基于真三轴强度准则分析页岩斜井坍塌风险

井壁稳定性分析常采用Mohr-Coulomb（M-C）准则，由于其忽略了中间主应力（σ₂）的影响，使得结果偏于保守。为此，给出一种考虑了σ₂影响的真三轴Mogi-Coulomb（MG-C）准则，通过拟合Yuubari页岩真三轴实验数据，评价了M-C、Drucker-Prager（D-P）和MG-C准则的适用性；基于孔隙弹性斜井井壁应力分布模型，采用3种强度准则进行了斜井相对坍塌风险的分析。研究表明，MG-C准则拟合结果最好，能够准确反映σ₂对强度的影响，岩石强度随σ₂增加，先增加后下降；采用3种强度准则计算的斜井相对坍塌风险分布规律基本一致，M-C准则计算结果偏高，D-P准则计算结果偏低，而MG-C准则比较准确地考虑了σ₂的影响，其计算结果比较适中，比较适合用于井壁坍塌压力分析。MG-C准则的材料常数可通过常规三轴岩石力学参数（内聚力和内摩擦角）计算得到，使得MG-C准则使用方便，采用测井资料分析井壁稳定可得到连续坍塌压力剖面。

Collapse Risk for Deviated Borehole in Shale Formations Based on True-triaxial Strength Criterion

The Mohr-Coulomb(M-C)criterion is commonly used in the analysis of wellbore stability. Consequently, the result of wellbore stability analysis is conservative because the influence of intermediate principal stress(σ₂) is ignored. Therefore, in this study, the influence of σ₂ is considered to derive a true-triaxial Mogi-Coulomb(MG-C) criterion, and the applicability of the M-C, Drucker-Prager(D-P), and MG-C criteria were evaluated by fitting the true-triaxial data of the Yuubari shale. The relative collapse risk was analyzed for these three criteria based on the poro-elastic stress distribution model of the deviated well. The results show that the fitting result of MG-C is the best as it can accurately predict the influence of σ₂, and the rock strength increases at first and then decreases with increasing σ₂. The distribution rule of the relative collapse risk for deviated wellbore is basically the same for the above three criteria. The result of M-C is slightly higher, while that of D-P is slightly lower, and that of MG-C is optimal, which can accurately predict the impact of σ₂. Thus, MG-C criterion is applicable to the analysis of collapse pressure. Moreover, the material constants of the MG-C criterion can be determined by conventional triaxial rock mechanics parameters(cohesion and friction angle), which led to MG-C convenience to apply. The wellbore stability can get a continuous collapse pressure profile with logging data analysis.