Search

• Inhibitory function in oil system is studied under pseudo multiphase flow. • Hydrate conversion divides in stages of entrainment, growth, aggregation, and plug. • Pectin has inhibitory effect on hydrate growth rather than crystal nucleation. • Excellent inhibition behavior of PVCap is verified under multiphase flow. With the successful application of kinetic hydrate inhibitors (KHIs) on hydrate plug prevention, there is a growing trend towards the development of affordable and environmentally friendly KHIs for solving challenges of high cost and inadequate biodegradability from their commercial KHIs. Therefore, further evaluation on the inhibitory effect of the developing inhibitors is needed under multiphase flow conditions. In this study, a high-pressure, fully visible rocking cell were used to simulate the multiphase transportation and both commercial KHIs and environmentally friendly KHIs were considered. Under the flow conditions of 60 % liquid loading and 20 % water cut, the results demonstrated that PVCap showed excellent inhibition performance with no hydrate formation at concentrations of 0.5 wt%, 0.2 wt% and 0.1 wt%. Although pectin exhibited certain inhibitory effect on hydrate growth rate and formation quantity, but it demonstrated a strong promoting effect on hydrate nucleation opportunities and rates. However, sodium alginate, which shares similar hydrate inhibition characteristics with pectin, exerted significantly promotional effects in terms of induction time, growth rate, and water conversion. Furthermore, the addition of pectin and sodium alginate led to the formation of hydrate balls in several test runs, resulting in complete pipeline blockage. Another noteworthy discovery was the relationship between the growth rate and final water conversion of hydrates, which exhibited an initial increase followed by a decrease, while the induction time exhibits an inverse trend, as the flow velocity increased. [ABSTRACT FROM AUTHOR]

With the increasing energy shortage and global warming, the oil/gas development and CO2 sequestration are moving toward the deep sea, and such a geological environment is conducive to gas hydrate formation. At present, for the gas solubility of a hydrate solution system, only Duan's simulation data are widely accepted, and a systematic experimental study is absent. The conventional measurement instruments for solubility of dissolved gas lack control of hydrate phase change, detailed regulation of temperature and pressure, and liquid–solid separation of sampling analysis. This paper describes the working principle, design, and use of a novel apparatus that can measure gas solubility in the solution system in the presence of hydrate. The application of constant pressure equipment avoids disturbing the phase equilibrium and dissolution equilibrium of the system in the sampling process. The apparatus is attractive for the continuous measurement of gas solubility and the guarantee of high accuracy. In addition, an isobaric method is proposed for gas solubility measurement, which promotes the measurement system to reach the target equilibrium state quickly and obtains highly regular data of gas solubility under environmental conditions. The experimental data obtained by this work are highly consistent with the Duan model, and the relative errors of measurements are within 2%. Gas solubility data from this apparatus will provide theoretical support for estimation of the marine CO2 sequestration capacity and prevention of hydrate blockage in oil/gas transportation. [ABSTRACT FROM AUTHOR]

Extensive efforts have been made regarding gas hydrate sample reconstruction in the laboratory for a better understanding and development of natural gas resources. Magnetic resonance imaging (MRI) is a useful method for directly observing the reconstruction of methane hydrate, yet relevant studies remain limited. In this study, a 9.4-T 400-MHz MRI instrument was employed to investigate CH 4 hydrate formation in porous media involving various initial water saturation levels and sand diameters. Pressure histories and MRI signal variations were monitored to discuss the process of CH 4 hydrate growth, and the three main formation stages of induction, rapid growth, and slow formation were determined. Furthermore, the liquid water performance in MRI micro-images was analyzed to predict the characteristics of CH 4 hydrate formation. The results indicated that CH 4 hydrate formed in a spatially and temporally random manner and that pore plugging occurred owing to the residual water encased in grown hydrate. Additionally, phase saturations, water conversion percentages, and formation rates were defined to evaluate the effect of sand diameter and initial water saturation on CH 4 hydrate formation. With the reduction in the diameter of quartz glass beads from 400 μm to 100 μm, the average hydrate formation rate increased from 0.0010 min−1 to 0.0034 min−1, respectively. When the initial water saturation decreased to the optimized value (0.22 in this study), the water conversion percentage and hydrate saturation increased. [ABSTRACT FROM AUTHOR]

An understanding of the nucleation and growth mechanism of methane hydrate in porous space is essential for exploitation and application of hydrates, but the mechanism is yet to be clarified. Magnetic resonance imaging (MRI) was employed to visually analyze the spatial and temporal formation behavior of methane hydrate in a porous media. Detailed information about the water distribution, initial nucleation sites, and hydrate growth was obtained, in addition to MRI images. The results demonstrated that the water molecules distributed in the vertical direction preferred the middle slice of a porous medium sample, and the decrease in the number of molecules in the middle slice and on both sides of the slice was similar during hydrate formation. The formation process are quite different in selected horizontal slices, which were contributed to the various distribution of water and gas in pore spaces and the randomness of methane hydrate formation. The extension of these predicted results could have important implications for optimizing the formation processes of gas hydrate in hydrate-based technologies. [ABSTRACT FROM AUTHOR]

The replacement of CH 4 by CO 2 in methane hydrates is a promising method for simultaneously achieving CO 2 storage and CH 4 recovery for global warming mitigation and energy production, respectively. However, gas replacement is restricted to the slow diffusion-limited transport of CO 2 caused by the formation of a mixed hydrate layer, and little attention has been paid to the storage of CO 2 . Therefore, this study proposed a combination of CH 4 /CO 2 replacement and thermal stimulation to enhance CH 4 recovery and CO 2 storage. The effects of the methane hydrate saturation level, replacement zone, and freezing point on the replacement were analyzed. The CH 4 replacement percentage and energy efficiency were obtained and compared using the replacement and combined methods. The results suggested that the combined method effectively improved CH 4 recovery, with the CH 4 replacement percentage exhibiting an upper limit of 64.63%. Moreover, In CH 4 /CO 2 replacement, the total number of moles of CO 2 stored is unequal to CH 4 recovered, because the replacement is sensitive to the free water in the pores of the hydrate sediments. In addition, the CO 2 storage efficiency was first discussed. The results proved that the CH 4 /CO 2 replacement has obvious advantages in CO 2 storage, and a maximum CO 2 storage efficiency of 96.73% was achieved by combined method. [ABSTRACT FROM AUTHOR]

