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• Methane hydrate production was assessed in South China Sea sediments. • Higher hydrate saturation increased the dissociation duration. • Dissociation duration barely affected the return time of sediment temperature. • Double depressurization is employed to increase the production efficiency. Marine methane hydrate is a considerable energy source for use in the near future. With great obstacles to spot production, researchers are focusing on the production characteristics of hydrates in various experimental systems, such as glass beads, clay, silica sand and so on. This study investigated the production behaviors of methane hydrate in the real South China Sea sediments using depressurization method. The hydrate saturations of the remolded hydrate-bearing sediments ranged from 10.10% to 23.76%. The results indicate that an 8% increase of hydrate saturation can prolong the dissociation duration under the same backpressure of 2 MPa by 120 min. In addition, the excess temperature drop caused by the depressurization may induce the unpredictable occurrence of hydrate reformation or icing; therefore, a double depressurization method that depressurizes to 2 MPa (second stage) after 20 min maintenance at 4 MPa (first stage) is employed in order to shorten the temperature drop in the first stage and increase the dissociation rate in the second stage. The results of this study are significant for the spot production of marine hydrates in order to achieve high efficient gas production. [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]

• Effective stress has cumulative damage effect on gas permeability of muddy core. • Dynamic permeability evolves with effective stress and hydrate saturation. • Intrinsic permeability is the predominant factor for dynamic permeability. • A model used dynamic permeability can predict gas production behaviors well. Methane hydrates with underlying gas are promising for gas production commercially in case of accurate prediction and timely effective measures taken to control dynamic permeability. In a core holder, marine sediments sampled from South China Sea were used to remold hydrate muddy cores. Three consecutive gas production experimental results confirmed that the effective stress has a cumulative negative impact on the reservoir permeability. Thus, a modified Masuda permeability model considering the evolution of effective stress (0.2–5.0 MPa) and hydrate saturation (36.6%–53.1%) was proposed to calculate the dynamic permeability during depressurization. On this basis, dynamic permeability was applied to a mathematical model to study the production characteristics of hydrates with underlying gas. Compared with experimental measurements, the error of underlying gas pressure predicted by the model can be less than 1.92%. Besides, the precise trajectory of the hydrate dissociation front in the core was predicted by the model. The results indicated that the dynamic permeability is dominated by intrinsic permeability, and the effect of hydrate saturation is more significant than that of effective stress. The mathematical model could provide a simple yet practical method for estimating dynamic permeability and predicting gas production performances of hydrate reservoirs with underlying gas. [ABSTRACT FROM AUTHOR]

The concentrations of CH4, SO, σCO2 and the carbon isotope compositions of ΣCO2 and CH4 in the pore-water of the GS sedimentary core collected from Guishan Island (Pearl River Estuary), South China Sea, were determined. The methane concentration in the pore-water shows dramatic changes and sulfate concentration gradients are linear at the base of the sulfate reduction zone for the station. The carbon isotope of methane becomes heavier at the sulfate-methane transition (SMT) likely because of the Raleigh distillation effect; 12CH4 was oxidized faster than 13CH4, and this caused the enrichment of residual methane δ13C and δ13C-ΣCO2 minimum. The geochemical profiles of the pore-water support the existence of anaerobic oxidation of methane (AOM), which is mainly controlled by the quality and quantity of the sedimentary organic matter. As inferred from the index of δ13C-TOC value and TOC/TN ratio, the organic matter is a mix of mainly refractory terrestrial component plus some labile alga marine-derived in the study area. A large amount of labile organic matter (mainly labile alga marine-derived) is consumed via the process of sedimentary organic matter diagenesis, and this reduces the amount of labile organic matter incorporated into the base of the sulfate reduction zone. Due to the scarcity of labile organic matter, the sulfate will in turn be consumed by its reaction with methane and therefore AOM takes place. Based on a diffussion model, the portion of pore-water sulfate reduction via AOM is 58.6%, and the percentage of ΣCO2 in the pore-water derived from AOM is 41.4%. Thus, AOM plays an important role in the carbon and sulfur cycling in the marine sediments of Pearl River Estuary. [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]

Marine methane hydrates have attracted global attentions as a considerable energy resource. The permeability of hydrate reservoirs critically affects the technical and economic feasibility of hydrate exploitation as well as the efficiency of gas production. In this study, marine sediments obtained from the South China Sea were used to remold core samples. Using a core holder, the gas permeability of marine sediments with and without methane hydrates were measured by injecting methane. In this study, the values of gas permeability range from 5.2 mD to 16.7 mD. The effects of confining pressure, hydrate saturation, and initial water saturation on gas permeability of cores were analyzed. The experimental results indicated that the gas permeability decreases (increases) with increasing (decreasing) confining pressure. In addition, the increase trend of confining pressure will significantly decrease on gas permeability in case the effective stress surpassed approximately 3.5 MPa. The deformation of silty-type marine sediments caused by increased confining pressure is irreversible to a certain extent. Relatively, the initial water saturation has little effect on gas permeability. This is attributed to that the water is bounded in the marine soil when the initial water saturation is less than 60%, resulting in a small water resistance effect. In addition, the hydrate dissociation induced by depressurization under confining pressure could result in a decrease of gas permeability. The results of this work revealed the effects of methane hydrates and confining pressure on the gas permeability of marine sediments, with great significance for the methane production from marine methane hydrate reservoirs. • Core permeability shows a decrease trend with increasing confining pressure. • The deformation of marine sediments caused by confining pressure is irreversible. • The low initial water saturation of core samples have little effect on permeability. • Hydrate dissociation under confining pressure causes a decrease in gas permeability. • Purge gas with high pressure can be used as a measure of reservoir stimulation. [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]

Clathrate hydrates are formed by water molecules and natural gas molecules under a certain temperature and pressure. Natural gas hydrates are mainly distributed in the marine sediments and the permafrost in nature. As a kind of potential clean energy, determining the structural characteristics of hydrates in sediments is the prerequisite for the safe and economic exploitation of natural gas hydrate. X‐ray computed tomography (CT) is a potential method to nondestructively detect the occurrence of natural gas hydrate. The basic physical properties such as porosity, hydrate saturation, and sediment permeability of the hydrate‐bearing sediments are analyzed by calculating the CT numbers or combining with the network model. Therefore, the X‐ray CT is used to study the hydrates produced in the laboratory and the hydrates formed in the nature by scientists all over the world. By summarizing the previous research, this article focuses on the important role of X‐ray CT in the hydrate research from the following three aspects: analyzing the basic physical properties of hydrate sediment, identifying the occurrence mode of hydrate in sediments, and measuring the hydrate formation and decomposition process. In response to the research progress and future research directions, relevant suggestions had been put forward. [ABSTRACT FROM AUTHOR]