. Furthermore, we validated mechanical testing results using ultrasonic velocity and NMR techniques. Energy dissipation analysis and damage factor calculations indicate that the level of damage and failure under alternating thermal loading is more than twice that of a single thermal treatment. In conclusion, this study provides valuable experimental data and theoretical guidance for understanding the thermomechanical behavior of HDR under alternating thermal loading. The findings are of significant importance for the field application of high-temperature steam and low-temperature liquid nitrogen cyclic injection, especially in the efficient development and usage of HDR geothermal energy.