Effect of wellhead depressurization on the stability of submarine hydrate-bearing reservoir using THMC coupling

Natural gas hydrate (NGH) has recently garnered global attention as a promising alternative energy source. However, extraction can alter the physical and mechanical properties of submarine hydrate-bearing reservoir (HBR), leading to various risks such as wellbore instability, sand production, and casing failure. To overcome these challenges and improve efficiency, it is essential to comprehend the impact of depressurization extraction on the mechanical behavior and stability of HBR. This study proposes a fully coupled thermal-hydraulic-mechanical-chemical (THMC) model incorporating an elasto-plastic Mohr-Coulomb failure criterion. Changes in effective porosity, effective permeability, volumetric strain, and stress state within HBR during depressurization extraction are analyzed using the finite element method (FEM). The study also focuses on wellhead yielding failure and instability mechanisms, as well as assessing top deformation and reservoir stability. Results indicate that reducing wellhead pressure (WHP) intensifies fluid-solid coupling, increasing reservoir permeability and porosity near the well. Lower WHP results in volume expansion, stress relaxation, and non-uniform settlement. Furthermore, the simulation predicts that local instability near the wellhead emerges during beginning of production and gradually expands into an oval shape. The findings have certain application value in the prevention of hydrate reservoir instability and ensuring safe, efficient, and stable extraction of seabed hydrates.