Study on the mechanical behavior of hot dry rock under coupled fatigue action of high temperatures and seawater

Utilizing inexhaustible seawater as a heat exchange medium to extract geothermal resources from deep hot dry rock (HDR) in coastal areas or marine islands may be one strategy to mitigate the energy crisis in the future. Analyzing the mechanical behavior of HDR geothermal reservoirs affected by seawater is critical for the innovative development of geothermal extraction methods and reservoir stimulation. Previous studies have not investigated the evolution of the mechanical parameters and the damage mechanisms of HDR reservoirs exposed to high temperatures and seawater dissolution. Therefore， this study conducted triaxial compression tests on HDR to assess the changes in key mechanical parameters， such as elastic modulus， Poisson's ratio， and compressive strength， under different temperatures (100， 200， 300， 400， and 500 ℃)， thermal shock cycles (5， 10， 15， and 20 cycles)， and confining pressures. The results showed that the compressive strength and elastic modulus significantly decreased with the increasing temperature and number of thermal shock cycles. The most significant effects occurred at 500 ℃ and 20 cycles; the compressive strength of the HDR was 125 MPa or 51.32% lower at 100 ℃ than at 500 ℃. The exponential relationship between the mechanical parameters (strength and modulus) and the temperature had a goodness of fit of 90%-99%. High-temperature thermal expansion， thermal shock damage， and seawater chemical dissolution were the dominant factors causing damage to the HDR. In addition， the compressive strength and modulus of the HDR were significantly lower under seawater than under freshwater conditions， which can be exploited may be beneficial for geothermal reservoir fracturing and the formation of fracture networks for heat exchange.