Large-Scale Experimental Investigation on the Production Characteristics of Marine Hydrate-Bearing Sediments

A self-developed large-scale 3D platform with a maximal volume of 1695 L and pressure of 30 MPa was employed to investigate the production behavior of hydrate-bearing sediment using depressurization and thermal stimulation technology. Moreover, a novel method involving the joint development of hydrate-bearing sediments and shallow gas was first carried out based on the established apparatus. Experimental results show that the temperature in the reactor exhibits a decreasing trend as a whole with the continuous heat adsorption of hydrate dissociation due to slow heat transfer in the large-scale device. Heat transfer is the main factor controlling the gas production rate in the whole process of depressurization. The temperature of the reactor almost approaches the equilibrium point corresponding to the internal pressure, and the hydrate keeps dissociating near the phase equilibrium curve. Meanwhile, local overpressure is first observed during depressurization, indicating that dissociated gas from hydrate cannot be fully released while some are trapped in the deposits. In addition, thermal stimulation may not be applicable for field-scale marine natural gas hydrate development owing to the large heat loss in the pipelines and the low heat transfer in the sediments. Investigation of the joint development of hydrate-bearing deposits and shallow gas indicates that hydrate hardly dissociates initially at the large production rate of shallow gas and starts to decompose gradually with the decreased value, showing strong interlayer interaction between hydrate layers and shallow gas. Therefore, it is necessary to reasonably allocate the production rate of different reservoirs at different depths to achieve the most economical development during the coproduction of multigas. Compared with that in small-scale experiments, the production behavior in the large-scale experimental apparatus is closer to field-scale development. Moreover, the first large-scale experimental investigation on the joint development of hydrate-bearing sediments and shallow gas provides novel insight for efficiently developing natural gas hydrate reservoirs.