极地冻土低温力学特性及井眼坍塌风险分析*

为解决极地冻土区油气资源储量极大但其力学特性和钻井井壁失稳机理尚不清晰的问题，对冻土岩样开展低温三轴力学特性实验，并基于此开展钻井井壁坍塌失稳相关数值模拟分析。结果表明:冻土的极限强度会随温度的降低和围压的增大而逐渐提高；冻土弹性模量尽管会随温度的降低而增加，但受围压影响较小，而泊松比基本不受温度及围压的影响；冻土内聚力会随实验温度的降低得到较大幅度提升，但不同实验温度范围内温度对冻土内聚力的影响程度存在差异，实验温度高于-12.5 ℃时温度对冻土内聚力的影响较强；极地冻土储层钻井过程近井地带冰的分解会随钻井作业的进行而逐渐变慢，冰的最终分解前缘位置为0.079 6 m，井眼扩大率达到63.51%。研究结果可为极地冻土层油气资源钻井作业设计提供参考与指导。

Analysis on low-temperature mechanical properties of polar frozen soil and borehole collapse risk

In order to solve the problems of extremely large reserves of oil and gas resources in the polar frozen soil area but its mechanical properties and the instability mechanism of drilling borehole wall are not clear,the low-temperature triaxial mechanical properties experiments were carried out on the frozen soil rock samples,and on this basis,the relevant numerical simulation analysis on the collapse and instability of drilling borehole wall was carried out.The results showed that the ultimate strength of frozen soil would gradually increase with the decreasing temperature and the increasing confining pressure.Although the elastic modulus of frozen soil would increase with the decrease of temperature,it was less affected by the confining pressure.However,the Poisson’s ratio was basically unaffected by the temperature and confining pressure.Moreover,the cohesion of frozen soil would greatly increase with the decrease of temperature,but the influence degree of temperature on the cohesion of frozen soil was different in different experimental temperature ranges,and the influence of temperature on the cohesion of frozen soil was stronger in the range of experimental temperature higher than -12.5 ℃.The ice decomposition in the near wellbore zone would gradually slow down during the drilling process of polar frozen soil reservoir,the final position of ice decomposition front was 0.0796 m,and the borehole expansion rate reached 63.51%.The results can provide reference and guidance for the design of drilling operation for the oil and gas resources in polar frozen soil layer.