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[Display omitted] • Various water saturations simulate Classes I and II hydrate deposits in nature. • Ambient temperature controls periphery dissociation in excess-gas hydrate deposits. • Sensible heat dominates spatially uniform dissociation in excess-water deposits. • Secondary hydrate formation are prevented by adjusted gas production pressure. • Optimized depressurization is crucial to improve water production and CH 4 recovery. Depressurization is considered as the most promising technique for hydrate exploitation, as it achieves the highest energy profit ratio and is the most technologically feasible. However, the exploitation of excess-water hydrate accumulations generates high water production, leading to increased cost, poor energy efficiency, and problems with sand during operation. Thus, water management is crucial to gas recovery by the depressurization of different classes of hydrate accumulations, yet relevant studies remain limited. In this study, synthetic hydrate samples were prepared to simulate two types of natural methane hydrate sediments: Class 1 accumulations (excess-gas hydrate) and Class 2 accumulations (excess-water hydrate). Hydrate dissociation was conducted using a variety of depressurization approaches, and MRI imaging was employed to characterize water performance and methane recovery. Methane hydrate preferentially dissociated along the peripheries of the excess-gas samples due to more efficient heat dissipation. Methane hydrate dissociated more uniformly in the excess-water samples because the high specific heat capacity of water enabled the supply of extra heat. Furthermore, pressure histories, mean intensity change in MRI images, and water variations were monitored to analyze the characteristics of hydrate dissociation, changes in porosity, intrinsic permeability and reservoir heat, water and gas production rates, and possible secondary hydrate formation. The results of this study suggest that an optimized depressurization approach, such as stepwise depressurization, could improve methane recovery from Class 1 and Class 2 methane hydrate accumulations. [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]

• CBCMs significantly improved the hydrate formation kinetics and storage capacity. • Carbonyl groups was identified as the active sites enhancing CH 4 hydrate formation. • Functional groups tuned hydrogen-bonding network results in the enhanced kinetics. The sluggish formation kinetics is a formidable challenge for the practical application of gas hydrate-based technologies for natural gas storage and transportation. Methane hydrates in both nature and labs exclusively form with the assistant of foreign additives via a heterogeneous process. Although the process is common and widely used, it remains unclear what makes a material a good methane hydrate promoter. Additionally, the molecular mechanisms underlying the promoted hydrate formation remain largely unknown. Herein, carbon monoliths (labeled as CBCM) with finely controlled surface functional groups were produced using cellulose as precursors. The oxygen-containing groups have been identified as the active sites enhancing the nucleating ability of methane hydrates. Carbonyl oxygen is pinned down as the most effective functional group in reducing the induction time and enhancing the formation kinetics of methane hydrate. The turned hydrogen bonds between water molecules, which are close to the surface of CBCM, contribute to the enhanced formation kinetics as confirmed by the Raman spectroscopy. The CBCM with optimized carbonyl oxygen significantly improve the methane hydrate formation kinetics and storage capacity with outstanding cycle stability, which are superior than most of the previously reported promoters, particularly in shorter induction time. The finding in this work paves the way for effectively designing promoters and unraveling the underlying mechanism for enhancing gas hydrate formation. [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]

Schematic diagram for N 2 /CO 2 gas enrichment process during hydrate growth process. Blue sticks and spheres: N 2 in the gas phase; yellow spheres: CO 2 in the gas phase or hydrate phase; red sticks: hydrate seeds. [Display omitted] • Proper subcooling can improve the hydrate growth rate and final occupancy of CO 2 molecules. • Mass transfer process could impact the hydrate growth rate, with a higher pressure improving the concentrations of N 2 and CO 2 in the free water. • Higher pressure decreased the selectivity of CO 2 for 51262, and lower subcooling assisted CO 2 in entering 51262 cages easily. The climate system is changing and becoming warmer due to the consumption of fossil energy, which results in a large quantity of CO 2 emitted into the atmosphere. The hydrate formation process can encapsulate guest molecules in hydrate cages, enabling a promising storage of CO 2 in the form of hydrate under the seafloor. Here, we performed molecular dynamics (MD) simulations to validate the hydrate-based gas storage method for low-concentration flue gas. The direct phase method was applied at four target pressures to obtain the melting temperature of mixed gas hydrate in the presence of SI hydrate seeds that are fully occupied by CO 2 molecules. Hydrate growth simulations showed that a proper subcooling can improve the hydrate growth rate and final occupancy of CO 2 molecules in newly formed hydrate cages. The mass transfer process could impact the hydrate growth rate, with a higher pressure improving the concentrations of N 2 and CO 2 in the free water. This enhanced the hydrate growth at both 260 K and low subcooling temperatures. The influence of pressure and subcooling on cage selectivity presented a different tendency for CO 2 and N 2. A higher pressure decreased the selectivity of CO 2 for 51262, and lower subcooling assisted CO 2 in entering 51262 cages easily. Interestingly, the CO 2 occupancy at 260 K is larger than that of the simulations at the melting temperature. The results could be of help in revealing the behavior of case filling during hydrate formation from a gas mixture. [ABSTRACT FROM AUTHOR]

