Liquid nitrogen is proposed as a new fracturing fluid to develop enhanced geothermal systems based on the thermal stimulation method. The large temperature difference between cryogenic liquid nitrogen and hot dry rock reservoirs induces high thermal stress, which quenches the reservoir and improves the heat transfer efficiency. Ultrasonic detection is a common non-destructive method for evaluating rock damage, but current analysis procedures mainly focus on wave velocities. This paper thus proposes a comprehensive ultrasonic characterization method. Granite cores were treated by cyclic heating at temperatures of 200-600 degrees C for 0-5 liquid nitrogen cooling cycles. The damage process was evaluated quantitatively by testing and analyzing the ultrasonic waves transmitted in the rock samples, and the effects of heating temperature and cold-shock times were compared. The results indicated that the velocity, amplitude, and dominant frequency of ultrasonic waves all attenuated with increasing rock damage, among which the velocity attenuated the most strongly, with maximum decrease of 85.01%. In the time domain, pores and fractures distorted the ultrasonic waveform and partially or completely reduced the amplitude. In the frequency domain, the dominant frequency shifted toward lower frequency and the intensity decreased. The fractured cores reduced the proportion of high-frequency energy and increased the proportion of low-frequency energy. The dynamic mechanical properties (elastic modulus, bulk modulus, shear modulus, and Poisson's ratio) were calculated, among which the bulk modulus was the most sensitive to rock damage, with maximum reduction of 15.86 GPa. The calculated damage variables showed an initially rapid and then slower increasing trend with cooling time and a linear increase with heating temperature. The damage variable values after five cold shocks ranged from 0.44 to 0.96. The cores generally experienced stages of micro-fracturing, single fracturing, and fracture networking, and both the heating temperature and liquid nitrogen cold-shock times had a significant effect on the extent of rock damage.
