Study on the Settlement Characteristics of Hydrate Bearing Sediment Caused by Gas Production with the Depressurization Method

The settlement of the reservoir caused by gas production from the hydrate reservoir is closely related to the safety of hydrate production. However, the settlement characteristics of a hydrate reservoir under a triaxial stress state are still unclear. In this study, the settlement characteristics of hydrate-bearing sediment (HBS) during the gas production process were experimentally investigated under a triaxial stress state. Additionally, numerical simulations were performed using the thermo-hydro-chemomechanical (THMC) fully coupled model for hydrate production implemented in COMSOL Multiphysics. Results show that the failure strength of the hydrate-bearing specimen increases significantly when hydrate saturation (Sh) is raised from 0 to 0.19 and 0.37 to 0.57, and only a slight change of failure strength is observed when Sh is raised from 0.19 to 0.37. The secant modulus and cohesion are positively correlated with hydrate saturation, and the internal friction angle is negatively correlated with hydrate saturation. In the gas production process, the settlement of the specimens can be divided into three stages. The first stage is characterized by rapid settling caused by free gas discharge, which has the largest settlement amount. Then, a relatively gentle settling stage is caused by hydrate decomposition. In the final stage, with continuous gas production, the settlement of specimens remains constant. A specimen with lower Sh or production pressure results in a greater settlement at the first stage. A larger settlement occurs at the second stage for a high Sh specimen. The confining pressure has little effect on the HBS.