sI Hydrate Research Articles

Thermodynamic assessment and microscale Raman spectroscopy of binary CO2/CH4 hydrates produced during replacement applications in natural reservoirs

The present research deals with the micro – scale characterization of sI hydrates containing a binary mixture of methane and carbon dioxide. The application of replacement strategies in natural hydrate reservoirs, always leads to the formation of “mixed” hydrates, whose mechanical and chemical properties are different from those of pure CH4 and CO2 hydrates. As a function of the technique used for the process and due to the variability of the systems, a wide range of different compositions and morphologies can be obtained and the current literature must be expanded, in order to achieve a wide and accurate experimental database of CO2/CH4 hydrate properties. In this work, binary CO2/CH4 hydrates binary CO2/CH4 hydrates were produced in a small – scale reactor and then supercooled, in order to favour their extraction from the reactor and their stability at environmental conditions for a certain period of time. The gas hydrates, prepared with CO2 hydrates of pure water and with CH4 and CO2 mixtures, also in the presence of specific sands, were ex situ analysed by the use Raman-spectroscopy that confirmed the gas uptake in the hydrate structures by identification of the fingerprint of CH4 and CO2 occupancy in the hydrates. The characteristic of water inside the gas hydrates and the interaction between the host molecules and the lattice of water molecules was clarified. The different gas hydrates, analysed by Field Emission Scanning Electron Microscopy instrument equipped with “Coolstage head” under high vacuum condition, differed in morphology and surface features.The analysis of water Raman spectra of the different GHs permitted to describe the relation between symmetric and asymmetric OHs bands, but also provided information about the characteristics of water inside the different GHs, showing that the least ordered water structure was that of GHs containing sand, while the most ordered one was present on binary CO2/CH4 hydrates.

Formation of the structure-II gas hydrate from low-concentration propane mixed with methane

It has been recognized that a small amount of propane mixed with methane can change greatly in not only the thermodynamics but also the structural properties of gas hydrate. However, its mechanism is still not well understood yet. In this research, structure-II (sII) hydrate is synthesized using a methane-propane gas mixture with an initial mole ratio of 99:1, and it is found that large (51264) cages are co-occupied by multiple gases based on the rigid structure analysis of neutron diffraction data. The first principles calculation and molecular dynamics simulation are conducted to uncover the molecular mechanism for sII methane-propane hydrate formation, revealing that the presence of propane inhibits the formation of structure-I (sI) hydrate but promotes sII hydrate formation. The results help to understand the accumulation mechanism of natural gas hydrate and benefit to optimize the condition for gas storage and transportation in hydrate form.

Hydrogen-bonded clathrate hydrate as tunable media for efficient methane storage

The tuning effect of gas hydrates, which creates additional nanospaces to accommodate gaseous guest while maintaining favorable formation conditions, is recognized as a promising strategy for energy gas (CH4 or H2) storage. Although the tuning behavior has been demonstrated on sI and sII gas hydrates, it has not been studied in sH hydrates which have the largest potential gas storage capacity. Here, we report the sH tuning behavior in 1-Methylpiperidine + CH4 hydrate systems under mild formation conditions (e.g., 263.15 K, and 8.0 MPa). The occurrence of sH tuning behavior is experimentally verified via 13C solid-state NMR with DFT calculations. The tuning patterns are observed below the eutectic composition of water + 1-Methylpiperidine, which strongly suggests that the supercooled aqueous phase existing on the surface of ice particles plays a significant role in the tuning phenomena. We expect these finding can open new strategies for enhancing capacity for gas storage, and provide critical insights into the fundamental nature of clathrate chemistry.

