考虑三向应力作用的渤南泥页岩地层坍塌压力

渤南区块罗家地区沙三储层以泥页岩为主，泥页岩厚度大、分布广，有机碳含量高，富含油气。但是该组地层微裂缝发育，岩石强度各向异性明显，加上水化作用影响，水平井钻井过程中井壁稳定问题突出，使用传统坍塌压力预测模型不能有效计算泥页岩储层的坍塌压力，普通考虑弱面的分析模型通常忽略了中间主应力对岩石强度的影响，预测结果也不如意。在对罗家地区泥页岩矿物成分和微裂缝分布情况分析的基础上，基于弱面强度理论，综合考虑三向应力的影响，建立了更精确的坍塌压力预测模型。实例分析表明，对于该区域的泥页岩钻井，一味地增大钻井液密度并不能保证井壁稳定，该区块岩石碳酸盐矿物含量较多，原始强度较高，且地层孔隙度低，如果使用控压钻井，使井底压力略低于地层压力，可增加岩石强度，增大井壁稳定性，并降低钻井成本，增加钻速。 

Determination of Formation Collapse Pressure of Bonan Shale Taking into Account Tri-axial Stresses

The lithology of the Shasan reservoir formations in Luojia, Block Bonan is mainly shales that are thick and widely spread. The shales are rich in organic carbon and oil and gas. Borehole instability has been encountered during horizontal drilling because of the developed fractures in those formations, the anisotropy of the formations, and the hydration of the formation rocks. Conventional models prognosing the collapse pressure of the formations cannot be used to calculate the collapse pressure. Those models that only consider the weak planes have no thought for the effect of intermediate principal stress on the strengths of rocks, and the prognoses are generally not done satisfactorily. Mineral composition and fracture distribution have been examined about the shale samples taken from Luojia, and based on the weak plane strength theory, a more accurate prognosis model, which takes into account the effects of triaxial stress, has been established for the prediction of collapse pressure. The application of the model shows that for wells penetrated the shale formations, borehole stability cannot be realized through only higher mud weight. Carbonate formations with higher original strengths and low porosity are not uncommon in this area. If managed pressure drilling (MPD) is used with bottom hole pressure slightly lower than formation pressure, the strength of the rocks can be increased, and hence the stability of the borehole wall. This, in turn, will help reduce drilling cost and increase ROP.