Figure TOC Comparison of methane hydrate nucleation process (a) without or (b) with EG molecules. Critical nucleus size increase under the influence of Thermodynamic hydrate inhibitors. [Display omitted] • Hydrophobic groups from the alcohols could approach the initial hydrate cages and destroy initial clusters. • The –OH groups were observed to form hydrogen bonds with the cage clusters and even initiate a new hydrate cage. • Alcohols can increase the critical nucleus size of gas hydrate. Thermodynamic inhibitors are intensively used in oil and gas pipelines to modify the formation conditions of gas hydrate, preventing the blockage of pipelines. However, their kinetic performance in hydrate nucleation is largely unclear. Herein, the molecular interactions of the small molecules of ethylene glycol and methanol with hydrate clusters were investigated. It was found that the hydrophobic groups from the glycols could approach the initial hydrate cages, thereby constricting the regional distribution of adjacent water molecules; thus, they functioned in the same way as the guest molecules that transiently helped to stabilize the water framework. However, these glycols would eventually dissolve into the solution to stay at the water/nanobubble interface; this would consequently destroy the initial cages upon losing the constraint from the surrounding hydrophobic molecules. In addition, the –OH groups were observed to form hydrogen bonds with the cage clusters and even initiate a new hydrate cage; however, they cannot stably reside there due to different atom coordination. Notably, these glycols can increase the critical nucleus size, which is considered the intrinsic mechanism of thermodynamic inhibition. Our results provide atomistic insights into the performance of glycols in the kinetic nucleation of gas hydrates and can help understand the interactions between guest and host molecules in the presence of hydrophobic and hydrophilic functional groups. [ABSTRACT FROM AUTHOR]

[Display omitted] • A magnetically recoverable nanoparticle inhibitor was synthesized. • The induction time with inhibitors was 3 times longer than pure water case. • The inhibitors exhibit good cycling performance and magnetic separation efficiency. • The inhibition mechanism based on in-situ Raman was proposed. Current schemes dealing with hydrate blockage problems in gas pipelines suffer a high dosage of inhibitors and thus are associated with a significant risk of environmental damage. Overcoming these issues requires the development of novel recoverable inhibitors with persistent cycle performance. In this work, a new approach involving coating poly(vinylcaprolactam) (PVCap) and poly(vinylpyrrolidone) (PVP) onto the surface of γ-methacryloxypropyltrimethoxy silane (MPS)-modified Fe 3 O 4 nanoparticles was proposed; this procedure enables the magnetic recovery of the inhibitory particles. It was found that the onset time of hydrate formation was extended 3 times compared to pure water with the help of the synthesized inhibitors showing better performance than commercial inhibitors. Excellent magnetic separation from sandy matrices in solution was achieved with a recovery efficiency of 86%. This was followed by a good cycle performance: the induction time remained still over 2 times longer than the pure water case after 5 cycles of recovery. The in-situ Raman spectra revealed that the recyclable inhibitors functioned through disturbing the construction of the large cages; a resulting mechanism was proposed with nanoparticles isolating the early hydrate patches preventing their further explosive bulk growth. In addition, the simple synthesis method makes it possible to coat various polymer inhibitors onto recovery particles. The results indicate that magnetically recovering the inhibitory particles and reusing them to hinder hydrate generation could be an alternative method to tackle the environmental and economic problems associated with current flow assurance procedures. [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]

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]