sI Hydrate Research Articles

The impact of the abnormal salinity enrichment in pore water on the thermodynamic stability of marine natural gas hydrates in the Arctic region

In this study, the thermodynamic and structural characteristics of natural gas hydrates (NGHs) retrieved from gas hydrate mounds (ARAON Mound 03 (AM03) and ARAON Mound 06 (AM06)) in the Chukchi Sea in the Arctic region were investigated. The gas compositions, crystalline structure, and cage occupancy of the NGHs at AM03 and AM06 were experimentally measured using gas chromatography (GC), 13C nuclear magnetic resonance (NMR), Raman spectroscopy, and powder X-ray diffraction (PXRD). In the NGHs from AM03 and AM06, a significantly large fraction of CH4 (> 99%) and a very small amount of H2S were enclathrated in small (512) and large (51262) cages of sI hydrate. The NGHs from AM03 and AM06 were almost identical in composition, guest distributions, and existing environment to each other. The salinity of the residual pore water in the hydrate-bearing sediment (AM06) was measured to be 50.32‰, which was much higher than that of seawater (34.88‰). This abnormal salinity enrichment in the pore water of the low-permeability sediment might induce the dissociation of NGHs at a lower temperature than expected. The saturation changes in the NGHs that corresponded with an increase in the seawater temperature were also predicted on the basis of the salinity changes in the pore water. The experimental and predicted results of this study would be helpful for understanding the thermodynamic stability of NGHs and potential CH4-releasing phenomena in the Arctic region.

Separation of coal mine methane gas mixture via sII and sH hydrate formation

The release of coal mine methane (CMM) or coal bed methane (CBM) to the atmosphere leads to the wastage of a valuable energy resource and contributes to the greenhouse effect. In the present work, CMM has been considered as a mixture of methane and nitrogen (CH4-N2, 30:70 mol%), and this gas mixture has been separated by the hydrate-based gas separation (HBGS) process. Formation of sI hydrate with 70% N2 in the mixture requires significantly higher pressure and thus not suitable for scale-up. Therefore, in this work with a suitable thermodynamic promoter, sII and sH hydrates were formed to study the methane separation from CMM gas mixture at moderate temperature and pressure conditions. sII hydrate formation was carried out using polar THF (Tetrahydrofuran) and non-polar cyclopentane (CP) at different molar concentrations and at 274.2 K temperature. sH hydrate formation was facilitated using polar tert-butyl-methyl-ether (TBME) and non-polar neo-hexane (NH) at different molar concentrations and at 274.2 K temperature. Further, water-soluble hydrate promoter sodium dodecyl sulfate (SDS) was used at 500 ppm to enhance the rate of hydrate formation and thus to achieve better separation efficiency in a given time. For the CMM gas mixture, sII hydrate formation showed better methane recovery compared to sH hydrate formation, whereas sH hydrate formation showed a better separation factor compared to sII hydrate formation. Hydrate dissociation was also carried out to recover the hydrated gas via depressurization and thermal stimulation to compare the effect of polar and non-polar hydrate formers.

Rapid Formation of Hydrogen-Enriched Hydrocarbon Gas Hydrates under Static Conditions

This work introduces a “hydrate seed solution”, a surfactant solution containing pre-constructed structure II (sII) hydrate crystals, for the rapid formation of hydrogen-enriched hydrocarbon mixed hydrates. We observed the instantaneous nucleation and fast growth of mixed gas hydrates for both CH4–H2 and C2H6–H2 mixtures with the cyclopentane (CP) hydrate seed. The CP hydrate seed, which is immiscible with water, dominantly induces the growth of CH4–H2 and C2H6–H2 mixed gas hydrates with thermodynamically favorable sI structure. However, the THF hydrate seed, which is miscible with water, induced the formation of seed-templated sII CH4–H2 or C2H6–H2 hydrate, and the overall reaction was much slower than the CP hydrate seed case. For the C2H6–H2 gas mixture, C2H6 molecules could not occupy the small cage of hydrates, but H2 mainly occupied the small cage. The THF hydrate seed provided a higher storage ratio of H2 than the CP hydrate seed because the seed-templated sII C2H6–H2 gas hydrates have a larger number of small cages than does the structure I (sI) hydrate. As the CP hydrate seed concentration decreased from 2.78 to 0.278 mol %, the growth of the mixed gas hydrates was extremely accelerated, and the overall conversion was completed within 1 h. The growth kinetics of hydrogen-enriched hydrocarbon mixed hydrates in the non-stirred system was much faster than that in prior agitation-applied studies. These findings suggest a way forward to an enhanced formation of hydrogen–natural gas clathrates for sustainable energy gas storage.

