The effect of weakness patches on wellbore stability in anisotropic media

Various laboratory experiments have shown that matching classical weak plane models to anisotropic rock strength test results can be challenging. Discrepancies between the applied models and laboratory observations are often explained by heterogeneities or local variations in both the intrinsic material and bedding planes, without physically addressing possible modeling flaws. Recent developments have however challenged this approach by introducing the patchy weakness concept. Here, local shear sliding is allowed to occur prior to macroscopic failure, thereby reducing strength at lower loading angles than predicted by the classical weak plane theory. In this paper the original 2D formulation is extended to describe failure under a fully 3D stress state. The updated criterion is applied to a borehole geometry, and based on laboratory tests on core plugs the full strength parameter suite of a caprock shale is derived. This allows for improved predictive modeling of wellbore stability in rock with anisotropic strength.