Acoustic emission evolution characteristics and constitutive model for damage of granite after high-temperature heating and liquid nitrogen cold shock treatment

By using the liquid nitrogen (LN₂) fracturing to create a massive network of fractures in the reservoir, the thermal energy extraction efficiency of hot dry rock reservoirs can be effectively improved. To investigate the effects of LN₂ cold shock treatment on the fracturing mechanism and fracture effects of reservoirs at different temperatures, the uniaxial compression tests are conducted on the granite samples undergone high-temperature heating (25℃~400℃) and liquid nitrogen cold shock treatment. The evolution characteristics of the mechanical strength and acoustic emission parameters of granite are analyzed, and an acoustic emission constitutive model for damage of granite is further established to evaluate and predict the deformation and strength characteristics of granite after high-temperature heating and liquid nitrogen cold shock treatment. The results indicate that the combined effects of high-temperature heating and LN₂ cold shock significantly degrade the mechanical properties of granite, with the gradual decrease in the peak strength and the maximum reduction of 32.8%. Meanwhile, with the increase in the heating temperature, there are differences in the thermal expansion deformation between different mineral particles, resulting in a lack of coordination in deformation between mineral particles. With the increase in the initial heating temperature, the average maximum b-value of acoustic emission significantly increases, with the maximum increase of 32.2%, and the strain corresponding to the initial silent stage of acoustic emission ringing counts decreases significantly, with the maximum reduction of 54.3%. With the increase in the heating temperature, the LN₂ cold shock treatment causes the microcracks to grow more densely. Under the external loading, the microcracks continuously expand and penetrate, making it easier for the granite to undergo shear deformation and for the initial stress level of shear failure to gradually decrease, with the maximum reduction of 62.3%. Meanwhile, the proportion of RA-AF scatter plot values in the shear zone increases, with the maximum increase of 29.5%. Additionally, an acoustic emission constitutive model is established using the accumulated ringing counts as a variable, which can describe the evolution characteristics of different mechanical parameters of granite during the deformation and failure process after high-temperature heating and LN₂ cold shock treatment.