Effect of cyclic heating-rapid cooling on fracture behavior of notched semi-circular bend granite

Cyclic liquid nitrogen (LN2) fracturing is a promising innovative technology expected to rapidly form fracture networks in geothermal reservoirs. The fracture characteristics are crucial for estimating the effect of the artificial fracturing. Therefore, a cyclic heating-rapid cooling (CHRC) experiment is conducted on the notched semicircular bend (NSCB) granite, where LN2 and water cooling are employed. The structural damage and fracture characteristics of the CHRC treated sample are analyzed using ultrasonic detection, three-point bending test, and acoustic emission (AE) technique. A grain-based model with thermo-mechanical coupling is constructed to explore the micro-cracking mechanism of the CHRC treatment. The results indicate that the damage caused by the CHRC increases significantly in the first five cycles, especially under LN2 cooling. The decline of fracture toughness with cycles under different cooling methods. LN2 cooling can dramatically enhance the AE energy release and fracture surface roughness in the initial cycles. LN2 cooling can promote the generation of tensile cracks and intra-granular microcracks. The fracture behaviors of the CHRC treated granite are mainly determined by the stable thermal stress, significantly affected by mineral distribution. These results can provide a comprehensive understanding of cyclic LN2 fracturing compared to hydraulic fracturing, supporting theoretical guidance for geothermal extraction.