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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]

Natural resins extracted from crude oil in the South China Sea have a significant inhibition effect on hydrate nucleation and growth due to the polar functional group of resins. [Display omitted] • Resins were extracted from the crude oil from the SCS. • The resins contained many polar heteroatomic functional groups. • Hydrate grew along gas flow in the microfluidic channels. • The resins could retard the hydrate formation kinetics. • The more polar resins showed a stronger inhibitory effect. Hydrate plugging in oil and gas transport pipelines is a crucial challenge to the safety of flow and maintenance of pipelines. Specifically, the resins, the strongest polar component in crude oil would significantly affect hydrate kinetics but the mechanism of the interaction remains unclear. Here, a microfluidic chip was used to monitor the hydrate formation under gas flow in-situ , with and without resins extracted from the crude oil. FTIR–ATR and NMR results indicate the presence of a large number of polar components in the resins. High-resolution mass spectrometry showed that two resins contain a variety of heteroatom compounds and sulfur-containing heteroatom. The results indicated that these resins played a surprisingly inhibitory role in the hydrate formation kinetics; this was ascribed to the presence of the polar functional groups. The results also showed that the hydrate grew along the direction of gas flow. Notably, the presence of resins could prolong the induction time and lower the growth rate; this effect could be enhanced with the addition of resins containing more heteroatoms. This study filled the gap in the effect of natural resins on the hydrate formation kinetics and provided guidance on preventing hydrate plugging in oil and gas transport pipelines. [ABSTRACT FROM AUTHOR]

• Metabolic molecules significantly promote hydrate formation. • Protein macromolecules could act as kinetic inhibitor of hydrate nucleation. • Water in the macropores is preferentially consumed during hydrate formation. Numerous efforts have been made to mimic the natural marine environments to understand the behavior of hydrate formation and accumulation in the sediments under seafloor. Yet it is found that the role of organic matters in the kinetics of hydrate formation is largely veiled, which should be much enriched as a result of the activities of the marine ecosystem. Therefore in this study, the kinetic effects of protein macromolecules and metabolic small molecules on CH 4 hydrate formation were examined; the organic compounds were extracted from the samples gathered from the hydrate reservoir located in the South China Sea. A total of 2058 species classified into 10 types of protein macromolecules and metabolic molecules were identified. Low-field nuclear magnetic resonance technique was used to in-situ monitor the hydrate formation process. The results showed that the metabolic molecules could remarkably promote hydrate formation, with more water converted into hydrate within a shorter time. Proteins have been widely studied to inhibit hydrate formation; here it is for the first time confirmed that the proteins extracted from the natural samples drilled in the hydrate reservoir could act as kinetic inhibitor, lowering the possibility of hydrate nucleation. It was further indicated that hydrate formation majorly consumed the water in the macro pores; this was followed by a diffusion-limitation process with the hydrate shell acting as mass transfer barrier slowing down the water conversion. The water in the micropores was yet found to be very difficult to participate in the reaction, potentially resulting from the unavailable gas-water contact. The findings could provide insights into the kinetics of hydrate formation in the presence of organic matters abundant in natural sediments and expand the current understandings on the occurrence of natural gas hydrate under marine environments. [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]

Schematic diagram for nanobubble formation in different hydrate dissociation stage. Methane supersaturation in water is the Prerequisite for nanobubble formation. [Display omitted] • Hydrate dissociation releases plentiful methane molecules to form gas supersaturation. • Closer nanobubbles coalesce to reduce system energy with one nanobubble surviving. • Nanobubble formation breaks mass transfer limitation, improveing dissociation rate. The formation condition of nanobubbles and its effect on the methane hydrate dissociation were studied using molecular dynamics (MD) simulations. To investigate the effect of liquid water proportion on the methane hydrate dissociation path and nanobubble formation conditions, two different initial configurations were built. Considering low liquid water proportion simulation, four main dissociation stages were identified, and nanobubbles formed when the methane supersaturation condition was met. During this period, hydrate dissociation rate decreases and fluctuates around zero, indicating that mass transfer limitation forms and hydrate cages undergo a long-term disappearance and rebuilding process. Nanobubbles can form in two distinct regions: the liquid water region and the final hydrate slice region. Hydrate dissociation rate increased after the first nanobubble formed, which broke the mass transfer limitation. Small nanobubbles formed at the end of the hydrate dissociation process also contributed to the final hydrate slice collapsing by shortening the diffusion distance of methane molecules to the gas phase. At the end of the simulation, only one nanobubble survived in the system with a mole percent of methane in water for two systems remaining at 0.4 and 0.9, respectively. [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] • 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]