Characteristic analysis of hot dry rock damage and failure under thermal shock

Wave propagation and thermal conduction properties can indicate structural damage to hot dry rock (HDR) caused by thermal shocks. Granite from the central paleo-uplift belt in the Daqing area in the northern Songliao Basin is the focus of this research. Wave propagation tests and thermal conduction tests were conducted on high-temperature specimens after natural cooling and water cooling. The influence of the cooling process with water on the apparent temperature and thermal shock velocity of rock masses is discussed. Changes in the P- and S-wave velocities, specific heat capacity, thermal diffusion coefficient, and thermal conductivity coefficient of specimens under different heat treatment conditions are analyzed and compared. The relationships between the wave propagation characteristics and the thermal conductivity values are established. The results indicate the following: (1) The apparent temperature and thermal shock velocity values of high-temperature rock masses show nonconstant rates of change from fast to slow. Rock masses with higher initial temperatures exhibit a continuous and strong thermal shock effect, and water temperature differences have only a limited effect on the thermal shock velocity over 20 s of cooling with water. (2) As the initial temperature increases or water temperature decreases, the P-wave velocity, S-wave velocity, thermal diffusion coefficient, and thermal conductivity coefficient values of high-temperature rock masses decrease overall, while the specific heat capacity values increase. The amplitude of change after water cooling is higher than that after natural cooling. When the initial temperature is below 100 degrees C, the free water content is the main factor influencing the wave propagation characteristics and changes in the thermal conduction characteristics. When the initial temperature is greater than 300 degrees C, the changes in both properties depend on the amount of thermal damage done to the rock mass. (3) The fitting results suggest that the wave propagation and thermal characteristics are closely related, and the damage factors are strongly consistent. The deviation of fitting coefficients is mainly due to the limited development of fractures in the small-sized specimens. The research results provide a reference for studies on thermal damage in HDR in the geothermal development process, and a method to determine the in situ thermal conductivity of reservoirs is proposed.