Morphological Evolution of a Plastic Zone Surrounding a Circular Wellbore in Natural Gas Hydrate-Bearing Sediments

Wellbore instability poses a significant challenge to the safe and long-term production of natural gas hydrates (NGHs). Characterizing the geometry of the wellbore-adjacent plastic zone is critical for evaluating geomechanical risks during hydrate exploitation. In this paper, an elastic–plastic analytical model incorporating the Mohr–Coulomb failure criterion is developed to describe the stress distribution around the borehole under non-uniform in situ stress conditions. Particular attention is paid to the role of hydrate saturation, which is integrated into the constitutive framework to reflect the cementation effect of NGH-bearing sediments (GHBS). Analytical solutions for the stress fields in both the elastic and plastic regions are derived, which are accompanied by a computational scheme for determining the plastic zone radius. Using site-specific mechanical parameters from the Shenhu area in the South China Sea, a parametric analysis is conducted to quantify the influences of hydrate saturation, reservoir depressurization, and stress anisotropy on the evolution of the plastic zone shape. The results indicate that elevated hydrate saturation enhances the load-bearing capacity of the formation, whereas depressurization significantly expands the plastic region, leading to severe wellbore instability. These findings provide theoretical insights for optimizing drilling strategies in deep-water hydrate reservoirs.