Experimental study on the effect of heating and liquid nitrogen-cooling cyclic treatment on mechanical properties and fracturing characteristics of granite

As an environmentally friendly anhydrous fracturing technology, liquid nitrogen (LN2)-assisted refracturing can effectively improve fracture complexity and build Enhanced geothermal systems by repeatedly cooling the hot dry rock. In this study, multiple heating and LN2-cooling cyclic tests were conducted on granite samples to evaluate the impact of LN2 cooling on the mechanical properties of granite. Subsequently, we performed uniaxial compression tests and acoustic emission (AE) detection to investigate the evolution of microcracks and fracture mechanisms in granite. The results show that heating and LN2-cooling cyclic stimulation aggravates the damage of granite, i.e., its physical and mechanical properties degrade in different degrees. After cyclic stimulation, there are regular changes in the types and sizes of microcracks in granite. The distribution range of the average frequency (AF) value of AE data decreases from 20–350 kHz to 20–250 kHz, while the rise time/amplitude (RA) value increases from 0–2 ms/v to 0–10 ms/v. Heating and LN2 cooling lead to an increased likelihood of mineral particles in granite undergoing shear deformation and generating shear microcracks. After three cycles of heating and LN2-cooling, the proportion of shear microcracks begins to exceed that of tensile microcracks, which are 65.3% and 34.7%, while the failure pattern of granite samples changes from tensile to shear fracturing. As the number of cycles increases, the proportion of AE counts with peak frequency exceeding 700 kHz generally show an upward trend. The appearance of high-frequency AE signals indicates that the fracture scale of granite is further increased. The research results provide a scientific foundation for the refracturing of high-temperature granite under LN2 cooling.