中文摘要：

液氮温度极低（-195.8℃）,当与储层岩石接触时,能够改变岩石物性并对岩石结构产生损伤致裂,因此,可用于储层压裂改造。为了研究液氮压裂时低温对岩石力学性能的影响,分别对不同含水状态（干燥与饱和）的不同类型（大理岩、砂岩和花岗岩）岩石进行液氮冻结处理,并对冻结前、后岩样进行抗拉及单轴抗压强度对比测试。结果表明,经液氮冻结后,岩石的单轴抗压强度、抗拉强度和弹性模量都降低;岩石在干燥状态下,液氮冻结对大理石强度的影响大于对红砂岩的影响;岩石饱和水状态下,液氮冻结对红砂岩强度的影响大于对大理岩的影响;饱和水状态岩石经液氮冻结后,其应力-轴应变曲线在弹性变形阶段出现一个拐点;对于同种类型岩石,饱和水状态能加剧液氮冻结并对岩石损伤,岩石强度影响显著;对3种岩样微观结构进行了电镜扫描（以大理岩为例进行分析）,发现经液氮冻结后在矿物颗粒之间生成了微裂隙。研究结果可为进一步研究液氮压裂机制提供试验依据。

英文摘要：

The petrophysical properties and structures of rock materials are changed when they contact with cryogenic nitrogen that has an extremely low temperature of-195.8℃, and thus the principle can be used to improve reservoir fracturing. To study the effect of cryogenic nitrogen freezing on rock strength and mechanical parameters, different types of rock are selected, such as marble, sandstone and granite with different moisture states（dry and saturated）, to freeze them with cryogenic nitrogen, and then to determine tensile strength and uniaxial compressive strength of these samples before and after freezing. The results show that the uniaxial compressive strength, tensile strength and elastic modulus of rocks are decreased after freezing with cryogenic nitrogen. When the rock sample is dry, the influence of cryogenic nitrogen freezing on the strength of marble is greater than that of red sandstone. However, when the rock sample is saturated, the influence on red sandstone is greater than that on marble. When the saturated rock is frozen with cryogenic nitrogen, there is a break point on the elastic deformation stage of stress-strain curve. For the same type of rock, the effect of cryogenic nitrogen freezing on the damage of saturated rock is more obvious than that of dry rock. Scanning electron microscope（SEM） tests are further conducted on these three types of rock and particularly the marble samples are analyzed in detail. It is found that intergranular fractures occur after cryogenic nitrogen freezing. This study provides an experimental basis for further studying the mechanism of induced fracturing by cryogenic nitrogen.