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Natural gas hydrate has attracted worldwide attention for its abundant availability and clean and nonpolluting combustion products. The actual environment of marine methane hydrates is water‐saturated, but present researches on water‐saturated hydrates are relatively few. This study prepares water‐saturated hydrate sediment of ≈70% saturation and mainly focuses on the thermodynamic and kinetic characteristics of decomposition process using different depressurization rates. Compared with gas‐saturated hydrates, both the thermodynamic evolution and the decomposition kinetics of water‐saturated hydrates get weakened due to the inhibition of water molecules surrounding hydrate crystals on heat and mass transfer. In all experimental cases of water‐saturated hydrates, the temperature still decreases at most 0.5 °C in several minutes after the pressure has become constant, indicating the delay effect of depressurization in water phase. The results show that the increase in depressurization rate strengthens both temperature decrease and pressure delay of water‐saturated hydrate‐bearing sediment. In addition, the decomposition rate of water‐saturated hydrates in depressurization stage is constant and positive to the depressurization rate, while the decomposition rate in constant pressure stage is a function of residual hydrate amount. This study is significant for decomposition control of hydrate mining under actual marine environment. [ABSTRACT FROM AUTHOR]

[Display omitted] • Production enhancement effect of underlying gas on hydrate reservoir is identified. • t 90 is reduced by nearly half as the underlying gas pressure increases by 3 MPa. • Underlying gas flow into the hydrate reservoir alleviates the temperature drop. • Underlying gas pressure shows no obvious effect on the average gas–water ratio. The marine hydrates with underlying gas show great commercial potential as a new cleaner alternative energy. Yet the effects of underlying gas on the production characteristics of marine hydrate reservoirs are still unclear. In this study, the clayey-silt marine sediments sampled in South China Sea were used to synthesize the excess-water methane hydrate samples with consistent porosity (38%) and three-phase saturation. Depressurization was conducted to produce gas from hydrate reservoirs and underlying gas with different pressures (3–6 MPa). The results indicate that the underlying gas pressure has a positive correlation on hydrate decomposition, gas production, and water production; yet its effect on the gas–water ratio is not significant. As the underlying gas pressure increased from 3 MPa to 6 MPa, the average temperature drop of the entire hydrate reservoir was reduced by more than 0.5 K, and the time required for 90% hydrate decomposition (t 90) was shorted by nearly half. The underlying gas flowing into the hydrate reservoir is beneficial to alleviate the drops of pressure and temperature and promote the formation of gas–water flow channels. Therefore, the production of underlying gas can not only improve the gas production potential but also effectively contribute to hydrate decomposition and avoid ice formation. Through comprehensive evaluation of hydrate decomposition time, reservoir temperature drops, and average gas–water ratio, the enhancement effect of underlying gas on hydrate reservoir exploitation is preliminarily demonstrated. The results of this study prove the importance of underlying gas and show the prospect of further research on marine hydrate reservoirs with underlying gas for commercial exploitation. [ABSTRACT FROM AUTHOR]

• Underlying gas pressure determines the hydrate saturation when f g − f eq < 2.6 MPa. • Hydrate dissociation front advanced with gradient pressure formed by underlying gas. • The production of hydrate dissociated gas and underlying gas are interdependent. • Underlying gas improves the gas recoverability and potential of hydrate reservoirs. Hydrate reservoirs with underlying gas are generally accepted as the most promising energy resource for commercial exploitation. The underlying gas is an important part of the energy recovery of hydrate reservoirs. However, the gas production characteristics of hydrate reservoirs with underlying gas are rarely discussed. This paper aims to analyze the effects of underlying gas on the formation of hydrate and methane production behaviors by depressurization. In a core holder, marine soil obtained from the South China Sea was used to remold muddy low-permeability cores. The hydrate saturation of the muddy cores ranged from 25.2% to 48.3%. In this work, when the pressure difference between the underlying gas and the hydrate phase equilibrium is less than 2.6 MPa, the underlying gas pressure is the decisive factor for hydrate saturation. Besides, underlying gas can provide a relatively slow pressure drop rate for hydrate decomposition. The low permeability of the core caused by high hydrate saturation weakens the production of underlying gas and leads to the pressure gradient in the core. Hydrate decomposition front advances with the pressure gradient. Therefore, the production of underlying gas and hydrate decomposition are interdependent during depressurization. The experimental results demonstrated the existence of underlying gas could boost the exploitation potential and gas recoverability of hydrate reservoirs. These findings expand the understanding of joint exploitation characteristics and provide guidance in assessing the production potential for further researches and field exploitation of hydrate reservoirs with underlying gas. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • A 0.026 °C/mm gradient in height is simulated for methane hydrate exploitation. • Local temperature without hydrates is affected by heat transfer and hydrates nearby. • Isothermal dissociation of hydrates is irrelevant to sediment temperature gradient. • Natural warming trajectory is effective to forecast the end of hydrate dissociation. [ABSTRACT FROM AUTHOR]

