Study on Damage Characteristics of Hot Dry Rock by Liquid Nitrogen Cyclic Cold Shocks Based on Ultrasonic Testing

Efficient exploitation of hot dry rock resources can alleviate global energy supply pressure. However, the "low-porosity, low-permeability" occurrence characteristics of hot dry rock severely restrict the exploitation efficiency. To achieve efficient exploitation of hot dry rock resources, artificial fracturing and permeability-enhancing measures must be taken to transform the reservoir and improve the heat-exchange efficiency. Liquid nitrogen cyclic cold shock is an effective fracturing method for geothermal reservoirs. The huge temperature difference between liquid nitrogen and hot rock can cause uneven contraction between mineral particles, inducing fracture networks. It is crucial to study the damage characteristics of hot dry rock under liquid nitrogen cyclic cold shock. Therefore, this paper carries out liquid nitrogen cold shock experiments on high-temperature granite. Granite samples heated to different temperatures (200, 300, 400, 500, and 600 °C) were subjected to cyclic cold shocks (five times) using liquid nitrogen. NMR testing and ultrasonic testing were performed on the treated samples to study the damage characteristics of hot dry rock. The results show that with the increase of heating temperature and the number of cold shocks, the porosity of the cores increases continuously. After five cold shocks, the porosity of the cores heated at 200–600 °C increases by 1.108, 1.154, 1.158, 3.080, and 6.896%, respectively. The wave velocity decreases continuously, and the time delay and distortion of the waveforms become more obvious. The frequency distribution gradually shifts toward lower frequencies, the wavelet packet energy distribution transfers to lower-frequency sub-bands, the total energy of the Hilbert energy spectrum decreases continuously, the duration of high instantaneous energy becomes shorter, and the frequency distribution interval shifts toward lower frequencies. The quality factor Q decreases with increasing heating temperature and the number of liquid nitrogen cold shocks and has a good linear relationship with the core porosity.