Abstract: 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.

Key words: wellbore stability, strength criterion, Mogi-Coulomb, shale formations, collapse pressure