Gas Hydrate Structure Research Articles

Synthesis and characterization of a new structure of gas hydrate

Atoms and molecules <0.9 nm in diameter can be incorporated in the cages formed by hydrogen-bonded water molecules making up the crystalline solid clathrate hydrates. For these materials crystallographic structures generally fall into 3 categories, which are 2 cubic forms and a hexagonal form. A unique clathrate hydrate structure, previously known only hypothetically, has been synthesized at high pressure and recovered at 77 K and ambient pressure in these experiments. These samples contain Xe as a guest atom and the details of this previously unobserved structure are described here, most notably the host-guest ratio is similar to the cubic Xe clathrate starting material. After pressure quench recovery to 1 atmosphere the structure shows considerable metastability with increasing temperature (T <160 K) before reverting back to the cubic form. This evidence of structural complexity in compositionally similar clathrate compounds indicates that the reaction path may be an important determinant of the structure, and impacts upon the structures that might be encountered in nature.

Tetrahydrofuran hydrate crystal growth inhibition by hyperbranched poly(ester amide)s

Kinetic hydrate inhibitors (KHIs) are water-soluble polymers designed to delay gas hydrate formation in gas and oilfield operations. Inhibition of growth of gas hydrate crystals is one of the mechanisms by which KHIs have been proposed to act. One class of commercial KHIs is the hyperbranched poly(ester amide)s. We have investigated the ability of a range of structurally different hyperbranched poly(ester amide)s to inhibit the crystal growth of tetrahydrofuran (THF) hydrate which forms a Structure II clathrate hydrate, the most common gas hydrate structure encountered in the upstream oil and gas industry. The results indicate that there is an optimum size of hydrophobic groups attached to the succinyl part of the polymer, which gives best crystal growth inhibition. However, total inhibition was impossible to achieve even at a concentration of 8000 ppm of one of the best polymers at a subcooling of 3.4 °C, tentatively suggesting that polymer adsorption onto natural gas hydrate crystal surfaces is probably not the primary mechanism of kinetic inhibition operating in field applications with this class of KHI.

Microstructures of structure I and II gas hydrates from the Gulf of Mexico

Gas hydrate samples from various locations in the Gulf of Mexico (GOM) differ considerably in their microstructure. Distinct microstructure characteristics coincide with discrete crystallographic structures, gas compositions and calculated thermodynamic stabilities. The crystallographic structures were established by X-ray diffraction, using both conventional X-ray sources and high-energy synchrotron radiation. The microstructures were examined by cryo-stage Field-Emission Scanning Electron Microscopy (FE-SEM). Good sample preservation was warranted by the low ice fractions shown from quantitative phase analyses. Gas hydrate structure II samples from the Green Canyon in the northern GOM had methane concentrations of 70–80% and up to 30% of C 2–C 5 of measured hydrocarbons. Hydrocarbons in the crystallographic structure I hydrate from the Chapopote asphalt volcano in the southern GOM was comprised of more than 98% methane. Fairly different microstructures were identified for those different hydrates: Pores measuring 200–400 nm in diameter were present in structure I gas hydrate samples; no such pores but dense crystal surfaces instead were discovered in structure II gas hydrate. The stability of the hydrate samples is discussed regarding gas composition, crystallographic structure and microstructure. Electron microscopic observations showed evidence of gas hydrate and liquid oil co-occurrence on a micrometer scale. That demonstrates that oil has direct contact to gas hydrates when it diffuses through a hydrate matrix.

Seismic analysis and distribution of a bottom-simulating reflector (BSR) in the Chilean margin offshore of Valdivia (40° S)

Multi-channel seismic (MCS) reflection data recorded offshore from Valdivia (40° S), in the Chilean margin, were processed to obtain a seismic image to establish structural characteristics and relate them to the presence of the bottom-simulating reflector (BSR). Seismic structure velocity of the BSR was determined using 1-D forward modeling. Recorded seismograms for two representative common mid-point (CMP) gathers were compared with synthetics, using different physical parameters to fit the waveforms. Our results confirm the presence of gas hydrates above the BSR. The BSR spatial continuity appears to be either interrupted or irregular due to the presence of faults. Tectonic movements can change the gas hydrate stability zone and consequently the BSR disappears or becomes weaker. Structural and topographic factors, differences in concentration, vertical distribution characteristics and internal structure of gas hydrates can influence BSR amplitude behavior. Variability in the concentration, volume, and extra supply of free gas coming from faults could be the main factors in the change of BSR amplitudes. The inclusion of the attenuation factor in the modeling supports the existence of free gas below the BSR. It is possible that the free gas below the BSR is distributed in bubbles or “bags”.