Carbon dioxide (CO 2) is a dominant greenhouse gas in the atmosphere that contributes to global warming. A promising approach to mitigate CO 2 emissions is CO 2 capture and storage (CCS) through clathrate hydrate crystallization under the seafloor; however, numerous issues regarding the mechanisms of concentration, efficiency, and stability of CO 2 hydrate sequestration in seafloor sediments remain under dispute. This study employed low-field nuclear magnetic resonance (NMR) measurements to observe the in-situ formation of CO 2 hydrate using the pressure oscillation method in porous media and evaluate the carbon sequestration efficiency. Our results indicate that CO 2 hydrates are preferentially formed in large pore spaces, further hindering the subsequent gas contact with water in isolated pores. Additionally, a high initial water saturation is more conducive to high-quantity CO 2 hydrate capture and sequestration in a pressure variation environment with a higher driving force. The proposed pressure oscillation method could effectively break the mass transfer barriers in the later stage of hydrate formation with the help of CO 2 solubility fluctuations, significantly increase the rate of later hydrate formation, and shorten the period of hydrate sequestration. [Display omitted] • Enhanced CO 2 sequestration method is proposed based on hydrate technology. • CO 2 hydrate formation characteristics are obtained by T 2 logmean time distribution analysis. • Pressure oscillation can effectively break the mass transfer barriers. • Pressure oscillation can significantly shorten the period of hydrate sequestration. [ABSTRACT FROM AUTHOR]

Depressurization is considered to be the most promising method for exploitation of natural gas hydrate. To analyze the characteristics of hydrate dissociation during depressurization, methane hydrate (MH) dissociation was performed at different depressurizing ranges and rates, and the hydrate dissociation process was directly observed using magnetic resonance imaging (MRI). The experimental results indicate that with increased depressurizing rate from 0.01 MPa/min to 0.1 MPa/min, the average dissociation rate increases for a given depressurizing range. Meanwhile, with an increase in depressurizing range from 0.3 MPa to 1.1 MPa, the average dissociation rate increases for a given depressurizing rate. Moreover, the hydrate dissociation process can be divided into two main stages: hydrate saturation remains constant with little fluctuation for several minutes after back-pressure decreases, and then the hydrate dissociates continuously until dissociation completes. In addition, excessively high depressurizing range and rate result in hydrate reformation and ice generation, which slow the rate of hydrate dissociation. Furthermore, it was also determined that MH reformation and ice generation always occur at the higher depressurizing range and rate due to insufficient heat transfer. [ABSTRACT FROM AUTHOR]

[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]

The optimization of pore structure for metal foam is considered a feasible approach for improving the overall heat transfer performance. Thus, we numerically investigated Kelvin cells with different throat areas and structures (elliptical Kelvin cell (EKC)) to characterize the influence on pressure drop and heat transfer coefficient using FLUENT 18.0. The standard k – ε model exhibited a better agreement with experimental data and required less time to achieve convergence. The results revealed that the throat area could not feasibly optimize the overall heat transfer performance. Moreover, the area goodness factor j / f that considered the influences of both heat transfer coefficient and pressure drop on the overall heat transfer performance of EKC with the higher than conventional Kelvin cell. Based on comparative analysis between pressure, velocity, turbulence kinetic energy, and temperature distribution, increasing the space and decreasing the angle between the skeleton and flow direction caused a significant pressure drop in the EKC samples. Owing to the existence of a lower temperature area at the leeward of skeletons and a small difference of impingement cooling on windward skeletons, the reduction of HTC was acceptable. Therefore, the EKC exhibited immense potential for enhancing the design of heat transfer devices. • Kelvin cells with varying long and short throat diagonal and axes ratios. • Pressure drop and heat transfer coefficient were investigated comprehensively. • Pressure, velocity, and turbulence kinetic energy distributions were compared. • Increasing the elliptical axes ratio can optimize the overall heat transfer. [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]

• Capillary pressures in different directions are presented in this study. • Hydrate formation makes the skewness of capillary pressure finer slanting degrees. • The heterogeneity in hydrate formation determines capillarity in anisotropy. • The stronger capillary heterogeneity occurs in the vertical direction. The random nucleation and growth of hydrates in the pore space contribute to the heterogeneity of hydrate sediments, which results in anisotropic fluid flow. An exact forecast for capillary pressure in different directions, which governs the gas/water distribution and the volume of residual water saturation in porous sediments, is of significant importance for hydrate exploitation. In this study, the index properties of the artificial hydrate sediment were characterized through X-ray computed tomography (CT) visualization scanning. The spatial structure feature of hydrate sediment was captured via a topological pore network, which was established from CT imaging. Then, the evolution of capillary pressure in anisotropy with the formation of hydrates was simulated and discussed. The results indicated that with hydrate formation, the capillary pressure curves became steeper, representing skewness with finer slanting degrees and poor sorting. Moreover, the differences in capillary pressure with the same hydrate saturation between different directions was enlarged. Stronger capillary heterogeneity in the vertical direction represents greater heterogeneity, according to the Leverett J-Function. [ABSTRACT FROM AUTHOR]

• Organic exclusions with hydrate growth were firstly simulated and in-situ observed. • Progressive organic exclusion caused substance migration and concentration gradient. • Higher driving formation force generated the precipitation of excluded organics. Inorganic substances and most organic compounds are typically excluded from crystalline hydrate structures. In this study, the phenomenon and mechanism of organic exclusions during hydrate formation were investigated using MD simulations and in-situ observations with an MRI experimental apparatus. The results showed that the organic macromolecules could not be trapped in the hydrate structure and formed a concentration gradient of dissolved organic substances from the growth front of the CO 2 hydrate into the aqueous solution. This study demonstrates the hydrate-based technology would be a promising method for organic wastewater treatment. [ABSTRACT FROM AUTHOR]

Natural gas hydrate is recognized as an ideal substitute for traditional energy resources. Gas production from hydrate reservoirs is accompanied by the phase transition of solid hydrate, which inevitably results in changes to the microstructure of hydrate sediments. Therefore, the interaction mechanism between the microstructure and permeation characteristics of hydrate sediments must be investigated, especially permeability anisotropy. In this study, we assess the variations in permeability anisotropy during krypton hydrate formation using a pore network model and X-ray computed tomography. The results showed that absolute permeability in the vertical (k v) and horizontal (k h) directions both decreased throughout the formation process, but that the decline in k v was steeper, thus resulting in a sharp increase in the degree of permeability anisotropy (k h /k v). The relative permeability to gas was always higher in the horizontal direction than of that in the vertical direction, whereas the relative permeability to water was the opposite. Moreover, the difference in the relative permeability to water and gas between the two directions increased with the increasing hydrate saturation (S h). When extrapolating to the field-scale for hydrate field trials, we consider that horizontal wells provide a better option for gas production, especially for hydrate reservoirs with a higher hydrate saturation. • Prediction of permeability anisotropy during hydrate formation is presented. • Hydrate formation increases the degree of permeability anisotropy (k h /k v). • Difference in gas/water flow between two directions increases with high S h. • Horizontal wells are found to be a better option for gas hydrate production. [ABSTRACT FROM AUTHOR]

