Experimental Evaluation of Physical and Mechanical Properties of Geothermal Reservoir Rock after Different Cooling Treatments

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.