Poroelastic response of inclined wellbore geometry in anisotropic dual-medium/media

Shale gas formations are typically characterized by anisotropic and fracture properties. Existing shale wellbore stability models have only considered the anisotropy of shale rocks and have ignored the effect of fractures. However, the mechanism of wellbore instability under the combined effects of shale anisotropy and dual-media (DM) remains unknown. Therefore, based on anisotropic poroelasticity, the DM theory, and the assumption of generalized plane strain, this study established a pseudo-3D hydraulic-mechanical coupling finite element model considering the DM and anisotropy of shale in wellbores. The effects of elastic anisotropy, fracture volume fraction, fracture permeability, mass transfer coefficient, and fluid pressure on wellbore stress were analyzed parametrically. The results showed that an anisotropic single-medium predicts that the wellbore will fail when the formation is drilled, whereas an anisotropic DM predicts periodic wellbore instability. The poroelastic effect of the matrix pore pressure is much greater than that of the fracture pore pressure. The matrix elastic anisotropy parameters control the distribution of mechanical parameters throughout the DM, further affecting the poroelastic response of the entire DM. When the drilling direction was mostly parallel to the bedding plane, the anisotropy of the matrix and fracture system of the elastic parameters were larger, and the wellbore was more stable. A larger fracture permeability leads to a larger fracture volume fraction and mass transfer coefficient, the matrix pore pressure and fracture pore pressure increase, and the instability of the wellbore also increases.