层理地层基质与弱面坍塌失稳规律分析

渤海深部地层中存在大量的硬脆性泥页岩，易发生井壁失稳问题。将硬脆性泥页岩当作一种横观各向同性介质，建立了横观各向同性地层井周基质及层理弱面应力计算模型，在此基础上，获取了弹性参数各向异性、井眼轨迹及层理产状对井壁坍塌破坏的影响规律。研究表明，各向异性对层理弱面应力分布影响显著，与各向同性模型相比，误差最大约为31%；随弹性模量各向异性程度增大，井壁坍塌风险增高；直井钻进时，井壁坍塌破坏方位不仅受水平地应力方位影响，还与层理弱面的倾斜角和倾向角、井眼轨迹等参数相关。随层理面倾斜角和倾向角增大，井壁坍塌形状依次为岩石只发生沿水平最小地应力方位的基质坍塌、层理面单对角坍塌及层理面双对角坍塌，基质坍塌方位对应最小地应力方位，层理面坍塌方位与弱面的倾向方位保持一致，最优钻井方位应结合层理弱面的倾向；当定向井钻进时，井眼沿井斜30°~75°，最大地应力方位附近，坍塌最为严重。对渤中一口深井进行井壁失稳实例分析，分析结果与实际情况吻合良好，研究成果对层理地层钻井作业有一定的指导意义。

Analysis of Collapse Matrix and Weak Plane of Bedding Strata

There are a lot of hard brittle shale in deep strata of Bohai Bay,which is prone to cause wellbore instability.Taking hard and brittle shale as a kind of transversely isotropic medium,the stress calculation model of well circumference matrix and bedding plane is established.On the basis,influence of elastic anisotropy parameter,wellbore anisotropy and bedding formation on wellbore collapse is obtained.The results show that anisotropy has a significant effect on the stress distribution of the weak surface.The maximum error is about 31%compared with the isotropic model.With the increase of elastic modulus anisotropy,the risk of wellbore collapse increases.The direction of wellbore collapse is not only affected by the horizontal stress orientation,but also by the inclination and squint angle of weak bedding surface and well trajectory when drilling in a vertical well.With the increase of inclination and squint angle of weak bedding plane,the collapse shapes of the wellbore are as follows:matrix collapse only along the minimum horizontal stress direction,single diagonal collapse of bedding plane and double diagonal collapse of bedding plane.The collapse orientation of the matrix corresponds to the minimum in-situ orientation,and the collapse orientation of the bedding plane is consistent with the dip direction azimuth.The optimum drilling azimuth should be combined with the squint angle of bedding plane.When matrix failure and weak surface failure occur simultaneously,the collapse risk increases with the increase of well inclination.The collapse is the most serious along well deviation 30°~75°,near the maximum in-situ stress azimuth when drilling in a directional well.An example of a deep well in Bozhong structure is analyzed.The analysis results of wall collapse pressure are in good agreement with the actual situation.The research results have certain guiding significance for drilling operation in bedding strata.