Experimental Study on Hydrate Structure Transition Using an In Situ High-Pressure Powder X-ray Diffractometer: Application in CO2 Capture

Clathrate (gas) hydrates as materials have received great interest due to their high-density gas storage potential and separation applications for their ability to preferentially separate a targeted component such as CO2 from waste streams. Among the three clathrate hydrate structures, only sH (structure H) hydrates require a large molecule such as neohexane (NH) or tert-butyl methyl ether (TBME) as well as a “help-gas” molecule such as methane (CH4) to form a stable structure. However, attempts to use CO2 as a help-gas came up with mixed results where it appeared that the sH hydrate formed was stable only at temperatures below the ice point, whereas the compound formed with CH4 was considerably more stable. sH hydrates have considerable potential for gas separation, for instance, of CH4–CO2 mixtures. Thus, in this work, several compositions were tested for their hydrate forming ability. The large cage guests tested were NH and TBME, the help-gas mixtures were a CO2-rich mixture (76% CO2 and 24% CH4), and a CO2 lean mixture (24% CO2 and 76% CH4). The phase behavior of hydrates formed from the various combinations was tested by measuring the endo- and exotherms associated with hydrate formation and decomposition in a high-pressure differential calorimeter. The different phases indicated from the DSC results were identified by employing an in situ high-pressure cell on a powder X-ray diffractometer. Powder patterns were recorded to identify the crystal phase arising from nucleation and possible re-crystallization events after annealing. It was confirmed that the CO2 lean mixture (24% CO2 and 76% CH4) forms structure H (sH) hydrate, while the CO2-rich mixture (76% CO2 and 24% CH4) forms structure I (sI) hydrate presenting a structure transition pattern across the gas mixtures investigated. Further, it was observed that the CO2 lean mixture, which forms sH hydrate, also starts as sI hydrate and gradually converts to the thermodynamically stable sH hydrate. This study, in essence, helps to understand the preference of CH4 and CO2 for three different types of cages in sH hydrates and presents a design framework for a suitable gas separation mechanism for this gas mixture of interest.

Recent advances in density functional theory and molecular dynamics simulation of mechanical, interfacial, and thermal properties of natural gas hydrates in Canada

AbstractGas hydrates are inclusion compounds of a water backbone that encloses gaseous molecules. Thanks to their applications in gas recovery, carbon capture and storage, gas storage, and flow assurance, generating high quality data and predictions of their properties is paramount. A review of novel techniques using first principles density functional theory and molecular dynamics simulations methods coupled to auxiliary simulations methods is presented herein. Structure I (sI), structure II (sII), and structure H (sH) hydrates have been studied extensively, with studies of their material strength showing that it is often misleading to use the properties of ice instead of the difficult‐to‐determine gas hydrate properties. Key differences between the three structures and their possible guests are presented. The interfacial properties of gas hydrates display key behaviours that control nucleation and growth, which are important phases in controlling and monitoring their formation. Gas hydrate thermal properties are also examined, with key differences existing between guests and some unusual alignment in the cages shown for carbon dioxide, ethane, and ethylene oxide sI hydrates. First principles infrared spectroscopy is also examined, with techniques showing that these signatures can be tied to and predict material properties to improve the speed of analysis. Therefore, by quantifying, modelling, predicting, and explaining their formation and dissociation, and linking these to their thermal, material, and interfacial properties, a database of reliable data for science and engineering methods and applications is formed to provide a basis for further work.

Revealing the growth mechanism of sH hydrate by molecular simulations

Clathrate hydrates are considered efficient materials for gas storage and transportation: sH hydrate is one of the promising candidates. Studying the growth of sH hydrate is critical for its use. Therefore, massive microsecond-long molecular dynamics simulations were performed in this work to investigate the growth mechanism of sH hydrate with the help of a soluble guest, methane–1-methylpiperidine (MPD). The impacts of temperature, pressure, and guest concentration on the growth processes were also evaluated. By tuning the simulation conditions, we observed the continuous growth of sH hydrate. The results show that the slow crystal growth of sH hydrate is mainly due to the unproper trapping of MPD molecules at the hydrate surface. Neither high nor low temperature is conducive to continuous growth of hydrates. Increasing pressure can greatly promote hydrate growth, however, sI and sII hydrates can cross-grow in the system when the pressure exceeds 50 MPa. In addition, our simulations show that neither too high nor too low MPD concentrations are favorable for the growth of sH hydrate. Our results are useful for designing efficient large-cage guests for storage and transportation of gases in the form of sH hydrate.

