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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]

Methane hydrates are considered as the future energy due to its vast resource volume and high energy density. The fluid production and thermal response of two types hydrate-bearing sediments (i.e. , excess-gas and excess-water) under controlled depressurization are still unclear and warrant investigation. In this study, we devised two different hydrate-bearing sediments (HBS) synthesis methods and synthesized excess-water (S A = ∼27.2%) and excess-gas (S G = ∼26.5%) HBS with S H of 72.0%. The hydrate dissociation kinetics and fluid production behavior were examined under three bottom-hole pressures, i.e. , 3.0, 5.0, and 7.0 MPa. Gas production from the excess-gas HBS follows two-stage profile, while continuous gas production was observed after S G reaches 6.0% in the excess-water HBS. Water production from excess-gas HBS was significantly delayed compared with excess-water HBS and only started when S A reached above 22.5%. A logarithmic water production profile was observed in all cases. Rapid temperature drop due to hydrate dissociation is significantly delayed in excess-gas cases. Heat transfer from surroundings is relatively slow due to its low composite thermal conductivity. The findings on the contrast fluid production behavior between excess-gas and excess-water cases shed light on optimizing production strategies for future field production trials from these two different types reservoirs. • Excess-water (EW) and excess-gas (EG) HBS synthesized using novel method. • Controlled depressurization under BHP at 3.0 MPa, 5.0 MPa and 7.0 MPa. • Comparison of fluid production and heat transfer behavior of EW and EG HBS. • EG HBS is favored over EW HBS for production due to high R G , low R W and WGR. [ABSTRACT FROM AUTHOR]

Buried depth, as an inherent occurrence feature of hydrate reservoir, plays a significant role in fluid production during hydrate dissociation. In this study, we experimentally investigate the production performance of hydrate reservoirs at various buried depths beneath the seafloor. The hydrate-bearing system is synthesized in quartz sand with grain size varying between 100 and 500 μm in a 0.98 L reactor. Similar hydrate saturation is obtained at different prevailing pressures between 5.6 and 8.8 MPa. Depressurization experiments are designed to investigate the effect of buried depths on fluid production behavior. The results show that gas and water production increase with elevated buried depths at the same production pressure. However, based on the gas to water ratio, a deep-buried reservoir has a higher production potential at the initial stage. In contrast, a shallow-buried reservoir is an ideal candidate for fluid production in the later stage. Depressurization to the equilibrium P - T condition could lead to potential hydrate dissociation, but the rate is less intensive with less than 46.0 vol% hydrates dissociated in a prolonged time. The experimental results also reveal that both the design of depressurization and the reservoir depths have a combined effect on the overall fluid production performance. • A series of hydrate-bearing sediments with different buried depths were synthesized. • An accurate calculation method quantifying multi-phase saturation was developed. • Stability of hydrate reservoir and fluid production performance were analyzed. • Different production pressures but with the same degree of pressure driving force were employed. [ABSTRACT FROM AUTHOR]

The field production test of gas hydrate conducted in South China Sea has been proven to be successful, but the production still cannot satisfy the requirement for commercial production. In order to achieve a higher production efficiency, the numerical simulation code Tough + Hydrate was employed to estimate the detailed production potential of four typical coring sites XX01∼XX04 in Shenhu area, which have distinct reservoir conditions and are considered as the focus for production tests. Our simulation results show that the hydrate deposit at Site XX03 featured with the highest permeability has the most promising gas production, despite the least thickness 11.56 m of the hydrate-bearing layer. In addition, we have also investigated the impact of the controllable engineering facts on the production, which suggests that the lower depressurization pressure and longer perforated interval would contribute to a higher gas production. And when the lower water production is considered, the bottom half perforated interval would be a relatively more promising design for the production. This research gives a forward sight for production prediction and our findings may provide a theoretical guideline in selection and design of the production targets. • Characteristics of four typical hydrate deposits are depicted by a schematic diagram. • Production potentials are evaluated just based on the analysis of geologic conditions. • The largest gas production is obtained though it has the thinnest hydrate layer. • Desirable production design is identified and can be directed in the field production. [ABSTRACT FROM AUTHOR]

