sI Hydrate Research Articles

Experimental and computational investigation of methane hydrate inhibition in the presence of amino acids and ionic liquids

Amino acids (glycine and alanine) and ionic liquids ([BMIM][BF4] and [BMIM][I]) were examined for their inhibition effects on CH4 hydrates with experimental and computational approaches. Both amino acids and ionic liquids functioned well as thermodynamic hydrate inhibitors, by shifting equilibrium curves of CH4 hydrates toward harsh conditions. However, powder X-ray diffraction patterns indicated that amino acids and ionic liquids did not affect the hydrate structure because they were not included in the hydrate cages due to their large molecular size. Gas uptake measurements showed that the conversion of water into gas hydrates and the formation rates of CH4 hydrate were significantly influenced by inhibitors. Density functional theory calculations indicated that [BMIM][BF4] had a larger potential than glycine to inhibit gas hydrate formation by giving a more negative interaction energy between a cage and an inhibitor molecule. The time-dependent Raman spectra collected during CH4 hydrate formation demonstrated that [BMIM][BF4] hindered CH4 molecules from occupying small (512) cages, whereas glycine had a strong influence on large (51262) cages of sI hydrates. The experimental and computational results provide a better understanding of inhibition mechanisms of gas hydrates and thus can contribute to the improved control of hydrate formation in oil and gas pipelines.

Unraveling the metastability of the SI and SII carbon monoxide hydrate with a combined DFT-neutron diffraction investigation.

Clathrate hydrates are crystalline compounds consisting of water molecules forming cages (so-called "host") inside of which "guest" molecules are encapsulated depending on the thermodynamic conditions of formation (systems stable at low temperature and high pressure). These icelike systems are naturally abundant on Earth and are generally expected to exist on icy celestial bodies. Carbon monoxide hydrate might be considered an important component of the carbon cycle in the solar system since CO gas is one of the predominant forms of carbon. Intriguing fundamental properties have also been reported: the CO hydrate initially forms in the sI structure (kinetically favored) and transforms into the sII structure (thermodynamically stable). Understanding and predicting the gas hydrate structural stability then become essential. The aim of this work is, thereby, to study the structural and energetic properties of the CO hydrate using density functional theory (DFT) calculations together with neutron diffraction measurements. In addition to the comparison of DFT-derived structural properties with those from experimental neutron diffraction, the originality of this work lies in the DFT-derived energy calculations performed on a complete unit cell (sI and sII) and not only by considering guest molecules confined in an isolated water cage (as usually performed for extracting the binding energies). Interestingly, an excellent agreement (within less than 1% error) is found between the measured and DFT-derived unit cell parameters by considering the Perdew-Burke-Ernzerhof (denoted PBE) functional. Moreover, a strategy is proposed for evaluating the hydrate structural stability on the basis of potential energy analysis of the total nonbonding energies (i.e., binding energy and water substructure nonbonding energy). It is found that the sII structure is the thermodynamically stable hydrate phase. In addition, increasing the CO content in the large cages has a stabilizing effect on the sII structure, while it destabilizes the sI structure. Such findings are in agreement with the recent experimental results evidencing the structural metastability of the CO hydrate.

Sodium Dodecyl Sulfate Preferentially Promotes Enclathration of Methane in Mixed Methane-Tetrahydrofuran Hydrates

SummaryMethane storage in mixed hydrates is advantageous due to faster kinetics and added stability. However, capacity needs to be improved. Here we study mixed hydrates of methane (CH4) and tetrahydrofuran (THF), in the presence of sodium dodecyl sulfate (SDS) as a kinetic promoter for hydrate formation. We report the co-existence of pure methane (sI) and mixed CH4-THF hydrates (sII) in the presence of SDS; however, in the absence of SDS, co-existence of pure THF (sII) and mixed CH4-THF hydrates (sII) was observed. Thus the presence of SDS preferentially promotes the enclathration of methane over that of THF. Furthermore, through in situ Raman spectrometry, complemented by high-pressure differential scanning calorimeter, we present temperature-dependent methane occupancy in small and large cages of sI and sII hydrates. Our findings offer new insights for enhancing the methane storage capacity in more stable sII hydrate configuration for large-scale methane storage via solidified natural gas technology.

