Gas Hydrate Structure Research Articles

Growth Kinetics and Porous Structure of Surfactant-Promoted Gas Hydrate.

Surfactants present in tiny amounts in the aqueous phase are known to be efficient gas hydrate promoters; yet, the promotion mechanisms are still not fully understood. Understanding and directing those mechanisms is key to the implementation of gas-hydrate-based applications such as gas storage and separation, secondary refrigeration or water treatment, and desalination. In this work, the growth at the water/gas interface and the porous structure of surfactant-promoted methane hydrate are observed by optical microscopy and Raman imaging in glass capillaries used as optical cells. Hollow crystals are continuously generated and expelled from the methane/water meniscus into the water or surfactant solution, where they ultimately form the skeleton of a porous medium filled with the solution. Unprecedented information is gathered over a range of scales from the molecular scale (crystal structure and cage filling) to the mesoscale (crystal morphologies, growth habits and pore sizes) and macroscale (rates and amounts of water and gas converted into hydrate and hydrate porosity). Following an early steady-state growth regime, a sudden order-of-magnitude increase of the conversion rate occurs, which is related to gaseous methane microbubbles being directly incorporated across the meniscus in the aqueous solution and later converted to methane hydrate. An assessment and comparison are made of the mechanisms and performance of two common anionic surfactants known to be efficient gas hydrate promoters, SDS (sodium dodecyl sulfate) and AOT (dioctylsulfosuccinate sodium or AerosolOcTyl). AOT provides a quicker but more limited conversion into hydrate than SDS, suggesting that it is more appropriate for continuous flow processes while SDS is better suited for gas storage applications. Raman spectra reveal that cage filling by methane of structure I methane hydrate is not affected by surfactants.

HTR+: a novel algorithm for identifying type and polycrystal of gas hydrates

In this work, the hierarchical topology ring (HTR+) algorithm, an extension of the HTR algorithm, was developed for identifying gas hydrate types, cage structures, and grain boundaries (GBs) within polycrystalline structures. Utilizing molecular dynamics trajectories of polycrystalline hydrates, the accuracy of the HTR+ algorithm is validated in identifying sI, sII and sH hydrate types, hydrate grains, and GBs in multi-hydrate polycrystals, as well as clathrate cages at GBs. Additionally, during the hydrate nucleation and growth processes, clathrate cages, hydrate type, hydrate grains and ice structures are accurately recognized. Significantly, this algorithm demonstrates high efficiency, particularly for large hydrate systems. HTR+ algorithm emerges a powerful tool for identifying micro/mesoscopic structures of gas hydrates, enabling an in-depth understanding of the formation mechanisms and properties of gas hydrates.

Application of Synthesized Hydrates in the National Economy

Abstract The article analyses the thermodynamic conditions for long-term storage of hydrates and proposes a methodology for calculating the main thermodynamic parameters that ensure long-term stability of the structure of gas hydrates. A criterion for optimizing thermodynamic parameters and heat and mass transfer parameters for effective introduction of new technology

Investigation of kinetics of semi-clathrate formation for CO2 capture in the presence of L-methionine amino acid

ABSTRACT Gas hydrates cater to several applications including methane/natural gas storage, CO2 capture, gas separation, desalination, etc. Semi-clathrates are a class of inclusion compounds in which a part of guest molecule (typically anion of tertiary butyl ammonium salts) involves in the formation of cage structure along with water and the other remaining part of guest molecule (cation) occupies and stabilizes the gas hydrate structure. This study is the first effort to explore the kinetics of the formation of tertiary butyl ammonium chloride (TBAC) semi clathrate in the presence of CO2 without and with the addition of a kinetic promoter (L-methionine amino acid). Experimental conditions of the study include starting pressure of 3.4 MPa and temperatures in the range of 279.2–283.2 K. Kinetics of TBAB-CO2 and TBAC-CO2 mixed semi-clathrates have been compared and reported along with morphology observations under similar experimental conditions. TBAC-CO2 mixed semi-clathrates showcase significant reduction in induction time in comparison to TBAB-CO2 mixed semi-clathrates. Also, gas uptake (CO2 storage capacity) is observed to be slightly better for TBAC mixed semi-clathrates at lower concentrations (5 wt%). Though l-methionine amino acid aids in the reduction of t90 (time taken for 90% completion of hydrate formation) for TBAB-CO2 system, such an effect was not observed for TBAC-CO2 system. Formed semi-clathrates can be easily dissociated with low-grade waste heat to release the stored CO2 (based on the observed dissociation by thermal stimulation in conducted experiments), making it an excellent contender for Carbon Capture Utilization and Storage (CCUS) Technology.

