The evolution of static liquefaction in hydrate-bearing clayey-silty sediments in the South China Sea by depressurization with a horizontal well

Marine gas hydrate extraction by horizontal well depressurization increases the risk of static liquefaction in sediments near the wellbore. However, few constitutive models are capable of describing the static liquefaction of hydrate-bearing sediments (HBS). Herein, we developed an elastoplastic constitutive model for static liquefaction of HBS by introducing a compression hardening parameter, cementation strength parameter, and dilatancy parameter. Subsequently, the model was implemented in ABAQUS through the user material (UMAT) and user-defined field (USDFLD) subroutines. A series of simulations were conducted to investigate the effects of depressurization rate on the mechanical response and static liquefaction of hydrate-bearing clayey-silty sediments (HBCSS) in the South China Sea (SCS). The transition from stable deformation to static liquefaction-induced instability was identified using the second-order work criterion. The results show that the depressurization produces a pressure-drop funnel, a spindle-shaped shear stress concentration zone, and an X-shaped cementation degradation zone around the wellbore. Under the simulated geological and production conditions, the critical depressurization rate for triggering static liquefaction is 0.64 MPa/d, which provides a quantitative reference for controlling mid- and late-stage pressure drawdown. The liquefied zone is approximately elliptical, extending 4.82 m horizontally and 4.30 m vertically. Moreover, the liquefaction boundary exhibits energy-buffering characteristics. These findings provide a theoretical and numerical reference for evaluating wellbore stability and optimizing depressurization strategies during gas hydrate production.