Permeability in hydrate-bearing sediment critically governs fluid flow and determines hydrate nucleation, growth, and distribution, making it important to characterize the evolution of permeability with respect to water and gas during hydrate formation. This study uses CT scanning of krypton hydrate formation in silica sand using the excess gas method, together with a pore network model, to investigate variations in hydrate morphology and associated permeation. The results show that during hydrate formation, the growth habit mainly varies from grain-coating to patchy with an increase in hydrate saturation; however, at relatively low saturation, hydrate preferentially grows as a grain-cementing habit. Theoretical models of a capillary tube and of a Kozeny grain both predict that permeability in grain-coating hydrates will be higher than in patchy hydrates. We thus recognise a correlation of permeability and hydrate saturation for multiple growth habits that is of interest for gas production from hydrate reservoirs. Under lower water saturation, there is a decrease in relative permeability to water but an increase in relative permeability to gas, due to the reduction in pore shape factors. Conversely, an upward shift in relative permeability to water and a downward shift in relative permeability to gas is found with hydrate formation under higher water saturation, owing to the Jamin effect. Steeper curves of relative permeability to gas and water with increasing hydrate saturation resulted in a reduction of the co-cementation zone for water and gas, even if or though no gas migrates. The results suggest that, due to the low relative permeability of gas, higher water saturation sediments result in an excess water yield accompanied by low gas production that is not desirable for natural gas hydrate production. Therefore, improving the permeability and weakening the Jamin effect are critical for gas production in marine hydrate reservoirs, especially in low permeability sediments. • CT scanning of Kr hydrate growth is used as input to modeling of multi-phase flow properties in hydrate bearing sediments • Hydrate pore habit shifts from coating to cementing as hydrate saturation increases. • Under lower S w , hydrate formation inhibits water flow due to reduced shape factor. • Under higher S w , hydrate growth impedes gas migration in fine pores due to the Jamin effect. [ABSTRACT FROM AUTHOR]

Kinetic enhancement for capturing and storing harmful gases into hydrates simultaneously to achieve low energy penalties and environmental mitigations. • Kinetic surfactants accelerate gas captures in pore spaces. • Oscillated pressure controls are first proposed to enhance CO 2 gas captures. • Enhanced gas capture processes change microstructural features. • The hydrate technology is a safely long-term storages of harmful gases. The use of hydrate-based technology for gas capture and storage is highly attractive for environmental mitigation, as it entails low energy penalties and provides gas storage density maximization and long-term storage stability. Although this method has been investigated in extensive researches, its development is restricted by the obscure underlying gas capture micro-mechanisms, elusive micro-structures of stored forms, and insufficient hydrate film growth rates. In this study, the Magnetic Resonance Imaging technique was employed to analyze the hydrate growth micro-processes for greenhouse gas (imitated by CO 2 , CH 4 , and various fractions of CO 2 -CH 4 mixed gases) and volatile organic compound (simulated by C 2 H 4 and C 2 H 2 gases) capture and storage. The hydrate film growth was enhanced with the addition of 288 ppm sodium dodecyl sulfate (SDS), which significantly improved the hydrate growth in the cases of hydrocarbon gases, but not CO 2 gas due to the competing adsorption of bicarbonate and dodecyl sulfate ions. With SDS, hydrocarbon gas hydrates grew via the patchy model at 65–105 mm/s, and 65–95% liquid water was converted into hydrates for gas capture and storage. However, only about 1.4% water was converted into CO 2 hydrates with SDS, at 10.4 mm/s. Thus, a multi-pressure control mechanism for secondary hydrate growth was developed to promote CO 2 capture and storage, based on a large amount of dissolved CO 2 gas compared to the other investigated gases. The enhanced CO 2 capture has important implications for the optimized harmful gas sequestration, due to preferentially patchy hydrate morphologies and associated impacts on permeability. [ABSTRACT FROM AUTHOR]

Hydrate‐saturation‐dependent permeability in sediments is largely affected by the pore morphology of hydrate. Hydrate pore morphology evolves from a single pore habit, which depends on its formation conditions, toward a sophisticated hybrid habit with increased hydrate saturation. Resulted permeability reduction curves in hydrate‐bearing sediments are diverse and lack universal models. By using a two‐parameter logistic function to link microscale hydrate pore habit evolution with macroscale permeability variations, this study establishes a pore‐morphology‐weighted permeability model that well captures the permeability evolution in hydrate‐bearing sediments at various conditions. The universality of this model is validated and the values of the two parameters in the model are calibrated using published laboratory and pressure core data. This newly proposed model offers a mechanistic understanding of the permeability in hydrate‐bearing sediments and an elegant expression that can be implemented in reservoir simulators for field‐scale gas production estimation. Plain Language Summary: Permeability of hydrate‐bearing sediments governs the gas and water flow, and thus plays important roles in many energy, environment, and climate related processes including the formation and evolution of hydrate deposits, the recovery of hydrate resources, and the release of methane from hydrates into the ocean and the atmosphere. Existing models of permeability in hydrate‐bearing sediments assume single and simplified hydrate pore morphology, and thus, fail to capture the dynamic evolution of permeability in sediments with changing hydrate saturation. With recent advances of direct observations of hydrate crystallization in sediments using microfocus X‐ray computed tomography, much has been learned about the dynamic evolution of hydrate morphology in sediment pores, based on which we established a physical model to quantitatively correlate the dynamic evolution of hybrid hydrate pore morphology to the permeability variation in sediments in this study. The robustness and universality of this new model are validated using published data from both laboratory synthesized and naturally occurred hydrate‐bearing samples. The results of this study enhance the physical understandings of flow in hydrate‐bearing sediments and shed light to hydrate reservoir evolution. Key Points: The evolution of hydrate pore morphology depends on hydrate formation conditions, exerting diverse controls over permeabilityThe relation between permeability and the saturation and pore morphology of hydrate is quantitatively described and analyzedThe new permeability model is universal and can capture measurements under excess‐gas, excess‐water, and dissolved‐gas conditions [ABSTRACT FROM AUTHOR]