THREE-DIMENSIONAL VELOCITY STRUCTURE OF THE CRUST IN CENTRAL LAKE BAIKAL FROM LOCAL SEISMIC TOMOGRAPHY

This work deals with the importance of studying seismicity and deep structure of the Earth’s crust in the region of the Baikal rift zone. The study presents a three-dimensional velocity structure of the Earth’s crust in the central part of Lake Baikal, obtained from the results of tomographic inversion of the travel times of P- and S-waves from more than 800 seismic events. Synthetic tests provide substantiation for the resolution of the tomographic inversion algorithm. The seismic structure of the crust was obtained to a depth of 35 km and has a direct relationship with the geological structure. The three-dimensional distributions of seismic P- and S-wave velocity anomalies are in good agreement with each other.The sharp contrast between the anomalies may indicate a difference in the material composition of the basement of the Central Baikal basin. At a 15-km depth below the Selenga River delta, there is observed a strong low-velocity anomaly which confirms the presence of a thick sedimentary cover therein. In the basement (at depths of 20 km or greater), to the northeast of the intersection between the Delta fault and the Fofanov fault, there occurs a high-velocity anomaly elongated towards the Olkhon Island. This anomaly is probably related to a rigid block in the earth’s crust. The same depths, on the western side of the Baikal-Buguldeika fault, show a reduced Vp/Vs ratio: 1.56–1.65 versus 1.70–1.75 in the adjacent areas. This indicates another type of basement rock composition and the presence of consolidated matter there.Besides, there has been made a more accurate hypocenter determination for further comparison between seismic events and active fault structures. For the central part of Lake Baikal, the distribution of seismicity mainly corresponds to depths of 10–22 km. The situation is different below the Selenga Delta – the only area where seismicity is observed at depths greater than 22 km, – which can be attributed to complex fault interactions.The velocity anomalies discussed herein are confined to reliably identified active faults and correlate well with the distribution of seismicity and gas hydrate structures.

Dissolution migration of gas, a mechanism to enrich ethane near the BSR and increase upwardly C1/C2 ratios in the hydrate-occurring zones: Insight from pore-scale experimental observation

Most experiments show that gas hydrates are often enriched in C2+ gases relative to the feeding gas source because of fractionation during hydrate crystallization directly from free gases and dissolved gases. However, sediments below and near the base of the gas hydrate stability zone (BGHSZ) in many ocean drilling program (ODP)/International Ocean Discovery Program (IODP) sites are relatively enriched in C2+ hydrocarbon gases, compared with the hydrate-occurring zone above. It is still unclear what kind of process causes the abrupt decreases in C1/C2+ ratios with the depth in headspace gas in sediments around seismic bottom-simulating reflector (BSR) and increasing upward C1/C2 ratios in the hydrate-occurring zone. To test the “dissolution/migration mechanism” and its links to the enrichment of ethane near the BSR and increasing upward C1/C2 ratios in the hydrate-occurring zone, we performed a series of pore-scale experimental observations, simulating the gas dissolution–migration–hydration processes, and investigated the effects of the composition of feeding gases and temperatures on the composition of the hydrate grown under the dissolution–migration mechanism. Hydrates are grown from aqueous fluids supplied by the migration of gases dissolved from the capillary-trapped free gas in a capillary high-pressure optical cell, with different supplying gases (90 mol% CH4 + 10 mol% C2H6, 80 mol% CH4 + 20 mol% C2H6) and a geothermal gradient (temperature from 278.15 to 293.15 K). The gas hydrate structure and composition were determined by quantitative Raman spectroscopy. Our study indicated that (1) under the dissolution–migration–hydration processes, the mole fraction of C2H6 in hydrates is depleted compared with gas sources, which confirms that the dissolution–migration of gases is a mechanism to enrich ethane near the BSR; (2) the proportion of C2H6 in structure I (sI) or structure II (sII) hydrates decreases with decreasing temperature, and decreasing temperature enlarges the difference of diffusion coefficient between methane and ethane and enhances the gas fractionation during migration, which could cause the increase upwardly C1/C2 ratios in the hydrate-occurring zone. A simplified geological model was proposed to explain the variability of hydrate composition with depth in the hydrate-occurring zone and the fractionation of gases near the BSR.