Structure and kinetics of gas hydrates from methane/ethane/propane mixtures relevant to the design of natural gas hydrate storage and transport facilities

AbstractThe kinetics of gas hydrate growth from binary CH4/C2H6 and CH4/C3H8 and ternary CH4/C2H6/C3H8 gas mixtures were obtained by the gas uptake method in a semibatch stirred vessel at constant pressure and a temperature of 273.7 K. These data are of interest for the design of facilities for natural gas storage and transportation in the solid (hydrate) state. During hydrate formation, samples from the gas phase were taken and analyzed by gas chromatography. It was found that the molar composition of CH4 in the vapor phase increased as hydrate crystallization progressed. The observed fractionation effect (enrichment of the hydrate phase with propane) complicates the natural gas storage process. The fractionation effect was also confirmed with molecular‐level studies where hydrate from the CH4/C2H6/C3H8 gas mixture was characterized by powder X‐ray diffraction (PXRD), NMR, and Raman spectroscopy. The hydrate phase composition and cage occupancy of each gas were calculated with the help of information obtained by Raman spectroscopy, gas chromatography, and PXRD. The results were consistent with those obtained by NMR. The composition of the gas phase and the hydrate are found to evolve over time, suggesting that kinetic and transport factors contribute in addition to thermodynamics. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Interaction Study of Guest with Host in Clathrate Hydrate

Lattice dynamical simulations of noble gas hydrate structures I and II have been performed. Potential energies were investigated to study the influence of guest species on the stability of the hydrate structure. Results show that when the diameter of inclusion molecules is between 3 Å and 4.2 Å, such as Ar and Kr, the critical role of the 5 12 cage in the stabilization of hydrates becomes effective. For Xe hydrates SI and SII, with the help of lattice dynamical calculations, the modes attributions are identified directly. We proposed the resonant effect of the fingerprint frequency at about 7 meV and 10 meV which arise from the coupling of Xe molecules in the 5 12 cage with the host lattice.

Gas Clathrate Hydrates Experiment for High School Projects and Undergraduate Laboratories

We present a laboratory procedure, suitable for high school and undergraduate students, for preparing and studying propane clathrate hydrate. Because of their gas storage potential and large natural deposits, gas clathrate hydrates may have economic importance both as an energy source and a transportation medium. Similar to pure ice, the gas hydrate structure consists of a lattice of water molecules held together by hydrogen bonds. However, the gas hydrate lattice is more complex than that of ice: cages are formed in which the molecules are trapped. Despite the fact that the gas hydrate lattice is less stable than the ice lattice, the van der Waals forces between the guest molecules and the host lattice stabilizes the overall structure. Propane clathrate hydrate is an especially interesting example because the solid is a mixture of two chemicals that are immiscible in the liquid state yet it has a higher melting point than that of pure water ice. A simple procedure is outlined for students ...

Phase formation and structure of high-pressure gas hydrates and modeling of tetrahedral frameworks with uniform polyhedral cavities

During the last decade, a number of high-pressure gas hydrates have been prepared and structurally characterized. One of the most interesting results obtained is the formation of compounds having tetrahedral water frameworks with uniform polyhedral cavities (e.g., clathrates THF·6H2O and 2Ar·6H2O), unprecedented among ambient pressure hydrates. In order to predict, reveal and perform effective investigations of such gas hydrates at high pressures it is plausible to know all possible tetrahedral frameworks of this kind. The solution of this problem demands to elaborate methods of a topological design of the tetrahedral frameworks represented as space-filling packings of uniform polyhedra with trivalent vertices (simple polyhedra). There are two related approaches to solve this problem: tiling of 3D space into symmetrically equal polyhedra (stereohedra) and generation of periodic four-connected nets. At present, the complete set of tetrahedral frameworks built of uniform simple 14-hedra (23 packings) was obtained and more than 800 frameworks were constructed from larger simple stereohedra. This article gives a discussion of structural and energetic characteristics of the frameworks generated, as well as the possibilities of using these results for interpretation of experimental structural data and a deliberate synthesis of clathrate compounds possessing new structures. Experimental and theoretical data show a high probability of finding a whole series of novel high-pressure gas hydrates with the tetrahedral water frameworks built of simple stereohedra. Of these, the most probable structures are those with 14-hedral cavities, these structures having the stoichiometry of six water molecules per cavity. In our opinion, the formation of such hydrates could be expected for guest molecules with van der Waals diameters from 5.8 to 7.2 Å. However, these hydrates cannot be excluded for substantially smaller guests as well, provided that the water framework cavities include two guest molecules. Packing polymorphism (different space filling arrangements of the same stereohedron) revealed in the search for tilings of 3D space into stereohedra offers experimental discovery of this phenomenon, which can be promoted by such delicate effects as the guest-guest interaction. The set of the derived water frameworks as stereohedra space-fillings gives opportunities to select starting structure models in the course of a diffraction study of polycrystalline high-pressure gas hydrate samples.

