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[Display omitted] Gas hydrates have gained great interest in the energy and environmental field as a medium for gas storage and transport, gas separation, and carbon dioxide sequestration. The presence of small doses of surfactants in the aqueous phase has been reported to enhance hydrate formation; however, the underlying mechanisms remain poorly understood. Thus, in situ high-resolution X-ray computed tomography measurements were performed to monitor the upward water migration and the resulting hydrate nucleation and growth. It was found that the presence of hydrate crystals at the gas–liquid–solid contact line triggered the enhanced growth of hydrates on the reactor wall. A time delay was observed between the disappearance of the bulk water reservoir and its transformation into hydrate. The lower interfacial tension between the hydrate surface and the solution facilitated its adsorption onto the reactor wall once a thin film of hydrate nucleated on the solid wall surface. These hydrate layers present on the reactor wall were found to be porous, wherein the porosity decreased with increased subcooling. These fundamental results will be of value in understanding the mechanism of hydrate growth in the presence of surfactants and its potential application in hydrate-based technologies. [ABSTRACT FROM AUTHOR]

Several trial productions have been conducted on marine natural gas hydrate reservoirs, demonstrating its enormous potential as an alternative energy source. Nonetheless, sustained and efficient production still remains challenging, with crucial issues on low gas production efficiency and blockages. The large amount of hydrate decomposition and resulting heat absorption can lead to prolonged subzero conditions close to the well; this can readily induce ice formation and blockage. This study aims to investigate the issue of ice blockages caused by insufficient energy supply during the recovery process. A systematic analysis of the energy consumption and ice blockage quantification at each stage of the traditional multi-stage direct depressurization and constant speed depressurization methods is conducted. A hybrid optimized scheme is proposed, which enhances energy utilization efficiency by appropriately utilizing reservoir sensible heat during the initial stage of production. Additionally, a multi-stage constant speed depressurization method is introduced in the later stages to mitigate production fluctuations and reduce blockage duration. Results showed a 69.5 % increase in energy efficiency and an average 67.1 % decline in gas blockage duration with comparable gas productivity. The proposed method could be a viable solution to mitigate challenges of icing blockages and fluctuating production during trial tests of marine hydrate reservoirs, achieving improved energy efficiency and sustainable production. • Ice blockage in hydrate reservoirs reduced production efficiency during direct and constant-speed depressurization methods. • Proposed a hybrid method to address gas production fluctuations caused by ice blockage through energy consumption analysis. • Multi-stage analysis revealed 70% energy efficiency improvement and 67% reduced ice blockage duration with the hybrid method. [ABSTRACT FROM AUTHOR]

Increasing the efficiency of gas production from marine gas hydrate still faces significant problems. Based on the geological survey of marine reservoirs, gas hydrates are generally associated with underlying free gas layers with high temperature and water saturation. Here the effect of this gas layer and its characteristics on the gas and water production from the hydrate layer are studied. It was found that a large temperature difference existed between the two layers during gas production due to the different heat demand and consumption in each layer. Slower gas production rate was observed in case of dense distribution of hydrates in the hydrate-bearing layer and the resulting sluggish gas permeation and enormous heat supply. Notably, this was not present when the free gas layer was more water-saturated. The resulting maximum gas production rate could be effectively enhanced by 41 % at most as well. This was attributed to the increased sensible heat available from the pore water and the interbedded heat exchange. It was thus indicated that a hydrate reservoir with an underlying water-rich layer could be more favorable for gas production in terms of heat supply. • Joint exploitation through pre-embedding compartment device and vertical well. • A significant temperature difference between the two layers during exploitation. • Highly water-saturated underlying shallow gas layer enhance hydrate decomposition. [ABSTRACT FROM AUTHOR]

Natural gas hydrates are a potential future energy modality with the advantages of clean burning and large resource reserves and have attracted worldwide attention. Natural gas hydrates formation and dissociation impact the skeletal structure and mechanical properties of porous media sediments. In addition, there is a synergistic effect between the migration of fine clay particles in porous media and hydrate behavior. In this study, methane hydrate was examined in-situ using a low-field nuclear magnetic resonance system in the presence of two suspensions of clay particles with different stability. The results showed that clay particles impacted methane hydrate formation and dissociation and the pore structure of porous media. Hydrate nucleated preferentially in large pores, causing them to split into smaller pores; the presence of clay particles, especially illite, improved the water conversion rate. The results indicated that water content in large pores increased after hydrate dissociation but was discontinuous among different pores. When illite was present, the distribution was more continuous; when montmorillonite was present, the water distribution of the large pores was similar to that of the original state. This work increases our understanding of the kinetics, water migration, and pore structure alteration of methane hydrate formation and dissociation in sediments with clay particles and provides support for the safe and efficient development of natural gas hydrates. • Clay particles impacted methane hydrate formation and dissociation • Hydrate nucleated preferentially in large pores • Clay particles, especially illite, improved the water conversion rate • Water content in large pores increased after hydrate dissociation but was discontinuous among different pores [ABSTRACT FROM AUTHOR]