Heavy metal pollution is an urgent challenge worldwide due to the acceleration of industrialization. While adsorption desalination is regarded as an innovative method for wastewater treatment, the current technologies have been impeded by high costs and intensive energy consumption. In this work, a novel low-temperature evaporation wastewater treatment apparatus based on hydrate adsorption was proposed. The water vapor from wastewater evaporation reacted with CO2 to form hydrate under the pressure of 3.3 MPa, constantly promoting wastewater evaporation due to the consumption of water vapor. The effect of feeding concentration on treatment effect was analyzed in terms of removal efficiency, water yield, and enrichment factor. Remarkably, a maximum removal efficiency of 97.4% can be achieved by treating an artificial solution with a Cu2+ concentration of 500 mg/L. Furthermore, compared with the control group that only depended on evaporation and condensation without forming hydrate, the maximum water yield of purified water in the experimental group increased to 310%. This innovative design concept for a low-temperature wastewater treatment apparatus based on hydrate adsorption presents a promising solution for the green and energy-efficient treatment of heavy metal wastewater. [ABSTRACT FROM AUTHOR]

[Display omitted] Carbon dioxide (CO 2) reduction is an urgent challenge worldwide due to the dramatically increased CO 2 concentration and concomitant environmental problems. Geological CO 2 storage in gas hydrate in marine sediment is a promising and attractive way to mitigate CO 2 emissions owning to its huge storage capability and safety. However, the sluggish kinetics and unclear enhancing mechanisms of CO 2 hydrate formation limit the practical application of hydrate-based CO 2 storage technologies. Here, we used vermiculite nanoflakes (VMNs) and methionine (Met) to investigate the synergistic promotion of natural clay surface and organic matter on CO 2 hydrate formation kinetics. Induction time and t 90 in VMNs dispersion with Met were shorter by one to two orders of magnitude than Met solution and VMNs dispersion. Besides, CO 2 hydrate formation kinetics showed significant concentration-dependence on both Met and VMNs. The side chains of Met can promote CO 2 hydrate formation by inducing water molecules to form a clathrate-like structure. However, when Met concentration exceeded 3.0 mg/mL, the critical amount of ammonium ions from dissociated Met distorted the ordered structure of water molecules, inhibiting CO 2 hydrate formation. Negatively charged VMNs can attenuate this inhibition by adsorbing ammonium ions in VMNs dispersion. This work sheds light on the formation mechanism of CO 2 hydrate in the presence of clay and organic matter which are the indispensable constituents of marine sediments, also contributes to the practical application of hydrate-based CO 2 storage technologies. [ABSTRACT FROM AUTHOR]

The separation of azeotropic mixtures has conventionally been one of the most challenging tasks in industrial processes due to the fact that components in the mixture will undergo gas–liquid phase transition at the same time. We proposed a method for separating azeotropes using hydrate formation as a solid–liquid phase transition. The feasibility of hydrate-based separation is determined by analyzing the crystal structure and chemical bonds of hydrate. Taking the azeotrope cyclopentane and neohexane in petroleum as an example, cyclopentane (95%) was purified to 98.56% yield using the proposed hydrate-based cyclopentane purification technology. However, this is difficult to achieve using conventional distillation methods. The proposed method is simple in operation and yields a good separation effect. This study provides a new method for separating cyclopentane and neohexane. [ABSTRACT FROM AUTHOR]

[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]

• CH 4 /CO 2 replacement mechanisms are investigated and compared using MD simulation. • The mass transfer limitation effect of surface mixed hydrate is considered. • Mechanism of cage complete rupture reacts more smoothly than cage incomplete rupture. • The situation of the increased number of surface mixed hydrate layers is studied. The CH 4 /CO 2 replacement method is a promising approach for the exploitation of natural gas hydrates, offering dual benefits of extracting CH 4 and sequestering CO 2. This technique not only facilitates the extraction process but also mitigates the greenhouse effect and maintains geological stability, thereby contributing to sustainable energy utilization and environmental preservation. Currently, there are multiple viewpoints about the replacement mechanism. Furthermore, mixed hydrates can form on the surface during the replacement process. The unclear mechanism of CH 4 /CO 2 replacement and the obstruction of mass transfer caused by the mixed hydrate impede its practical applicability. Clarifying the effects of surface mixed hydrate in replacement mechanisms can offer valuable insights for identifying optimal operating conditions in practical exploitation applications. Therefore, the present study aims to reveal the significant roles of surface mixed hydrates in the replacement mechanism by employing the molecular dynamics simulation methodology. The results showed that under the mechanism of cage complete rupture, the surface mixed hydrate decomposes rapidly to form a thickening water zone. Additionally, each layer of the hydrate cage structure undergoes a process from deformation or partial rupture to sudden complete disintegration, which is concomitant with an abrupt change of potential energy. Under the mechanism of cage incomplete rupture, the surface mixed hydrates hinder mass transfer and decelerate the reaction rate. A similar situation arises when the number of the surface mixed hydrate layers is increased. Therefore, the key to distinguishing the replacement mechanism lies in whether the environmental conditions are sufficient to break the potential energy barrier of the surface mixed hydrate and disrupt its cage structure. [ABSTRACT FROM AUTHOR]

Significant advances in seawater desalination have been made to meet the world's increasing demand for clean water. However, the accompanying hypersaline concentrate discharge has negative environmental impact and is challenging for treatment. Herein, we demonstrate the potential of hydrate-based desalination (HBD) in treating brine with concentrations up to 16 wt%. It is difficult for hydrate nucleation to occur in strong brine without nucleation agents, as this results in lengthy induction time, the challenge can be resolved using carbon materials. We found that crystallinity and surface compositions have critical impact on cyclopentane hydrate formation and desalination efficiency. Carbon material with high crystallinity is more effective in inducing hydrate nucleation in NaCl solution due to the properly tuned interface hydrogen-bonding network as confirmed by Raman spectra. The improved hydrophilic property would degrade the desalting efficiency due to the tightly trapped brine among carbon materials and hydrate crystals. Although high concentration of NaCl would hinder hydrate formation, leading to a lower water recovery, desalting efficiency unexpectedly increases due to interfacial pre-melting in the presence of salt, reaching up to 79% in 16 wt% NaCl. The findings would help to identify and fabricate the good hydrate promoters used for HBD for treating hypersaline brine. • Hydrate-based desalination treats brine with NaCl concentration of 16 wt%. • Crystallinity of carbon has critical impact on enhancing hydrate formation. • The water state tuned by carbon materials results in the enhanced kinetics. [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]

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]

