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[Display omitted] • The phase equilibria of 72CH 4 /28CO 2 hydrate with DIOX no more than 5.56 mol% were measured. • Kinetics and morphology were acquired for CH 4 /CO 2 + DIOX hydrate varying 1.00–5.56 mol% • DIOX below 5.56 mol% induces CH 4 /CO 2 + DIOX hydrate formation with heterogeneous compositions. • CH 4 /CO 2 + DIOX hydrate growth for DIOX below 5.56 mol% yields DIOX concentrated in hydrate phase with a lean DIOX solution. • Co-storage of CH 4 /CO 2 in hydrate was suggested based on GC measurement and Raman analysis. Hydrate technology could provide a novel approach to deliver convenient CO 2 -rich natural gas. The identification of a suitable and environmental-friendly promoter for enhancing CO 2 /CH 4 hydrate kinetics is critical. In this study, we evaluated the thermodynamics, kinetics, and morphology of CH 4 /CO 2 + DIOX mixed hydrates in the presence of a thermodynamic promoter 1,3-dioxolane (DIOX) at and below its stoichiometric concentration (SC, C DIOX = 5.56 mol%). Due to the thermodynamic preference of DIOX enclathration in mixed hydrates formation, a decrease in C DIOX in the unconverted solution was observed post nucleation below SC and the hydrates formed exhibit significant heterogeneity in cage occupancy. As C DIOX in the unconverted solution decreases, the DIOX concentration in hydrate (R DIOX) and the thermodynamic stability of the newly formed hydrates decreases. Based on the step-wise heating design, the thermodynamic stability of the hydrates formed below SC with high- R DIOX is close to that of hydrates formed at SC, while hydrates with low -R DIOX undergo dissociation at their P-T trajectories above the measured phase equilibria. Raman measurement indicates that the lower- R DIOX hydrates possess a higher CO 2 composition. This study provides new insights into the tuning effect of DIOX on mixed hydrates formation and the basis for using low thermodynamic promoter concentration in hydrate-based technologies. [ABSTRACT FROM AUTHOR]

Oceanic hydrate-based CO 2 sequestration (HBCS) holds great promise for achieving carbon neutrality. However, the presence of inorganic salts, particularly MgCl 2 besides NaCl, in seawater can significantly impact the formation rate and the stability of CO 2 hydrate. In this study, experimental investigations were conducted to examine the thermodynamics, kinetics, and the resulting morphological features of CO 2 hydrate in the presence of MgCl 2 , covering mass fractions ranging from 0 to 5.0 wt%. The experimental findings reveal that MgCl 2 exerts a thermodynamic inhibitory effect with its inhibitory capacity increasing with higher mass fractions. The solubility model of CO 2 in MgCl 2 solution was modified, demonstrating a gradual weakening of CO 2 solubility as MgCl 2 mass fraction increases. Additionally, the growth kinetics of CO 2 hydrate decreases with increasing MgCl 2 mass fraction. Regarding CO 2 hydrate morphology, it was observed that at low mass fractions of MgCl 2 (<1.0 wt%), a dense hydrate film rapidly formed at the gas-liquid interface after CO 2 hydrate nucleation, hindering the further conversion of CO 2 into hydrate. Conversely, at higher mass fractions (>3.0 wt%), CO 2 hydrate exhibits a more porous and slurry-like structure, facilitating more gas-liquid contact and mass transfer, thereby enhancing the conversion of CO 2 into hydrate. During hydrate dissociation, a salt-removal effect associated with CO 2 hydrate formation was observed, leading to the accumulation of concentrated electrolyte (MgCl 2) and facilitating CO 2 hydrate dissociation. These findings have implications for understanding the CO 2 hydrate formation and dissociation in the presence of MgCl 2 relevant in the subsea environment and can contribute to the development of effective hydrate-based CO 2 sequestration strategies. [Display omitted] • Measured phase equilibria of CO 2 hydrate in the presence of 0–5.0 wt% MgCl 2. • Increasing MgCl 2 reduced CO 2 solubility in H 2 O by 22 % for 5.0 wt% MgCl 2. • Increasing MgCl 2 yielded sluggish CO 2 formation kinetics but enhancement observed at 1.0 wt% MgCl 2. • CO 2 hydrate morphology transits from surface film growth to slurry with increasing MgCl 2. • CO 2 hydrate dissociation triggered by concentrated MgCl 2 after CO 2 hydrate formation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Depressurization followed CO 2 -N 2 injection for CH 4 recovery and CO 2 sequestration. • Effect of MH dissociation ratio on subsequently mixed hydrate formation. • CH 4 hydrate re-formation and dissociation occurred depending on CH 4 hydrate left. • CO 2 hydrate formation depended on the CH 4 left in both gas phase and hydrate phase. • Increasing CH 4 production decreased the CO 2 sequestration and hydrate restoration. Natural gas hydrates (NGHs) buried below the seafloor are globally abundant, energy-dense, and essential to the world's future energy mix. The combination of depressurization and CO 2 -N 2 injection processes for NGHs recovery has been identified as an environmental method to harvest energy, CO 2 sequestration, and to maintain the mechanical stability of sediment. In this regime, partial CH 4 was produced by depressurization followed by CO 2 -N 2 injection to form CH 4 -CO 2 -N 2 mixed hydrate (Mix-H). In this study, a series experiments were designed to examine the key factors in the depressurization process (i.e. bottom hole pressure BHP and CH 4 production ratio) on the subsequent processes of Mix-H formation, CO 2 sequestration and hydrate restoration. It was observed that Mix-H formation occurred in all cases, but the final amount of Mix-H were all lower than the initial amount of MH before depressurization. Particularly, the formation of CO 2 /N 2 hydrate in Mix-H is positively correlated with the amount of remaining CH 4 in the combined phases of gas and hydrate after depressurization. The formation of CH 4 hydrate in Mix-H during the reformation process occurred in five cases when MH dissociation ratio (D MH) reaches above 45.1 % during depressurization. MH was only observed to dissociate continuously after CO 2 /N 2 injection when D MH is low at 30.8 %, which indicates possible "CH 4 -CO 2 replacement reaction". Overall, increasing CH 4 production at the same BHP decreases the CO 2 sequestration and hydrate restoration. We first verified the feasibility of the combination method for synergistic CH 4 recovery and CO 2 sequestration in marine conditions. The experimental results provide possible guidance on the optimal design of the coupled processes and shed light on the relationship between CH 4 recovery and CO 2 sequestration that need to be wholistically balanced. [ABSTRACT FROM AUTHOR]