Kinetics of sII and Mixed sI/sII, Gas Hydrate Growth for a Methane/Propane Mixture Using Neutron Diffraction

Gas hydrates are solid solutions formed from water and enclathrated gas molecules. The hydrate types sI and sII are formed from pure methane or propane, while mixtures of these gases can form either type. In this study, eight experiments are presented of gas hydrates grown under semibatch conditions using D2O ice powder and a feed gas of methane (90%)–propane (10%). In situ neutron scattering was used to measure the phase fractions of sI and sII hydrates, ice, and liquid–water components during hydrate growth and dissociation. The composition of the free gas and the methane/propane cage occupancies were derived from the observed phase fractions using mass-balance equations and Gibbs energy calculations. In all cases, the sII hydrate was formed before, or concurrent with, the sI phase. After an initial period of hydrate growth from ice, the temperature was increased to release liquid water by melting the ice particles. Different reaction mechanisms were observed; two occurred while the ice was melting and ...

Unexpected formation of sII methane hydrate in some water-in-oil emulsions: Different reasons for the same phenomenon

The structures of methane hydrate obtained from water emulsions in oils of four types, n-heptane and n-decane were studied. Surfactant Span 80 was used to stabilize emulsions of water in n-heptane and n-decane. Hydrate synthesis was carried out by two methods, namely rapid cooling of a water-in-oil emulsion saturated with methane and long-term isothermal holding of this emulsion. It was shown that different methods of hydrate preparation may result in formation of gas hydrates with different structures. Rapid cooling of three of these emulsions (in two oils and n-heptane) saturated with methane to a temperature below −35 °C leads not only to the formation of the expected methane hydrate of cubic structure I (sI) but also to the cubic structure II (sII) hydrate. In case of oils, the formation of the hydrates in the emulsions seemed to occur at a temperature below the pour point of the corresponding oil. Experiments were carried out with the cooling rate about 14 °C/min at initial methane pressures near 12, 10 and 7 MPa. More detailed investigation showed that in two of these emulsions (in one oil and n-heptane) only sI hydrate is formed during long-term synthesis at 1 °C and methane pressure of 12 MPa. The formed sII hydrate must be metastable. In the case of the emulsion in second oil, the formation of sII hydrate can be related either to the kinetic factor (the formation of metastable hydrate) or to the presence of propane and butanes in the corresponding oil in rather high concentrations. The reason of the metastable phase appearance in the systems under consideration is most likely to be that Span 80 and some kinds of crude oil can inhibit nucleation of sI gas hydrate at the oil – water interface. Thus, some emulsions saturated with methane can be overcooled to a temperature at which the nucleation of sII hydrate is preferable. The data obtained are of interest to understand mechanisms of gas hydrate inhibition/promotion and may provide fresh insight into the influence of crude oils and surfactants on gas hydrate nucleation in water – oil – gas systems.

Adsorption Behaviors and Phase Equilibria for Clathrate Hydrates of Sulfur- and Nitrogen-Containing Small Molecules

In order to understand the role for different components of flue gases in the replacement of CH4 from natural gas hydrates, in this work, the cage occupancies of SO2, H2S, N2O, NO, and CS2 gases in the structure I (sI) and structure II (sII) hydrates have been investigated by grand canonical Monte Carlo + molecular dynamics simulations. The results show that SO2 and N2O molecules prefer to occupy the large cages (LCs) of sI and sII hydrates, and H2S and NO molecules almost have no preference to cage types, whereas the CS2 molecule can only enter the LCs of the sII hydrate. The adsorption isotherms are used to predict the gas hydrate phase equilibria by statistical thermodynamics, which are in good agreement with the experimental measurements. The dissociation pressures of these clathrate hydrates at given temperatures are in the order: NO > N2O > H2S > SO2. Moreover, it is found that the SO2, H2S, N2O, and even CS2 gases have the ability to replace the CH4 gas from natural gas hydrates. Results from this ...