[Display omitted] • An approach of warm brine injection to promote CH 4 /CO 2 replacement is proposed. • Warm brine injection provides a 2.0 times enhancement of the cumulative gas yield. • A potential risk of hydrate reformation is alleviated by increasing salinity. • Gas production is insensitive to injected heat as the significant heat loss. • Brine injection scheme should be optimized based on energy harvest and CO 2 storage. CH 4 /CO 2 replacement for natural gas hydrates (NGHs) exploitation is a promising method for CO 2 geological sequestration and energy recovery simultaneously. However, the puzzles of low replacement efficiency and slow reaction rate caused by the mass transfer obstacle of gas exchange are fatal bottlenecks for field application of gas replacement method. Therefore, we propose a method that uses warm brine injection during CH 4 /CO 2 replacement process to break the barrier of CO 2 diffusion and enhance CH 4 recovery as well as CO 2 storage. Benefiting from the synergistic influence of salt effect and thermal stimulation as well as water flow erosion, warm brine injection provides three dimensionally connected channels in hydrate for subsequent mass transfer, improving CH 4 /CO 2 replacement in the deep layer of hydrate. CO 2 sequestration efficiency of the newly proposed method reaches 76.46%, and the maximum amounts of CH 4 recovery is nearly treble than that achieved by single CO 2 replacement method. Notably, a potential risk of secondary hydrate formation could occur upon the pressure surge and fluid migration attached to warm brine injection, which could be effectively alleviated by increasing salinity to destabilize hydrate lattices by reducing water activity. The gas production is insensitive to the injected heat as the significant heat loss during the transportation of injected brine in pipelines and thermal diffusion through boundaries. The introduction of free water increases the complexity of the reactions in hydrate reservoirs, and the formulation of brine injection regimes should be synergistically optimized based on energy harvest, energy efficiency and CO 2 sequestration. This work extends the previous knowledge on CH 4 /CO 2 replacement and shows important practical significance for future field studies of NGHs exploitation and CO 2 geological sequestration. [ABSTRACT FROM AUTHOR]

• A new hydrate formation promoter was selected for methane storage. • The R141b was confirmed to improve the equilibrium condition of methane hydrate. • Thermodynamic characteristics of mixture hydrate was analysed. • Mechanism of mixture hydrate nucleation was proposed. • Gas storage procedure was optimized. Solidified natural gas is a promising alternative for improving natural gas storage owing to its safe and environmentally friendly properties. However, harsh conditions limit its application. In this study, the R141b was used as a thermodynamic promoter to enhance methane hydrate formation under mild conditions. Addition of R141b to the water system significantly shifted the phase equilibrium boundary to lower pressures and higher temperatures, in which the R141b-CH 4 hydrate can be formed at 0.1 MPa and 287.2 K. Notably, the final pressure of the cell after hydrate formation was near the atmospheric pressure even at an initial pressure of 5.384 MPa, suggesting that the R141b-CH 4 mixed hydrate had milder storage conditions and lower costs than liquefied natural gas (approximately 110 K). The higher initial pressure significantly reduces the induction time and increases gas storage. The mechanism of unusual kinetic characteristics of the R141b-CH 4 hydrates that were observed can be attributed to the different nucleation behaviour of CH 4 , R141b, and R141b-CH 4 hydrates. This study provides evidence of a new promoter using the solid hydrate method under mild conditions that can be alternatively used for the storage and transport of natural gas, thereby increasing the supply of clean energy without the high economic costs and safety issues. [ABSTRACT FROM AUTHOR]

• Presence of fine particle could block the pores and hinder gas production. • Water content determines the behavior of fine particle aggregation. • Distribution pattern of fine particles directly affects the gas production efficiency. Natural gas hydrates are widely distributed in silty marine sediments containing fine particles less than 75 μm in size. The effects of WC (water content) on the behavior of fine particle migration and their role in gas and water production do not appear well understood. This work found that the WC slightly affected the gas production pattern in uniform sandy reservoirs. Yet upon a mixture of coarse and fine particles, the fine particles would instantly aggregate at the nearby pore throat reducing the reservoir permeability and prolonging the production by more than 3 times. Surprisingly, 16% of the fine particles at the edge and 19% at the middle were found to migrate with the fluid flow upon increasing water content, resulting in a less hindered initial gas production but a following more complete blockage or even production failure. This behavior can be consolidated in the scenario of a sand-kaolin mixture. The findings could provide new knowledge on the particle migration behavior upon gas and especially water production and guide the strategy design on dealing with the sand problem in the field test. [ABSTRACT FROM AUTHOR]

