Damage Analysis of High-Temperature Rocks Subjected to LN2 Thermal Shock

Liquid nitrogen (LN2) fracturing is a technology that can dramatically enhance the stimulation performances of high-temperature reservoirs, such as hot dry rock geothermal and deep/ultra-deep hydrocarbon reservoirs. The aim of the present study was to investigate the damage characteristics of high-temperature rocks subjected to LN2 thermal shock, which is a critical concern in the engineering application of LN2 fracturing. In our work, the rocks (granite, shale and sandstone) were slowly heated to different temperatures (25 °C, 150 °C and 260 °C) and maintained at the target temperatures for 10 h, followed by LN2 quenching. After thermal treatments, we tested the physical and mechanical properties of the rocks to evaluate their damages. Additionally, sensitivities of the three rocks to thermal shock were also compared and analyzed. According to our experiments, LN2 thermal shock can enhance the permeability of the rocks and deteriorate their mechanical properties significantly. Increasing rock temperature helps strengthen the effect of LN2 thermal shock, leading to more severe damage. Inter-granular cracking is the primary contribution to the rock damage in the LN2 cooling process. Compared with granite and shale, sandstone is less sensitive to LN2 thermal shock. The lower sensitivity of sandstone to thermal shock is mainly attributed to its larger pore spaces and weaker heterogeneity of mineral thermal expansion. The present paper can provide some guidance for the engineering application of LN2 fracturing technology.