CO2 storage behavior via forming hydrate from N2/CO2 gas mixtures in the presence of initial SI CO2 hydrate seeds

The climate system is changing and becoming warmer due to the consumption of fossil energy, which results in a large quantity of CO2 emitted into the atmosphere. The hydrate formation process can encapsulate guest molecules in hydrate cages, enabling a promising storage of CO2 in the form of hydrate under the seafloor. Here, we performed molecular dynamics (MD) simulations to validate the hydrate-based gas storage method for low-concentration flue gas. The direct phase method was applied at four target pressures to obtain the melting temperature of mixed gas hydrate in the presence of SI hydrate seeds that are fully occupied by CO2 molecules. Hydrate growth simulations showed that a proper subcooling can improve the hydrate growth rate and final occupancy of CO2 molecules in newly formed hydrate cages. The mass transfer process could impact the hydrate growth rate, with a higher pressure improving the concentrations of N2 and CO2 in the free water. This enhanced the hydrate growth at both 260 K and low subcooling temperatures. The influence of pressure and subcooling on cage selectivity presented a different tendency for CO2 and N2. A higher pressure decreased the selectivity of CO2 for 51262, and lower subcooling assisted CO2 in entering 51262 cages easily. Interestingly, the CO2 occupancy at 260 K is larger than that of the simulations at the melting temperature. The results could be of help in revealing the behavior of case filling during hydrate formation from a gas mixture.

Dimethyl sulfoxide as a novel thermodynamic inhibitor of carbon dioxide hydrate formation

• Dimethyl sulfoxide decreases the thermodynamic stability of carbon dioxide hydrate. • Obtained CO 2 hydrate dissociation data are reliable and thermodynamically coherent. • Phases of sI hydrate, crystalline ice I h , and solid CO 2 were identified by PXRD. • DMSO avoids inclusion into the CO 2 hydrate framework. • DMSO is an effective thermodynamic inhibitor of CO 2 hydrate formation. The paper investigates dimethyl sulfoxide (DMSO) as a promising inhibitor of gas hydrates. The equilibrium conditions of the CO 2 hydrate were measured over a broad range of temperatures (from 243 K to 283 K) and DMSO concentrations (up to 50 mass%). DMSO decreases the thermodynamic stability of the CO 2 hydrate. Thermodynamic coherence of new hydrate equilibria data was demonstrated. The sI hydrate, crystalline ice I h , and solid CO 2 phases were identified using PXRD at 143 K. Measured unit cell parameter for sI hydrate (11.86 Å) coincided with the pure sI CO 2 hydrate literature value. The DMSO inhibition activity grows much faster in water solution in comparison to lower alcohols. We explain this behavior by the more pronounced non-ideality of DMSO water solutions. Thus, DMSO can be considered an effective thermodynamic inhibitor of the formation of CO 2 hydrate, superior in anti-hydrate activity to alcohol THIs within a specific range.

First-Principle Molecular Dynamics Study of Methane Hydrate

Currently, much attention of the scientific community and the gas industry is focused on the structural, physical and thermodynamic properties of gas hydrates. This interest is explained by the fact that there is a prospect of using natural gas hydrates as a new fuel source. This article presents the results obtained during the first-principle molecular dynamic study of the thermal and electronic properties of hydrates. For hydrates of methane with cubic sI and hexagonal sH configurations, the average heat capacity Cv was computed. The densities of electronic states are studied for filled and unfilled configurations of sI and sH hydrates. The spectra of electron energy were calculated for sI hydrate, which has unfilled molecular cages. Also, for methane and ethane hydrates, the binding energies between the framework and the gas molecule are calculated.

Methane Hydrate Formation in the Salty Water Confined in Clay Nanopores: A Molecular Simulation Study

