Wellbore stability analysis based on a new strength criterion

Wellbore stability is critical to the overall safety of drilling. The adoption of a better strength criterion greatly improves the accuracy of wellbore stability prediction. Although the Mohr Coulomb strength criterion is widely used for wellbore stability analysis, it is difficult to use its linear form to accurately describe the nonlinear characteristics of the rock strength under different confining pressures. In the present study, we developed a new criterion, which we refer to as the single-parameter parabolic criterion, for wellbore stability analysis based on the Mohr strength criterion with a parabolic failure envelope. The fitting precisions of the Mohr-Coulomb criterion, the Hoek-Brown criterion, and the developed single-parameter parabolic criterion were statistically analyzed. It was found that the single-parameter parabolic criterion accurately describes the relation between the rock strength and confining pressure. Based on the single-parameter parabolic strength criterion, a collapse pressure calculation model for vertical and inclined wells was developed and applied to a field case, and the new criterion was further demonstrated to be superior to the Mohr Coulomb criterion. (C) 2015 Elsevier B.V. All rights reserved.