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There are four main classes of methane hydrate deposits in nature. These classes differ in their distributions of the methane hydrate layer, free water layer and free gas layer. The gas production rate, water production rate, gas-to-water ratio, gas and water production recovery during the depressurization dissociation process are different for hydrate samples of different classes. Water-excess hydrate samples of Classes 2 and 3 were prepared in the present work, and they were dissociated at 2.0 MPa, 2.3 MPa, 2.6 MPa and 3.0 MPa. Ice formation was observed when the backpressure was 2.0 MPa. The gas-to-water ratio for Class 3 samples was higher than that of Class 2 samples, regardless of the backpressure. The average gas production rate for the 90% overall gas amount of Class 2 samples was higher than that of Class 3 samples for a backpressure of 2.0 MPa, whereas the opposite result was found for backpressures equal to or greater than 2.3 MPa. Furthermore, the water recovery value of Class 2 samples was higher than that of Class 3 samples by approximately 10%, and the gas recovery value of Class 2 samples was lower than that of Class 3 samples by approximately 20% at the same backpressure. These experiments provided some basic data for the exploitation of Class 2 and Class 3 deposits and find out the influence of backpressure on methane hydrate dissociation of the two class deposits by depressurization. [ABSTRACT FROM AUTHOR]

[Display omitted] • CO 2 hydrates formed above and inside a water phase differ in morphology and stability. • The dissociation of hydrates inside the water phase is affected by both heat and mass transfer. • A 0.2 mm thick low porosity hydrate film at the original water-CO 2 interface was detected by MRI. • CO 2 hydrate film are supported by hydrate dendrites and sink when these dissociate. Gas hydrates are widely considered as promising candidates for gas storage, energy transportation and seawater desalination. Critical to such applications is a detailed 3D understanding of hydrate formation. To this end, we employ magnetic resonance imaging (MRI) to non-invasively image the gradual formation of opaque hydrate from CO 2 and water in a cylindrical vessel at 1 °C and as a function of pressure between 2.0 and 3.5 MPa. A 200 µm thick dense hydrate layer is consistently observed to form at the gas-water interface accompanied by a ~1.4 mm thick porous hydrate layer above it and frequently complex dendritic hydrate formation in the water phase below it. Dissociation is observed to occur preferentially via the thick hydrate layer with the initial hydrate film retained largely intact for an extended period of time. The sequential images of hydrate dissociation inside the water phase are most consistent with a vertical heat and mass transfer controlled hydrate dissociation process. The observed difference between the hydrates formed above and inside the water phase is of mechanistic value in understanding these complex interfacial phase transitions. [ABSTRACT FROM AUTHOR]

Methane hydrate as a clean energy source has attracted worldwide attention. Yet, thermodynamic evolution of methane hydrate-bearing deposits during long depressurization are still unclear. In this study, five methane yields (1.17, 0.77, 0.41, 0.21 and 0.13 L s /min) for exploitation were conducted to simulate long depressurization of thousands of minutes. The rates of both depressurization and hydrate dissociation are linearly enhanced by increasing methane yield. Icing is always observed when the pressure reaches approximately 2.4 MPa unless the hydrate dissociation already finished. The accelerating effect of icing on hydrate dissociation can be strengthened by high methane yields. Thermodynamic responses at different exploitation stages are different because of the varied combinations of some non-equilibrium processes including gas expansion, hydrate dissociation, icing and melting as well as heat transfer. In this study, there are four thermodynamic non-equilibrium phenomena, respectively are gas expansion production, non-isothermal heat transfer, hydrate stationary dissociation and ice isothermal melting. The thermodynamic controlling mechanism and influencing factors of different exploitation stages are analyzed qualitatively or quantitatively, which reveals the temperature response of methane hydrate exploitation during long depressurization process. These results are helpful for understanding complex coupling mechanism of phase change and heat and mass transfer in methane hydrate exploitation. [Display omitted] • Five methane yields are employed to simulate long depressurization process. • Hydrate dissociation rate is linearly enhanced by increased methane yield. • Four thermodynamic stages of phase change and heat and mass transfer are found. • Thermodynamic priorities are ice melting, hydrate dissociation and gas production. [ABSTRACT FROM AUTHOR]