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Methane hydrates (MHs) have been considered a promising future energy source due to their vast resource volume and high energy density. Understanding the behavior of MH formation and dissociation at the pore-scale and the effect of MH distribution on the gas–liquid two phase flow is of critical importance for designing effective production strategies from natural gas hydrate (NGH) reservoirs. In this study, we devised a novel high-pressure microfluidic chip apparatus that is capable of direct observation of MH formation and dissociation behavior at the pore-scale. MH nucleation and growth behavior at 10.0 MPa and dissociation via thermal stimulation with gas bubble generation and evolution were examined. Our experimental results reveal that two different MH formation mechanisms co-exist in pores: (a) porous-type MH with a rough surface formed from CH4 gas bubbles at the gas–liquid interface and (b) crystal-type MH formed from dissolved CH4 gas. The growth and movement of crystal-type MH can trigger the sudden nucleation of porous-type MH. Spatially, MHs preferentially grow along the gas–liquid interface in pores. MH dissociation under thermal stimulation practically generates gas bubbles with diameters of 20.0–200.0 μm. Based on a custom-designed image analysis technique, three distinct stages of gas bubble evolution were identified during MH dissociation via thermal stimulation: (a) single gas bubble growth with an expanding water layer at an initial slow dissociation rate, (b) rapid generation of clusters of gas bubbles at a fast dissociation rate, and (c) gas bubble coalescence with uniform distribution in the pore space. The novel apparatus designed and the image analysis technique developed in this study allow us to directly capture the dynamic evolution of the gas–liquid interface during MH formation and dissociation at the pore-scale. The results provide direct first-hand visual evidence of the growth of MHs in pores and valuable insights into gas–liquid two-phase flow behavior during fluid production from NGHs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Reveals the inhibition effect of representative clays from South China Sea on CH 4 hydrate experimentally. • Thermodynamic inhibition is more significant for Na-montmorillonite than kaolinite and illite. • Develops a thermodynamic model accounting for the reduced water activity on clay surface. • Implications provided on the stability and fluid production from South China Sea clayey hydrate-bearing sediments. Clay minerals are widely distributed in hydrate-bearing sediments worldwide, especially in the clayey-silty sediments in the South China Sea. The debate on the influence of clay on the thermodynamics of natural gas hydrate (NGH) remains and impedes the fundamental understanding of hydrate-clay interactions and the design of effective production strategies for energy recovery. Specifically, the phase equilibria of MH in the presence of clay and how different types of water associated with clay particles affect NGH thermodynamics and stability are not fully elucidated and warrant investigation. In this study, the phase equilibria of methane hydrate (MH) were measured in the presence of typical clays from the South China Sea, i.e., montmorillonite, kaolinite, and illite. The shift of MH phase equilibria curve to a more stringent pressure was identified with Na-montmorillonite showing the most significant inhibition. The leftward shift (inhibition) of phase equilibrium temperature (T eq) is up to 3.9 K at a pressure of 10.40 MPa for Na-montmorillonite compared with pure water. Furthermore, we develop a thermodynamic model based on the classical Chen-Guo model first accounting for the change in water activity (a w) due to cation exchange of Na-montmorillonite. The developed model exhibits good predictability with the measured phase equilibira data. The inhibition on the phase equilibria of MH is linked to the reduction of a w due to the clay-bound water for clays with a swelling nature. The findings of this study offer direct evidence and explanations on how clay minerals thermodynamically inhibit CH 4 hydrate. It provides valuable insights into the resource estimation of clayey-silty NGH in nature and facilities the design of suitable production strategies targeting clay-rich hydrate-bearing sediments. [ABSTRACT FROM AUTHOR]

[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]

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]

[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]

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]

