Thermodynamics analysis and temperature response mechanism during methane hydrate production by depressurization

Methane hydrate is expected as a future energy to satisfy the long-term energy needs worldwide. Yet, the thermodynamics behaviors and temperature response mechanism during methane hydrate production by depressurization are still unclear. In this study, four methane hydrate-bearing sandy sediments with different initial pressures (6.0, 5.5, 4.8 and 4.2 MPa) are depressurized to 3.0 MPa at a slow rate, and the temperature decreases with the depressurization progresses and will be accelerated from the hydrates start to dissociate. The dissociation rate of hydrates gradually increase to and keep at a highest value under a rated production flux, when the temperature is strictly response to the pressure and the pressure and temperature trajectories are parallel to the hydrate phase equilibrium line. At this stage, the external heat transfer is almost used for the hydrate dissociation, and it is called stationary state of hydrate dissociation in this study. When the pressure turns constant, the hydrate dissociation changes to a lower rate than the depressurization stage, and then the declined dissociation rate consumes less heat and therefore can warm the whole sediment. The results reveal the temperature response differences before, in and after the hydrate dissociation process and provide direct thermodynamic criterion for the field monitoring of methane hydrate production.