目前研究井壁失稳问题时,考虑的因素为井周应力状态与强度的相对关系.当最大主应力与最小主应力之差差值跃过莫尔圆包络线,即认为井壁发生剪切破坏,产生平行于井眼轴线的纵向裂缝,一般取切向应力与径向应力分别为最大最小主应力.但是在某些特殊构造区域,井壁轴向应力成为最大主应力,如果仍按照传统的切向应力与径向应力进行计算,得出的坍塌压力与实际相比偏小,会低估井壁失稳的风险.研究了计算坍塌压力时考虑轴向应力的判断条件,给出了解析判别式,提高了坍塌压力计算精度.当地层强度低于该临界值时,应选取轴向应力为最大主应力；可以为直井钻井设计提供参考依据.%The research of borehole instability problem focus on the relative relationship between stress state and strength. Once the difference between tangential and radial stresses leapfrog Mohr Circle envelope, then the borehole will breakout. However, in some special structure area, the axial stress becomes the maximum stress. The calculated collapse pressure can be underestimated, if the tangential stress is deemed as maximum stress. The criterion condition is determined, and the analytical expression is obtained. If the formation strength is less than the critical strength UCS0, the maximum stress should be axial stress. It provides a reference for drilling design of vertical well.
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