A model for characterization of crack closure stage in stress–strain curves of Suizhou granite after exposure to various thermal shocks

Reservoir rocks have an obvious initial nonlinear crack-closure stage during geothermal energy exploration using enhanced geothermal systems (EGSs). Therefore, the purpose of this research is to establish a quantitative model considering the influence of initial nonlinear characteristics on geothermal reservoirs. According to effective medium theory (EMT), deformation of reservoir rocks after thermal shocks can be divided into rock frame and void sections, and the effect of microcracks is considered in the establishment of the proposed model. Compressive tests conducted on Suizhou granite specimens after cyclic water cooling were used to validate this model. Furthermore, the influence of microcracks on initial crack closure part of Suizhou granite specimens after thermal shocks was revealed by scanning electron microscope (SEM) observation. There is a negative exponential relationship between the axial stress–strain in void section of specimens after cyclic water cooling. The crack closure stage in the stress–strain relation of Suizhou granite after thermal shocks is more apparent with heating temperature and cyclic time. The proposed model well characterizes the crack closure part in the stress–strain curves of specimens after cyclic water cooling. The proposed model describes the initial nonlinear stage of surrounding geothermal reservoir granite and provides theoretical support for the evaluation of stability in EGS engineering design.