Predicting the Hydrate Stability Zones of Natural Gases Using Artificial Neural Networks

A feed-forward artificial neural network with 19 input variables (temperature, gas hydrate structure, gas composition and inhibitor concentration in aqueous phase) and 35 neurons in single hidden layer has been developed for estimating hydrate dissociation pressures of natural gases in the presence/absence of inhibitor aqueous solutions. The model has been developed using 3296 hydrate dissociation data gathered from the literature. The reliability of the method has been examined using independent experimental data (not used in training and developing the model). It is shown that the results of predictions are in acceptable agreement with experimental data indicating the capability of the artificial neural network for estimating hydrate stability zones of natural gases.

Appearance and preservation of natural gas hydrate from Hydrate Ridge sampled during ODP Leg 204 drilling

The state of preservation of natural gas hydrate samples, recovered from 6 sites drilled during ODP Leg 204 at southern summit of Hydrate Ridge, Oregon Margin, has been investigated by X-ray diffraction (XRD) and cryo-scanning-electron-microscopy (cryo-SEM) techniques. A detailed characterization of the state of decomposition of gas hydrates is necessary since no pressurized autoclave tools were used for sampling and partial dissociation must have occurred during recovery prior to the quench and storage in liquid nitrogen. Samples from 16 distinct horizons have been investigated by synchrotron X-ray diffraction measurements at HASYLAB/ Hamburg. A full profile fitting analysis (“Rietveld method”) of synchrotron XRD data provides quantitative phase determinations of the major sample constituents such as gas hydrate structure I (sI), hexagonal ice (Ih) and quartz. The ice content (Ih) in each sample is related to frozen water composed of both original existing pore water and the water from decomposed hydrates. Hydrate contents as measured by diffraction vary between 0 and 68 wt.% in the samples we measured. Samples with low hydrate content usually show micro-structural features in cryo-SEM ascribed to extensive decomposition. Comparing the appearance of hydrates at different scales, the grade of preservation seems to be primarily correlated with the contiguous volume of the original existing hydrate; the dissociation front appears to be indicated by micrometer-sized pores in a dense ice matrix.

Fabric of gas hydrate in sediments from Hydrate Ridge—results from ODP Leg 204 samples

Drilling on Hydrate Ridge, offshore Oregon, during ODP Leg 204 enabled us to investigate fabrics of gas hydrate samples in a wide depth range of the gas hydrate stability zone (GHSZ). X-ray computerized tomographic imaging on whole-round samples, frozen in liquid nitrogen, revealed that layered gas hydrate structures are related to variable processes occurring at different sediment depths. Shallow gas hydrates often form layers parallel or sub-parallel to bedding and also crosscut sedimentary strata and other gas hydrate layers, destroying the original depositional fabric. The dynamic processes interacting with this complicated plumbing system in this shallow environment are responsible for such highly variable gas hydrate fabrics. Gas hydrate layers deeper in the sediments are most often dipping with various angles, and are interpreted as gas hydrate precipitates filling tectonic fractures. These originally open fractures are potential candidates for free gas transportation, and might explain why free gas can rapidly emanate from below the bottom-simulating reflector through the GHSZ to the seafloor.