[Display omitted] • Hydrate-bearing sediments under excess-water condition were synthesized systematically. • Fluid production induced by depressurization below the quadruple point were examined. • Effect of pressure drawdown rate and hydrate phase saturation were investigated. • Both gas production rate and recovery ratio increased under ultra-deep depressurization. Driven by the large resource volume, extraction of CH 4 from hydrate reservoirs has received worldwide attention. Depressurization is a feasible method and has been widely used in past field tests. However, fluid production behavior from water-saturated hydrate-bearing sediments below the quadruple point of CH 4 + H 2 O has not been well understood and warrants investigation. In this study, we designed a series of experiments using ultra-deep depressurization with bottom hole pressure (BHP) of 1.0 MPa, 1.5 MPa and 2.1 MPa in water-saturated hydrate-bearing sediments to investigate the fluid production behavior, particularly in the region with potential ice or secondary hydrate formation. The effect of hydrate saturations and pressure drawdown rates on fluid production below the quadruple point were systematically examined. Our experimental results show that ice formation was likely based on the temperature response, however, the self-preservation effect by ice formation prevents a further drop in temperature to its equilibrium temperature at the corresponding BHP. The gas and water recovery ratios increased by 6.5% and 49.1%, respectively for BHP below the quadruple point. No significant benefit of water trapping due to ice formation was observed. Faster pressure drawdown rate results in a more substantial temperature drop, which could trigger ice formation. Still, interestingly gas production was continuous and not affected in the phase diagram of ice + CH 4. Higher hydrate phase saturation improves gas production rate and final gas recovery ratio, yielding a lower water-gas ratio for more economic-viable CH 4 recovery. Our results indicate that ultra-deep depressurization could be one technical feasible option for increasing both gas production rate and final recovery ratio. The findings provide valuable information for designing an ultra-deep depressurization scheme based on S H of interest in future production tests. [ABSTRACT FROM AUTHOR]

Implementing efficient measures to increase production is crucial for the commercial development of hydrate. Currently, several potential stimulation methods, including well type optimization, depressurization amendment and reservoir reconstruction, have been proposed. However, most studies tend to focus on a single measure, and there are few reports on the stimulation performance under multiple means. Therefore, our study concentrated on constructing production models for simplified layered hydrate reservoirs in the Nankai Trough using four different horizontal well systems. The production enhancement resulting from aggressive depressurization and various well patterns were explored. Additionally, the production performances under conditions of burden sealing and reservoir permeability enhancement were further evaluated respectively. Lastly, the common impact of employing both burden sealing and reservoir permeability enhancement was assessed. The prediction results show that: (i) Decreasing the bottomhole pressure effectively enhances gas recovery and can be combined with other techniques for better expectation; (ii) There exists an optimal burden sealing ratio that maximizes gas production while increasing the permeability enhancement ratio consistently stimulates gas recovery; (iii) Both single and dual well systems, assisted by aggressive depressurization, are expected to achieve commercialization whereas the triple well pattern is unfavorable for achieving a significant increase in yield. • Stimulation performance of multiple means for NGH was systematically evaluated. • The optimal sealing ratios under different situations were firstly obtained. • Effects of well type, production pressure and permeability increase were studied. • Some promising schemes reaching commercial standard were suggested. [ABSTRACT FROM AUTHOR]

Kinetic hydrate inhibitor laboratory testing before field application is one of the key priorities in the oil and gas industry. The common induction-time-based technique is often used to evaluate and screen for kinetic hydrate inhibitors (KHIs). However, the main challenge relates to the stochastic nature of hydrate nucleation observed in fresh systems, which often results in scattered data on hydrate formation with unacceptable uncertainties. A much more precise KHI evaluation method, called crystal growth inhibition (CGI), provides comprehensive insights into the inhibitory behavior of a kinetic hydrate inhibitor, including both hydrate formation and decomposition. Given that industry does not require this much information, it is not feasible to expend either much time or cash on this strategy. This study aims to provide a cost-effective technique that presents maximum data accuracy and precision with relatively little time and cost expenditure. Hence, the impact of water-hydrate memory on improving the accuracy and repeatability of the results of the induction-time-based technique (IT method) was examined. First, the concept of water-hydrate memory, which contains information about how it is created, was reviewed, and then, the factors influencing it were identified and experimentally investigated, like the heating rate of hydrate dissociation and the water-hydrate memory target temperature during heating. Finally, a procedure was developed based on the background information in the earlier sections to compare the consistency of the results, originating from the conjunction of water-hydrate memory with the IT technique. The results of replications at KHI evaluation target temperatures of 12.3-12.4°C and 11.5-11.7°C showed that more repeatable data were obtained by applying water-hydrate memory, and a more conclusive decision was made in evaluating KHI performance than with an IT method. It seems that combining the IT method with water-hydrate memory, introduced as the "HME method", can lead to more definitive evaluations of KHIs. This approach is expected to gain in popularity, even surpassing the accurate but complex and time-consuming CGI method. [ABSTRACT FROM AUTHOR]

