Fracture behavior and fracture surface roughness of high-temperature granite subjected to liquid nitrogen and water cooling

The thermal damage mechanism of water and liquid nitrogen on granite is a key scientific concern in reservoir fracturing. This study conducted fracture tests on high-temperature granite specimens subjected to water and liquid nitrogen cooling. The fracturing process was monitored and the fracture surface roughness was evaluated. The results indicated that the P-wave velocity, peak load, and fracture toughness exhibited a decreasing trend with increasing heating temperature. Specifically, at 150 °C, liquid nitrogen cooling samples exhibited higher fracture toughness than untreated ones. Acoustic emission events at peak load were more prominent in liquid nitrogen cooling samples, indicating higher brittleness and concentrated energy release. Conversely, water cooling samples displayed prolonged post-peak stages, suggesting greater ductility. The size of fracture process zones (FPZ) generally increased with temperature, with water cooling samples consistently exhibiting larger FPZ dimensions compared to liquid nitrogen cooling samples. Despite this, liquid nitrogen cooling resulted in significantly rougher fracture surfaces and numerous small cracks, while water cooling formed more extended through-going fractures. This suggests that high-temperature granite reservoirs by liquid nitrogen fracturing can enhance heat exchange areas more effectively. Additionally, different types of granite exhibit varying responses to temperature changes and cooling treatments, contingent on their unique mineral composition and structure.