There have been several field tests worldwide to recover natural gas from marine gas hydrate reservoirs. The resulting intrinsic characteristics of the sediments are recognized as the major challenge hindering the large-scale gas production; of particular interest is the heterogeneous spatial distribution of natural gas hydrates in the sediments. Open issues still remain on mimicking the locally varying reservoir and revealing its role in the production process. Here in this work, reservoirs with a heterogeneous distribution of gas hydrates were prepared by controlling the uneven distribution of moisture in the fine natural sediments drilled from the South China Sea; special interest was put in the effects of the well location and depressurization schemes on the gas production performance. The results showed that the production efficiency in the heterogeneous reservoirs was significantly weakened compared with the homogeneous case; this was ascribed to the ununiform distribution of methane hydrates and pore fluids and a resulting locally different energy demand. Consequently, the production behavior showed a strong dependence on the depressurization location. A wellbore arranged in the layer with a high permeability can help improve the gas production efficiency by up to 38.25%. This yet resulted in an uneven temperature distribution; the rapid gas production and methane hydrate dissociation decreased the reservoir temperature to −3.33 °C. Further applying a cycling and step-wise depressurization could raise the minimum reservoir temperature up to 1.12 °C. Whereas, relieving temperature decline only through consuming the sensible heat in the reservoir and the surroundings was found to seriously weaken the gas production rate; an external heat supply was thus suggested in the hydrate-abundant regions for efficient gas production. Our findings expand the understanding of the spatially varying gas production behavior from the vertically heterogeneous hydrate-bearing fine marine sediments and may guide the operation of well location and depressurization schemes for field production in similar reservoirs. [Display omitted] • Heterogeneous hydrate samples were prepared by unevenly distributing the moisture. • Production behavior from the vertically heterogeneous hydrate reservoir was studied. • The optimized wellbore location would increase the production efficiency by 38.25%. • Heat supply was suggested in the hydrate-rich area to compensate the sensible heat. [ABSTRACT FROM AUTHOR]

• 3D morphologies and time-dependent parameters of the hydrate shell are obtained. • Water is more mobile than gas in hydrates in the diffusion-controlled growth stage. • Protrusions growth on the outer surface of a hydrate shell is due to water permeation. • Effective diffusion coefficients of gas and water in hydrate are determined. Understanding the transport properties of water and guest molecules in dense hydrate structures is pivotal in controlling their formation and stability; this is nevertheless hindered by challenges in characterizing the time-varying diffusion and permeation process. This paper reports experimental results on the mass-transfer characteristics of water and guest molecules across hydrate shells during their formation from water droplets. The time-dependent 3D morphologies and parameters of hydrate shells were in-situ obtained via X-ray computed tomography to reveal the transport mechanism in hydrates. It was found that in the mass-transfer-limited stage, hydrate growth occurred primarily at the hydrate-gas interface, indicating a more mobile characteristic of water molecules in those hydrate shells than gas. The resulting outward permeation of water led to the growth of hydrate protrusions on the outer surface of the hydrate shell, suggesting a transition of the water transport schema through the hydrate shell from diffusion to permeation. Consequently, a diffusion-based shell growth model was developed to quantify the diffusivity of gas and water molecules in hydrates. Combining the measurements in the temperature range of 275.15–283.15 K, the effective diffusion coefficient of gas through the hydrates was estimated to be in the range of 1.34 × 10−14 − 1.90 × 10−13 m2/s, with that of interstitial water in the range of 2.87 × 10−12 − 3.83 × 10−11 m2/s. These results are of fundamental value in developing an improved understanding of the kinetics of hydrate formation from gas-water interfaces, which is essential in the optimization of hydrate-based techniques. [ABSTRACT FROM AUTHOR]

[Display omitted] • CH 4 /CO 2 replacement behaviors in natural marine sediments are investigated. • CO 2 diffusion is weakened by clayey sediment during replacement process. • NGH reservoir with moderate water content has excellent potential for replacement. • Optimized P-T conditions are crucial to improve CH 4 recovery from hydrates. The use of CH 4 /CO 2 replacement from hydrate bearing sediments for CH 4 recovery and CO 2 storage is an alternative option to mitigate energy shortage and global warming. Fine marine sediments are highly attractive for abundant gas hydrate reserves and tremendous CO 2 sequestration potential. However, the CH 4 /CO 2 replacement regularity previously obtained from coarse sands may not be suitable for fine marine sediments because of their distinct differences in physical properties. In this study, fine natural marine sediments obtained from the South China Sea were used as porous media to investigate the CH 4 /CO 2 replacement characteristics. The results indicated that the pressure and the temperature were the main controlling factors affecting the replacement efficiency. The content of initial methane hydrate and water in reservoir had a more significant effect on CO 2 storage than CH 4 recovery. The gas exchange kinetics in hydrates presented in fine marine sediments were significantly different from those in coarse grained sediments. CH 4 /CO 2 replacement in fine marine sediment seemed to be inhibited by weak CO 2 diffusion as peculiarities of the sediment including fine grain size, clay swelling and high proportion of bound water. According to the observed experimental results, the pressure and temperature conditions should be comprehensively optimized to enhance the mass transfer effect and improve economic benefits. This work provided greater insights into future marine NGH exploitation and contributed to carbon sequestration with applying CH 4 /CO 2 replacement method. [ABSTRACT FROM AUTHOR]