[Display omitted] • The amount of dissociation of natural gas hydrates below the quadruple point is non-linearly correlated with the depressurization rate. • The existence of a metastable state could be beneficial for increasing the gas production rate of methane hydrates and for improving the recovery rate of reservoir temperatures. • When hydrates disengagement the metastable region, the gas production rate of hydrates is improved. • The time that the hydrate is in the metastable region needs to be kept at the right time interval. The depressurization method has been proven to be an energy-efficient and relatively feasible production technique for natural gas hydrates. However, during the depressurization process, especially when the hydrates reservoir approaches the quadruple point, it is inevitable to experience phenomena such as freezing and hydrates secondary formation. Therefore, most researchers think that the P-T near the quadruple point may not be suitable for the dissociation of hydrates and a series of depressurization control methods were adopted to avoid this. However, according to other researchers, the effect of depressurization near the quadruple point on the dissociation behavior of hydrates is not inhibitory, but even a promoting effect to a certain extent. In addition, A special region called metastable state appears with the decrease of pressure near the quadruple point, which makes the dissociation behavior of hydrate below the quadruple point different from that above it. Therefore, in this paper, to reveal more clearly the dissociation behavior of natural gas hydrates near the quadruple point and the influence of the existence of metastable state on the depressurization dissociation of hydrate, the dissociation experiment of natural gas hydrates in the porous medium peri -freezing point is investigated in different depressurization rates and amplitudes, peri -freezing point and pressure stabilization times quadruple point. Experimental studies have shown that the amount of dissociation of natural gas hydrates below the quadruple point is non-linearly correlated with the depressurization rate. This correlation is synergistically regulated by the depressurization magnitude per unit of time and the environmental heating efficiency. Compared with the dissociation of hydrates above the freezing point, the metastable region does not show an obvious inhibitory effect on the dissociation of hydrates, and it even effectively shortens the dissociation time of hydrates and improves the heat transfer efficiency of the reservoir. Under the dissociation pressure of 2.5 MPa (metastable state), the reservoir temperature rose from 0 °C to 1 °C in 2.9 h, 3.6 h less than that under 2.6 MPa (above the quadruple point), and 0.7 h more than that under 1.0 MPa (below the metastable state). In addition, the gas production rate of hydrates is improved when hydrates disengagement the metastable region, increasing by 65.12 % compared with those above the quadruple point. However, pressure stabilization times when hydrates are in a metastable state need to be kept within a certain interval, too long or short pressure stabilization time will inhibit the dissociation of hydrates, and reduce the promotion effect on the dissociation of hydrates. [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]

[Display omitted] • Coupling L -Val with THF enhanced the kinetics of H 2 -THF hydrate formation. • The optimal concentration of L -Val is 0.3 wt% to achieve a maximum H 2 uptake of 29.83 v/v (0.26 wt%) • The reusability of L-Val as a kinetic promoter was demonstrated by five consecutive cycles. • Lower cooling rate promotes H 2 gas uptake with an optimal cooling rate at 0.05 K/min. • A H 2 -THF hydrate growth mechanism is proposed explaining the enhanced H 2 gas uptake using slow cooling. Hydrogen (H 2) emerges as a promising future clean energy for the low-carbon and sustainable future due to its high energy density and no pollutant or greenhouse gas emissions after combustion. The advantage of H 2 storage in solid-hydrate form is the requirement of only moderate pressure and temperature compared to compressed H 2 storage and the relatively lower energy consumption compared to liquid H 2 storage. However, the slow kinetics of H 2 hydrate formation is one major challenge to breakthrough for the large-scale development of hydrate-based H 2 storage. In this study, we introduced an environmental-benign kinetic promoter (L -valine, L -Val) and coupled it with a thermodynamic promoter tetrahydrofuran (THF) using stoichiometric concentration (5.56 mol%) to enhance H 2 hydrate formation kinetics. 0.3 wt% L -Val coupled with 5.56 mol% THF yields the optimal kinetic enhancement, and the maximum volumetric H 2 uptake (V H2) is 29.83 ± 1.22 volume of H 2 /volume of H 2 -THF hydrate (v /v). The reusability of L -Val is proved in five consecutive cycles of H 2 -THF hydrate formation and dissociation with consistently superior H 2 uptake. Furthermore, it was identified that cooling rate (R c) was an important factor determining the H 2 -THF hydrate formation kinetics. Lower R c (0.05 K/min) shows an optimal promoting effect of V H2 , which was 105.29% higher than that at higher R c (0.3 K/min). A mechanism is proposed to explain the effect of R c on H 2 -THF hydrate formation kinetics. The maximum gravimetric H 2 (G H2) reported in our study reaches 0.26 ± 0.01 wt%, which was superior in current literature below 15.0 MPa. The results provide insights into speeding up H 2 uptake in solid-hydrate form and have potential applications in hydrate-based hydrogen storage. [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]

