Effect of material anisotropy on time-dependent wellbore stability

• Development of a numerical solution using the FEM for anisotropic poroelasticity.
• The generalized plane strain assumption allows for fast and efficient analysis.
• Model incorporates a 3D stress tensor and a complete elastic anisotropy.
• The pseudo-3D method enables the model to analyze inclined boreholes.
• Time-dependent wellbore stability analysis is conducted.
• Stability analysis includes matrix and slip failures along bedding planes.
• Study includes estimation of borehole collapse and fracture initiation pressures.

The paper presents a numerical wellbore stability model using the finite element method (FEM). The core driver of the model uses Biot's theory of poroelasticity. The model uses a time-dependent analysis that accounts for the poroelastic effects of pore pressure and deviatoric stress, which are not accounted for in the common static linear elasticity analyses. The use of pseudo-3D FEM based on the theory of generalized plane strain, allows the freedom to analyze complex geometries such as inclined boreholes in anisotropic formation under three dimensional stress field. Shear failure prediction is demonstrated using isotropic and anisotropic failure criteria. Available analytical solutions in literature for anisotropic poroelasticity assume transversely isotropic material with the plane of isotropy always perpendicular to the borehole axis. The presented numerical model removes the above assumption and accounts for the mechanical anisotropy and formation dipping effects that are difficult to deal with using an analytical-based analysis. Finally, juxtaposition between isotropic and anisotropic analyses shows significantly dissimilar failure prediction and mud weight recommendations.