Objective The surrounding rocks of wellbores remain in a state of stress release during the drilling and heat extraction of deep geothermal reservoirs. Therefore, investigating the characteristic stresses of heat-treated granites under loading and unloading conditions is critical to understanding the crack evolution in deep hot dry rock (HDR) reservoirs during their stimulation.

Methods Targeting the granite specimens after heat treatment at varying high temperatures (e.g., 200 ℃, 300 ℃, 400 ℃, 500 ℃, and 600 ℃), this study conducted triaxial loading and unloading tests under a confining pressure of 60 MPa, aiming to investigate the impacts of high temperature and unloading stress on the mechanical properties of granites. Subsequently, the evolution mechanisms of microcracks in the granite specimens were revealed in combination with observations under the polarizing microscope. Finally, the evolutionary patterns of the granite specimens where mesocracks transitioned to macrocracks and then to ultimate failure under loading/unloading conditions were summarized.

Results and Conclusions  The test results indicate that under loading and unloading conditions, the granite specimens after heat treatment at varying temperatures exhibited decreases in the four characteristic stresses with increasing temperature. The water cooling under high temperature led to decreased threshold stresses for crack closure, initiation, and coalescence in the granite specimens, while the unloading stress further reduced the threshold stress for crack evolution in the specimens. With an increase in temperature, high temperature caused the uneven thermal expansion of mineral crystals and changes in both mineral composition and structure. These processes led to the gradual initiation, propagation, and intersection of intergranular and intragranular cracks in the heat-treated granite specimens. Both the density (ρ_f) and average width (W_a) of cracks in the specimens increased with the temperature, corresponding to the trends in the characteristic stresses varying with temperature. During the unloading process, the confining pressure gradually decreased, with the unloading stress state essentially equivalent to the superimposition of lateral tensile stress on the loading stress state. The tensile cracks induced by the lateral tensile stress experienced gradual expansion and coalescence, leading to more pronounced radial expansion. Consequently, the characteristic stresses decreased, which ultimately caused multiple shear failures in the granite specimens after heat treatment at varying high temperatures. The test results of this study can provide theoretical support for the analysis, calculation, and numerical simulation of the fracturing and heat extraction of deep hot dry granite reservoirs.