A wellbore stability model for formations with anisotropic rock strengths

Shale formations, due to the presence of laminations and weak planes, exhibit directional strength characteristics. In most conventional wellbore stability analyses, rock formations are typically assumed to have isotropic strength. This may cause erroneous results in anisotropic formations such as shales which show strength variations with changing loading directions with respect to the plane of weakness. Therefore a more complex wellbore stability model is required. We have developed such a model in which the anisotropic rock strength characteristic is incorporated. Applying this model to two case studies shows that shear failures occur either along or across the bedding planes depending on the relative orientation between the wellbore trajectories and the bedding planes. Additionally, the extent of failure region around the wellbore and the safe mud weights are significantly affected by the wellbore orientation with respect to the directions of bedding plane and in-situ stress field.