Analysis of subsidence patterns of the formation around the wellbore during deepwater natural gas hydrate test production conditions

Deep-water natural gas hydrates are commonly found in muddy siltstone at shallow seabed depths. The land subsidence caused by the continuous dissociation of hydrate can influence the stability of the subsea wellhead and facilities. The analysis of formation subsidence pattern around wellbore is helpful to avoid the risk of field operation. A multi-field coupled model for hydrate extraction experiments was established, considering the coupling effects of seepage, mechanical, thermal, and chemical fields based on the storage characteristics of hydrates and the properties of submarine soils. Based on the formation physical parameters and wellbore parameters of the Shenhu area in the South China Sea, a numerical simulation method was used to analyze the formation subsidence pattern around the wellbore under hydrate production test conditions, and a sensitivity analysis of subsidence factors was completed. According to the subsidence patterns obtained, combined with the field operation conditions, the hazardous loads on the conductor throughout its lifecycle during the hydrate production test process were analyzed. A design method for the minimum mud depth of the conductor under hydrate production test conditions was provided. The study of seabed subsidence patterns during natural gas hydrate production tests in deep-water sea is helpful in reducing the instability risk of subsea wellheads and ensuring operation security, which provides a certain of reference for the design of natural gas hydrate production test engineering.