Abstract Borehole instability problem is one of the key problems in drilling engineering, especially the shale formation with bedding planes and natural fractures, the failure of shale rock could be divided into two parts, shear failure across intact rock matrix and slippage failure in shale bedding, resulting in more serious borehole instability problem during drilling process. In this study, combining with rock strength anisotropy, a method of failure region around wellbore is developed for layered rocks on the basis of isotropic borehole stability model. The effect of shale bedding occurrence on failure region around wellbore, what was hardly conducted by former researchers, is examined systematically. The results show that, polar plot of critical lower mud weight with rock strength anisotropy considered reflects the actual situation of a shale formation; the shale bedding strength, pore pressure and intrinsic cohesion are the three most impact parameters on wellbore stability. Moreover, the maximum failure depth is no longer in the minimum principal stress, but locates in four corners at top and bottom of wellbore, which is induced by slippage failure in laminated formations; the collapse of wellbore will be aggravated due to the overlapping of failure in rock matrix and bedding. This proposed model not only can provide guiding suggestions for well constructions and cementations, but also can be used as an effective complement to provide richer ground stress inversion data.

中文翻译：

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鄂尔多斯盆地弱层理面页岩地层井筒稳定性及破坏区分析

摘要 井壁失稳问题是钻井工程中的关键问题之一，尤其是具有层理面和天然裂缝的页岩地层，页岩的破坏可分为两部分，完整岩石基质上的剪切破坏和页岩层理中的滑移破坏，导致钻井过程中井眼失稳问题更为严重。本研究结合岩石强度各向异性，在各向同性井筒稳定性模型的基础上，提出了一种层状岩石井筒周围破坏区方法。系统地研究了页岩层理的出现对井筒周围破坏区域的影响，以前的研究人员几乎没有进行过这些研究。结果表明，考虑岩石强度各向异性的临界低泥重极坐标图反映了页岩地层的实际情况；页岩层理强度、孔隙压力和内聚力是对井筒稳定性影响最大的三个参数。此外，最大破坏深度不再在最小主应力处，而是位于井筒顶部和底部的四个拐角处，这是由层状地层滑动破坏引起的；岩体和层理破坏的重叠会加剧井筒的坍塌。该模型不仅可以为建井和固井提供指导建议，还可以作为有效补充，提供更丰富的地应力反演数据。孔隙压力和内聚力是对井筒稳定性影响最大的三个参数。此外，最大破坏深度不再在最小主应力处，而是位于井筒顶部和底部的四个拐角处，这是由层状地层滑动破坏引起的；岩体和层理破坏的重叠会加剧井筒的坍塌。该模型不仅可以为建井和固井提供指导建议，还可以作为有效补充，提供更丰富的地应力反演数据。孔隙压力和内聚力是对井筒稳定性影响最大的三个参数。此外，最大破坏深度不再在最小主应力处，而是位于井筒顶部和底部的四个拐角处，这是由层状地层滑动破坏引起的；岩体和层理破坏的重叠会加剧井筒的坍塌。该模型不仅可以为建井和固井提供指导建议，还可以作为有效补充，提供更丰富的地应力反演数据。岩体和层理破坏的重叠会加剧井筒的坍塌。该模型不仅可以为建井和固井提供指导建议，还可以作为有效补充，提供更丰富的地应力反演数据。岩体和层理破坏的重叠会加剧井筒的坍塌。该模型不仅可以为建井和固井提供指导建议，还可以作为有效补充，提供更丰富的地应力反演数据。