Relationships between P-wave velocity and mechanical properties of granite after exposure to different cyclic heating and water cooling treatments

The bedrocks of deep geothermal reservoirs are exposed to cyclic water cooling during the exploitation of deep geothermal energy. Therefore, it is important to understand the physico-mechanical parameters of geothermal reservoir rocks. This paper reports on the P-wave velocity (Vp), uniaxial compressive strength (UCS) and elastic modulus (E) of granite specimens after exposure to different cyclic heating and water cooling treatments based on laboratory tests, and the relationships between Vp, UCS and E established through regression analysis. The physico-mechanical parameters of granite specimens all decrease remarkably in the first few thermal cycles, and their rates of decrease gradually diminish with thermal cycles, which is beneficial for the long-term exploitation of deep geothermal resources. Both UCS and E show a logarithmic correlation with Vp of granite under different high temperatures. There is a transformation from a linear relation (1 and 5 cycles) to an exponential relation (from 10 to 30 cycles) between Vp, UCS and E with thermal cycles. Such a correlation can provide a good estimation and avoid the costly, time-consuming and tedious mechanical tests. SEM observation reveals the change mechanism of the deterioration of physico-mechanical parameters, which can guide the well borehole stability during the deep geothermal energy exploitation.