A sound knowledge of the effects of clay surfaces and salt ions on CH4 hydrate formation is vital to understand the formation of natural gas hydrate resources and develop hydrate-based technologies, such as seawater desalination. Herein, microsecond simulations are conducted to investigate CH4 hydrate formation from fresh water and NaCl solution in kaolinite nanopores and the outside bulk phase. Simulation results show that the gibbsite surface shows strong affinity for water and salt ions, while the siloxane surface exhibits certain affinity for CH4 and hence affects hydrate formation. With two siloxane surfaces in the nanopore, hydrate formation is severely inhibited by the significantly lowered aqueous CH4 concentration, caused by surface adsorption of CH4 to form nanobubbles; salt ions greatly enhance such an inhibitory effect by promoting surface nanobubble formation and suppressing CH4 dissolution from nanobubbles. However, with one siloxane surface and one gibbsite surface presented in the nanopore, no nanobubble forms on the siloxane surface, no matter salt ions are present or not, and the inhibitory effects caused by the siloxane surface are not observed. In contrast, gibbsite surfaces are more beneficial to hydrate formation, and salt ions have little effect as most ions adsorb to the surface. Hydrate formation outside of the nanopores is obviously inhibited by salt ions, especially at high salt concentrations. Some interesting phenomena are observed, such as stochastic formation of large sI and sII hydrate domains, which then blocks the nanopore throat and hinders the mass transfer of CH4 and water, and sustained growth of large hydrate solids outside of the nanopore causes decomposition of small hydrate solids in the nanopore. Additionally, densification of salt ions in solution and formation of ion-containing hydrate cages are systematically analyzed to figure out the surface effects of kaolinite on the desalination process. These molecular insights are of scientific and engineering significance to sustainable chemistry related to exploitation of natural gas hydrate and hydrate-based seawater desalination technology.

Carbon Capture and Storage Toward Industrialization: A Novel Continuous Process for the Production of Carbon Dioxide Clathrates

Gas hydrates have been potentially recognized for developing new technologies for CO2 capture and storage; however, the respective industrialization faces difficulties. Hydrate's crystallization is highly exothermic, 1.3 kJ tonne−1 of captured CO2, and it only occurs in a narrow window of temperatures, typically 1–5 °C. Previous works have systematically reported low space–time yields (STY) due to low specific heat and mass transfer rates of the technologies tested. Herein, NetMIX, a novel mixing technology, is used for the continuous production of CO2 hydrates. NetMIX is a structured mixer consisting of a network of unit cells comprising mixing chambers interconnected by channels. The device used here has specific heat transfer rates ranging from 107 to 108 . The setup proves to be capable of producing hydrates at a STY of 200 , two orders of magnitude larger than other technologies, resulting in a slurry with more than 20 wt% of CO2 inside the hydrates lattice. The solid is characterized, and a cubic structure I (sI) hydrate structure is detected, with no ice traces. Moreover, results indicate that the process is stable, and no plugging occurs, crucial for industrialization.

Raman Micro‐Imaging of the Coexistence of sI and sII Hydrates Formed from a Mixed Methane‐Propane Gas in a Confined Space

AbstractNatural gas hydrate contains a specific amount of heavy hydrocarbons, such as ethane, propane, etc., aside from the primary guest gas of methane. Although the coexistence of two or even three hydrate structures has been discovered at several hydrate sites, the requisite formation mechanism is still not well understood. In‐situ observation of the formation process of mixed methane‐propane hydrate in a confined space was conducted using confocal Raman imaging microscopy. The Raman imaging results reveal that sI methane hydrate and sII mixed methane‐propane hydrate are formed and coexist in the reaction system. In the confined space, the sI hydrate originates from the dissolved gas in water, while the sII hydrate is formed from free gas. The results obtained can help explain the coexistence of sI and sII hydrates found in natural hydrate samples, as well as providing insights into a possible dynamic scenario of hydrate reservoirs in geological history.

Superheating of Structure I Gas Hydrates within the Structure II Cyclopentane Hydrate Shell.

The superheated state of methane (CH4) hydrate that exists under the surface ice layer can persist for considerable lengths of time, which showed promise as a method for storing and transporting natural gas. This study extends this further by coating sI CH4 hydrate with one of several sII hydrates, thus eliminating the need for a defect-free continuous interface between the sI and sII hydrates. Gas hydrate crystals were kept intact above their dissociation temperature by immersing them in liquid cyclopentane (CP), as observed with powder X-ray diffraction and X-ray CT methods. It was observed that placing the CH4 hydrate in CP converted the outer layer of CH4 hydrate to a thin layer of CP hydrate at around 270 K under atmospheric pressure, which is ∼80 K higher than the usual dissociation temperature. It was also observed that sI CO2 hydrate and C2H6 hydrate could be preserved by CP hydrate.

