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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]

• The phase equilibria of natural gas hydrate in real marine sediments were determined. • The endothermic peak bottom was proved corresponding to the phase equilibrium point. • Moisture difference affected decomposition pressure via changing sediment salinity. • The sediment salinity and gas components should be considered in spot production. Marine natural gas hydrate production has become a hot topic in recent years. There is still a long way to achieve the commercial production of marine hydrate reservoirs. The purpose of this work is to investigate the natural gas hydrate decomposition characteristics in the real marine sediments obtained from the South China Sea. A high-pressure micro-differential scanning calorimeter was employed to study the thermodynamic properties of natural gas hydrates. The results indicated that the increase in gas pressure or sediment salinity decreased the ice melting temperature by changing the vapor pressure of water. In addition, the decrease in sediment moisture from 40% to 30% increased the decomposition pressure of hydrates by 4.4 MPa at 278 K of sea bottom temperature, due to the water absorption capacity of the sediments increases the actual salinity furtherly. The influence of mixed gases on the decomposition pressure of hydrates was also investigated. The findings of this work on the effects of salinity and gas components on the decomposition pressure are significant to the spot safety production of marine natural gas hydrate reservoirs. [ABSTRACT FROM AUTHOR]