Multiscale analysis of the hydrate based carbon capture from gas mixtures containing carbon dioxide

To reveal the kinetic performance of gas molecules in hydrate growth, hydrate formation from pure CO2, flue gas, and biogas was measured using in-situ Raman and macroscopic methods at 271.6 K. In the in-situ Raman measurements, Raman peaks of gases in the hydrate phase were characterised and normalised by taking the water bands from 2800 to 3800 cm−1 as a reference, whose line shapes were not found to have a noticeable change in the conversion from Ih ice to sI hydrate. The hydrate growth was suggested to start with the formation of unsaturated hydrate nuclei followed by gas adsorption. In hydrate formed from all tested gases, CO2 concentrations in hydrate nuclei were found to be 23–33% of the saturation state. In the flue gas system, the N2 concentration reached a saturation state once hydrate nuclei formed. In the biogas system, competitive adsorption of CH4 and CO2 molecules was observed, while N2 molecules hardly evolved in hydrate formation. Combined with micro- and macroscopic analysis, small molecules such as N2 and CO2 were suggested to be more active in the formation of hydrate nuclei, and the preferential adsorption of CO2 molecules took place in the subsequent gas adsorption process.

Mixed gas sources induced co-existence of sI and sII gas hydrates in the Qiongdongnan Basin, South China Sea

Crystal structure and gas source are two important aspects in studying natural gas hydrate systems. In 2018, Guangzhou Marine Geological Survey carried out its fifth natural gas hydrate drilling expedition (GMGS5) in the Qiongdongnan basin of the South China Sea. In this paper, analyses of gas composition, stable carbon isotopes and Raman spectroscopy were conducted on twelve gas hydrate samples recovered at drill site W08, at depths of 17–170 m below seafloor. The results show that the main component of the hydrate-bound gas is methane (72.5–97.5%), mixed with other hydrocarbon gases (ethane: 1.9–12.1%, propane: 0.1–11.3%, i-butane: 0.0–3.5%, n-butane: 0.0–0.2% and C5+: 0.0–0.2%). The δ13C of methane, ethane and propane in the hydrate-bound gases are −63.6 ~ −51.9‰, −29.2 ~ −26.6‰ and −25.6 ~ −23.7‰, respectively. The hydrate-bound gas is inferred to be a mixture of microbial and thermogenic gases. Raman spectroscopic analysis indicates that structure I and structure II gas hydrates co-exist in the samples. Structure II gas hydrate is increasingly being discovered in the South China Sea, and it is suggested that natural gas hydrate resources might be underestimated and need to be re-evaluated.

Kinetic inhibition of structure I and II hydrate using novel modified poly(N-vinylcaprolactam)s in methane-water and methane-THF-water systems

In the present study, poly(N-vinylcaprolactam)s (PVCaps) as potent kinetic hydrate inhibitors (KHIs) were synthesized to unveil their effect on the structure I (sI) and structure II (sII) hydrate nucleation and growth rate. With the aim of enhancing the KHI performance of PVCap, 3-mercaptopropionic acid, mercaptoacetic acid, and L-cysteine as chain transfer agents (CTAs) were applied to produce polymers with different end groups. The synthesized polymers were evaluated in methane-water system and methane-THF-water system. All of the modified PVCaps outperformed the non-modified PVCap in terms of cloud point temperature (Tcl) and decreasing both sI and sII hydrate growth rate. The most effective PVCap to reduce the sI growth rate was mercaptoacetic acid-modified PVCap, while mercaptopropionic acid-modified PVCap was the strongest to slow down the sII growth rate. Furthermore, the synthesized polymers acted better than commercial Luvicap EG and PVP to retard the induction time in a way that the relative inhibition power (RIP) value for PVCap was 1.5 times greater than the RIP when Luvicap EG was used, also the maximum subcooling that synthesized PVCap could tolerated, was 2.7 and 1.5 K greater than that of PVP and Luvicap EG, respectively. Besides, it was found that a blend of PVCap (MWav = 4308 g/mol) with a higher molecular weight PVCap (6900 g/mol) had the best performance to increase the RIP values to 4.07 and 22.87 on sI and sII hydrate, respectively.