Methane Hydrate Intrinsic Dissociation Kinetics Measured in a Microfluidic System by Means of in Situ Raman Spectroscopy

The present work investigates the dissociation kinetics of methane (sI) hydrate in a thermoelectrically cooled microfluidic system with in situ Raman spectroscopy. The dissociation profile of methane (sI) hydrate was measured under different Reynolds numbers (0.42–4.16), pressures (60.2 to 80.1 bar), and temperature driving force ([Teq – 0.1 K] to [Teq + 0.3 K]). A theoretical model was derived from first-principles to describe the contributions of heat transfer and intrinsic kinetics on the dissociation rate of methane hydrate. It was observed that the dimensionless ratio of heat transfer to the intrinsic dissociation rate depended on the initial thickness of methane hydrate, temperature driving force, pressure, and the time. Intrinsic kinetics dominated where the initial thickness of methane hydrate was in the range of 10 μm and the temperature driving force was low. Increasing initial thickness of methane hydrate resulted in a switch to a heat-transfer-limited dissociation. Our results support that the “memory effect” previously reported is the result of dissociation limited by intrinsic kinetics. Furthermore, this work introduces a new laboratory method of unusually sensitive microscale control of the dissociation conditions, while generating molecular-level insight. Our findings support that microfluidics with in situ Raman spectroscopy are excellent laboratory tools to understand methane hydrate dissociation with potentially much broader utility to the field. Methane hydrates play an important role in energy production and storage and the atmospheric and oceanic sciences.

Grand Canonical Monte Carlo Simulations on Phase Equilibria of Methane, Carbon Dioxide, and Their Mixture Hydrates

The cage occupancy plays a crucial role in the thermodynamic stability of clathrate hydrates and is an important quantity for understanding the CO2-CH4 replacement phenomenon. In this work, the occupancy isotherms of pure CH4, pure CO2, and their mixture in sI and sII hydrates are studied by GCMC + MD simulations. The adsorption of CH4 and CO2 + CH4 in the sI and sII hydrates can be categorized as the one-site Langmuir type. The calculated occupancy ratio θL/θS and the abundance ratio of CO2 to CH4 vary with the temperature and pressure, which provide the prerequisite information for the prediction of CH4 recovery yield at different conditions in the CO2-CH4 gas exchange process. The phase equilibria of clathrate hydrates of pure gases and mixtures are explored and the corresponding heat of dissociation and hydration numbers are determined. The current investigation provides new perspectives to understand the mechanism behind the gas adsorption behavior of clathrate hydrates.

Distribution and characteristics of natural gas hydrates in the Shenhu Sea Area, South China Sea

Abstract Structure type, gas composition and cage occupancy are important gas hydrate characteristics which play an important role in calculating gas hydrate stability and estimating the amount of global hydrate-bound methane. China conducted the Fourth gas hydrate drilling campaign (GMGS4) in the Shenhu Sea Area (SSA) of the South China Sea in 2016, during which comprehensive Logging-While-Drilling (LWD) data and natural gas hydrate samples were obtained. The LWD data shows high electrical resistivity and high sound velocity at various depth, indicating the occurrence of gas hydrate in this area. Coexistence of structure I (sI) and structure II (sII) gas hydrates in the South China Sea was confirmed for the first time by using Raman spectra analysis. The methane cage occupancies of large cages and small cages in sI hydrate are 0.95 and 0.96, respectively. The hydration number of sI hydrate is 6.03. Gas composition analysis shows that methane (>98.7%) is the dominate gas, with small amount of heavier hydrocarbon gases (ethane: 0.006%–0.976%, propane: 0.033%–0.216%, isobutane: 0.004%–0.047%) and other non-hydrocarbon gases (carbon dioxide:

Experimental investigation into gas recovery from CH4-C2H6-C3H8 hydrates by CO2 replacement