Tetra- n -butylammonium bromide (TBAB) semi-clathrate hydrate is an attractive phase change material for cold energy storage and transport. The high viscosity of hydrate slurry is a major challenge for process design and causes large pumping power consumption, not to mention the significant discrepancy of viscosity data observed in the literature. In this study, the discrepancy was clarified by studying the rheology of TBAB semi-clathrate hydrate slurry (SHS) using a rheometer with seed-induced in-situ formation of hydrate up to a high hydrate fraction (∼0.51). TBAB SHS exhibited a non-Newtonian shear-thinning behavior. Its apparent viscosity increased exponentially with the increase of hydrate fraction. Type B TBAB SHS was recommended due to its lower apparent viscosity than type A TBAB SHS. Energy analysis revealed that TBAB SHS with appropriate hydrate fractions could outperform chilled water to provide the same cooling capacity but consume less pumping power. Furthermore, to reduce the viscosity, an environmentally benign additive l -tryptophan was identified and employed for the first time. Addition of l -tryptophan up to 1.0 wt% in TBAB SHS significantly decreased the apparent viscosity and lead to a 68.7% reduction of pumping power consumption compared with chilled water. This work demonstrated the potential of TBAB SHS as a more efficient next-generation secondary refrigerant and the addition of a small dosage of l -tryptophan would considerably enhance its economic strength in cooling applications. [Display omitted] • Rheological study of TBAB hydrate slurry up to 0.51 hydrate fraction. • Apparent viscosity for type A TBAB hydrate slurry was greater than type B. • Energy analysis of TBAB hydrate slurry flow and benchmarked with chilled water. • Viscosity-reducing agent l -tryptophan (1 wt%) reduces energy consumption by 68.7%. [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]

The transition to a low-carbon economy requires the implementation of effective carbon capture and sequestration (CCS) strategies. One of the potential CCS strategies is to capture industrial CO 2 emissions and inject them into the oceanic sediments to be stored as CO 2 hydrates. However, the success of this technique depends on a few key factors such as the type of sediments where CO 2 is injected, the kinetics of CO 2 hydrate formation and dissociation, the accuracy of the models for prediction of formation kinetics, and the CO 2 hydrates morphology. So, in this first-gen work, a highly complex set of experiments was carried out to examine the CO 2 hydrate formation and dissociation processes by injecting CO 2 via injection tube into different size wet sediments, i.e., coarse (diameter: 0.5–1.5 mm), granules (diameter:1.5–3.0 mm) and dual-layered sand (coarse + granules), embedded inside high-pressure reactor as the artificial seabed. The experiments were carried out at 3.5 MPa at T = 1.5–2.0 °C with 500 ppm of the eco-friendly hydrate promotor (l -tryptophan). The images of morphological changes during hydrate formation/dissociation, the Scanning Electron Microscope analysis of the sediments, and the estimated water-to-hydrate conversations have been reported in this work. A novel mathematical four-parameter-based CO 2 hydrate kinetics model was also developed. A set of 32,843 experimental data points was used to train a supervised machine learning algorithm using two parameters with the other two taken from published literature. The water-to-hydrate conversion was estimated and follows the order of dual-layered sand [88.26 (±4.62) %] > coarse [77.77 (±5.72) %] > granules [65.36 (±2.3) %]. The proposed ML-based model predicted the water-to-hydrate conversion with an Average Absolute Relative Deviation [%AARD] of 4.23–13.29 %. This work serves as a step forward in developing a sustainable hydrate-based oceanic carbon storage technology. [Display omitted] • CO 2 hydrate kinetics and morphology in dual-layered sediments. • Effect of injecting CO 2 inside the sediments directly via vertical injection tube. • Developed and trained CO 2 hydrate formation kinetics model in sediments via a supervised machine learning (ML) algorithm. • ML model can predict CO 2 hydrate formation kinetics in sediments with accuracy in terms of AARD of 4.23–13.29 %. [ABSTRACT FROM AUTHOR]