Joint-production characteristics of typical marine composite reservoir with water-saturated methane hydrates and higher-pressure gas by depressurization

Abstract

Marine natural gas hydrate is water-saturated and usually accompanied with higher pressure underlying gas, and this typical composite reservoir is greatly economical for production. Yet, the experimental simulation on the typical marine composite reservoir is hard, and its joint-production studies are quite rare. In this work, the same 7.5 MPa methane hydrate reservoirs with an approximately 20% saturation underlain with four different pressures (7.1, 8.0, 8.1 and 9.0 MPa) gas were remolded. During the depressurization process from the top of hydrate layer, the water-saturated hydrate layer can allow the overpressure of underlying gas as highest as 6.0 MPa because of the presence of the hydrate seal. The early production stage is independent and lasts for several minutes until the hydrate seal is broken due to the dissociation of some hydrates, and then the massive inflow of underlying gas into hydrate layer induces the dissociation pause or even reformation of hydrates in return. After that, the production of underlying gas become synchronous and in a same depressurization process with the hydrate reservoir. As a result, the joint-production process of water-saturated methane hydrate and higher-pressure gas can be divided into independent and synchronous stages. In addition, the decisive factors of underlying gas and hydrate production processes are gas seepage and hydrate dissociation, respectively, and the production rate of underlying gas is much faster than that of hydrate reservoir. It is therefore suggested that the key of improving joint-production efficiency of the composite reservoir is to accelerate the hydrate dissociation. This work is of great significance for the guidance of spot production of marine natural gas hydrate with higher-pressure underlying gas.