Pressure oscillation controlled CH 4 /CO 2 replacement in methane hydrates to enhance CH 4 recovery and CO 2 storage. [Display omitted] • The oscillated pressure control is proposed to enhance CH 4 /CO 2 replacement. • Inner replacement cause CH 4 hydrate decomposition and mixed hydrate regeneration. • Optimized depressurization is crucial to improve CH 4 production and CO 2 storage. The recovery of CH 4 from natural gas hydrates via CH 4 /CO 2 replacement is a highly attractive method for achieving the recovery of CH 4 and storing CO 2. However, although extensive simulation and experimental studies have been conducted, applying CH 4 /CO 2 replacement is currently hampered by a lack of in-depth understanding about the replacement mechanism and the insufficient replacement efficiency and rate achieved. Therefore, to enhance CH 4 recovery and CO 2 storage, we propose a method that uses pressure oscillation during CH 4 /CO 2 replacement. The effects of depressurization pressure, depressurization time, and depressurization frequencies were analyzed in this study. The characteristics of pressure and temperature were analyzed during the depressurization process, and the decomposition and regeneration of hydrates relating to the oscillation pressure were observed on a macro level. The results show that this newly proposed combined method effectively improves CH 4 /CO 2 replacement, and the maximum amounts of CH 4 recovered and the CO 2 storage efficiency are nearly double than those achieved without pressure oscillation. CH 4 /CO 2 replacement is promoted by the pressure oscillation mechanism by breaking the balance of the hydrate layer, which results in the dissociation of CH 4 hydrates and finally forms a CO 2 /CH 4 mixed hydrate layer with free water. The results of this study provide a valuable method that can be used to enhance CH 4 recovery and CO 2 sequestration. [ABSTRACT FROM AUTHOR]

[Display omitted] • Inhibitors are used to prevent gas hydrates from clogging pipelines. • Effects of clay particles on polyvinylcaprolactam hydrate inhibition were studied. • Clay particles adsorbed the inhibitor, and the particle surface potential changed. • In the presence of clay particles, the inhibitory effects were weakened. Kinetic hydrate inhibitors are widely used during upstream activities in the oil industry to prevent gas hydrates from clogging pipelines. Yet their performance in the pipelines transporting natural gas from marine environment is largely veiled where clay particles and their adsorbability are generally present. In this study, it was found that the fine clay particles from a hydrate rich area in the South China Sea are majorly composed of layered silicate with a negative surface charge. Consequently, the clay particles would adsorb the inhibitor through Van der Waals attractions and ion–dipole interactions. The resulting weakened surface potential of the particles would trigger an aggregation of the clays together with the inhibitors attached on their surfaces. This will thereby result in less active components in the solution, significantly weakening the inhibition effect. It is therefore suggested that the effects of clay-rich conditions on the performance of the inhibitors should be carefully considered for an efficient dosage in the marine environments. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrate formation induced by cold seeps is first investigated by MRI. • Local heat release by hydrates formation leads to the dissociation of nearby hydrates. • Cold seeps with high gas flow rate and pressure are more valuable for hydrate resource surveys. Gas hydrates could occur as outcrop resources overspreading on the seafloor, coexisting with bubbling gas in activated cold seeps. This involves a complicated process of hydrate nucleation and accumulation in the sedimentary matrices as well as in the bulk water with an underlying gas plume. In addition, the escaping methane gas could also be a major concern in terms of its greenhouse effect. Consequently, the behaviors of hydrate formation in the sediments with a successive gas flow to mimic the cold seeps were monitored through magnetic resonance imaging technique in this work. It was found that the initial hydrate nucleation could occur universally throughout the sedimentary matrices resulted from an enhanced gas–water contact upon the gas flow. Specially, the substantial heat released upon hydrate formation could even locally trigger dissociation of adjacent hydrates. A faster hydrate formation was observed under higher gas flow rates and formation pressures, consolidating the dominant role of a sufficient gas–water contact in the hydrate formation. Nevertheless, large amount of residual water could still remain till the very end of the formation process; the subsequent reaction would be significantly limited by the gas diffusion barrier effect of the existing hydrate shell. The findings could provide insights into the kinetic process of hydrate formation in the cold seeps and expand our knowledge on the diverse occurrence of gas hydrate under the sea. [ABSTRACT FROM AUTHOR]