The constructions of algebraic geometry and symmetry laws of gas hydrate structure

The constructions of algebraic geometry and symmetry laws of gas hydrate structure

Slide structure and role of gas hydrate at the northern boundary of the Storegga Slide, offshore Norway

The Storegga Slide off the coast of Norway is one of the largest underwater slide complexes known and has been proposed as a significant source of past methane release into the atmosphere. We present pre-stack depth-migrated images from a new multi-channel seismic data set in the Storegga Slide. The northern scarp of the Storegga Slide has previously been interpreted as a single, large recent slope failure; however, our images show strong evidence for a composite structure consisting of a much older event and recent slumping. We observe onlapping features onto slide deposit highs, and layer thickening as post-slide sediments fill in accommodation space created at the slide scarp, both of which support this conclusion. Displaced fault blocks are overlain by undeformed, flat-lying sediments, also indicating considerable time between slide events. According to dating of the base of the Naust at this location, this older slide event occurred at a minimum of ∼ 250 ka. The causes of submarine slope failure are poorly understood, but previous studies have proposed both earthquakes and dissociation of gas hydrates as triggering mechanisms. Pressure/temperature modeling shows that, assuming steady-state conditions, the bottom simulating reflector (BSR) would have been deeper than the glide plane at the time of slope failure. The base of the gas hydrate stability zone, and any gas that may have been present, likely played only a minor role, if any, in slide initiation at this locale.

The Geological Structure and Prospect of Gas Hydrate over the Dongsha Slope, South China Sea

The area around the Dongsha Island in the northeastern South China Sea was generally known for the occurrence of gas hydrate related features, like BSR, cold seep carbonates and chemoautotrophic bacteria. To precisely evaluate the potential of gas hydrate occurrence in the NE South China Sea, marine geology of the large Dongsha slope region is studied based on more geophysical and geological data including multiple-channel seismic and high resolution sub-bottom topographic images. The east Dongsha slope area represents the largest slumping zone in the northern slope of the South China Sea, and might exhibit the highest potential of gas hydrate occurrences. The central Dongsha slope area, with very thin Cenozoic sediments, was tectonically disturbed by the latest Cenozoic uplifting and magmatism events, therefore is unfavorable for gas hydrate occurrence. The west Dongsha slope area with the deepest Cenozoic depression and moderate to minor slumping bodies might be favorable for gas hydrate occurrence.

Compositional and structural identification of natural gas hydrates collected at site 1249 on ocean drilling program leg 204

In contrast to the structural studies of laboratory-grown gas hydrate, this study has been performed on naturally grown clathrate hydrates from the sea floor. The PXRD pattern of natural gas hydrate shows that the sample had a structure I hydrate. The13C NMR spectrum was obtained for the natural gas hydrate sample in order to identify the cage occupancy of guest molecules and determine the hydration number. The NMR spectrum reveal that the natural gas hydrates used in this study contain only methane with no noticeable amount of other hydrocarbons. The existence of two peaks at different chemical shifts indicates that methane molecules are encapsulated in both large and small cages. In addition, Raman spectroscopic analysis is also carried out to identify natural hydrates and compared with the NMR results. Investigating the composition and structure of natural gas hydrates is essential for applying natural gas hydrates as a novel energy source.

Novel compounds Sn 20Zn 4P 22− vI 8 ( [formula omitted]), Sn 17Zn 7P 22I 8, and Sn 17Zn 7P 22Br 8: Synthesis, properties, and special features of their clathrate-like crystal structures