The replacement of CH4-C2H6-C3H8 ternary hydrates in sandy sediments by gaseous and liquid CO2 was studied to explore the gas recovery from structure II (sII) gas hydrate reservoirs. It was found that the swapping pattern in sII mixed hydrates is significantly different from that in the pure methane hydrate, which is a structure I (sI) hydrate. CH4, C2H6 and C3H8 can be recovered heterogeneously from the mixed hydrates by CO2 replacement. The results showed CH4 was the easiest to replace in all of the experimental runs. In addition, there was a preferential release of C3H8 over C2H6 with higher mole ratio of CO2 to initial hydrate. The guest-to-cavity size ratio was used to qualitatively study the gas exchange kinetics in the mixed hydrates. It was proposed that the isostructural conversion and structural transition coexisted in the mixed hydrates replacement. The spatial distribution of multiple components in the replaced hydrate particles was obtained by coupling gas production measurements with compositional analysis of the hydrate dissociation. It was found that further hydrate structure-transition from sII to sI occurred in deeper layers of the hydrate particles in replacement with higher mole ratio of CO2 to initial hydrate. A power function relation was found to exist between the replacement efficiency and mole ratio of CO2 to hydrate. These results are of significance for guiding successful sII hydrate recovery and CO2 sequestration.

Structural transition induced by cage-dependent guest exchange in CH4 + C3H8 hydrates with CO2 injection for energy recovery and CO2 sequestration

This study investigated a structural transition induced by cage-dependent guest exchange in the CH4 + C3H8 hydrate with CO2 injection for CH4 recovery and CO2 sequestration. The influence of the CO2 replacement on the crystalline structure of initial CH4 + C3H8 hydrates and the cage-dependent distribution of guest molecules were quantitatively investigated using powder X-ray diffraction, 13C nuclear magnetic resonance spectroscopy, and gas chromatography. The quantitative analyses demonstrated that the CO2 occupation caused the depletion of C3H8 molecules in the large 51264 cages of structure II hydrates, thereby resulting in the subsequent transformation into CO2-rich sI hydrates and the coexistence of structure I and structure II hydrates after the replacement. The guest-exchange behavior observed from time-dependent Raman spectra indicated that the replacement rate was increased with an increase in pressure of injected CO2 and that the extent of the replacement was enhanced at higher pressure of injected CO2. Overall experimental evidence of the partial structural-transition replacement suggests that CO2 molecules first occupied structure II hydrates predominantly with the rapid guest exchange at the surface and that the initial structure II hydrates were subsequently converted to the CO2-rich structure I hydrates from the surface to the inner side. Precise identification of the mechanism responsible for the partial structural transition occurring in the CH4 + C3H8 - CO2 replacement will be very helpful in developing a strategy for actual CO2 injection into structure II gas hydrate reservoirs for energy recovery and CO2 sequestration.

Diffusion of gas mixtures in the sI hydrate structure.

Replacing methane with carbon dioxide in gas hydrates has been suggested as a way of harvesting methane, while at the same time storing carbon dioxide. Experimental evidence suggests that this process is facilitated if gas mixtures are used instead of pure carbon dioxide. We studied the free energy barriers for diffusion of methane, carbon dioxide, nitrogen, and hydrogen in the sI hydrate structure using molecular simulation techniques. Cage hops between neighboring cages were considered with and without a water vacancy and with a potential inclusion of an additional gas molecule in either the initial or final cage. Our results give little evidence for enhanced methane and carbon dioxide diffusion if nitrogen is present as well. However, the inclusion of hydrogen seems to have a substantial effect as it diffuses rapidly and can easily enter occupied cages, which reduces the barriers of diffusion for the gas molecules that co-occupy a cage with hydrogen.

Thermodynamic phase equilibria and cage occupancy of NF3 hydrate

F-gases are man-made gases that are utilized mainly in the semiconductor industry and in refrigeration systems. Because F-gases have a high potential of contributing to global warming, various methods, including gas hydrate-based F-gas separation, for separating and recovering F-gases have been widely studied. However, gas hydrate formation with NF3 has not been well studied in spite of its extremely high global warming potential (12,700) and its long atmospheric lifetime (740 years). In this study, the enclathration of NF3 in gas hydrate lattices was investigated, with a focus on phase equilibria and guest distribution. The three-phase (gas hydrate (H)–liquid water (Lw)–vapor (V)) equilibria of NF3 hydrate were measured to observe the stability conditions for NF3 hydrate. The crystal structure of NF3 hydrate was identified as cubic sI (Pm3n) with a lattice parameter of 11.87 Å through powder X-ray diffraction (PXRD). In addition, the cage-filling behavior of NF3 hydrate was examined through both PXRD and in situ Raman spectroscopy. This revealed that the large (51262) cages were fully occupied by NF3 molecules whereas the small (512) cages were less populated. The results obtained in this study would be helpful for understanding the cage-specific occupation of F-gas molecules in sI hydrate and devising possible gas hydrate-based F-gas separation methods.