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]

[Display omitted] • Phase equilibrium for CH 4 -DIOX hydrates in DIOX concentrations 0.15 mol%–5.56 mol%. • Low dosage DIOX induces more CH 4 in sII hydrate large cage but with inferior gas uptake. • The kinetics and morphology of mixed CH 4 + DIOX hydrate formation were observed. • An evaluation of DIOX promoting effect based on thermodynamics and kinetics. Hydrate-based CH 4 storage is a promising technology for safe and economically-viable natural gas storage and transportation. Solid CH 4 hydrate possesses advantages in both energy density and storage stability. The quest for an effective and economic-friendly thermodynamic promoter for CH 4 hydrate formation is imminent with the aim of (a) reducing CH 4 hydrate formation P - T condition and (b) increasing both the CH 4 gas uptake rate and final CH 4 storage capacity. In this study, a non-carcinogenic thermodynamic promoter 1,3-dioxolane (DIOX) was systematically investigated to quantify the effect of DIOX concentrations (0.15–5.56 mol%) on the process of mixed CH 4 + DIOX hydrate formation. A series of thermodynamic, kinetic, Raman spectroscopy measurement, and morphology observation experiments were conducted to elucidate the mechanism of the promotion effect of DIOX at concentrations below the stoichiometric concentration (5.56 mol%). DIOX with various concentrations shifts the thermodynamic phase equilibrium of binary hydrate to the region of moderate temperature and pressure. Raman spectra provided evidence of more CH 4 molecules filling in the large cages of sII hydrate with the decrease of DIOX concentration. The ratio of CH 4 in large cages to CH 4 in small cages of sII hydrate increases from 0.05 to 0.38 with the decrease in DIOX concentration. The gas uptake rate of CH 4 declined with the decrease in DIOX concentration. We also observed that DIOX concentration is an important factor affecting the morphologies during hydrate formation. These findings provide fundamental evidence for improving the CH 4 storage capacity and environmental friendliness in the application of DIOX in hydrate-based CH 4 storage. [ABSTRACT FROM AUTHOR]

[Display omitted] • Electrical resistivity can quantify the real-time TBAB hydrate fraction in slurry. • Sequential phase transition from type A to type B TBAB hydrate was observed. • From 278.2 K to 274.2 K, the hydrate growth rate was enhanced by 2.5 times. • L-tryptophan enhanced TBAB hydrate formation at 500 ppm but inhibited at 1000 ppm. Tetra-n-butylammonium bromide (TBAB) semi-clathrate hydrate has a large latent heat and suitable phase transition temperatures to be used as a phase change material for cold energy storage and transport. The mass fraction of hydrates in the semi-clathrate hydrate slurry is a key parameter determining the cold carrying capacity and flow properties. We developed a new experimental methodology based on electrical resistivity to quantify the hydrate fraction in semi-clathrate hydrate slurry during hydrate formation. Relationship between electrical resistivity and hydrate fraction of semi-clathrate hydrate slurry was established based on Bruggeman's effective-medium approximation. After validation, this method was employed to quantitatively investigate the effect of temperature on TBAB hydrate formation from 20 wt% TBAB/water system. As the temperature was lowered from 278.2 K to 274.2 K, the induction time was reduced by 97.8% and the hydrate growth rate was enhanced by over 2.5 times. At 274.2 K and 276.2 K, type A hydrates were preferentially formed followed by a structural transition to type B. At 278.2 K, only type A hydrates were observed. Furthermore, given an appropriate concentration, amino acid L-tryptophan was identified to be a good kinetic promoter for TBAB hydrate formation. The presence of 500 ppm L-tryptophan reduced the induction time by 60% and boost the hydrate growth rate by more than 32%. The electrical resistivity-based method developed in this work has shown simplicity, low cost, high accuracy, and repeatability. It would enable precise investigation of semi-clathrate hydrate kinetics in the future for cold energy applications and beyond. [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]