[Display omitted] • Phase equilibria of CO 2 + DIOX hydrate for C DIOX between 0.05 mol% and 5.56 mol% • The kinetics of CO 2 + DIOX hydrate is quantified with morphology observation. • Phase separation of DIOX/H 2 O above 3.1 MPa weakens the promotion effect of DIOX. • A comprehensive evaluation of DIOX in the application of hydrate-based CO 2 storage. To reduce anthropogenic CO 2 emissions for the mitigation of climate change require novel CCUS solutions. Hydrate-based CO 2 sequestration (HCS) is a novel carbon-neutrality technology that aims to store CO 2 in solid hydrate form with long-term stability. However, imminent issues exist for the application of HCS in terms of demanding thermodynamic conditions, slow formation kinetics, and low CO 2 gas uptake. These challenges necessitate the quest for an efficient and eco-friendly CO 2 hydrate promoter. In this study, 1,3-dioxolane (DIOX) as a low-toxicity CO 2 hydrate promoter was systematically examined. The phase equilibria, cage occupancy, and the kinetics of binary CO 2 + DIOX hydrate were measured for DIOX concentrations (C DIOX) varying from 0.05 mol% to 5.56 mol%. It was confirmed that DIOX is a dual-function promoter for CO 2 hydrate, but its promotion effect is weakened for C DIOX between 0.60 mol% and 1.00 mol% and for all C DIOX at relatively high pressure. The CO 2 uptake in the hydrate phase increases with C DIOX above 2.00 mol% and is the highest for C DIOX = 5.56 mol% (57.08 ± 6.39 mmol/mol). Based on the morphology observation, hydrate transits from ice-like to slurry to mushy-like and finally to snow-like with increasing C DIOX. Interestingly, we observed that the aqueous phase separates into two phases (i.e., DIOX-rich and H 2 O-rich) at pressure above 3.09 MPa, which explains the gradual loss of the promotion effect. The results of our study provide a comprehensive evaluation on DIOX as a possible promoter for CO 2 hydrate in HCS application. [ABSTRACT FROM AUTHOR]

