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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. [ABSTRACT FROM AUTHOR]

A comprehensive understanding of the physico-mechanical behavior of rocks in hot dry rock (HDR) reservoir after different stimulation treatments is essential for the safe and effective exploitation of geothermal energy. In this study, the physico-mechanical properties of high-temperature granite (25–600 °C) subjected to slow cooling, water cooling, and liquid nitrogen (LN2) cooling were experimentally investigated, and the damage evolution and damage mechanism of the rock were discussed from the macro- and microscopic perspectives. According to the experimental results, the increase in thermal treatment temperature aggravates the deterioration of the physico-mechanical properties of granite specimens. It is found that 400 °C is the threshold temperature of the tested granite, after which the physico-mechanical properties of the rock present more prominent changes. Since LN2 can induce a more intense thermal shock within rocks, it has the most significant damage to the specimens compared with other two cooling methods, especially at a higher thermal treatment temperature. Acoustic emission (AE) monitoring can well reflect the failure process and the associated microcrack behavior of the specimens during loading. The results of thin slice analysis indicate that the generation and extension of microcracks are responsible for the macro-properties degradation of rocks. Both grain boundary and intra-grain microcracks are more common near quartz boundaries and inside quartz grains. The results in this study would shed light on performing HDR reservoir stimulations assisted with cryogenic LN2. [ABSTRACT FROM AUTHOR]