Modelling the hydrate formation condition in consideration of hydrates structure transformation

Accurate prediction of hydrate formation conditions is the basis of hydrate related research. The Chen-Guo hydrate model could accurately predict the formation conditions of hydrates. However, the misjudgment of basic hydrates structure makes it unsatisfactory to calculate the system with sI and sII hydrates coexisting. Consequently, we proposed that sI and sII basic hydrates will be formed simultaneously under specific conditions. The structure composition of basic hydrates is influenced by three factors: structure induction, driving force of hydrate independent nucleation and composition of gas mixture. On this basis, we improved the Chen-Guo hydrate model by introducing new parameters and adjusted the true structure composition of basic hydrates. The calculation results show that the average absolute deviation of 201 sets of data can be reduced from 9.33% to 2.74%. The calculation precision of modified method is better than that of the original Chen-Guo hydrate model.

Physical–Chemical Properties of Compressible Clathrates: A Natural Pressure Shift by Extending the van der Waals and Platteeuw Model

We have developed a new model for compressible clathrates that extends the well-known van der Waals and Platteeuw model. The new model is derived by dispensing with the assumption of constant cage radii in the partition function level, resulting in new thermodynamically consistent expressions relating the thermodynamic properties of the hydrate phase and the empty lattice isochoric reference. One set of additional parameters to the clathrate modeling framework is introduced, consisting of a scaling factor for each cage radius relative to the edge length of the unit cell. No additional guest-dependent empirical parameters are required. The model exhibits two features not previously reported in the literature: (i) a pressure shift between the clathrate being described and the empty lattice isochoric reference and (ii) differences in the edge length of the unit cell and in the cage radii for different guest species at the same temperature and pressure, as a consequence of the sorption of guests. We also propose a test for thermodynamic consistency at high pressure, based on the multicomponent and multiphase Clapeyron equation. Using this test, we show that the proposed model solves an inconsistency issue observed in phase equilibrium calculations with some of the compressible clathrate models currently in use. We have performed parameter optimization for methane, ethane, and xenon in sI hydrates. Two sets of results are presented: three-phase equilibrium conditions and lattice size versus temperature or pressure for each of these substances, along with available experimental data.

Crystal Structure and Guest Distribution of N2O Hydrate Determined by Powder X-ray Diffraction Measurements

Research and development of control technology of N2O greenhouse gas are crucial for sustainable development. The crystallographic structural model of structure I (sI) for N2O hydrate, with a composition of N2O·6.1H2O and a density of 1.127 g/cm3 at 263 K, was obtained using powder X-ray diffraction data. In addition, the distributions of the disordered N2O molecules in 512 and 51262 host water cages of sI hydrate were explored. N2O in the large 51262 cage had an off-center distribution and it was out-of-plane to the long axis of the cage, and N2O in the small 512 cage had an isotropic distribution around its center. These crystallographic features of the N2O hydrate were identical to those of the CO2 hydrate, and an increase in cage occupancy depending on the formation pressure was observed. The hydrate composition and dissociation kinetics were discussed pertaining to the information required for the gas storage of N2O.

Distortion of the Host Water Cages of Structure I Gas Hydrates: Structural Analysis of C2H4 Hydrate by Powder X-ray Diffraction

This study investigated the crystal structure and composition of C2H4 hydrate, prepared from finely powdered ice and C2H4 gas, using powder X-ray diffraction (PXRD). A crystallographic structural model of C2H4 hydrate, derived using the Rietveld and direct-space methods, shows that the occupancy of the 512 cages is 38%, assuming that the 51262 cages are fully occupied. Furthermore, similar to C2H6 hydrate, the centers of guest molecules (C2H4) lie out of the centers of the equatorial planes of the 51262 cages and near the centers of the 512 cages (carbon atoms are spherically arranged around these cores). However, the 512 and 51262 cages of C2H4 hydrate are larger and smaller, respectively, than the corresponding cages of C2H6 hydrate, despite the fact that C2H4 molecules are slightly smaller than C2H6 molecules. This difference is attributed to the high occupancy of the 512 cages of C2H4 hydrate, which leads to the expansion and concomitant contraction of the 512 and 51262 cages, respectively. Evidently, the relative distortion of the 51262 and 512 cages of the three-dimensional sI hydrate is a crucial factor determining its stability. In addition, the kinetic stability of C2H4 hydrate under low-temperature conditions was investigated by temperature-dependent PXRD.