Stability analysis of methane hydrates for gas storage application

Solidified Natural Gas (SNG) technology offers an attractive option for compact and safe, large-scale natural gas storage. Gas storage technologies are gaining traction due to the increasing demand for natural gas and the expected transition to hydrogen gas-based economies in the near future. Introduction of thermodynamic promoters like THF into the classical methane-water system enables rapid gas uptake at moderate pressure and temperature due to sII hydrate formation, with highly stable storage characteristics. Herein, we compare the stability of cylindrical mixed methane-THF (sII) and pure methane (sI) hydrate pellets, produced using freshwater. Storage pressure for both types of pellets was 1 atmosphere (atm) whereas the storage temperature was 268 K for sII hydrate and 268 K and 253 K for sI hydrate. While only 7.06% gas evolution occured from a methane-THF (sII) hydrate pellet stored at 1 atm and 268 K over a 10 day period, % gas evolution from two pure methane (sI) hydrate pellets stored at 268 K for similar duration was 41.45% and 37.05% individually. Correspondingly, % gas evolution from an sI pellet stored at 253 K was 51.09%. We also report 2 year uninterrupted stability testing of methane-THF (sII) hydrate pellets produced using both fresh and seawater, stored at atmospheric pressure and temperatures of 271 K and 268 K, respectively. To the best of our knowledge, this is the longest successful storage demonstration of any type of methane hydrates ever reported, showcasing the extraordinary stability of sII hydrates produced using both fresh and seawater. The results obtained in the present study resolve the challenge of hydrate stability in SNG technology and strengthen its commercial viability by reducing downstream complications related to gas hydrate storage.

Kinetic and Morphology Study of Equimolar CO2–CH4 Hydrate Formation in the Presence of Cyclooctane and l-Tryptophan

This study investigates the kinetics and morphology of an equimolar CO2–CH4 gas mixture by inducing sI and sH hydrates in gas–water and gas–liquid hydrocarbon (LHC)–water systems using pure water a...

Structural Transition of the Methane–Ethane Mixture Hydrate in a Hydrate/Water/Hydrocarbon Three-Phase Coexistence System: Effect of Gas Concentration

Hydrate formation of a natural gas mixture is fascinating. Whereas both pure methane and ethane form a structure I (sI) hydrate, their mixture may form a structure II (sII) hydrate at certain compo...

Influence of Large Molecular Guest Species on the Formation of CH <sub>4</sub> and CO <sub>2</sub> Hydrates

Methane hydrates in nature are considered as a future energy resource for natural gas due to their abundance in nature. The presence of all three structures of clathrate hydrates in the nature have been reported. Structural transition in clathrate hydrates in particular with regards to sH hydrates and the influence of large guest molecules is still not well understood. In the current work, we synthesize mixed hydrate of CO2 & CH4 mixture in presence of sH formers, neo-hexane (NH) and tert-butylmethylether (TBME). High-pressure in-situ differential scanning calorimetry was employed to investigate the phase transformation between soild-liquid-gas. We confirmed that CO2 lean mixture (24% CO2 & 76% CH4) forms structure H (sH) hydrate, while CO2 rich mixture (76% CO2 & 24% CH4) forms structure I (sI) hydrate. This study presents new insights to understand the dynamic preference of methane and carbon dioxide in three different types of cages in sH hydrate.

RETRACTED ARTICLE: Modeling and molecular simulation of natural gas hydrate stabilizers

ABSTRACT Uncontrolled decomposition of natural gas hydrate may lead to serious marine geological disasters and air pollution. The model of natural gas hydrate and lecithin was established. The stability mechanism of lecithin to structure hydrate was studied by molecular dynamics simulation. The consistent valence force field (CVFF) and TIP3P potential models are used to define the interaction between CH4-CH4 and water-water species, respectively. The simulations are performed on a combination of a 2 × 2 × 4 unit cell of sI hydrate and a water liquid phase with lecithin. The results of the simulations indicate that lecithin molecules adsorb on the hydrate surface with their hydrocarbon chains crossing and forming a net structure, easily producing the hydrate memory effect, which will narrow the available space for hydrate methane and water movement. Compared to the pure water-hydrate model, the mean square displacement (MSD) values of hydrate methane and water molecules are much lower, indicating that the hydrate dissociates more slowly.

Molecular Dynamics Simulation of Decomposition of Methane Hydrate and Interfacial Characteristics in Nanostructure Region

Methane hydrate decomposition is crucial to its safe and efficient exploitation and utilization. The water-methane hydrate–water system in the nanostructure region during decomposition process was studied by molecular dynamics simulation. Taking the decomposition of 3 × 3 × 6 SI hydrate at an initial temperature of 310 K as an example, the decomposition processes of different layers at different moments were studied. The density distribution of methane molecules, radial distribution function of water molecules in each layer and F3 order parameter of each layer during the decomposition process were analyzed. The locations of hydrate–water interfaces at different temperatures and different moments were estimated, and furthermore the decomposition rate of hydrate was obtained. In addition, this work is also concerned with the influence of hydrate–water interfacial contact area on decomposition. It is noticed that hydrate decomposes row by row from outside to inside. The formation of methane nano-scale bubbles is observed during decomposition. Due to the increase of the heat and mass transfer resistance, the decomposition rate becomes slower with time. However, higher initial temperature is helpful to promote the decomposition rate. Besides, expanding hydrate–water interfacial contact area can enhance the hydrate decomposition efficiently.

