Experimental Investigation on Physical and Mechanical Properties of Granite Subjected to Cyclic Heating and Liquid Nitrogen Cooling

Cyclic liquid nitrogen (LN₂) fracturing is an innovative technology for reservoir stimulation with great potential in geothermal energy exploitation. Understanding of the mechanical responses of high-temperature rock to repetitive LN₂ cooling at the laboratory scale is of great relevance to facilitate the field application of cyclic LN₂ fracturing. In this work, granite samples were subjected to 0–24 heating–cooling cycles in two manners (i.e., slow heating to 300 °C followed by air cooling or LN₂ cooling), and subsequent a series of laboratory tests with the aim of investigating the damage characteristics and mechanisms of rock samples after different heating–cooling cycles. The results concluded that increasing the number of cycles aggravates the internal structural damage of granite, which further deteriorates its physico-mechanical properties. Compared with air cooling, the cold shock effect of LN₂ is more capable of facilitating the growth of microcracks within the rock, thereby inducing a more severe initial damage to granite sample. However, the damage of granite is prominent only in the first few cycles, and basically no longer intensified after more than about 12 cycles. This is related to the fact that the increase in micro-defects provides more space for thermal deformations of minerals and reduces the level of thermal stress within the rock. The results in this study are instructive for evaluating the effectiveness of cryogenic fracturing and determining the reasonable number of LN₂ fracturing during reservoir stimulation.