Energy Recovery from Natural Gas Hydrate and Shallow Gas Reservoirs: Exploring the Impact of Interlayer Gas Cross-Flow Behaviors

The coproduction of natural gas hydrate (NGH) and shallow gas (SG) represents a promising pathway toward the commercialization of marine hydrate resources. However, there is still a gap in exploring the impact of interlayer gas cross-flow behaviors on the energy recovery from NGH-SG reservoirs and coproduction characteristics with different reservoir properties. In order to address the gap, we conducted numerical simulations to study the production characteristics and interlayer gas cross-flow behaviors based on an NGH-SG reservoir model. The results demonstrate that reservoir properties affect both gas production and interlayer gas cross-flow. Interlayer gas cross-flow during the production process reduces production efficiency. The increase in permeability of shallow gas layer (SGL) is more favorable to improve the gas yield of coproduction compared with the increase in permeability of hydrate-bearing layer (HBL) and thickness of interlayer. Higher HBL permeability improves the gas cross-flow, while increased SGL permeability accelerates gas cross-flow during the early stages of production. Nevertheless, the increase in interlayer thickness mitigates interlayer gas cross-flow. The low-pressure zone of the HBL under the depressurization effect of the wellbore during coproduction is more extensive than that of the SGL, leading to the formation of a large interlayer pressure difference between the two layers. The pressure difference serves as a decisive factor in determining the occurrence of gas cross-flow, in addition to gas permeability, which significantly influences cross-flow velocity. To assess the efficiency of gas production and energy loss during coproduction of NGH-SG reservoirs, we have established an evaluation panel based on the gas-water ratio and yield-loss ratio. Combined energy recovery efficiency and economic efficiency, high SGL permeability is more favorable for coproduction. The findings of this study significantly enhance our understanding of interlayer gas cross-flow behaviors during the development of multilayer reservoirs and provide valuable guidance for the efficient coproduction of NGH-SG reservoirs.