Characterization of Coextraction Sequences in Multilayer Gas Hydrate Reservoirs with Deep and Shallow Gas Layers

The presence of underlying gas in hydrate reservoirs provides significant commercial potential due to the feasibility of dual-gas coproduction from both hydrate and free-gas zones, leading to enhanced overall gas yields. This study experimentally investigates gas production characteristics in multilayer hydrate reservoirs containing shallow gas, hydrate, and deep conventional gas layers under different extraction sequences. A systematic analysis evaluates how different gas extraction sequences affect pore pressure, temperature, and production characteristics during the process. Results demonstrate that initiating extraction from the deep conventional gas layer yields a higher initial gas production rate. In this scenario, early gas production primarily originates from deep conventional gas, with its contribution gradually declining as hydrate decomposition and shallow gas contributions increase. Conversely, initiating extraction from the shallow gas layer leads to a lower initial production rate but accelerates temperature recovery. Here, early production is dominated by shallow gas beneath the hydrate layer, followed by increasing contributions from deep conventional gas and hydrate decomposition. Furthermore, initiating production from the deep gas layer requires 3.42 times longer to achieve zero pressure differential compared to the shallow-first approach. Therefore, prioritizing shallow gas extraction supports rapid temperature recovery to mitigate hydrate reformation risks, while prioritizing deep gas extraction maximizes short-term gas output. For practical hydrate reservoir exploitation, the choice of the initial extraction layer should be tailored to specific conditions.