Gas hydrate-based CO 2 storage, using only water cages, provides an environmentally friendly and energy-efficient solution to reduce greenhouse gas emissions. However, the sluggish formation kinetics of gas hydrate is hindered by the limited reaction interfaces, thereby impeding the practical utilization of hydrate-based greenhouse gas storage. Herein, we reported a novel strategy to fabricate natural vermiculite and tannic acid composite aerogels as the effective substrates for boosting CO 2 hydrate formation. The results show that the formation kinetics of CO 2 hydrate in the aerogels exhibits composition dependence and an outstanding CO 2 storage capacity of 130.1 v/v (corresponding to 0.114 mol CO 2 /mol water). Inverse gas chromatography was used to analyze the surface energy and its components of the aerogels. Raman spectra and nuclear magnetic resonance were used to unravel the water molecule assembly and states. The CO 2 uptake capacity can be further improved by compressing the aerogel to 75 % of the original volume, showing an impressive storage capacity of 146.8 v/v. The as-made aerogels demonstrated outstanding cycle stability and short induction time for CO 2 hydrate formation. The results of this study pave the way for designing effective CO 2 hydrate promoters and facilitating promising CO 2 capture and storage technologies via gas hydrate. [Display omitted] • Monolithic aerogel can be easily fabricated by cross-linking vermiculite nanosheets. • The surface compositions and pore structures of aerogels can be finely tuned. • Aerogels have excellent CO 2 storing capacity by boosting hydrate formation. • Hydrogen bond arrangement plays the most vital role in hydrate formation. [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]

The detailed knowledge of the low thermal conductivity of host–guest compounds is essential to improve our fundamental understanding of heat conduction in complex solids and develop high-performance thermoelectric materials. In this Letter, the intrinsic ultralow thermal conductivity (0.44 ± 0.06 W m − 1 K − 1 in 140–190 K) of the tetrahydrofuran (THF) clathrate hydrate is characterized by the time-domain thermoreflectance technique. The underlying heat conduction mechanism is further investigated by non-equilibrium molecular dynamics simulations. We find that trapped THF molecules do harmonic motions and behave as parts of a crystalline structure, thus playing negligible roles in thermal conductivity reduction. The large unit cell and complex cage-like host structure dominate the low thermal conductivity of the THF hydrate. [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]

The utilization and transportation of clean energy require efficient energy storage solutions. Gas hydrate represents a promising way for high-density storage under mild conditions. In particular, hydrate induced by confined space has the advantage of being environmentally friendly with rapid nucleation and high mass transfer efficiency. However, the cost of artificial pore-construction methods has hindered its widespread application. In this study, we report a novel approach of hydrate storage in the -SO 3 − modified flexible balsa wood as a naturally porous material. The surface sulfonate groups were successfully grafted by coupling agents which was verified by various techniques. The material's natural porous hierarchical structure allows for efficient fluid flow in porous media, enabling a reduction in induction time by ∼88% and a storage capacity of up to 150.6 v/v by adjusting the load water amount. The 100 wt% water-loaded wood materials exhibited the highest water conversion efficiency. Moreover, the recoverable mechanical properties make it reusable without performance degradation. The inner pore structure and hydrate morphologies were further investigated by X-ray microscopy to clarify the hydrate growth mechanism. The interconnected pores and channels make the hydrate grow in layers inside. In addition, the performance could be adjusted by simply changing the hydrophobicity to regulate the gas flow which may contribute to the large storage systems. The use of natural biomass porous materials provides an environmentally friendly and economically feasible strategy for gas storage. [Display omitted] • The modified flexible balsa wood was synthesized and applied in energy storage. • Methane hydrate induction time was reduced by ∼88% and the storage capacity was enhanced to 150.6 v /v. • The 400 wt% water loaded case exhibited the best storage performance. • The hydrate started to form in axial parenchyma structures verified by X-ray microscopy. • Tunable storage performance based on hydrophobic and hydrophilic modification. [ABSTRACT FROM AUTHOR]

The vast hydrocarbon resources (oil, gas, and gas hydrate) in deepwater areas are considered one of the most significant resources of the future. However, the dangerous flow patterns and blockage accidents caused by hydrates threaten the safety and efficiency of the transportation process. A mutual interaction exists between hydrate growth and multiphase flow. In this study, hydrate formation and gas–slurry two–phase flow tests were conducted in a pure water system with a constant pressure (4 MPa) and various liquid loadings (40–80%) in a visual flow loop. It was found that the hydrate growth process in the dynamic system included bubble promotion, subcooling promotion, and mass transfer inhibition. Meanwhile, two atypical slug patterns (rapid conversion slug and deposition slug) in the vertical upward flow were determined, which corresponded to the promotion and inhibition regions. In the rapid conversion slug flow, the churn segments replaced the original Taylor bubbles when the liquid loading was less than 60%. In the deposition slug flow, slurry shedding phenomena was observed. In addition, the hydrate particle deposition behavior was found to inhibit hydrate growth and cause flow pattern shifts, but did not trigger the differential pressure surge. The gas isolation caused by the liquid plug and energy dissipation due to the collision between hydrate particles and the tube wall may be responsible for the sudden increase in pressure drop. Furthermore, the feasibility of predicting hydrate blockage using the differential pressure method was explored. The experiment results showed that this warning method was effective when the liquid loading was below 76%. And an effective warning can improve the transport capacity by approximately 34%. • The hydrate formation and flow behaviors are complicated in deepwater risers. • Hydrate formation and multiphase flow behaviors were observed by a visual flow loop. • Three regions of growth kinetics and two atypical slug patterns were determined. • The differential-pressure hydrate blockage warning method was proposed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Maximum energy storage density (57.4 kWh/m3) of hydrate cold storage was obtained. • Increasing the solution concentration boost the cold charge capacity by 2–3 times. • Rapid dilution of dissociated solution was key to enhancing the cold discharge. • Self-regulation of driving force was relied on temperature and concentration. Low energy storage density, intermittent phase changes, and heat transfer barriers have posed significant challenges in the implementation of hydrate energy storage systems. Based on the heterogeneous nucleation mechanism for tetrabutylammonium bromide (TBAB) hydrate phase change energy storage, a novel cold storage system with internally circulating gas disturbance was constructed for energy evaluation and thermodynamic evolution. The hydrate cold storage efficiency, response time, dynamic driving force, unique force pattern of the coils and guest molecule diffusion were analyzed for the first time. The bubbles generated by gas internal circulation provided numerous nucleation sites for hydrate formation, efficiently promoting the energy storage process. The disturbances caused by the bubbles also rapidly transmitted the energy generated by the phase change. A hydrate storage density of 57.4 kWh/m3 for this system was the maximum among known hydrate storage systems. Notably, a change in solution concentration (from 10 wt% to 40 wt%) resulted in a 2–3-fold increase in the cold charge capacity. Rapid dilution of local solutions and rapid release of cold were the ways to promote the cold discharge. Less damage to the stress structure of the coil was due to the loose accumulation of hydrate particles under gas disturbance. The microbubbles generated by the gas disturbance enabled heterogeneous nucleation and heat-mass transfer in the hydrate cold storage. The gas disturbance-based hydrate energy storage process holds significant guiding value for various applications such as refrigerated transport, building cooling systems, and grid peak shaving. [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]