Theoretical Investigation of the Fusion Process of Mono-Cages to Tri-Cages with CH4/C2H6 Guest Molecules in sI Hydrates.

Owing to a stable and porous cage structure, natural gas hydrates can store abundant methane and serve as a potentially natural gas resource. However, the microscopic mechanism of how hydrate crystalline grows has not been fully explored, especially for the structure containing different guest molecules. Hence, we adopt density functional theory (DFT) to investigate the fusion process of structure I hydrates with CH4/C2H6 guest molecules from mono-cages to triple-cages. We find that the volume of guest molecules affects the stabilities of large (51262, L) and small (512, s) cages, which are prone to capture C2H6 and CH4, respectively. Mixed double cages (small cage and large cage) with the mixed guest molecules have the highest stability and fusion energy. The triangular triple cages exhibit superior stability because of the three shared faces, and the triangular mixed triple cages (large-small-large) structure with the mixed guest molecules shows the highest stability and fusion energy in the triple-cage fusion process. These results can provide theoretical insights into the growth mechanism of hydrates with other mono/mixed guest molecules for further development and application of these substances.

Kinetic studies of structure-H hydrate using a Langmuir adsorption model: Effects of methyl cyclohexane and methyl cyclopentane as large-molecule guest substances on methane hydrate

The present study investigates the kinetics of structure-H hydrate formation in the presence of large molecule guest substances (LMGS), Methyl cyclohexane (MCH) and Methyl cyclopentane (MCP), as well as methane as a help gas. The experiments were done at the temperatures range of 274.1–277.1 K. For studying the effect of LMGS on the kinetics of the hydrate formation, the initial pressure of each experiment was considered to be higher than the equilibrium pressure of structure-I methane hydrate formation. The molar consumptions of the help gas and guest substances during hydrate formation were calculated by using an algorithm based on the Langmuir adsorption theory. It was found that first step of a two-step process of hydrate formation in the presence of MCH and MCP is more related to the structure-I methane hydrate formation, so that the transition of sI to sH occurs after that. The results showed that the molar consumption of methane in the presence of MCH is more than that of MCP, and more structure-H hydrate is formed at all operational conditions. MCH could lower the gas/water interfacial tension dramatically and also affect the rate of sI hydrate formation. However, the MCP is more effective LMGS on the rate of sH hydrate formation as having the higher equilibrium pressures at all conditions. Finally, the consumption of help gas depends on the consumption of LMGS that it causes a constant participation ratio.

Rapid screening method for hydrate agglomeration and plugging assessment using high pressure differential scanning calorimetry

Gas hydrate formation during subsea oil and gas production can result in flowline blockages, which present enormous economic and safety risks. Certain crude oils have natural anti-agglomerant (AA) properties, which facilitate their non-plugging behavior over a defined operating window. Thus, it is crucial to be able to easily screen a crude oil system for its tendency to form hydrate plugs. In this paper, we present a Hydrate Plugging Assessment (HPA) technique that can be used as a rapid screening method to evaluate the plugging tendency using a high pressure differential scanning calorimeter (HP-DSC). The technique requires a small crude oil sample (~15 mg), and the experimental time is relatively short. In this work, HP-DSC tests were conducted on 13 crude oils (22 to 45 ˚API). Tests were conducted at various water contents, salinity, chemical dosage (without/with AAs), and different hydrate structures. The results show that an increase in water content destabilizes the emulsion, and increases the hydrate plugging risks. An increase in salinity decreases hydrate plugging tendency. Different hydrate crystal structures showed that for a similar water content, salinity, and driving force, structure II (sII) hydrates have a slightly greater agglomeration tendency compared to structure I (sI) hydrates. Finally, the effect of combining a low stability oil with a high stability oil showed that the resulting emulsion retained the high stability property, paving the possibility for natural oil treatment. In summary, these studies show that the HPA technique is a cost-effective and attractive method to evaluate hydrate plugging tendency.