[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]

• Effect of hydrate saturation, morphology and particle size on HBS permeability. • Analysis of pore-scale flow behavior at pore throat on HBS permeability. • Abrupt permeability reduction identified at S H = 33.9% for pore-filling HBS. • A new model proposed for HBS permeability calculation considering tortuosity. • HBS permeability comparison between uniform and non-uniform settings. The distribution pattern of CH 4 hydrate in hydrate-bearing sediments (HBS) significantly impacts the permeability, which is one of the critical flow properties controlling the fluid production in the exploitation of natural gas hydrate (NGH) reservoirs. Thus, it warrants investigation on the key influencing factors, i.e., (a) hydrate saturation; (b) hydrate distribution patterns; (c) sediment particle size on the associated permeability of HBS especially at pore-scale. In this study, a series of two-dimensional geometric models were constructed that best represent the commonly-observed hydrate distribution patterns. The pore-scale flow behavior of the resulting models was simulated through computational fluid dynamics. The simulation results suggest that the reduction of permeability is dependent on the distribution pattern of hydrate. The permeability of grain-coating HBS decreases by a maximum of five orders of magnitude at S H = 0.3. Additionally, increasing S H results in new pore throats and induces clogging in fluid flow that controls permeability reduction. A strong log-log linear relationship was identified between sediment particle size and the simulated permeability. A modified empirical model for the estimation of HBS permeability was proposed considering both tortuosity and effective porosity. Moreover, a heterogeneous sediment particle distribution model was constructed for the first time that accurately described the particle size distribution of a hydrate-bearing core sample recovered from Japan Nankai Trough. Compared to the homogeneous model, a heterogeneous model results in a more substantial reduction of permeability with S H and better represents the permeability of the HBS core sample. The finding of this study can be significant in understanding the pore-scale flow behavior in HBS and are meaningful for the design of safe and effective production methods in the exploitation of NGH reservoirs. [Display omitted] [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]

[Display omitted] • Shorter nucleation time but slower growth kinetics observed for MH in clay suspension. • Clay-bound water induced by diffuse double layer results in slow MH formation kinetics. • Migration of water and the agglomeration of clay resulted in gas hydrate-clay stratification. • Implications for hydrate occurrence and energy recovery in clay-rich geological sediments. Silty-clayey host sediments have been widely identified in naturally-occurring hydrate-bearing sediments in the South China Sea. However, the effect of clay minerals on the kinetics of CH 4 hydrate (MH) formation and dissociation remains unknown and warrants further investigation. The objective of this paper is to elucidate the effect of montmorillonite (MMT) clay particles on the formation and dissociation kinetics of MH. A series of experiments involving kinetics and morphological observations were designed to investigate the kinetics of MH in MMT suspensions with mass fractions ranging from 0.1 wt% to 9.0 wt%. The experimental results reveal that MMT promotes nucleation but delays the growth kinetics of MH. The diffuse double layer (DDL) on clay minerals is quantified to explain the observed kinetic behavior. The electric field induced on the clay surface resulted in a shorter induction time. The morphology observed indicated that MH initially forms at the gas–liquid interface and develops upward along the reactor surface, resulting in the hydrate-clay stratification behavior. The upward migration of water due to hydrate growth promotes the agglomeration of MMT clay and reduces the conversion of free water to hydrate. The findings of this study are essential in understanding the mechanisms of the naturally occurring negatively-charged clay particles on the kinetics of MH. Moreover, the results have implications for the occurrence of MH in clay-rich sediments in nature and the development of energy-efficient recovery strategies from silty-clayey hydrate-bearing sediments. [ABSTRACT FROM AUTHOR]