Mechanisms and numerical simulation of mudstone wellbore instability considering hydro-mechanical coupling and weak bedding planes

Wellbore instability in deep hard–brittle mudstones poses a severe challenge due to the complex interplay of hydration damage and weak bedding planes. Previous models often oversimplify fluid diffusion and neglect dynamic rock property evolution. This study proposes a novel fully coupled 2D hydro-mechanical (HM) model incorporating dynamic porosity-permeability evolution, strength degradation, and a composite matrix-weak‑plane failure criterion. Based on comprehensive experiments (XRD, environmental scanning electron microscope, mercury intrusion porosimetry, and soaking tests) on Dong 3 Member hard–brittle mudstone, we identified a critical 4-day hydration window causing a 37.66% drop in compressive strength. Numerical results reveal that fluid invasion is highly anisotropic, forming elliptical pore‑pressure fronts parallel to bedding. The wellbore failure mode bifurcates with bedding dip: narrow slip-shear bands dominate at 0°~45°, whereas a massive butterfly-shaped composite failure emerges at 60°. Model predictions were validated against field data from Well PG-2, demonstrating high accuracy in determining the safe mud weight window. This integrated approach provides a rigorous theoretical framework for optimizing drilling fluid density and soaking time in highly anisotropic formations.