Purification of seawater and wastewater by solar-driven interfacial evaporation is a promising way to alleviate water scarcity. However, the high cost, complex fabrication methods, and short service life of the materials limit their practical application. Herein, a novel three-dimensional (3D) photothermal aerogel composited of vermiculite nanosheets and polyvinyl alcohol (PVA) precursors have been prepared via freeze-drying and treated by quick pyrolysis for highly efficient solar-driven interfacial evaporation. The vermiculite nanosheets exfoliated from natural clay were used as the skeletal support of the composite aerogels, which are cheap and abundant natural materials. The introduced PVA improves the mechanical properties and produces carbon derivatives after pyrolysis, enhancing the light absorption capacity. The as-prepared aerogels have hierarchical porous structures, impressive mechanical properties, great light absorption capacity, and outstanding salt-rejection ability. The optimized aerogel not only exhibits a high evaporation rate of 2.64 kg m−2 h−1 (normalized to top and side surfaces) under 1.0 sun irradiation with excellent cost-effectiveness at 361.64 g h−1 $−1 but also shows superior performance in treating high-salt (15.0 wt%) brine water and organic dye-contained wastewater. The proposed method provides an attractive and effective way to prepare photothermal aerogels for efficient solar steam generation for mitigating water scarcity issues. • 3D clay-based solar evaporators for desalination were made. • Scalable and low-cost PVPA was prepared via a simple and quick pyrolysis strategy. • The as-prepared PVPA shows excellent evaporation performance and reusability. • The PVPA can treat different concentrations of brine and dye-contained wastewater. [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]

The gas production from marine gas hydrate has always been plagued by low productivity; the complex geological environment poses a significant obstacle to its commercialization. Horizontal wells are thus getting increasing attention due to their advantages in facilitating pressure propagation. Here a pre-embedded dual horizontal well was used to probe its effects in the local temperature distribution and gas production behavior. By doubling the number of boreholes, the flow channels of gas were increased enlarging the decomposition zone. Specifically, this was found to raise the reservoir temperature from −1 °C in a single well to approximately 0 °C. The temperature decline was more moderate due to a weakened Joule-Thomson effect. Consequently, almost 90% of the cumulative gas yield was produced before the lowest temperature occurred, compared to ∼30% in the single vertical well case. This indicates that the gas production from a dual well case was proceeding at a relatively higher temperature, potentially benefitting an enhanced gas production efficiency. An enlarged depressurization region was thus suggested in the field test for a controlled temperature decline to make more use of the sensible heat of the reservoir. • Simulation of horizontal wells by the scheme of pre-embedding. • Increase the minimum temperature during gas production by the dual horizontal wells. • Multiple gas flow paths can alleviate the rapid drop in reservoir temperature. [ABSTRACT FROM AUTHOR]

Tetrabutylammonium bromide (TBAB) semi-clathrate hydrate possesses a unique clathrate structure for capturing and sequestering small-molecule gases, such as CH 4 , H 2 and, CO 2 and the advantage of phase change energy storage. Elucidating the diversified reactions and determining the optimal phase change characteristics of TBAB hydrate is crucial to discovering and overcoming specific limitations hindering the large-scale applications. In this study, in situ macroscopic visualization, three-dimensional reconstruction via X-ray Computed Tomography, and differential scanning calorimeter measurements were performed to discover, verify, and analyze the distinct characteristics of TBAB hydrate phase change, including phase separation, three-dimensional structure, and thermal effects. The optimal concentration of TBAB hydrate energy storage was found and verified experimentally for the first time. A peculiar liquid layer (liquid-liquid phase separation) caused by hydrate-solution phase separation was observed after multiple formation-dissociation cycles. The liquid layer separated a region that optimized the hydrate phase change. The solutions with different initial concentrations could evolve an optimized phase-change region of the same concentration (37 wt%) via multiple cycles. The region of the same concentration was shown to be the result of optimal aggregation of molecules. In proving the particularity of the concentration in the optimized region, TBAB hydrate dissociation exhibit thermal effects that can be easily ignored (TBAB molecules ionization and ionic bond synthesis), which affected the TBAB hydrate latent heat measurement. Finally, the optimal concentration of TBAB hydrate phase change was determined as 37 ± 1 wt%, and the characteristics of the concentration, including the volume fraction, complete conversion of water molecules and TBAB molecules, and latent heat, were confirmed by the liquid layer. The experimental results obtained for the first time in this study are important guidance for TBAB hydrate in the field of gas capture and energy storage. [Display omitted] • Solid-liquid phase separation leads to liquid-liquid phase separation. • Guest molecule optimum aggregation hypothesis of memory effect was proposed. • Changes in ionic bonds play an important role in TBAB hydrate phase change. • Hydrate volume fraction could be up 99% by XCT 3D reconstruction. • 37 wt% was considered to be the optimal concentration. [ABSTRACT FROM AUTHOR]

With more and more exploration of the marine environment, a series of underwater monitoring equipment such as underwater vehicles are used more widely. Reducing the operation cost of underwater equipment and improving work efficiency have become the focus of attention. Superhydrophobic coatings have excellent applications in drag reduction, antifouling and other fields due to their unique surface wettability. This paper reports a superhydrophobic polyurea/TiO 2 composite coating with rapid self-healing ability by simple sprinkling or spraying of modified TiO 2 nanoparticles on the brushed polyurea coating. The modified TiO 2 nanoparticles were driven to migrate to the surface of the coating by the synergistic self-healing effect of the disulfide and hydrogen bonds inside the polyurea, contributing to restoring the superhydrophobicity. This superhydrophobic polyurea/TiO 2 composite coating possesses ~11.28 % drag reduction efficiency after being applied to underwater navigation, and it shows ~11.04 times higher water loading capacity than its weight. Moreover, this coating possesses excellent antifouling properties and chemical durability. This work provides new clues for future applications of drag reduction and pollution prevention of underwater exploration equipment. • A fluoride-free self-healing superhydrophobic coating was prepared. • The synergistic action of disulfide bonds and hydrogen bonds achieves self-healing ability. • The coating shows excellent loading capacity and underwater drag reduction efficiency. • The coating shows remarkable liquid repellency and self-cleaning property. [ABSTRACT FROM AUTHOR]