Estimation of the gas hydrate saturation from multichannel seismic data on the western continental margin of the Chukchi Rise in the Arctic Ocean

A multichannel seismic survey was conducted to investigate the geophysical characteristics of gas hydrates along the western continental margin of the Chukchi Rise around an ARAON mound cluster, which was first recovered in 2016. In the seismic data, gas hydrate-related bottom simulating reflection was widely distributed along the western continental margin of the Chukchi Rise. High-precision seismic P-wave velocity was obtained to investigate the geophysical characteristics of the gas hydrate structures in the BSR areas. Iterative migration velocity analysis was used to construct a detailed P-wave velocity model from the acquired seismic data. The gas hydrate and free gas layers have abnormally high- and low-seismic P-wave velocities; the precise velocity model allows us to understand the detailed spatial distribution of gas hydrate and free gas structures. The effective medium theory model enables estimations of the gas hydrate saturation from constructed seismic P-wave velocity model. We propose the P-wave velocity and gas hydrate saturation models from acquired multichannel seismic data in the western continental margin of the Chukchi Rise for the first time.

Mini Review on Application and Outlook of In Situ Raman Spectrometry in Gas Hydrate Research

Natural gas hydrates have drawn much attention as a result of their considerable reserves, clean energy, and global distribution. Some new technologies, such as energy storage and transportation, gas separation, and desalination, have been developed via forming hydrates. The structure, composition, and cage occupancy of gas hydrates and kinetics of hydrate formation and dissociation are crucial in flow assurance and the applications mentioned above. In view of its dependable, efficient, and non-destructive features, in situ Raman spectroscopy is a typical and popular measuring technique in gas hydrates. In this mini review, the application and microscopic characterization using in situ Raman spectroscopy in the gas hydrate field were reviewed, including how to use Raman spectroscopy to identify the structure and molar composition of hydrate and how to offer in situ information on the kinetics of formation, decomposition, replacement, and molecular diffusion at the molecular level. The challenges and perspectives on future studies of in situ Raman spectrometry have been proposed.

Gas hydrates in nature and in the laboratory: necessary requirements for formation and properties of the resulting hydrate phase

Clathrate hydrates—also known as gas hydrates—are ice-like compounds consisting of gas and water molecules. They occur wherever elevated pressures and low temperatures prevail; and where enough water and hydrate-forming gas molecules are available. Therefore, natural gas hydrates occur at all active and passive continental margins, in permafrost regions, in some deep lakes, and under unfavorable circumstances, also, in pipelines. This article provides an overview of the (thermodynamic) requirements and various models for the nucleation and growth of gas hydrates and the different gas hydrate structures that may occur and which have been detected in nature. Furthermore, this study also shows the influence of the properties of the enclosed gas molecules such as size and shape on the structure and thermodynamic properties of the resulting hydrate phase. Finally, the complexity of a natural environment with regard to the various influences of sediments, microbial activity, and salinity of the pore fluid on hydrate formation is also discussed.

