Effects of temperature gradient on methane hydrate formation and dissociation processes and sediment heat transfer characteristics

Commercial exploitation of methane hydrate has become an essential potential to issue global energy shortage. Yet, the temperature gradient of hydrate sediments is always ignored, and laboratory-scale methane hydrate formation and dissociation investigations with temperature gradient are rarely conducted. In this study, a 240 mm height reactor was used to simulate hydrate sediment with the temperature gradient of approximately 0.026 °C/mm in height. Methane hydrates were first formed under constant-volume condition with three initial pressures of 6, 8 and 10 MPa. The presence of temperature gradient caused the difference in the height of hydrate-bearing region, and the height expanded with formation pressure. Then, the hydrates were dissociated by depressurization to 2.5 MPa in three production rates of 0.1, 2.3 and 60 ln/min. The temperature changes of hydrate-bearing and non-hydrate areas were obviously distinguished, affected by hydrate dissociation. Due to the limited heat transfer, an isothermal period at approximately −1 °C, irrelevant to the temperature gradient, was observed during the dissociation process at 2.5 MPa. After comparing with the natural warming trajectory, it is confirmed that the temperature response of hydrate-bearing sediments agrees with thermodynamic relationship, rather than conventional heat transfer. These results are significant for the efficiency improvement of actual methane hydrate exploitation.