Abnormal variation of P-wave velocity of red sandstone after cyclical thermal shock in water

Hot dry rock (HDR) has great development potential because of its advantages of clean, environmental protection and renewable. The study of physical and mechanical properties of HDR is one of the important links in the process of geothermal energy development. Previous studies have mainly focused on the granite thermal reservoir but research on the sandstone thermal reservoir, especially the physical properties of sandstone after multiple thermal shocks, is scarce. Therefore, in this study, cyclic thermal shock experiments of sandstone at different temperatures are carried out, and the variation law of P-wave velocity of sandstone after heat treatment is revealed. It is found that the P-wave velocity of red sandstone decreases with the increase of temperature and cycle times, especially when the temperature is higher than 400 °C, the maximum change rate of wave velocity reaches 52.6%. It is particularly noteworthy that the P- wave velocity of sandstone increases abnormally at about 600 °C. And this article puts forward three hypotheses to explain the wave velocity anomaly. In addition, the nonlinear relationship model among P-wave velocity, temperature and number of cycles is established for the first time, and the correlation coefficient R 2 is more than 0.9. This study serves as a reference for the development and utilization of the sandstone geothermal reservoir. Article highlights The effects of temperature and cycle times on wave velocity of red sandstone are discussed. The threshold temperature of wave velocity change after cold heat cycle is 400 °C. The nonlinear fitting relationship among temperature, thermal shock times and wave velocity is established.