液氮循环冷冻对岩石力学性能强度影响试验研究

Experimental Study on the Effect of Liquid Nitrogen Cycling Freezing on the Mechanical Properties of Rock Mass

液态氮气温度非常低(-195.8℃),在与地层岩石接触时,低温冷冻效应导致岩石内部结构产生损伤,物理力学指标降低,可作为压裂液对储层进行压裂改造。为研究液氮低温对岩石力学性能的影响,选择自然干燥样品、全饱和水样品(新疆灰岩、四川地区灰岩、四川灰色砂岩),在液氮浸泡前后及不同循环浸泡后对岩样进行抗拉及单轴抗压强度对比测试。结果表明,1)经液氮冻结后,岩石的单轴抗压强度、抗拉强度都降低;2)岩石在干燥状态下,液氮冻结对新疆地区灰岩强度的影响大于其它地区岩样的影响;3)在岩石水饱和状态下对于同种类型岩石而言,水饱和状态下能够加剧液氮冷冻效果,并对岩石产生损伤,随着循环次数增加岩石强度影响显著。研究结果可为进一步研究液氮压裂机制提供试验依据。

The temperature of liquid nitrogen state is very low(-195.8℃).When it contacts with the rock mass,the cryogenic effect will cause damage to the structure of the rock mass and reduce its physical and mechanical strength parameters.Therefore,the liquid nitrogen can be used as a fracturing fluid for the fracturing of the storage reservoir.In order to study the effect of the low temperature of the liquid nitrogen on the mechanical properties of rock mass,the naturally dried rock samples and fully saturated water samples(Xinjiang limestone,Sichuan limestone and Sichuan grey sandstone)were selected and compared with the tensile and uniaxial compressive strength of rock samples before and after soaking into the liquid nitrogen with different soaking cycles.The results show that,1)The uniaxial compressive strength and the tensile strength of the rock samples are reduced after being frozen by the liquid nitrogen.2)Under the natural dry state,the effect of the liquid nitrogen freezing on the strength of the Xinjiang limestone is greater than that of rock samples in other regions.3)For the saturated same type of rock samples,the effect of the liquid nitrogen freezing can be intensified under the state of water saturation and the rock damage can be induced.The impact on rock strength is significant with the increase of soaking cyclic times.The results can provide the experimental basis for further research on the liquid nitrogen fracturing mechanism.