In gas capture/storage and energy storage, tetrabutylammonium bromide (TBAB) hydrates have been regarded as superior carriers owing to their unique molecular selectivity and high latent heat. Hydrate conversion amount is a significant parameter in evaluating gas selection and cold storage characteristics. This study proposes a method for calculating hydrate conversion amount proposed based on the correlation between electrical conductivity and solution concentration. The correlation index (R2) between electrical conductivity and TBAB solution concentration was above 0.99 at concentrations less than below 5 wt%. The conversion amounts of different hydrate types were obtained using a unique hybrid calculation method. The differences in hydrate conversion amount, conversion ratio, and memory effect were compared at different mass fractions. The results indicated that the hydrate morphology and accumulation patterns were influenced by the TBAB solution ionization degree. Regular hexagonal TBAB hydrate crystals were observed at a low ionization degree. The maximum conversion ratio of TBAB hydrate reached 96% (1 °C/30 wt%). The variation trend of TBAB hydrate phase equilibrium temperature was consistent with that of the electrical conductivity of the solution. The investigation of the memory effect revealed that the phase change process of TBAB hydrate is more predictable in high-concentration initial solutions. [Display omitted] • Regular hexagonal hydrate was observed for the first time. • Insufficient ionization of TBAB slowed hydrate conversion. • TBAB hydrate phase equilibrium temperature was consistent with ionization degree. • Temperature, induction factor, and residual solution volume affected hydrate conversion. [ABSTRACT FROM AUTHOR]

Overcoming the gas production efficiency issues from marine gas hydrate reservoirs requires a better understanding of the interactions between the depressurization scheme and hydrate decomposition behavior. In this work, special attentions were paid to the time-varying relationships between the local temperature/pressure conditions and hydrate phase equilibrium. It was found that the routine step-wise depressurization scheme with equal gradient per step will significantly sacrifice the production efficiency. Whereas through dividing the overall production process into two stages according to the evolving boundary and handling them separately, we found that a straightforward depressurization at the free gas release stage would contribute to a maximum of 43.59% increase in the production efficiency. Once hydrate decomposition was intervening, the pressure should be carefully managed to avoid temperature drop problem. This could be improved by carrying out a further step-wise depressurization and regulating the duration of the constant pressure stage. Consequently, a positive dependence was identified between the production efficiency and the phase-equilibrium-based thermodynamic driving force. This could be an indicator to guide the design of the pressure schemes in the field test via simply monitoring the local pressure and temperature conditions to balance between the production efficiency and reservoir temperature. [Display omitted] • The time-varying relationships between the P-T conditions and phase equilibrium was established. • A direct depressurization at the free gas release stage contributed to a 43.59% increase in the production efficiency. • A positive dependence was identified between the production efficiency and the thermodynamic driving force. [ABSTRACT FROM AUTHOR]

[Display omitted] • The max total interaction potential between the montmorillonite particles (358 k b T) was 8.3 times that of the illite particles (43 k b T) in seawater. • The presence of clay particles and especially illite particles could promote CO 2 hydrate formation. • The pore structure would be changed during hydrate formation and dissociation. CO 2 storage in marine sediments in the form of solid hydrates with high storage capacity and stability is a promising approach for the control of greenhouse gas pollution. Specifically, clay is much abundant in marine minerals, and multiple forces exist between fine clay particles suspended in seawater, significantly affecting the hydrate formation and dissociation behavior. This study investigates the phase transition performance of CO 2 hydrates with or without montmorillonite and illite particles using low-field nuclear magnetic resonance. According to the DLVO theory, the total interaction potential between the montmorillonite particles reached 358 k b T, with that between illite particles being only 43 k b T. It was found that water in large pores was preferentially consumed during hydrate formation, with clay particles significantly promoting the conversion of water into hydrate. In seawater-montmorillonite and seawater-illite solutions, a higher number of water molecules was detected in the small and medium pores, respectively. Notably, the initial pore structure changed upon hydrate dissociation, with more medium pores formed. This effect was enhanced in the seawater-montmorillonite solutions and weakened in seawater-illite solutions. The findings could further clarify the formation and dissociation kinetics of CO 2 hydrates in the presence of clay particles, as well as the geological structure evolution and safety of CO 2 hydrate storage under the seafloor. [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]

Compatibility problems are observed during the co-injection of corrosion and gas hydrate inhibitors inside oil and gas pipelines, which reduces their performance. In this study, the newly synthesized dual-purpose inhibitors (DPIs) were developed to overcome the compatibility challenge between the inhibitors. A detailed experimental and computational study was performed to investigate the inhibition activity of DPIs. The results of constant cooling experiments showed that the inhibitors significantly prevented natural gas hydrate formation. DPI2 with a propyl pendant group was the best sample by providing a subcooling temperature of 18.1 °C at 5000 ppm. DPI1 and DPI3 decreased gas consumption by 2.6 and 2.4 times, respectively, compared to pure water. In addition, molecular dynamics simulation revealed that the transportation of gas molecules to the growing hydrate cages was disrupted due to DPI2 adsorption on the surface of the hydrate, which partially covered it and acted as a mass transfer barrier. Furthermore, the interaction of the anion part of the inhibitor with the nearest neighbor water molecules lowered the water activity to form the hydrogen-bonding networks for the hydrate formation. According to corrosion measurements, DPIs suppressed the corrosion rate of mild steel in H 2 S–CO 2 saturated oilfield-produced water, and a maximum inhibition efficiency of 96.3% was obtained by adding 1000 ppm of DPI2. Moreover, the estimated adsorption energy of DPI2 were relatively high and matched with experimental data, implying that the inhibitor has a high degree of adsorption on the metal for forming a protective layer on the mild steel surface. These findings signified that DPIs provide a potential hybrid inhibition of corrosion and gas hydrate formation for flow assurance applications and reduce the operation costs. [Display omitted] • An effective hybrid inhibition of corrosion and gas hydrate formation was developed for flow assurance applications. • DPI2 provided the maximum subcooling temperature of 18.1 °C. • DPI2 significantly inhibited steel corrosion by 96.3% protection. • The hydrogen-bond network of water was distrusted in the presence of the inhibitor. • DPIs were found to serve dual functions as hydrate and corrosion inhibitors. [ABSTRACT FROM AUTHOR]

