Characterization of real-time thermal response during rupture of hot dry rock under the shock of liquid Nitrogen circulation

Circulating liquid nitrogen (LN₂) fracturing technology significantly enhances the remodeling performance of geothermal reservoirs in hot dry rock (HDR). This study examines the real-time thermal response characteristics of hot dry rock subjected to thermal shock from the LN₂ cycle. The dynamic evolution and real-time rupture behavior of internal cracks in HDR under LN₂ cycle impacts at varying heating temperatures are captured using acoustic emission technology. The damage mechanism and crack development process of HDR under the LN₂ cycle shock are discussed. Findings indicate that during the early stage of LN₂ cooling, the extreme temperature differential between the HDR surface and liquid nitrogen induces a sharp increase in thermal stress, resulting in notable damage effects, peak acoustic emission energy counts occur. In the middle and late stages of cooling, acoustic emission activity is minimal at heating temperatures of 200 °C and 300 °C, while at 400 °C, high acoustic emission energy is observed. 300 °C-400 °C is the temperature threshold interval for significant changes in damage characteristics of HDR under heating- LN₂ rapid cooling conditions. Additionally, the cumulative energy of the total number of cycles at 300 °C is 46.2 % higher than at 200 °C, and at 400 °C, it is 186.9 % and 96.2 % higher compared to 200 °C and 300 °C, respectively. The degree of damage to HDR escalates with increasing heating temperature and cycle number. The LN₂ cooling process primarily manifests as shear damage, with small amount of tensile damage occurring at higher heating temperatures.