Numerical investigation of production characteristics and interlayer interference during co-production of natural gas hydrate and shallow gas reservoir

The coexistent system of natural gas hydrate and shallow gas (NGH-SG) enhances stability compared to single resource reservoirs, rendering it a promising target for co-production and commercial exploitation. In this study, we established a reservoir model of the NGH-SG system based on actual geological conditions. Subsequently, a long-term production simulation was performed to contrast the production behaviors of three exploitation methods: co-production of NGH-SG (CP), single production of hydrate-bearing layer (SP-HBL), and single production of shallow gas layer (SP-SGL). The study revealed that the presence of free gas in SGL can significantly increase the overall gas production capacity, with SGL contributing over 85% of the total gas production during the entire co-production period. On the 5000th day, the cumulative gas production of CP exceeded that of SP-SGL by 1.07 times and surpassed SP-HBL by 9.60 times. Furthermore, the study extensively investigated the impact of interlayer permeability on production behaviors. Lower interlayer permeability was found to be more conducive to releasing the natural gas production capacity from the reservoir. Notably, the phenomenon of interlayer interference should be carefully considered during CP. The influence degree of interlayer interference during CP becomes increasingly pronounced with increasing interlayer permeability. Fundamental contributors to interlayer interference encompass heat transfer, pressure diffusion, and interlayer cross-flow. As interlayer permeability rises, mass transfer between the layers intensifies, thus promoting pressure diffusion. Interlayer pressure difference and permeability are critical factors that affect interlayer cross-flow. For NGH-SG reservoirs with higher interlayer permeability, the advantages of CP are more obvious during the early production period. The implementation of the findings of this study may improve production efficiency and contribute to the development of effective management strategies for offshore NGH-SG reservoirs.