Temperature effects on the C-H symmetric stretching vibrational frequencies of guest hydrocarbon molecules in 512, 51262 and 51264 cages of sI and sII clathrate hydrates.

C–H symmetric stretching vibrational frequencies of CH4, C2H4 and C2H6 molecules encapsulated in 512, 51262 and 51264 cages of structures I (sI) and II (sII) clathrate hydrates measured by Raman spectroscopy in the temperature range of 93–183 K was analysed. The slopes of the symmetric stretch vibrational frequencies under changing temperatures (Δv/ΔT) for CH4, C2H4 and C2H6 molecules encapsulated in sII 51264 cages were smaller than those for molecules in sI 51262 cages, although sI 51262 cages are smaller than sII 51264 cages. We compared the results of Δv/ΔT in this study with the geometrical properties of each host water cage, and these comparisons suggest that the geometry of host water cages affects Δv/ΔT.

Effect of temperature and large guest molecules on the C-H symmetric stretching vibrational frequencies of methane in structure H and I clathrate hydrates.

Large molecules such as 2-methylbutane (C5H12) or 2,2-dimethylbutane (C6H14) form structure H (sH) hydrates with methane (CH4) as a help gas. In this study, the Raman spectra of the C–H symmetric stretch region of CH4 enclathrated within various sH hydrates and structure I CH4 hydrates were analyzed in the temperature range 137.7–205.4 K. Thermal expansions of these sH hydrate samples were also measured using powder X-ray diffraction. Symmetric stretch vibrational frequencies of CH4 in host–water cages increased because of varying temperature, and the sizes of the host–water cages also increased; variation of CH4 in small cages was less than in larger cages. Comparing the variations of the C–H symmetric stretching frequencies of CH4 in gas hydrates with varying pressure and temperature, we suggest that the observed trend is caused by thermal vibrations of the CH4 molecule in water cages.

Study of the Surface Morphology of Gas Hydrate

Study of the surface morphology of gas hydrate is of great importance in understanding its physical properties and occurrence. In order to investigate the surface morphology of different types (sI and sII) and occurrences (pore-filling and fracture-filling) of gas hydrate, both lab-synthesized and drilled-gas hydrate samples were measured using cryo-scanning electron microscopy (cryo-SEM). Results showed that the surface of sI hydrate was relatively smooth, and spongy nano-pores (200–400 nm) gradually occurred at the surface during continuous observation. The surface of sII hydrate was more compact, showing a tier-like structure. Hydrate occurred in quartz sand and usually filled the pores of the sediments and both hydrate and sediments were cemented with each other. SEM observation of the gas hydrates collected from the South China Sea showed that the surface morphology and contact relation with sediments varied with hydrate occurrence. For instance, hydrates dispersed in sediments mainly filled the pores of the sediments. The existence of microorganism shells, such as foraminifera, was beneficial to the formation of gas hydrate. When hydrate occurred as a massive or vein structure, it was easily distinguished from the surrounding sediments. The surface of hydrate with massive or vein structure showed two distinct characters: one was dense and smooth, the other is porous (several to tens of micrometers in diameter). The occurrence of different hydrate morphologies was probably caused by the supplement rates of methane gas.

Observation of distinct structural transformation between sI and sH gas hydrates and their kinetic properties during CO2 capture from N2 + CO2

sH hydrate has a deal of attention as an attractive medium of gas storage and separation due to its relatively milder formation condition and higher storage capacity, and thus sH hydrate has focused on the potential application in the post-combustion CO2 capture. Nonetheless, in-depth investigations on the thermodynamic stability of sH hydrate of N2 + CO2 and the possible occurrence of structural transformation during the process have not yet been addressed. Since the gas hydrate-based CO2 capture technology involves stepwise enrichments of CO2 through iterative formation-dissociation processes of CO2 hydrate, knowledge about the thermodynamic stability between sI and sH hydrates in the presence of various N2:CO2 ratios and their kinetic properties is important for designing the gas hydrate-based CO2 separation process. Here, we explored sI and sH hydrates of 3,3-dimethyl-1-butanol + N2 + CO2 focusing on their structural stability and structure-dependent kinetic properties. The experimental data including P–T phase equilibria, induction time, gas uptake, and CO2 separation factor are provided with a structural analysis via synchrotron high-resolution powder diffraction.