Retracted Article: 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 83–183 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.

The molecular mechanism of the inhibition effects of PVCaps on the growth of sI hydrate: an unstable adsorption mechanism.

The inhibition properties of kinetic hydrate inhibitor (KHI) molecules on the dynamic growth of a hydrate/water interface are investigated by using molecular dynamics simulations. The shape of the hydrate interface is transformed from laminar to funnel by PVCaps. Results indicate that the inhibition effects not only depend on the adsorption capacity which was believed to determine inhibition, but also on the fact that PVCaps must have some non-binding-hydrate sites that don't tend to combine with hydrate. By observing the time evolution of the distance between each component of PVCaps and hydrate, the heterocyclic ring of PVCaps mainly contributes to adsorption and can preferentially adjust itself to come into contact with a hydrate semi-large-cage. The distance between the amide of PVCaps and hydrate is about 4 Å and exceeds the range of a general hydrogen bond (3.5 Å), which proves that the non-binding-hydrate sites of PVCaps exist. On the other hand, the amide of PVCaps is at the intersection of the solid-liquid interface but has no adsorption affinity for hydrate, so this adsorption pattern indicates that the PVCaps at the hydrate interface are not stable. Due to this unstable adsorption, a repeated hydrate destruction phenomenon was revealed by the identification algorithm of hydrate and the calculation of the local number density of methane. The statistical evolution of water rings further proved the existence of non-binding-hydrate sites in PVCaps and the inhibition mechanism to destroy the hydrate cages by PVCaps. This unstable adsorption mechanism may shed light on the development of novel efficient KHIs.

Adsorption of Kinetic Hydrate Inhibitors on Growing Surfaces: A Molecular Dynamics Study.

We investigate the mechanism of a typical kinetic hydrate inhibitor (KHI), polyvinylcaprolactam (PVCap), which has been applied to prevent hydrate plugs from forming in gas pipe lines, using molecular dynamics simulations of crystal growth of ethylene oxide hydrate. Water-soluble ethylene oxide is chosen as a guest species to avoid problems associated with the presence of the gas phase in the simulation cell such as slow crystal growth. A PVCap dodecamer adsorbs irreversibly on the hydrate surface which grows at supercooling of 3 K when the hydrophobic part of two pendent groups are trapped in open cages at the surface. The amide hydrogen bonds make no contribution to the adsorption. PVCap can adsorb on various crystallographic planes of sI hydrate. This is in contrast to antifreeze proteins, each of which prefers a specific plane of ice. The trapped PVCap gives rise to necessarily the concave surface of the hydrate. The crystal growth rate decreases with increasing surface curvature, indicating that the inhibition by PVCap is explained by the Gibbs-Thomson effect.

Thermodynamic modeling of phase equilibria of clathrate hydrates formed from CH4, CO2, C2H6, N2 and C3H8, with different equations of state

Abstract A thermodynamic model to predict three phase (L-H-V and I-H-V) equilibria of gas hydrates is presented. In this model we have employed a fugacity based approach where the hydrate phase is modeled using van der Waals-Platteeuw solid solution theory and the liquid phase activity coefficients are determined from the modified UNIFAC method. For the vapour phase fugacity calculations we have investigated three equations of state (EOS): Peng-Robinson-Stryjek-Vera (PRSV), Patel-Teja (PT) and Soave-Redlich-Kwong (SRK). This model employs only parameters reported in the literature. The coexistence pressures predicted by our model for the sI hydrates of methane, carbon dioxide and ethane are in reasonable agreement with experiments, whereas our model overestimates the coexistence pressures for the sII clathrates of nitrogen and propane. The predicted cage occupancies are found to increase with increasing temperature in the L-H-V equilibria. For I-H-V equilibria the cage occupancy is observed to decrease with temperature. We have also estimated the solubility of each guest in the liquid phase (for L-H-V equilibria) using the Henry’s law. The solubilities predicted using all three EOS are in good agreement for all guest molecules, with the exception of nitrogen where at relatively higher temperatures the estimates from the PRSV EOS are noticeably lower than the corresponding predictions from the PT and SRK EOS.