Advances in Characterizing Gas Hydrate Formation in Sediments with NMR Transverse Relaxation Time

The formation process, structure, and distribution of gas hydrate in sediments have become focal points in exploring and exploiting natural gas hydrate. To better understand the dynamic behavior of gas hydrate formation in sediments, transverse relaxation time (T2) of nuclear magnetic resonance (NMR) is widely used to quantitatively characterize the formation process of gas hydrate and the change in pore characteristics of sediments. NMR T2 has been considered as a rapid and non-destructive method to distinguish the phase states of water, gas, and gas hydrate, estimate the saturations of water and gas hydrate, and analyze the kinetics of gas hydrate formation in sediments. NMR T2 is also widely employed to specify the pore structure in sediments in terms of pore size distribution, porosity, and permeability. For the recognition of the advantages and shortage of NMR T2 method, comparisons with other methods as X-ray CT, cryo-SEM, etc., are made regarding the application characteristics including resolution, phase recognition, and scanning time. As a future perspective, combining NMR T2 with other techniques can more effectively characterize the dynamic behavior of gas hydrate formation and pore structure in sediments.

A New Method for the Rapid Synthesis of Gas Hydrates for their Storage and Transportation

Abstract This presents the analysis of the main reasons for a significant decrease in the intensity of diffusion processes during formation of gas hydrates; solutions to this problem are proposed in a new process flow diagram for the continuous synthesis of gas hydrates. The physical processes, occurring at corresponding stages of the process flow are described in detail. In the proposed device, gas hydrate is formed at the boundary of gas bubbles immersed in cooled water. The dynamic effects arising at the bubble boundary contribute to destruction of a forming gas hydrate structure, making it possible to renew the contact surface and ensure efficient heat removal from the reaction zone. The article proposes an assessment technique for the main process parameters of the synthesis of gas hydrates based on the criterion of thermodynamic parameters optimization.

Differentiation of communities of macroinvertebrates and cottoid fish associated with methane seeps of different bottom landscapes of Lake Baikal

The paper presents the results of the study of communities of macroinvertebrates and cottoid fish inhabiting methane seeps of Lake Baikal. For the analysis, we used video surveillance and collection of animals carried out with the help of "Mir" deep-water submersible, as well as NIOZ-type box-corer samplers from the board of a research vessel. Posolskaya Bank and Saint Petersburg methane seeps are located in different basins (southern and middle) and at different depths (300–500 m and ~ 1400 m), characterized by the different underwater landscapes (slope of underwater upland and hills formed by gas hydrates), by the structure of gas hydrates and their depth location in sediments, as well as the composition of microbial mats and communities of microorganisms of bottom sediments. Both seeps are characterized by bubble discharge of methane gas and the formation of highly productive communities of large invertebrates and cottoid fish on seep habitats. Seep animal communities consisted of species-depleted invertebrates and fish of the surrounding deep-water benthal of the Lake. We showed the similarities and differences in the composition of the faunas of two seeps, as well as the quantitative characteristics of taxonomic groups of macroinvertebrates and cottoid fishes. Obligate species have not been revealed on the methane seep Posolskaya Bank. For the methane seep Saint Petersburg, the gastropod species Kobeltocochlea tamarae Sitnikova, Teterina et Maximova, 2021 (Caenogastropoda: Benedictiidae) was designated as an obligate species; among bottom cottoid fishes, Neocottus werestschagini (Taliev, 1953) (Cottoidei: Abyssocottidae) had possible a transitional state to obligate. We presented the data on the assimilation by seep animals of mixed photo- and chemosynthetic food with different proportions of methane-derived carbon. A hypothesis has been substantiated that deep-water seep areas could serve as refugium for the preservation of endemic fauna during the Pliocene-Pleistocene glaciations of Lake Baikal.

