The Role of Layer Heterogeneity on Productivity, Geomechanical Responses, and Wellbore Stability of a Marine Gas Hydrate Deposit in South Korea

We numerically study the role of heterogeneity in hydrate saturation for a gas hydrate site located in the Ulleung Basin, investigating geomechanical and flow responses such as wellbore slip, production, and stress changes during depressurization. We take a newly updated geological model of the Ulleung Basin and consider more detailed heterogeneous distributions of hydrate saturation and porosity. We perform two-way coupled flow and geomechanics simulations based on the fixed-stress sequential method, also employing interface elements between the well casing and reservoir formation for the slip modeling. From the numerical results, the thin layers that contain low hydrate saturation yield high productivity due to high effective permeability. Simultaneously, a large amount of production of both gas and water induces fast reservoir depressurization and stress changes, followed by substantial reservoir deformation and subsidence. As severe compaction and subsidence are concentrated in the vicinity of the wellbore, we further investigate possible failure and well instability caused by unbearable axial stress. From the parametric study, the wellbore failure such as slip or detachment can occur depending on bonding strength at the interface, although the axial stress distribution and the total slip along the well casing do not seem severe. This study can contribute to practical design of well completion to ensure wellbore stability for the gas hydrate development projects from drilling to production.