A Fugacity Approach for Prediction of Phase Equilibria of Methane Clathrate Hydrate in Structure H

In this communication, a thermodynamic model is presented to predict the dissociation conditions of structure H (sH) clathrate hydrates with methane as help gas. This approach is an extension of the Klauda and Sandler fugacity model (2000) for prediction of phase boundaries of sI and sII clathrate hydrates. The phase behavior of the water and hydrocarbon system is modeled using the Peng-Robinson equation of state (PR-EoS) with Wong-Sandler mixing rule, while the excess Gibbs free energy of the mixture is calculated using the UNIFAC activity model. The van der Waals-Platteeuw (vdWP) solid solution theory is used to model the hydrate phase. Results are compared with the results of the models proposed by Mehta and Sloan (1996) and Chen et al. (2003) for prediction of dissociation conditions of structure H hydrates. According to the results, the overall average absolute deviation of dissociation temperatures between experiments and predictions are 0.18 (K), depending on the promoter, and the accuracy of the model proposed in this study is found to be better than the accuracies of the aforementioned models.

Characterization of hydrate-bearing sediments recovered from the Shenhu area of the South China Sea

The characteristics of the hydrate-bearing layer are important parameters for gas hydrate exploration and exploitation, which can be provided by laboratory analysis. We have carried out a systematic analysis of modern instruments, including the laser grain size analyzer, X-ray computed tomography, scanning electron microscopy, X-ray diffraction (XRD), Raman spectroscopy, gas chromatography, and isotope mass spectrometry, on the hydrate-bearing sediment samples recovered from the Shenhu area in the South China Sea in 2016. We focused on the structure, hydration number, and gas composition of the gas hydrate samples. We also evaluated the effects of sediments on the occurrence and distribution of gas hydrate. Our results showed that the sediments are much finer than those recovered from the Shenhu area in 2007. Some samples are found to contain calcareous microfossils and foraminifera, which are beneficial for dispersed gas hydrate formation. Whereas the only visible hydrate sample (FC4) has not found any bioclast in the sediments. Based on the Raman data and the XRD analysis, the gas hydrates demonstrate typical structure I (sI) hydrates, with cage occupancies of methane molecules higher than 99.3% in large cages and 81.5%–91.4% in small cages, respectively. The crystal lattice parameter is 11.89 Å with the hydration number being approximately 6.0. Methane is obviously the dominant component, ranging from 97.6% to 99.95% in these samples. Comprehensive study of carbon and hydrogen isotopes suggests that the hydrate-bound gases are from biogenic and thermogenic gases in the study area.

CH4-CO2 replacement occurring in sII natural gas hydrates for CH4 recovery and CO2 sequestration

The CH4-CO2 replacement occurring in sII natural gas hydrates for CH4 recovery and CO2 sequestration was investigated with a primary focus on thermodynamic, microscopic, and kinetic aspects. The guest-exchange behavior during replacement and the end-state composition analysis of replaced hydrates demonstrated that the extent of the replacement after CO2 injection into the sII CH4+C3H8 hydrate was significantly enhanced with an increase in the injected CO2 pressure (PCO2). The structure identification using powder X-ray diffraction (PXRD) suggested that the higher extent of replacement at higher PCO2 was closely related with the higher portion of sI hydrate after replacement. 13C NMR spectroscopy confirmed that most of the CH4 molecules resided in the small cages of the replaced sII hydrate while small amount of them also existed in the CO2-rich sI hydrate after replacement. The dissociation behavior and dissociation enthalpies of the replaced hydrates examined using an HP μ-DSC also verified the structural coexistence of sI and sII hydrates after replacement. The overall results can offer the first experimental evidence of the relationship between the replacement efficiency and the partial structure-transition in the CH4+C3H8−CO2 replacement, and can provide further insights into the cage-specific occupation of external gas molecules and thermal property changes for the actual replacement occurring in sII natural gas hydrates.