Mechanical behavior and fracture mechanism of high-temperature granite cooled with liquid nitrogen for geothermal reservoir applications

Liquid nitrogen (LN2) fracturing technology is an innovative method for enhancing geothermal energy extraction from hot dry rock reservoirs. This study investigates the impact of LN2 cooling on the mechanical properties of granite subjected to varying pre-heating temperatures. Experiments including ultrasonic testing, Brazilian splitting tests, and 3D contour scanning were conducted to examine the effects of LN2-induced low-temperature fracturing. Results show that LN2 cooling significantly reduces the physical and mechanical properties of high-temperature granite, including wave velocity, tensile strength, and elastic modulus. Notably, the P-wave velocity decreases, with a reduction in the peak amplitude from 102.42 dB to 20.98 dB. The damage effect of LN2 is minimal at room temperature (25 °C) but becomes pronounced when the granite is heated above 200 °C. Additionally, an increase in initial temperature leads to a greater increase in fracture surface roughness, scanning height, and box dimension. At 400 °C, the average scanning height increases to 2.82 mm, reflecting a 125.63% increase, which highlights the enhanced fracture behavior induced by LN2 cooling. These findings demonstrate the potential of LN2 cooling to improve fracture formation and could have important implications for geothermal resource extraction.