Damage Evaluation and Statistic Constitutive Model of High-Temperature Granites Subjected to Liquid Nitrogen Cold Shock

Liquid nitrogen (LN2) fracturing is a promising waterless technology that can effectively enhance the modification performances of hot dry rock (HDR) geothermal reservoir. The physical and mechanical damage characteristic of high-temperature reservoir rock subjected to LN2 cold shock is a fundamental and critical concern in the engineering application of LN2 fracturing. In this paper, granite samples subjected to heating treatment and LN2 cold shock were tested experimentally with the aim of evaluating the damage by physical and mechanical properties from the macro- and microscopic perspectives. According to experimental results, the treatment of heating and LN2 cold shock can aggravate granite internal structural damage and deteriorate physical and mechanical properties significantly. Increasing the heating temperature of granite samples contributes to the enhancement of LN2 cold effect, causing more serious damage. Based on the skeleton compression coefficient proposed and the generalized strain equivalent principle, a coupled thermal-mechanical damage statistical model considering initial crack closure stage was established. Compared with experimental result, the proposed model can fit the deformation behavior of the rock well, especially the initial nonlinear characteristics of the crack closure stage. The results in this paper are expected to provide some guidance and reference for the application of LN2 fracturing technology in the stimulation treatment of geothermal reservoir.