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• 3D xenon hydrate shell meshes on water particle and ice sphere created from micro XCT. • Time-dependent hydrate shell thickness and inner/outer diameter were determined. • Novel 3D mesh method can quantify heterogeneous hydrate shell growth. Gas hydrates have gained increasing attention in energy and environmental fields and have potential applications in gas storage and transport, carbon dioxide sequestration, and flow assurance. Nevertheless, the kinetics of hydrate formation are still not well understood. In situ high-resolution X-ray computed tomography measurements were performed to monitor xenon hydrate formation on water droplets and ice spheres. For the first time, three-dimensional thickness meshes were used to quantify and visualize the kinetics of hydrate formation. The evolution of the hydrate morphology was investigated, and the time-dependent kinetic parameters were obtained, including the hydrate shell thickness and inner and outer diameters and the effective volume fraction of hydrate particles. The results indicate that the formation of gas hydrates undergoes an initial reaction-controlled stage followed by a mass-transfer-limited growth stage. For hydrate formation from a water droplet with an initial diameter of 1.66 mm, the hydrate shell thickness was approximately 30 μm and the effective volume fraction of hydrate particles was approximately 11% at 12 h after hydrate formation began. The standard deviation of the shell thickness, which indicates the surface roughness of the hydrate shell, increased with time for hydrate formation from water droplets. The results presented in this study renew our knowledge on the kinetics of hydrate formation, which is essential for their use in flow assurance and other hydrate-related technologies. [ABSTRACT FROM AUTHOR]

• Saturates, aromatics, and resins were extracted from the crude oil from the SCS. • Saturates accelerated the rates of both hydrate nucleation and growth. • The more polar aromatics and especially resins retarded hydrate formation kinetics. Gas hydrate has been a major problem for flow assurance due to its potential to cause pipelines clogging inside subsea multiphase transportation systems. Of special interest is that polar components in crude oil should play a significant role in the hydrate formation kinetics, but the detailed understanding of their interaction remains unclear. Here, the individual component in the crude oil collected from the South China Sea was for the first time separated through the saturates, aromatics, resins, and asphaltenes (SARA) fractionation method. The promotion effect of saturates on hydrate nucleation and growth, and the surprising inhibition effect of aromatics and resins were discovered. Specifically, the average induction time was shortened by 45.13 % with the addition of the saturates; while the aromatics and resins would prolong the induction time by 2.20 and 20.83 times, respectively. Results also indicated that gas flow direction determined the behavior of hydrate growth; hydrates preferentially nucleated at the cell walls with a higher subcooling. It was further illustrated that the average growth rate of hydrate films was enhanced by 32.65 % with the addition of the saturates; while it was reduced by 54.01 % and 84.52 % with aromatics and resins, respectively. Our findings provided the first knowledge on the effect of single polar component in crude oil on hydrate formation kinetics; natural fraction such as resins was also suggested for a potential application in preventing hydrates plugging in pipelines. [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]

• 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]