Evaluation of rock strength criteria for wellbore stability analysis

Polyaxial strength test data of five rocks are used to examine the Mohr-Coulomb, Drucker-Prager, modified Lade, Mogi-Coulomb and three-dimensional (3D) Hoek-Brown criteria regarding their ability, with parameters determined based on the triaxial compression test data, to represent the rock behavior under polyaxial stress states. Then the five strength criteria, with parameters determined based on the triaxial compression test data, are used to analyze wellbore stability of both vertical and inclined boreholes. The results show that the Mohr-Coulomb criterion under-predicts the polyaxial strength and estimates the highest minimum mud pressure required for wellbore stability while the Drucker-Prager criterion over-predicts the polyaxial strength and estimates the lowest minimum mud pressure. The modified Lade, Mogi-Coulomb and 3D Hoek-Brown criteria, with parameters based on triaxial test data, can either over-predict or under-predict the polyaxial strength. The over-prediction of the modified Lade criterion can be very large, and thus it may be unsafe to use it to estimate the minimum mud pressure. Both the over-prediction and under-prediction are relatively small for the 3D Hoek-Brown and Mogi-Coulomb criteria. Therefore, the 3D Hoek-Brown and Mogi-Coulomb criteria are recommended for wellbore stability analysis.