Experimental study on mode I fracture characteristics of heated granite subjected to water and liquid nitrogen cooling treatments

Abstract

Liquid nitrogen (LN2) cryogenic fracturing, as a kind of waterless fracturing, is being considered for its potential to enhance the permeability of hot dry rock (HDR) reservoirs. The model I fracture behavior of HDR plays a pivotal role in the HDR fracturing. Therefore, this study focuses on the fracture behavior of the heated granite subjected to water cooling (WC) and liquid nitrogen cooling (LNC) under mode I load. The micro-cracking mechanism of the high-temperature granite under the rapid cooling treatment is analyzed based on the thermo-mechanical coupling grain-based model (GBM). The results indicate that the change in the fracture toughness with the granite temperature has obvious segmental characteristics and the temperature of 200 ℃ is a critical point. Before the critical temperature, the trend of fracture toughness is indistinct with the granite temperature for both cooling methods. However, beyond the critical temperature, the fracture toughness obviously decreases with the increase in the granite temperature, and the LNC is more effective at reducing fracture toughness compared to the WC. With increasing the granite temperature, shear cracks occur earlier during the three-point bending test. A higher proportion of tensile fractures after the LNC treatment is induced by mode I load than after the WC treatment, especially at high granite temperatures. Compared to the WC, the LNC can generate a greater number of microcracks inside the granite, which further results in smaller fracture damage of the LNC-treated granite during mode I load. Ultimately, this leads to improved stimulation performance in terms of reduced fracture resistance and decreased fracture energy requirements for the high-temperature granite cooled by LN2. The results can enhance the understanding of distinctions between LN2 cryogenic fracturing and hydraulic fracturing in geothermal exploitation.