Effect of rock strength variation on the estimated borehole breakout using shear failure criteria

One of the primary goals of wellbore stability analysis is the estimation of the shear failure onset or borehole breakout. Estimation of borehole shear failure requires selecting an appropriate failure criterion. Numerous failure criteria have been used for rock failure analysis, but there is no common agreement of which criterion to select for wellbore stability analysis. In general, rock failure criteria mainly depend on rock mechanical properties and in situ stresses. This paper investigates the effect of rock strength variation and strength anisotropy on the estimated borehole shear failure using thirteen different criteria. A rock strength database was created from different US unconventional shale plays. Rock failure criteria were ranked based on the estimated borehole breakout for different shales. According to the results for different level of rock strength, there are some failure criteria that are highly responsive to variation in rock mechanical properties. Circumscribed Drucker-Prager, Modified Griffith, Inscribed Drucker-Prager criteria have shown great response to the change in the internal angle of friction. Murrell, Stassi D'Alia, Hoek-Brown, and Griffith are sensitive to variation of uniaxial compressive strength. Mogi-Coulomb, Modified Wiebols-Cook, Modified Lade, and Mohr-Coulomb failure criteria did not show any significant response to the variation of rock mechanical properties and strength anisotropy.