Effects of cage occupancy in hydrate by first-principles calculation and modification of the van der Waals–Platteeuw hypothesis

The changes of methane hydrate lattice with the decrease of cage occupancy were calculated by first-principles methods. The calculation results show that the decrease of the cages occupancy in sII and sH hydrate does not lead to large deformation in the lattice. Even if all the methane molecules are removed so that the hydrates have become new types of ice, the sII and sH lattices remain stable. The same conclusion is also true when the occupancy of the small cages in sI hydrate is reduced. However, the sI hydrate lattice will deform and almost collapse as the large cage occupancy decreases. These calculation results suggest that sI hydrate cannot exist with empty cages. Since the van der Waals–Platteeuw theory is based on the assumption that the stability of host lattice is independent of the occupancy of guest molecule, it would be applicable to sII and sH lattices, but not to sI hydrates. We propose a modification to the van der Waals–Platteeuw hypothesis so that the theory seems more reasonable.

Electric-Field Control of Neon Uptake and Release to and from Clathrate Hydrates

The thermodynamic and kinetic properties of neon uptake and release to and from a sII clathrate hydrate were evaluated using molecular dynamics simulation in the absence and presence of an applied static electric field. In the case of neon “leakage”, neon clathrate in contact with a vacuum was simulated at temperatures ranging from 200 to 225 K for 0.5 μs, with progressive neon-emptying of cages monitored. Activation energies for release and uptake were 16.4 and 14.9 kJ/mol, respectively—consistent with experimentally determined values; for neon release in a 0.7 V/nm electric field, the value declines to 6.5 kJ/mol, suggesting its use as a control agent in facilitating gas release from already-loaded clathrates. For neon uptake into an initially empty hydrate, loading free energies for 512 and 51264 cages, computed via thermodynamic integration, was lower than that previously reported for methane uptake into sI hydrates.

Structural properties of sH hydrate: a DFT study of anisotropy and equation of state

ABSTRACTStructure-H (sH) hydrate is one of the canonical gas hydrates with significant potential applications and scarce characterised material properties despite the wide knowledge available on other gas hydrates. In this work we characterise some of the important physical properties of this hydrate at the atomistic level using Density Functional Theory. Two exchange-correlation functionals (revPBE and DRSLL) were used to simulate six sH hydrate systems encapsulating neohexane and different help gas molecules. The important role of dispersion forces is quantified. The density and isothermal bulk modulus of sH hydrate are higher when dispersion interactions are considered. The presence of those interactions imposes a direct relationship between the hydrate density and its bulk modulus, while their absence reveals the bulk modulus dependency on hydrogen bond density. Anisotropy is a distinguishing feature of this hydrate in distinction to nearly isotropic sI and sII hydrates. Structure-H hydrate experiences a compressional anisotropy in which the a-lattice and the c-lattice constants respond differently to applied pressure showing less compressibility along the c-axis. This compressional anisotropy was found dependant on the chemistry of help gas molecules. Taken together, these property characterisation results and analysis are a significant and novel contribution to the material physics of sH hydrates.

CO2/N2 injection into CH4 + C3H8 hydrates for gas recovery and CO2 sequestration

CO2 injection into CH4 hydrate (sI) is a promising method for simultaneous energy production and global warming mitigation. In fact, the natural abundance of sII hydrates have been also confirmed. The gas exchange kinetics in sII hydrates were significantly different from those in CH4 hydrate. In this work, the injection of CO2 with or without N2 into CH4-C3H8 hydrate-bearing sediments was experimentally investigated to explore gas recovery and CO2 sequestration during the replacement. The results indicated that the CH4-CO2 exchange in large cages was always controlled by gas diffusion through newly-formed mixed hydrate layer. Differently, the CH4-N2 exchange in small cages changed from gas exchange-controlled to gas diffusion-controlled with the increase of N2 partial pressure. It was found that the recovery of CH4 was much more continuous than that of C3H8 in gaseous CO2/N2 replacement while CH4 and C3H8 were released synchronously in liquid CO2 replacement. CH4-C3H8 hydrate particles soaked in liquid CO2 may undergo almost complete structural transitions from sII to sI hydrate rich in CO2. The analysis of heterogeneous compositions of the replaced hydrates indicated that the presence of N2 may have inhibition effects on the structural transition of CH4-C3H8 hydrates. With the injection of gaseous CO2/N2, there was only a surface structure-transition followed by an iso-structural conversion in the deeper layers of hydrate particles. The structural transition played an important role in the replacement of sII hydrate, which made sII gas hydrate reservoirs to be promising target regions for efficient gas production and CO2 sequestration.