X‐ray synchrotron diffraction study of natural gas hydrates from African margin

Bourry, Christophe; Charlou, Jean-Luc; Donval, Jean-Pierre; Brunelli, Michela; Focşa, Cristian; Chazallon, Bertrand

doi:10.1029/2007gl031285

Cited by 17 publications

(12 citation statements)

References 36 publications

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“…The situation is illustrated in Figure 1 and Figure 2 for CH 4 -and CO 2 -hydrates. As an example we quote the large discrepancies between the works of [3][4][5][6]. For methane hydrate none of these agrees with any other, while the discrepancies are much larger than the respective estimated standard deviations.…”

Section: Introductionmentioning

confidence: 79%

“…For methane hydrate none of these agrees with any other, while the discrepancies are much larger than the respective estimated standard deviations. As the natural variation of lattice constants for methane hydrate was found to be fairly small with respect to the inter-experimental disagreement [3], the reason for this behavior can be suspected to be systematic errors in the data collection procedures of which we will discuss some of the most important ones in the following. The lattice constants quoted in literature can be affected by uncertainties of the measured lattice constants themselves but also by uncertainties of the temperature measurement.…”

Section: Introductionmentioning

confidence: 99%

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Lattice constants and expansivities of gas hydrates from 10 K up to the stability limit

Hansen

Falenty

Kuhs

2016

The Journal of Chemical Physics

107

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In a combination of neutron and synchrotron diffraction the lattice constants and expansivities of hydrogenated and deuterated CH 4 -, CO 2 -, Xe-(structure type I) and N 2 -hydrate (structure type II) from 10 K up to the stability limit under pressure were established. Some important results emerge from our analysis: (1) Despite the larger guest-size of CO 2 as compared to methane, CO 2 -hydrate has the smaller lattice constants at low temperatures which we ascribe to the larger attractive guest-host interaction of the CO 2 -water system. (2) The expansivity of CO 2 -hydrate is larger than for CH 4 -hydrate which leads to larger lattice constants for the former at temperatures above ~ 150 K; this is likely due to the higher motional degrees of freedom of the CO 2 guest molecules (3) The cage filling does not affect significantly the lattice constants in CH 4 -and CO 2 -hydrate in contrast to Xe-hydrate for which the effect is quantitatively established. (4) Similar to ice Ih, the deuterated compounds have slightly larger lattice constants for all investigated systems which can be ascribed to the somewhat weaker H-bonding; the isotopic difference is smallest for the Xesystem, in which the large Xe atoms lead to an increase of averaged H-bond distances. (5) Compared to ice Ih the high temperature expansivities are about 50% larger; in contrast to ice Ih, there is no negative thermal expansion at low temperature.

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Section: Introductionmentioning

confidence: 79%

Section: Introductionmentioning

confidence: 99%

Lattice constants and expansivities of gas hydrates from 10 K up to the stability limit

Hansen

Falenty

Kuhs

2016

The Journal of Chemical Physics

107

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“…where n H 2 O is the number of water molecules per unit cell in the hydrate structure ͑sI, sII, or sH͒, M is the molecular weight, N AV is the Avogrado's number, and V c is the volume of the unit cell. In Table III the experimental lattice parameters [74][75][76][77][78] for two different temperatures are given. The experimental densities of the empty gas hydrate as estimated from Eq.…”

Section: Resultsmentioning

confidence: 99%

“…͑8͒ are also presented. Obviously the unit cell parameters depend on temperature, 74,77 and thus so does the density. As can be seen the densities of the empty hydrates are lower than that of ice I h ͑which is about 79 0.92 g / cm 3 ͒, and for this reason these phases are ideal candidates to occupy the phase diagram of water at negative pressures 18 ͑i.e., at constant temperature any phase transition always decreases the density when decreasing the pressure, as stated by Bridgmann 80 ͒.…”

Section: Resultsmentioning

confidence: 99%

Can gas hydrate structures be described using classical simulations?