As potential alternative energy sources, natural gas hydrate has attracted international attentions. In order to optimize its exploitation process, the thermodynamics characteristics during hydrate dissociation were investigated in this study. Eleven reservoirs with three ratios of gas amounts in gas and hydrate phases (2.1, 2.9 and 4.6) were employed, and four exhaust rates (2.3, 4.7, 7.1 and 8.8 ln/min) were conducted to pressurize the reservoirs to 2.0 MPa. Hydrates dissociated at a constant rate during the depressurization stage, and the dissociation rate increased with higher depressurization rate, while the effect is enhancing with the increase of gas space. The temperature decreased with the dropped pressure in a similar trend, affected by both depressurization rate and hydrate saturation. During constant-pressure stage, the hydrate-bearing reservoir temperature kept at approximately −2.5 °C until ice formation, making the temperature increase to −1.5 °C. Ice formation can nearly quintuple the instantaneous rate of hydrate dissociation, while no obvious changes in hydrate dissociation before and after the end of ice melting. In addition, the induction time and existence duration of ice obey good stepwise spatial and temporal sequences. These results are great significant for pressure controlling and efficiency optimization of later period of methane hydrate exploitation. [Display omitted] • Depressurization rate controls the rates of both hydrate dissociation and temperature change. • Temperature with hydrate dissociation and ice melting is variable and constant, respectively. • Spatiotemporal distribution of ice behavior in hydrate dissociation is plotted for first time. • Quintuple increase in hydrate dissociation rate can be caused by ice formation. [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]

Gas permeability and the depressurization path are decisive factors affecting the efficiency of gas hydrate reservoir exploitation. However, there was a lack of quantitative studies on the gas permeability and dynamic optimization of real-time depressurization paths under different in-situ conditions. Therefore, hydrate-bearing cores within the core holder were remolded using South Sea clay under different in-situ conditions. Then, the gas permeability was continuously measured using a constant flow rate. Next, a gas permeability model coupling effective pressure, hydrate saturation, and absolute permeability were established. It was used to predict the gas permeability under different in situ conditions, with deviations mainly in the range of 1–2 %. In addition, the optimum outlet pressure and positive outlet pressure range during depressurization under different in-situ conditions were proposed. The real-time gas flow rate of the optimal depressurization path was larger than other depressurization paths. Also, the logic reasoning process for real-time optimal outlet pressure and positive outlet pressure range was given. Finally, the inlet pressure and the optimum outlet pressure were also accurately predicted. None of the predicted values deviated by more than 4 %. The dynamic optimization of the outlet pressure behavior during depressurization can provide some scientific guidance for hydrate development and utilization. [Display omitted] • Permeability model including hydrate saturation, effective pressure was established. • Optimum outlet pressure and pressure range during depressurization were found. • Real-time inlet pressure of the core was accurately predicted. • Real-time optimal outlet pressure and pressure range were accurately predicted. [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]

Natural gas hydrate-bearing sediment permeability, which influences the flow behavior of fluids, is a key physical parameter used to determine the exploitation efficiency of hydrate. However, no comprehensive overview of existing research related to its measurement and application development has been conducted to date. In this review, the related advances in sediment permeability are systematically summarized in terms of experiments, models, numerical simulations, and its influence on hydrate exploitation. The sediment permeability measurement and their influencing factors have been comprehensively analyzed. In particular, the effects of hydrate phase transition on sediment permeability are discussed in detail. In addition, the normalized models of sediment permeability and numerical simulations of sediment structure are investigated. However, no universal normalized models of sediment permeability and numerical simulation of hydrate phase transition are available. The mechanism by which sediment permeability magnitude and anisotropy influence the hydrate exploitation efficiency has also been discussed. Finally, future efforts should focus on dynamic evolution, high-precision measurement, multifactor coupling effect, generalization of models, and optimization of numerical simulations, which are beneficial to improve guidance for the commercial exploitation of hydrate. • The influence of permeability on hydrate exploitation from experiments, models, numerical simulations are reviewed. • The sediment permeability measurement and its influencing factors have been comprehensively analyzed. • The normalized models of sediment permeability and numerical simulations of sediment structure are investigated. • The effect mechanism of sediment permeability magnitude /anisotropy on hydrate exploitation efficiency has been discussed. • The challenges of current research and future research prospects are discussed. [ABSTRACT FROM AUTHOR]

• The water conversion rate in large pores of porous media was higher than that in small pores. • Gas injection from top to bottom was beneficial to the hydrate-based gas storage and transportation. • The kinetics of methane hydrate formation in size-varied porous media was explored. • The contribution of 2nd hydrate growth to total gas consumption exceeds 33.7%. Using porous media to store and transport natural gas by the hydrate method can significantly improve gas-liquid contact and increase the hydrate formation rate. Therefore, it is particularly important to understand the characteristics of hydrate formation in porous media. In this study, the formation characteristics of hydrate in porous media with different particle sizes were quantitatively investigated by low-field 1H NMR from the microscopic scale to macroscopic characteristics. The experimental results show that hydrate always tends to grow in the larger pore space area caused by the heterogeneity of the porous media, and the water conversion rate in large pores is significantly higher than that in small pores. Due to the water accumulation under the action of gravity, the distribution of hydrate shows obvious spatial heterogeneity, and the lower part is more prone to clogging. Analysis of the macroscopic characteristics of hydrate formation shows that the induction time of hydrate has no obvious relationship with the particle size of the porous media, while the formation rate and gas consumption of hydrate increase with the decrease of the particle size of porous media. The gas consumption and formation rate of hydrate in BZ01 increase by 16.85% and 174.7% respectively compared with those in BZ02/04. In addition, due to gas diffusion – hydrate film rupture – capillary force, hydrate growth has the characteristics of 2nd growth, resulting in more than 33.7% of the final gas consumption. [ABSTRACT FROM AUTHOR]