A new 3D stability model for the design of non-vertical wellbores

The Mohr-Coulomb failure criterion is known to be too conservative in estimating the critical mud pressure required to maintain wellbore stability, due to ignoring the strengthening effect of the intermediate principal stress. To eliminate this deficiency, we have developed a new analytical model to estimate the mud pressure required to avoid shear failure at the wall of non-vertical boreholes (i.e., the collapse pressure). This has been achieved by using a linear elastic and isotropic constitutive model for the stresses, and the Mogi-Coulomb criterion to predict failure. For comparison, the stability analysis has been also carried out using Mohr-Coulomb, the modified Lade criterion, and the Drucker-Prager criterion. We have found that implementing the Mogi-Coulomb law instead of Mohr-Coulomb does indeed lessen the conservative nature of the wellbore stability analysis. In general, the Mogi-Coulomb criterion gives an estimate of the collapse pressure that is similar to that given by the modified Lade criterion in polyaxial stress states, and similar to Mohr-Coulomb in triaxial stress states. By comparison, the Drucker-Prager criterion always underestimates the required mud weight. The developed 3D stability model has been applied in a field case study.