Experimental study of behavior of hydrate-bearing sediments during servo depressurization

Natural gas hydrate in deep sea exists in a certain temperature and pressure condition. The depressurization rate during hydrate dissociation by depressurization has a great impact on the gas production rate and hydrate-bearing sediment deformation characteristics. In order to investigate the influence of depressurization rate on temperature field, pore pressure field, deformation characteristics, and gas production rate of hydrate-bearing sediment, a group of depressurization tests with different depressurization rates was carried out on the apparatus independently developed by Zhejiang University that can perform linear gradient servo depressurization for simulating the hydrate decomposition process. The results show that the temperature decreases first from the perimeter of the shaft where the decomposition region starts, and then gradually spreads to the surrounding sediment at the initial stage of depressurization. Increasing the depressurization rate appropriately can improve the production efficiency of the reservoir, but the higher depressurization rate may cause the hydrate regeneration, which is not conducive to gas production. Optimal gas production efficiency can be obtained by selecting a specific depressurization rate. In the process of hydrate exploitation, the pore shape of the hydrate-bearing sediment can be divided into three types, according to the connection degree between pores and the surrounding area: completely sealed, partially sealed, and open. After hydrate exploitation, the shallow surface soil of reservoir can be divided into three areas based on the deformation characteristics: Zone I is the soil layer around the shaft, showing a funnel-shaped subsidence structure; the soil layer in Zone II is flat with no obvious disturbance; Zone III is the boundary soil layer, where the upward migration of water and gas production is blocked, leading to a mound like uplift zone. These deformation characteristics are related to the migration paths and modes of gas production in hydrate-bearing sediment. Through similarity analysis, the corresponding relationships between the decomposition time and gas production of the model and prototype are given.