Advances in research on the mechanical responses of hot dry granites under varying cooling conditions in China

Significance  Enhanced geothermal systems (EGSs) serve as a key technique for exploiting hot dry rocks (HDRs) presently. During the construction and operation of EGSs, rocks of the HDR reservoirs will experience various cooling conditions, such as natural cooling, cyclic cooling, and water cooling, rendering it greatly significant to investigate the mechanical response characteristics of granites—typical HDRs—under varying cooling conditions.

Advances  This study offers a summary and analysis of experimental data about the mechanical responses, i.e., uniaxial compressive strength (σ_c), elastic modulus (E), tensile strength (σ_t), and Poisson's ratio (ν), of typical granites in China under varying cooling conditions including natural, water, and cyclic cooling. The results reveal that after natural cooling, the σ_c, E, and σ_t values of the granites decrease slightly at temperatures below 200 ℃ or 300 ℃ but decrease nearly linearly with an increase in temperature at temperatures greater than 200 ℃ or 300 ℃. After water cooling, these values decrease nearly linearly with an increase in the temperature at any temperature. Under cyclic cooling, these values decrease rapidly after cooling for the first time and then gradually tend to remain constant after more than five times of cooling. Under natural cooling, the value of ν decreases with a rise in temperature, with the decreasing amplitude greater than that under water cooling. The deterioration of the mechanical properties of high-temperature granites under varying cooling conditions is primarily due to the generation and propagation of microcracks in them. Based on the statistics and the internal mechanism analysis of the mechanical response data on hot dry granites under varying cooling conditions, this study conducts the fitting of normalized mechanical parameters with temperature and proposes relevant empirical formulas.

Prospects This study proposes that future research on the mechanical properties of high-temperature granites will focus on the mechanical responses of HDR during their exploitation combined with CO₂ sequestration, the mechanical responses of HDRs under the coupling effects of multiple fields, phases, and processes, and experimental studies based on the actual conditions of HDR exploitation. All these are expected to provide some theoretical support for the design, calculation, and numerical simulation of HDR exploitation.