[Display omitted] • The hydrate blockage control methods in oil and gas flowlines are reviewed. • Growth kinetic models and blockage mechanisms in various systems are summarized. • Anti-hydrate surface is analyzed in depth as a promising passive prevention method. • Hydrate blockage remediation methods and decomposition models are checked. With the development of oil and gas resources moving from onshore to offshore, hydrate blockage issues in subsea flowlines have become increasingly prominent. Hydrate control methods are critical and could be created from prediction, prevention, and remediation. The industrial strategy has gradually shifted from traditional complete avoidance to risk management in the past two decades. Hydrate blockage prediction simulators are reviewed in detail as a promising risk management method. The key factors for blockage prediction include formation/blockage mechanism, growth rate, hydrate amount, and slurry viscosity. The anti-hydrate surface, a potential prevention method, is discussed from the perspectives of "hard to form, weak to deposit, easy to remove" for hydrate. The remediation methods of inhibitor injection, depressurization, thermal methods, and decomposition models for developing safety simulators are summarized. In conclusion, hydrate blockage prediction simulators and anti-hydrate surfaces show good application potential. A remediation safety simulator is also necessary for determining decomposition pressure. By checking the main features of all the models, weak applicability owing to empirical parameters is the primary problem. Although surface modification studies have progressed, systematic analyses have not been conducted sufficiently. Future efforts should be devoted to improving the applicability of the models by developing dimensionless parameters, as well as systematically studying and designing for surface chemical and physical properties. [ABSTRACT FROM AUTHOR]

In this work, a coupled lattice Boltzmann model is proposed to study the gas hydrate dissociation with consideration of ice evolution in microporous media. The reliability of this model is verified by simulating the dissociation of xenon hydrate and the freezing of water droplets, respectively. The initial temperature is one of the influences on methane hydrate dissociation. In this paper, methane hydrate dissociation and ice evolution characteristics have been analyzed at the initial temperature of 270.5 K–278 K. Within this temperature range, a maximum percentage of methane hydrate dissociation can be obtained near the freezing point. Ice formation inhibits and delays the methane hydrate dissociation at the initial stage. It is demonstrated that for the whole process, heat release caused by the ice-water mixture formation facilitates methane hydrate dissociation. However, the ice formation stage has negative impacts on hydrate dissociation time. Additionally, the whole dissociation can be divided into four processes by analyzing the evolution of dissociation percentage and ice saturation. Especially, three types of ice formation positions are concluded. They are related to the hydrate structure and occurrence state. This paper provides a reference of the effect of ice evolution on methane hydrate dissociation in practical applications. • A lattice Boltzmann model considering gas hydrate dissociation and ice evolution. • Methane hydrate dissociation characteristics at different initial temperatures. • Quantification of ice amount and visualization of microstructure. • The effect of ice-water mixture formation on methane hydrate dissociation. • Local positional distributions of ice formation during methane hydrate dissociation. [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]

• Aerogels can be easily fabricated via cross-linking vermiculite and MXene. • The pore structures and surface compositions of aerogels can be finely tuned. • Monolithic aerogel has excellent CO 2 storing capacity by boosted hydrate formation. • Degree of water saturation plays the most vital role in hydrate formation. Gas hydrate provides an ideal way for CO 2 capture and storage using water by forming cages via an environment-friendly and energy-efficient hydrate formation process. However, the practical utilization and upscaling of hydrate-based gas storage are impeded by the slow formation kinetics of gas hydrate due to the limited multiphase interface for mass transfer and reaction. Herein, we demonstrated a simple strategy to fabricate composite aerogels assembled of natural vermiculite and MXene nanosheets as the ideal substrate for boosting CO 2 hydrate formation. The structure and surface compositions of vermiculite/MXene composite aerogels were analyzed by XRD, FT-IR, SEM, and XPS. It shows that monolithic pore structures and surface functional groups can be finely tuned by controlling the mass ratio of vermiculite and MXene, leading to an outstanding CO 2 storage capacity of 136.9 v/v (corresponding to 0.121 mol CO 2 /mol water) with enhanced hydrate formation kinetics. The degree of water saturation plays the most vital role in controlling formation kinetics and gas storage capacity. This work provides a reliable method to synthesize aerogels for boosting CO 2 storage via enhanced hydrate formation and sheds light on the structure performance relationship of porous materials for enhancing gas hydrate formation. [ABSTRACT FROM AUTHOR]

• Aerogels can be easily fabricated via cross-linking vermiculite and MXene. • The pore structures and surface compositions of aerogels can be finely tuned. • Monolithic aerogel has excellent CO 2 storing capacity by boosted hydrate formation. • Degree of water saturation plays the most vital role in hydrate formation. Gas hydrate provides an ideal way for CO 2 capture and storage using water by forming cages via an environment-friendly and energy-efficient hydrate formation process. However, the practical utilization and upscaling of hydrate-based gas storage are impeded by the slow formation kinetics of gas hydrate due to the limited multiphase interface for mass transfer and reaction. Herein, we demonstrated a simple strategy to fabricate composite aerogels assembled of natural vermiculite and MXene nanosheets as the ideal substrate for boosting CO 2 hydrate formation. The structure and surface compositions of vermiculite/MXene composite aerogels were analyzed by XRD, FT-IR, SEM, and XPS. It shows that monolithic pore structures and surface functional groups can be finely tuned by controlling the mass ratio of vermiculite and MXene, leading to an outstanding CO 2 storage capacity of 136.9 v/v (corresponding to 0.121 mol CO 2 /mol water) with enhanced hydrate formation kinetics. The degree of water saturation plays the most vital role in controlling formation kinetics and gas storage capacity. This work provides a reliable method to synthesize aerogels for boosting CO 2 storage via enhanced hydrate formation and sheds light on the structure performance relationship of porous materials for enhancing gas hydrate formation. [ABSTRACT FROM AUTHOR]

Hydrate-based desalination has attracted considerable attention as an innovative desalination process without extra pollution; however, the slow kinetics of hydrate formation and the difficulty of separating solid hydrate from liquid brine hinder the industrialization of this technology. Hydrates exhibit water absorption effects and can be formed above silica sand beds to promote separation. Unfortunately, the mechanism underlying the self-driven and continuous directional transport of water during hydrate formation is unclear. In this study, the spatial and temporal formation behavior of hydrate in quartz glass beads were observed using a nuclear magnetic resonance system. The results revealed that not all types of guest molecules capable of forming hydrates exhibited the hydrate water absorption effect. The interfacial tension between the hydrate and bound water provided capillary driving force for water migration. Large particle sand had larger pore channels for water migration and low initial water saturation will inhibited water migration. The results of this study improve the understanding of water migration during hydrate formation in porous media. Our findings have significant potential for the application in desalination, sludge dewatering processes, and gas capture. • Not all types of hydrate guest molecules exhibit the hydrate water absorption effect. • Mechanism underlying the self-driven and continuous directional transport of water is analyzed. • Gases with low solubility tend to nucleate at the gas–liquid interface. • The capillary force of water in a porous structure drives water migration. • Sands with large particle sizes have large pore channels, which promote migration. [ABSTRACT FROM AUTHOR]