Numerical study of failure in anisotropic formations: insights from experiment and field observations on Mancos and Marcellus shales

ABSTRACT: In increasingly challenging geological environments, drilling operations have become significantly demanding. As extended reach wells are becoming more common, inclined wellbores drilled in laminated formations can pose risks and result in damage with characteristics different to the common symmetric breakouts. While the stability of these structures needs to be assessed, borehole breakouts may also provide useful information regarding the stress field in terms of direction and magnitude if the failure mode is well understood. Understanding and predicting failure patterns observed in shales for both wellbore stability analysis and stress determination requires the consideration of an appropriate material model accounting for strength anisotropy in the case of laminated formations. Common failure patterns such as strain localisation or buckling in the presence of bedding planes cannot be properly predicted using a conventional framework. 3-dimensional numerical modelling combined with advanced constitutive material models can capture material anisotropy in a continuum model using a smeared approach. In this paper, a numerical model of this kind is initially verified against experimental data on Mancos shale samples where breakout characteristics and the failure modes are reproduced. The same approach is then used to predict the location and width of breakouts formed while drilling in the Marcellus shale, in the presence of weak bedding planes. Results suggest that rupture modes captured in the model are in good agreement with experimental and field data under conditions commonly encountered while drilling in unfavourable conditions. The combination of such modelling and results assessment techniques available in Elfen wellbore, aims to enhance current wellbore stability assessments and limit the risks associated with drilling.