• Depressurization followed by CO 2 -N 2 injection for CH 4 recovery and CO 2 sequestration. • A pilot-scale investigation on the combined processes at South China Sea NGH condition. • Spatio-temporal evolution of mixed CH 4 /CO 2 /N 2 hydrate formation was investigated. • Hydrate saturation reverts back to 50%-70% of its original after mixed hydrate formation. • Employment of horizontal well enhanced the storage of CO 2 after CO 2 /N 2 injection. To ensure sustainable energy supply while mitigating climate change, combining CH 4 recovery with CO 2 storage in marine natural gas hydrate (NGH) reservoirs is a promising carbon–neutral technology. In this study, a pilot-scale reactor (V = 22 L) was used to study the combined processes of CH 4 recovery from the methane hydrate-bearing sediment by using depressurization followed by in-situ CO 2 /N 2 injection to form mixed CH 4 /CO 2 /N 2 hydrate (Mix-H) at marine NGH reservoir conditions (T = 283.8 K and P = 11.0 MPa). The bottom hole pressure (BHP = 3.2 MPa, 5.3 MPa, and 6.8 MPa) during depressurization and the setting of horizontal wellbore (HW), were systematically investigated to optimize the synergistic CH 4 recovery and CO 2 storage. The CH 4 recovery ratio (55.4–80.6%) and the CH 4 production rate (5.15–134.98 mol/d) increased as BHP decreased. After CO 2 /N 2 injection, Mix-H formed first in the CH 4 -rich region in the upper section of the reactor and propagated downward to the lower. The Mix-H formation kinetics were significantly enhanced with an increase in BHP, leading to a decrease in CO 2 composition in the Mix-H. The use of a horizontal wellbore (HW) facilitates a more even distribution of the CO 2 /N 2 within sediments, resulting in a 13.5% increase in CO 2 composition of Mix-H compared with no HW setting. The CO 2 conversion ratio and hydrate restoration ratio reaches 52.8% and 69.4% at BHP = 5.3 MPa with HW, respectively. The technical feasibility of CO 2 storage in exploited marine NGH reservoirs was first proved at pilot-scale. The results provided insights into the synergistic method of NGH exploitation and CO 2 storage at the marine NGH reservoirs. [ABSTRACT FROM AUTHOR]

Hydrate-based desalination is a promising technology that offers low energy consumption. However, the low hydrate conversion rate and sluggish hydrate formation still hinder its applicability and therefore has not been commercialized yet, as it is necessary to enhance hydrate formation kinetics and obtain more freshwater. Hence, this study introduces a dual function (thermodynamic and kinetic) promoter cyclopentane and hydrophobic graphite and investigates the effect of cyclopentane/water volume ratio from 0.05 to 0.325, graphite size (75 μ m − 1. 6 μ m), and graphite/water volume ratio (ranging from 0.625 mg/mL to 3.25 mg/mL) on the CO 2 + C 3 H 8 hydrate formation kinetics at 275.6 K and 2.0–3.0 MPa. The results indicated that cyclopentane could shift the hydrate phase equilibrium condition to low pressure and high temperature and accelerate CO 2 + C 3 H 8 hydrate formation. Additionally, the experimental findings highlighted that the cyclopentane/water volume ratio of 0.175 afforded a faster hydrate formation rate and water conversion to hydrate. Moreover, we demonstrated that hydrophobic graphite with a smaller size provided more sites for hydrate nucleation and growth, and the synergy of graphite and cyclopentane reduced the hydrate induction time from 8.25 (±4.42) mins to 1.95 (±1.7) mins and increased the hydrate conversion rate from 35.62 (±1.76) % to 43.25 (±0.14) %. Overall, this study provides a guide for selecting the appropriate experimental condition, promoter, and porous media and lays a foundation for the hydrate-based desalination technology's feasibility and sustainable development. [Display omitted] • Cyclopentane combined with graphite to enhance hydrate formation rate was studied. • Synergy of cyclopentane and graphite shortens the hydrate induction time. • Cyclopentane/water volume ratio of 0.175 shows the fastest hydrate formation rate. • Graphite size of 6. 5 μ m indicates the highest gas uptake and hydrate conversion rate. [ABSTRACT FROM AUTHOR]

The prediction of gas productivity and reservoir stability of natural gas hydrate (NGH) reservoirs plays a vital role in the exploitation of NGH. In this study, we developed a THMC (thermal-hydrodynamic-mechanical-chemical) numerical model for the simulation of gas production behavior and the reservoir response. The model can describe the phase change, multiphase flow in porous media, heat transfer, and deformation behavior during the exploitation of NGH reservoirs. Two different production scenarios were employed for the simulation: depressurization and depressurization coupled with CO2 exchange. The simulation results suggested that the injection of CO2 promotes the dissociation of NGH between the injection well and the production well compared with depressurization only. The cumulative production of gas and water increased by 27.88% and 2.90%, respectively, based on 2000 days of production simulation. In addition, the subsidence of the NGH reservoir was lower in the CO2 exchange case compared with the single depressurization case for the same amount of cumulative gas production. The simulation results suggested that CO2 exchange in NGH reservoirs alleviates the issue of reservoir subsidence during production and maintains good reservoir stability. The results of this study can be used to provide guidance on field production from marine NGH reservoirs. [ABSTRACT FROM AUTHOR]

