Experimental study on the mechanical properties of granite after circulating liquid nitrogen subjected to real time high temperature

Liquid nitrogen (LN2) fracturing can effectively create more complex fracture networks in hot dry rock (granite) reservoirs to effectively construct artificial geothermal reservoirs. However, after the cooling effect of LN2, the granite will return to a high-temperature state. The changes in the mechanical properties of the granite under real-time high-temperature will affect the fractures permeability and wellbore stability in the reservoir. Therefore, the macro-mechanical properties and micro-mechanical properties of granite after cyclic LN2 cooling under real-time high-temperature and room-temperature were studied to analyze the effects of real-time high temperatures and cyclic LN2 cooling on the mechanical properties. The experimental results show that, compared with room temperature, granite treated with LN2 under real-time high-temperature exhibits more pronounced plastic characteristics and fails at larger strains. Under real-time high-temperature, the strength and elastic modulus of granite treated by cyclic LN2 cooling are lower than those under room temperature. Localized cracking is a common phenomenon observed in granite during its failure under real-time high temperature. Conversely, continuous cracking is relatively more common in granite when it fails at room temperature. Micro-mechanical experiments results show that the elastic modulus of minerals remains unchanged under room temperature, but it decreases significantly under real-time high-temperature, indicating that the deterioration of granite's mechanical properties after LN2 cooling under room temperature mainly due to thermal fracturing, and the deterioration of granite's mechanical properties after LN2 cooling under real-time high temperature is caused by both thermal fracturing and mineral softening. •The macro-mechanical and micro-mechanical properties of granite after circulating LN2 cooling subjected to RHT were studied.•The self-developed RHT uniaxial rock testing machine was used to study the mechanical properties.•The mechanical properties under RHT is lower than that under RT.•The thermal fracturing is the main reason for the deterioration of mechanical properties after LN2 impact under RT.•The thermal fracturing and mineral softening are the main reason for the deterioration of mechanical properties under RHT.