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• 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]

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]

[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]

• Hydrate reformation during exploitation is caused mainly by insufficient heat supply. • Hydrate reformation is likely to occur in the depressurization process. • The depressurization process should be combined with thermal stimulation. • A new system is designed to investigate hydrate reformation under various conditions. Natural gas hydrates are considered as some of the most promising energy sources. However, economical, safe, and commercial exploitation of marine hydrates has not yet been achieved. One of the main obstacles is hydrate reformation in the process of hydrate exploitation, which needs to be properly addressed. The processes of hydrate dissociation and reformation under depressurization and thermal stimulation are discussed in this review. It is necessary to understand the manner in which various factors affect hydrate reformation during dissociation. Therefore, the effects of critical factors on hydrate reformation during thermal stimulation and depressurization dissociation are analysed to provide a comprehensive explanation of the hydrate reformation mechanism. In addition, several measures used to prevent hydrate reformation during the exploitation process are discussed. Finally, an experimental system was designed to investigate the exploitation methods and optimization of various hydrate-bearing sediments. Future efforts should focus on finding effective methods and evaluating the combined sequences, which can prevent hydrate reformation with acceptable exploitation efficiency. [ABSTRACT FROM AUTHOR]