The growth kinetics and macroscopic morphology evolution of CO 2 hydrate promoted by l -Methionine (L -Met) in NaCl aqueous solutions in a wide range of mass fractions were systematically investigated. 0.05 wt% was the threshold mass fraction of L -Met to have prominent promotion effects in a batch mode. The addition of NaCl could suppress its promotion performance, while it could be regained by further addition of L -Met to 1.0 wt%. Porous feature and wall-climbing phenomena of CO 2 hydrate were obvious in the presence of L -Met while it could be suppressed by NaCl. The tangential (lateral) growth rate of the hydrate film on planar pure water surface was the highest while both L -Met and NaCl could decrease it. For L -Met, it was the highest at 0.05 wt%. Special ball-like, mosaic-like and sword-like appearances of CO 2 hydrates were found. Both the adsorption-capillary effects of L -Met and the hydration effects of L -Met and NaCl were thought to account for all the phenomena and a detailed scenario was proposed and discussed. A macroscopic growth model based on the tangential growth rates of hydrate films was proposed and evaluated. • Excellent kinetic promotion effects of l -Methionine at 0.05 wt% were found. • NaCl suppressed the promotion effects by competing for water molecules. • CO 2 hydrate films first formed with different tangential growth rates. • Both the adsorption-capillary effects and hydration effects played a role. • Growth model was proposed and the control factor was evaluated. [ABSTRACT FROM AUTHOR]

[Display omitted] • Feasibility of CH 4 recovery and CO 2 storage by gas injection at marine condition. • CH 4 hydrate co-growth and dissociation occurred depending on the gas composition. • High CO 2 composition is beneficial for co-growth of mixed hydrate. • Continuous CH 4 hydrate dissociation was observed with high N 2 composition. CH 4 hydrate have been considered as the future clean energy resource. Recovery of CH 4 from natural gas hydrates by CO 2 replacement is a promising method for energy harvest and CO 2 sequestration. Field production tests have proved the feasibility of gas production by CO 2 replacement in the permafrost region. However, most of the hydrate reservoirs are located at marine conditions where pressure and temperature conditions are not favorable for the replacement process. In this work, we identified a possible marine condition, where mixed hydrates could potentially form. We investigated the technical feasibility of CH 4 recovery by a depressurization process followed by the injection of CO 2 -N 2 gas. To quantify the dynamic behavior of mixed hydrates formation and dissociation, we developed a numerical method to estimate the phase partitioning during the replacement process. The effects of mixed gas compositions in the reactor on the replacement efficiency were investigated. Our experimental results showed that rapid co-growth of mixed hydrates occurs when the injected gas composed of a lower N 2 concentration (<25.2%). On the other hand, continuous CH 4 hydrate dissociation was observed when the injected gas was composed of a higher N 2 concentration (28.5%). During the prolonged replacement stage, the amount of CH 4 hydrate decreased and the amount of CO 2 -N 2 hydrates increased only slightly, which indicated that possible replacement occurred but at a very localized level. The results proved the technical feasibility of CH 4 production by gas injection at marine conditions, but the compositions of the mixture gas injected need to be well controlled. [ABSTRACT FROM AUTHOR]