Insights into the porous structure of surfactant-promoted gas hydrate

We show how insights into the porous structure of surfactant-promoted gas hydrate can be obtained from two types of experiments that probe very different length scales. On the one hand, the observation of an existing porous hydrate being imbibed with the aqueous surfactant solution gives information as to the porosity – inferred from the volume increase – and the average pore size – inferred from the imbibition rate. On the other hand, direct pore visualization and determination of the fluid and hydrate contents at the micron scale are obtained by optical microscopy and Raman microspectroscopy. We observe a porosity in the range of 60–70% and pore sizes of about 20–30 μm for a porous methane hydrate formed from a 500 ppmw SDS (sodium dodecyl sulfate) solution under moderate subcooling.

Intensification of Gas Hydrate Formation Processes by Renewal of Interfacial Area between Phases

This paper presents the analysis of the main reasons for a significant decrease in the intensity of diffusion processes during the formation of gas hydrates; solutions to this problem are proposed in a new process flow diagram for the continuous synthesis of gas hydrates. The physical processes, occurring at the corresponding stages of the process flow, have been described in detail. In the proposed device, gas hydrate is formed at the boundary of gas bubbles immersed in cooled water. The dynamic effects arising at the bubble boundary contribute to the destruction of a forming gas hydrate structure, making it possible to renew the contact surface and ensure efficient heat removal from the reaction zone. The article proposes an assessment technique for the main process parameters in the synthesis of hydrates based on the criterion of thermodynamic parameters optimization. The optimization criterion determines the relationship of intensity of heat and mass transfer processes at the phase contact interface of reacting phases, correlating with the maximum GH synthesis rate, and makes it possible to determine optimum thermodynamic parameters in the reactor zone.

Numerical Simulation on the Dissociation, Formation, and Recovery of Gas Hydrates on Microscale Approach.

Investigations into the structures of gas hydrates, the mechanisms of formation, and dissociation with modern instruments on the experimental aspects, including Raman, X-ray, XRD, X-CT, MRI, and pore networks, and numerical analyses, including CFD, LBM, and MD, were carried out. The gas hydrate characteristics for dissociation and formation are multi-phase and multi-component complexes. Therefore, it was important to carry out a comprehensive investigation to improve the concept of mechanisms involved in microscale porous media, emphasizing micro-modeling experiments, 3D imaging, and pore network modeling. This article reviewed the studies, carried out to date, regarding conditions surrounding hydrate dissociation, hydrate formation, and hydrate recovery, especially at the pore-scale phase in numerical simulations. The purpose of visualizing pores in microscale sediments is to obtain a robust analysis to apply the gas hydrate exploitation technique. The observed parameters, including temperature, pressure, concentration, porosity, saturation rate, and permeability, etc., present an interrelationship, to achieve an accurate production process method and recovery of gas hydrates.

Local structure analysis of low-temperature neutron pair distribution function coupled with molecular dynamics simulations of CH4 and CO2 hydrates from 2 to 210 K

CH4 hydrates occur naturally and are an abundant potential fuel source with a corresponding risk of potent greenhouse gas release, due to their stability conditions at low temperature and high pressure. Byproduct CO2 can be exchanged with CH4 in these natural deposits and can potentially stabilize the hydrates at higher temperatures. CH4, CO2, and mixed CH4-CO2 hydrates are studied with in situ neutron pair distribution function experiments from 2 to 210 K to investigate the impact of varying the CH4-CO2 guest composition in the gas hydrate structure. These experiments combined with Reverse Monte Carlo analysis allow for the characterization of intermolecular CH4 and CO2 interactions with the water molecules which form the hydrate lattice and how they impact the local structure of the lattice itself. Results indicate that when CH4 and CO2 co-occupy the hydrate, the host is more strongly distorted than in the pure CH4 and CO2 hydrates, but this becomes less defined with increasing temperature. The presence of CO2 in mixed hydrate increases the stability range and creates a barrier for CH4 to completely leave the structure.

