Comparison of rock failure criteria in predicting borehole shear failure

Selection of the appropriate rock failure criteria is one of the key steps in determining minimum required mud weight in wellbore stability analysis. Numerous failure criteria have been used for rock failure analysis, but there is no common agreement of which failure criterion to select. In this paper, thirteen failure criteria used in predicting borehole shear failure were evaluated for four field cases. In a comparison of the results with actual field failure cases, Tresca, Von Mises, and Inscribed Drucker-Prager overestimated the rock breakout and predicted the highest required minimum required mud weight for all cases. Also the results of these criteria are significantly higher than the actual borehole shear failure. Circumscribed Drucker-Prager underestimated the rock breakout and predicted the lowest bound of the minimum required mud weight in most cases which is mainly less than actual onset of borehole breakout. The minimum required mud weights determined by Modified Lade, Modified Wiebols-Cook and Mogi-Coulomb is above, but close to, the onset of breakout based on the field reported failure cases. This means that using of any of these three criteria in wellbore stability analysis could be a safe approach. Furthermore, Modified Lade, Modified Wiebols-Cook and Mogi-Coulomb provided similar results for all studied cases, so these failure criteria may be used interchangeably.