Liquid nitrogen has been proposed to promote fracture networks generated by hydraulic fracturing in geothermal reservoirs. To compare the deterioration in mechanical properties due to water and liquid nitrogen cooling under conditions of high temperatures and confining pressures, three stress thresholds (crack initiation stress, crack damage stress, and peak stress) of granite were studied by performing triaxial compression tests (under 0, 20, 40, 50, and 60 MPa) after heating and rapid cooling. The relationships between the three stress thresholds and the confining pressures were analyzed. The effects of different cooling treatments on the stress thresholds were analyzed under different confining pressures. The thermal damage mechanism and confining pressure effect were revealed. For both rapid cooling treatments, the three stress thresholds showed convex nonlinear relationships with the confining pressure. The temperature sensitivity on the stress thresholds can be weakened by confining pressure. Under low confining pressures (< 40 MPa), the three stress thresholds of samples cooled by liquid nitrogen were higher than those by water, and the differences were suppressed at higher confining pressures. The higher mechanical properties observed under liquid nitrogen were due to the heat transfer restriction by the Leidenfrost effect and the crack closure induced by volumetric contraction. A higher thermal expansion heterogeneity contributed to the complexity of cracks induced by liquid nitrogen. Under high confining pressures (> 40 MPa), there was a trend that the stress thresholds of samples cooled by liquid nitrogen were lower than those by water, which was related to cohesion.

已经提出液氮促进地热储层中水力压裂产生的裂缝网络。为比较高温高围压条件下水冷和液氮冷却对力学性能的影响，通过三轴压缩试验研究了花岗岩的三个应力阈值（裂纹萌生应力、裂纹破坏应力和峰值应力）。 0、20、40、50 和 60 MPa) 加热和快速冷却后。分析了三个应力阈值与围压的关系。分析了不同围压下不同冷却处理对应力阈值的影响。揭示了热损伤机理和围压效应。对于两种快速冷却处理，三个应力阈值与围压呈凸非线性关系。通过围压可以削弱应力阈值的温度敏感性。在低围压（< 40 MPa）下，液氮冷却样品的三个应力阈值均高于水冷却样品的三个应力阈值，并且在较高围压下差异得到抑制。在液氮下观察到的较高机械性能是由于莱顿弗罗斯特效应的传热限制和体积收缩引起的裂纹闭合。较高的热膨胀异质性导致液氮引起的裂缝的复杂性。在高围压（> 40 MPa）下，液氮冷却样品的应力阈值有低于水冷却样品的趋势，