Структура, морфология и состав природных газовых гидратов отобранных на грязевом вулкане Кедр-1 (оз. Байкал)

Изучена структура, морфология и состав газа для природных газовых гидратов, отобранных на грязевом вулкане Кедр-1 (оз. Байкал). Показано, что все эти гидраты имеют кубическую структуру II, а связанный газ содержит около 14 % этана (остальное метан). Один из образцов представлял собой пористый монолитный слой гидрата с прилегающими к нему сверху и снизу слоями гранул гидрата. И в слое, и в гранулах гидрат имел одну и ту же структуру и состав. Насколько нам известно, ранее такие особенности морфологии природных гидратов не описаны. Обсуждаются возможные пути формирования подобных объектов.

Experimental investigation of the exact role of large-molecule guest substances (LMGSs) in determining phase equilibria and structures of natural gas hydrates

This study examined the complex phase behavior and structures of natural gas hydrates containing large-molecule guest substances (LMGSs). Methylcyclopentane (MCP) and 2,2-dimethylbutane (neohexane, NH) were selected as LMGSs for CH4 (90%) + C3H8 (10%) hydrates. To verify the exact role of LMGSs, the four-phase (hydrate (H) – liquid water (LW) – liquid LMGS (LLMGS) – vapor (V)) equilibria of the CH4 + C3H8 + LMGS + water mixtures were measured. The hydrate phase equilibria demonstrated that the presence of LMGSs did not contribute to shifts in the equilibrium curves of the CH4 + C3H8 hydrates. The structures and guest distributions of the gas hydrates containing LMGSs were analyzed using nuclear magnetic resonance (NMR) and powder X-ray diffraction (PXRD). 13C NMR spectra and PXRD patterns revealed the coexistence of structure II (sII) and structure H (sH) hydrates in the CH4 + C3H8 + LMGS + water mixtures. CH4 and LMGSs were enclathrated in sH, while CH4 and C3H8 were encapsulated in sII. The experimental results covering complex phase equilibria and structural characteristics of natural gas hydrates containing LMGSs will contribute to a better understanding of the exact role of LMGSs in natural gas hydrates.

Characteristics of hydrate-bound gas retrieved at the Kedr mud volcano (southern Lake Baikal)

We reported the characteristics of hydrate-bound hydrocarbons in lake-bottom sediments at the Kedr mud volcano in Lake Baikal. Twenty hydrate-bearing sediment cores were retrieved, and methane-stable isotopes of hydrate-bound gases (δ13C and δ2H of − 47.8‰ to − 44.0‰ V-PDB and − 280.5‰ to − 272.8‰ V-SMOW, respectively) indicated their thermogenic origin accompanied with secondary microbial methane. Powder X-ray diffraction patterns of the crystals and molecular composition of the hydrate-bound gases suggested that structure II crystals showed a high concentration of ethane (around 14% of hydrate-bound hydrocarbons), whereas structure I crystals showed a relatively low concentration of ethane (2–5% of hydrate-bound hydrocarbons). These different crystallographic structures comprised complicated layers in the sub-lacustrine sediment, suggesting that the gas hydrates partly dissociate, concentrate ethane and form structure II crystals. We concluded that a high concentration of thermogenic ethane primarily controls the crystallographic structure of gas hydrates and that propane, iso-butane (2-methylpropane) and neopentane (2,2-dimethylpropane) are encaged into crystals in the re-crystallisation process.

On a new nitrogen sX hydrate from ice XVII.

Recently, a new gas hydrate structure has been discovered. This structure, sX, is unique in a sense that it is so far the only gas hydrate with chiral channels. It is formed by hydrogen-water or carbon dioxide-water mixtures at pressures above 0.300 GPa, and it has been shown that it is the only clathrate hydrate that is refillable with hydrogen. This property makes it a possible storage material for gases. By analyzing neutron diffraction data and calculations based on density-functional theory, we show that sX is also refillable with nitrogen; the guest:host ratio will be shown to be 2.6(3). Furthermore, we report sX's decomposition behavior and give evidence that it undergoes several transitions into the exotic hydrates sH and sIII that have not been observed at these pressure and temperature conditions-before forming the stable nitrogen hydrate sII.