[Display omitted] • Delamination and the electric field induced by clay particle shortens induction time. • Delayed growth kinetics observed in clay suspension with increasing mass fraction. • Water migration and clay agglomeration yielded the stratification of CO 2 hydrate and clay layer. • Heterogeneous spatial distribution of CO 2 hydrate observed in formation and dissociation. Hydrate-based CO 2 sequestration in marine sediments has emerged as a novel sustainable technology for long-term and stable CO 2 sequestration. However, the role of clay minerals in CO 2 hydrate formation and dissociation in clay-rich sediments, which are widely distributed in the South China Sea, remains controversial due to a lack of experimental evidence. This study investigates the effect of a representative clay mineral, sodium montmorillonite (Na-MMT), on the nucleation and growth kinetics of CO 2 hydrate in suspensions with a msass fraction below 20.0 wt%. The kinetic experiments reveal that Na-MMT significantly reduces the induction time due to the additional nucleation sites provided by the delamination of clay particles and the induced surface electric field. While the average growth rate of CO 2 hydrate is reduced by ∼72 % for Na-MMT mass fraction above 5.0 wt%. Morphologically, hydrate-clay stratification and gas-tunneling behaviors were observed, providing explanations for the retarded kinetics. The mass transfer of CO 2 from the gas phase to the liquid phase is impeded by the high viscosity of the suspension and the clay-induced strongly-polarized water layer, which retards the overall kinetics of CO 2 hydrate formation. Upon thermal stimulation, partial CO 2 hydrate dissociation was observed within the pure CO 2 hydrate stability region for 10.0 wt% Na-MMT suspension, indicating a possible impact of Na-MMT on CO 2 hydrate thermodynamics. Moreover, the addition of Na-MMT clay promotes CO 2 hydrate dissociation kinetics significantly. This study provides fundamental insights into the interaction between Na-MMT clay and CO 2 hydrate, offering new perspectives for designing effective strategies for CO 2 sequestration in abundant clay-rich marine sediments. [ABSTRACT FROM AUTHOR]

[Display omitted] • Revealed the completion behavior between CO 2 hydrate and ice by experiments carried out at both micrometer-scale and mesoscale. • The morphological characteristics of the co-formation of ice and CO 2 hydrate were observed in-situ in optical cells. • Hydrate forms preferably and replaces ice at small subcoolings; while, at large subcoolings, ice forms first and hydrate emerges subsequently as a fibrous envelope around ice crystals. • Experimental results shed new light on hydrate stability cycle in permafrost caused by temperature fluctuations. As climate change and industrial requirements intensify, the potential of CO 2 hydrates for carbon storage has attracted increasing attention. However, despite continuous progress, the subzero phase behavior of the CO 2 -H 2 O system is not well understood and merits further research. To investigate the competing and coexisting growth mechanisms of CO 2 gas hydrate and ice below the quadruple point, we conducted experiments at both the micrometer-scale and mesoscale scales. The growth morphology of hydrate and ice at large and small subcooling temperatures was observed in situ in a high-pressure optical cell, and the thermodynamic behavior of the competitive process after hydrate formation was investigated in a high-pressure reactor with sands. When hydrates and ice are formed together at small subcooling temperatures, preferential hydrate formation leads to ice dissolution or even disappearance. At large subcooling temperatures, ice forms first, and the hydrate mainly forms a fibrous envelope. A small decrease in the subzero temperature after hydrate formation partially decomposes the hydrates owing to ice formation, followed by more rapid hydrate production. This study provides a thorough and expanded understanding of the competition between ice and hydrates by connecting the evolution of crystal frameworks with the thermodynamic features of the process. [ABSTRACT FROM AUTHOR]

[Display omitted] • A comparison of the promoting effect of SDS and L -Met on CO 2 hydrate formation. • Morphology of CO 2 hydrate in the presence of SDS and L -Met with stirring. • A calculation method quantifying CO 2 distribution in gas, liquid and hydrate phase. • Verification of L -Met promoting effect in repeated tests and at various G-L ratios. The continuously increasing CO 2 emissions since industrialization has sparked widespread concerns on environmental and climate change issues. Carbon sequestration and storage technologies (e.g., hydrate-based CO 2 sequestration) present promising potential for the disposing of excessive CO 2 in geological media. High gas uptake capacity and rapid CO 2 hydrate formation kinetics are required for the application of such a technology. In this study, we compared the kinetic promotion effects of sodium dodecyl sulfate (SDS) and L -methionine (L -Met) on CO 2 hydrate with the objective of identification of an effective and eco-friendly CO 2 hydrate kinetic promoter. The experimental results suggested that L -Met (0.1 wt%) promotes CO 2 hydrates formation significantly with a gas uptake in CO 2 hydrate five times more than SDS at the same concentration. The hydrate morphology observations indicated that the wall-climbing hydrates associated with the addition of L -Met induced by the capillary driving force offer a possible explanation for the difference in the observed kinetics. A novel calculation method was developed to quantify the partition of CO 2 in gas, liquid and hydrate phases. Increasing the initial gas–liquid ratio promotes CO 2 stored in the hydrate phase as solid rather than in the liquid phase as dissolution. We further demonstrated the reusability of L -Met in five consecutive cycles of CO 2 hydrate formation and dissociation. It was assessed that L -Met as a CO 2 hydrate kinetic promoter is superior to SDS and can be employed as an efficient, reliable and eco-friendly kinetic promoter for the hydrate-based CO 2 sequestration technology. [ABSTRACT FROM AUTHOR]