Tin–zinc pnictidehalides Sn 20Zn 4P 22− v I 8 ( v = 1.2 ), Sn 17Zn 7P 22I 8, and Sn 17Zn 7P 22Br 8 have been prepared as single-phase samples by a standard ampoule procedure. The compounds are isotypic and possess the structure of type-I gas hydrate (clathrate-I type). They all crystallize in the cubic space group Pm 3 ¯ n (no. 223) with the unit cell parameters a = 10.883 ( 2 ) Å (Sn 20Zn 4P 22− v I 8), a = 10.8458 ( 3 ) Å (Sn 17Zn 7P 22I 8), a = 10.7449 ( 2 ) Å (Sn 17Zn 7P 22Br 8), and Z = 1 . The crystal structures of all compounds have been solved from single-crystal X-ray diffraction data sets and refined to R = 0.032 (Sn 20Zn 4P 22− v I 8), R = 0.027 (Sn 17Zn 7P 22I 8), and R = 0.020 (Sn 17Zn 7P 22Br 8). In all structures tin, zinc, and phosphorus atoms form the clathrate-I framework while the guest halogen atoms are encapsulated in the cavities of the framework. The compositions of the synthesized compounds are in agreement with those predicted by using the Zintl–Klemm concept. Sn 20Zn 4P 22− v I 8 contains two partially filled metal sites and vacancies in one of the phosphorus sites and can be formulated as Sn 20Zn 4P 20.8□ 1.2I 8 (or Sn 20Zn 4P 22−1.2I 8). Sn 17Zn 7P 22I 8 and Sn 17Zn 7P 22Br 8 are vacancy-free and reveal three partially filled positions of metal atoms. Thus, Sn 17Zn 7P 22 X 8 and Sn 20Zn 4P 20.8□ 1.2I 8, being the limiting and the intermediate compositions, respectively, of the solid solutions of a general formula Sn 24− n Zn n P 22− v X 8 ( v = ( 14 − 2 n ) / 5 , X=Br, I), show different mechanisms of the structural stabilization. The 119Sn Mössbauer together with the 31P and 119Sn NMR spectra confirm the local environment of tin and phosphorus atoms in the structures of Sn 17Zn 7P 22I 8 and Sn 17Zn 7P 22Br 8. The compounds Sn 17Zn 7P 22I 8 and Sn 17Zn 7P 22Br 8 display diamagnetic and semiconducting behavior. The heat capacity has been measured for Sn 17Zn 7P 22I 8 in a temperature range of 6 K to 400 K. No low-temperature phase transitions have been detected in the measurements.

A wide variety of structures of gas hydrates; methane hydrate under high pressure.

A wide variety of structures of gas hydrates; methane hydrate under high pressure.

Filled ice structure of gas hydrates—a density functional study

The structural, electronic, and spectroscopic properties of a high-pressure phase of methanehydrate (MH-III) are studied by first-principles electronic structure calculations. A detailedanalysis of the atomic positions suggests that ionization of hydrogen-bonded watermolecules occurs around 40 GPa and centring or symmetrization of hydrogen bonds occursaround 70 GPa. These pressures are much lower compared with ionization around 55 GPaand centring around 100 GPa in pure ice. The transition may be observed withlow-temperature IR/Raman spectroscopy of OH stretching modes or neutron diffraction.

Experimental study on isotopic fractionation in water during gas hydrate formation

Isotopic fractionation of oxygen and hydrogen in water caused by gas hydrate formation was investigated experimentally. Two different gas hydrates in structures, Structure I hydrate and Structure II hydrate, were formed with methane and krypton gases in a NaCl solution. Isotopic fractionation during gas hydrate formation was observed by measuring oxygen and hydrogen isotopic compositions in the solutions sampled before and after gas hydrate formation. Heavy isotopes of oxygen and hydrogen in water were depleted in the solution resulting from that those isotopes were concentrated in the gas hydrate. The isotopic fractionation was larger as increasing amounts of gas hydrates that were calculated from both the decrease of gas pressure and increase of NaCl concentration in solution. The isotopic fractionation factors of oxygen and hydrogen in water between gas hydrate and liquid water were determined to be 1.0023-1.0032 and 1.014-1.022, respectively. The significant difference of the gas hydrate structures was not observed beyond the analytical errors. These factors are similar to those between ice and liquid water.

Geochemistry of gas hydrate accumulation offshore NE Sakhalin Island (the Sea of Okhotsk): results from the KOMEX-2002 cruise

Geochemical properties of gas hydrate accumulation associated with an active gas vent on the continental slope offshore northeast Sakhalin Island in the Sea of Okhotsk have been investigated. The pore water chemistry data suggest that the gas hydrates (GHs) were formed in an environment of upward-migrating fluid combined with a mechanism of pore water segregation. The upward infiltration of water enriched mainly by Cl− and K+ species appears to occur on the background of earlier diagenesis processes within the gas vent sediments. The GHs were formed from water with chlorinity ranging from 530 to 570 mM. The δ18O and δD of GH water varied from −1.4 to −1.8‰ and from −13 to −18‰, respectively, representing a mix of seawater and infiltrating fluid water. A complex interaction of pore water, water of ascending fluid and segregated pore water during hydrate formation is also supported by water content measurements and observed gas hydrate structure. The direction of segregated water is opposite to upward fluid migration. Decreasing activity of the gas vent is inferred by comparing the present top of the recovered hydrate layer with previous observations.