Short- and Long-term Wellbore Stability Analysis for a Horizontal Well in a Transversely Isotropic Poroelastic Formation

Sedimental oil/gas reservoirs often exhibit transversely isotropic poroelastic characteristics. Neglecting this anisotropic nature may lead to suboptimal predictions of wellbore stability. The time-dependent wellbore failure phenomenon is closely associated with pore pressure variation. Therefore, two extreme cases were considered to constrain the time-dependent safe mud window: (1) undrained Short-term immediately after drilling, and (2) drained Long-term (assuming steady-state pore pressure field). In this study, analytical Short- and Long-term stress solutions for a horizontal well drilled along the minimum horizontal principal stress direction in a transversely isotropic formation are derived. Then combined with the 3D shear and tensile failure criteria, the Short- and Long-term safe mud windows are obtained and compared with the common impermeable model neglecting pore pressure variation. Model results indicated that the impermeable model incorrectly estimates the Short- and Long-term safe mud windows, highlighting the importance of considering anisotropic pore pressure variation and poroelastic effects. In the Short-term case, stress redistribution induces abnormal undrained pore pressure and then wellbore pressure influences the anisotropic undrained response, differing from that in isotropic formations. In the Long-term case, anisotropic permeability significantly aggravates wellbore shear and tensile failure potentials. Ultimately, the Long-term case gets the narrowest allowable wellbore pressure bounds compared to the impermeable and Short-term cases. The analytical method can serve as a reference for time-dependent wellbore stability in transversely isotropic poroelastic formations.