Extraction of Methane Hydrate from Clayey Silt via Temperature–Pressure Oscillation Combined with Flue Gas Displacement

Gas displacement in natural gas hydrates (NGHs) represents a viable strategy for methane recovery and CO2 sequestration. However, its practical implementation is limited by low displacement efficiency and slow reaction kinetics. This study investigated the enhancement of flue-gas-driven methane displacement through the combined application of temperature and pressure oscillation. Key operational parameters─including the amplitude, interval, frequency, oscillation sequence, and synchronous mode─were systematically evaluated. The results demonstrate that optimizing these parameters can significantly enhance the displacement efficiency. Specifically, a higher amplitude, increased frequency, and extended intervals contribute to improved methane recovery. Following three oscillation cycles, the displacement rate reached 64.05%, doubling that under nonoscillatory conditions, although CO2 retention decreased simultaneously. Temperature oscillation also exerts a notable effect on the process. When the amplitude exceeded 4 K, the interval was extended to 2 h, or high-frequency oscillation was applied, and methane recovery increased to 96.23%─nearly complete─after three cycles. Different oscillation sequences were found to enhance methane recovery to varying extents and exhibited distinct impacts on CO2 retention. Under the pressure-then-temperature sequence, the retention of CO2 initially increased and subsequently decreased, whereas under the temperature-then-pressure sequence, it continuously declined. In synchronous oscillation mode, methane recovery displayed a steady upward trend, reaching 88.94% after three cycles. CO2 retention initially increased and then gradually declined; however, the CO2/N2 molar ratio in the hydrate phase remained consistently high, indicating effective CO2 sequestration. These findings provide important insights for the optimization of methane extraction and long-term storage of CO2 in submarine NGH reservoirs.