Effects of Liquid Nitrogen on Mechanical Deterioration and Fracturing Efficiency in Hot Dry Rock

Geothermal energy can be obtained from hot dry rock (HDR). The target temperatures for heat extraction from HDR range from 100 to 400 °C. Artificial fracturing is employed to stimulate HDR and create a network of fractures for geothermal resource extraction. Liquid nitrogen (LN2) is environmentally friendly and shows better performance in reservoir stimulation than does conventional fracturing. In this study, triaxial compression experiments and acoustic emission location techniques were used to evaluate the impacts of temperatures and confining pressures on the mechanical property deterioration caused by LN2 cooling. The numerical simulation of LN2 fracturing was performed, and the results were compared with those for water and nitrogen fracturing. The results demonstrate that the confining pressure mitigated the deterioration effect of LN2 on the crack initiation stress, crack damage stress, and peak stress. From 20 to 60 MPa, LN2-induced reductions in these three stress parameters ranged between 7.73–18.51%, 3.46–12.15%, and 2.51–8.50%, respectively. Cryogenic LN2 increased the number and complexity of cracks generated during rock failure, further enhancing the fracture performance. Compared with those for water and nitrogen fracturing, the initiation pressures of LN2 fracturing decreased by 61.54% and 68.75%, and the instability pressures of LN2 fracturing decreased by 20.00% and 29.41%, respectively. These results contribute to the theoretical foundation for LN2 fracturing in HDR.