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[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]

• Confining pressure surrounding the core was adverse to gas production. • Hydrate and free gas were easily trapped in sediment pores under confining pressure. • Hydrate and water in sediment pores severely slowed down the gas production rate. • Water content increased water production while hardly affected water conversion. The abundant methane hydrates stored in marine sediments have been widely evaluated as a potential energy source. Understanding the gas and water production characteristics of methane hydrate-bearing marine sediments is critical for hydrates commercial exploitation. In this study, confining pressure was applied to simulate a sub-seafloor environment. Methane was repeatedly injected into cores to remold hydrate-bearing marine sediments with different hydrate saturation. The hydrate saturation increased from 10.6% to 21.6% as the water content increased from 8.9% to 22.2%. The results indicate that the higher the core water content, the greater the hydrate saturation and the longer the hydrate dissociation time. The gas production characteristics of hydrate-bearing sediments were severely affected by water and hydrates in pores. The results indicated that some hydrates and free gas were easily trapped by the surrounding soil, meaning that the hydrates were isolated and disconnected with the pore channels under confining pressure during depressurization. Thus, a second depressurization was conducted to achieve further gas production. For cores with different water contents, their water conversion percentage is approximately 20%. When the water content exceeded 16.7%, the water production was observed. The results of this study are meaningful for further related research and field production of marine hydrates. [ABSTRACT FROM AUTHOR]

• Simultaneous investigation on the gas-water consumption of methane hydrate formation. • The formation process of methane hydrate under different driving forces were studied. • The hydration numbers of methane hydrate under different conditions were determined. Methane hydrate is an important energy source and a good form of methane storage and transportation. The limit of gas storage capacity in hydrates is still not clear. In this study, we employed a low-field nuclear magnetic resonance to study the gas-water consumption characteristics quantitatively at various formation conditions. Nine methane hydrate formation experiments of different temperatures pressures were conducted in porous media. The consumption of gas and water for hydrate formation displayed as asynchronous trend, due to the change in hydrate cage occupancy. The results indicated that higher pressures or lower temperatures improved both the amount of hydrate formation and the cage occupancy of hydrate. In addition, the driving forces affected the formation of methane hydrate more obviously under a lower temperature. Ultimately, the hydration number of methane hydrate under different driving forces were determined. This study is of great significance to the estimation of hydrate and gas capacity in the hydrate based technology application in methane storage and transportation. [ABSTRACT FROM AUTHOR]

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]

Natural gas hydrates (NGHs) are a new, clean and effective energy source with great potential for exploitation. The permeability of hydrate-bearing sediment is one of the crucial factors for evaluating the efficiency of gas hydrate production. Many researchers have investigated the variation in permeability, but there is still no effective method for identifying the variation in permeability during hydrate dissociation. In this study, a new method was proposed by using a constant step of 0.5 K in the temperature-increasing process to control hydrate dissociation, which achieved a continuous measurement of permeability. Different initial water saturations (0%, 25%, 30%, 40%, 50%, 55%, and 60%) were used to investigate water phase permeability in the porous media during hydrate dissociation. The results showed that there was little MH in the reservoir after 277.65 K under a constant volume in the simulation process of hydrate extraction. Additionally, the distribution of permeability tended to be gradually centralized by dispersal during the increase in temperature under various experimental conditions. There is a variation in the permeability curve from an exponential distribution to a logarithmic distribution with an increase in temperature. The higher the initial water saturation, the lower the initial water phase permeability. Furthermore, in the process of hydrate dissociation, the permeability decreased exponentially with the increase in saturation. But when the hydrate saturation was less than 3%, the exponential correlation is not valid. [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]

• 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]