Expansion of Horizontal Wellbore Stability Model for Elastically Anisotropic Shale Formations With Anisotropic Failure Criteria: Permian Basin Case Study Available to Purchase

The analytical wellbore stability model proposed in this study allows for a more accurate calculation of the safe drilling window for horizontal wells drilled in anisotropic shale formations. The stress distributions around the horizontal wellbore, based on Green and Taylor equations for stress concentrations around holes in elastically orthotropic plates, are compared with the result from the traditional Kirsch equations. The comparison reveals that the elastic anisotropy of shales induces larger magnitudes of tangential stress concentrations at the potential shear and tensile failure locations, i.e. p/2 or 0 from the maximum in-situ stress direction. The anisotropic wellbore stability (WBS) model is expanded with a failure criterion that accounts for the anisotropy of shale. The critical stresses for the safe drilling window depend on (1) the anisotropy of the in-situ stress conditions, (2) the elastic anisotropy of the local formation, and (3) local strength anisotropy, due to anisotropy of the shale fabric. The model can be practically applied in real-time wellbore stability analyses, to determine the stress concentrations and trajectories near the wellbore, as well as for calculation of the safe drilling window. Two case study wells for the Permian Basin (West Texas and New Mexico) are included in our study.