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In order to investigate the failure region around wellbore under a given mud pressure, three different strength criteria are fit to triaxial experimental data, then the strength criterion with the smallest fitting error is selected to analyse wellbore stability. Based on the investigate of rock strength criteria, a prediction model for the shear failure region around borehole is established. The influence of wellbore trajectory on the shape and position of shear failure region around wellbore, and collapse pressure variation at wellbore wall is analysed. Results indicated that, the area of shear collapse around the well predicted by Mohr-Coulomb criterion (MC) is elliptic, while the same results predicted by extended Mogi-Coulomb criterion (EMG-C) is approximately quadrilateral. The intermediate main stress decreases the collapse pressure and its anisotropy at the wellbore wall, but increases the failure region around the wellbore. The EMG-C takes into account the effect of intermediate principal stress and the nonlinear failure characteristics of rocks in high stress environment, and the prediction position of the shear collapse region around the borehole is quite different from the shape predicted by MC criterion.

Investigation of failure region around the wellbore based on the extended Mogi-Coulomb criterion for rock matrix