Study on the co-production strategies of natural gas hydrates and conventional natural gas multiple gas reservoirs by depressurization

The commercial-scale development of natural gas hydrates is constrained by low single-well productivity and short production duration, making it difficult to achieve economic viability. Based on the accumulation features of the hydrate-bearing layer (HBL), shallow gas layer (SGL) and deep gas layer (DGL), the three-gas co-production is expected to promote the commercial development of marine hydrates by substantially increasing total gas production. A new reservoir preparation method that explicitly accounts for the geothermal gradient is proposed. The effects of well position and mining pressure on pressure response, the evolution of temperature field and gas/water production were systematically studied. The results show that placing the mining well beneath the SGL improves production efficiency, but it will sacrifice the thermal energy available from the shallow and deep gas. Reducing the mining pressure, particularly below the quadruple point, enhances gas production but introduces safety risks, including excessive pressure differences, icing, and hydrate re-formation. A multi-stage depressurization strategy effectively mitigates these issues, reducing the maximum pressure difference by 32% and increasing the average minimum temperature by 1.71 °C. Further optimization is possible by adjusting the depressurization stages and duration. This study expands the understanding of joint production characteristics for formulating mining strategies.