Conde

Vega

McBride

et al. 2010

The Journal of Chemical Physics

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Quantum path-integral simulations of the hydrate solid structures have been performed using the recently proposed TIP4PQ/2005 model. By also performing classical simulations using this model, the impact of the nuclear quantum effects on the hydrates is highlighted; nuclear quantum effects significantly modify the structure, densities, and energies of the hydrates, leading to the conclusion that nuclear quantum effects are important not only when studying the solid phases of water but also when studying the hydrates. To analyze the validity of a classical description of hydrates, a comparison of the results of the TIP4P/2005 model (optimized for classical simulations) with those of TIP4PQ/2005 (optimized for path-integral simulations) was undertaken. A classical description of hydrates is able to correctly predict the densities at temperatures above 150 K and the relative stabilities between the hydrates and ice I(h). The inclusion of nuclear quantum effects does not significantly modify the sequence of phases found in the phase diagram of water at negative pressures, namely, I(h)-->sII-->sH. In fact the transition pressures are little affected by the inclusion of nuclear quantum effects; the phase diagram predictions for hydrates can be performed with reasonable accuracy using classical simulations. However, for a reliable calculation of the densities below 150 K, the sublimation energies, the constant pressure heat capacity, and the radial distribution functions, the incorporation of nuclear quantum effects is indeed required.

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“…The structural and compositional data of natural gas hydrates are important for developing technologies to extract this energy resource from the sea floor and to establish their formation/dissociation rates and stability field. Molecular and isotopic composition of the main hydrate-bound gases are plentifully detailed (Milkov, 2005), but only few physical characterizations have been performed on natural samples from the Gulf of Mexico (Davidson et al, 1986;Yousuf et al, 2004), Black Ridge (Uchida et al, 1999), Mallik (Tulk et al, 2000), Northeast Pacific continental margin off Oregon (Gutt et al, 1999), Cascadia Margin (Yousuf et al, 2004), the Okhotsk sea (Takeya et al, 2006), offshore Vancouver Island (Lu et al, 2005), the Congo-Angola basin (Charlou et al, 2004;Chazallon et al, 2007) and the Nigerian margin (Chazallon et al, 2007;Bourry et al, 2007). The present work extends these physical and chemical studies of natural gas hydrates to new specimens recovered from the Sea of Marmara, during the Marnaut cruise (May-June 2007).…”

Section: Introductionmentioning

confidence: 99%

Free gas and gas hydrates from the Sea of Marmara, Turkey

Bourry¹,

Chazallon²,

Charlou³

et al. 2009

Chemical Geology

Self Cite

115

131

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Gas hydrates and gas bubbles were collected during the MARNAUT cruise (May-June 2007) in the Sea of Marmara along the North Anatolian Fault system, Turkey. Gas hydrates were sampled in the western part of the Sea of Marmara (on the Western High), and three gas-bubble samples were recovered on the Western High, the Central High (center part of the Sea of Marmara) and in the Çinarcik Basin (eastern part of the Sea of Marmara). Methane is the major component of hydrates (66.1%), but heavier gases such as C 2 , C 3 , and i-C 4 are also present in relatively high concentration. The methane contained within gas hydrate is clearly thermogenic as evidenced by a low C 1 /C 2 + C 3 ratio of 3.3, and carbon and hydrogen isotopic data (δ 13 C CH4 of − 44.1‰ PDB and δD CH4 of − 219‰ SMOW). A similar signature is found for the associated gas bubbles (C 1 /C 2 + C 3 ratio of 24.4, δ 13 C CH4 of − 44.4‰ PDB) which have the same composition as natural gas fromK. Marmara-af field. Gas bubbles from Central High show also a thermogenic origin as evidenced by a C 1 /C 2 + C 3 ratio of 137, and carbon and hydrogen isotopic data (δ 13 C CH4 of − 44.4‰ PDB and δD CH4 of − 210‰ SMOW), whereas those from the Çinarcik Basin have a primarily microbial origin (C 1 /C 2 + C 3 ratio of 16,600, δ 13 C CH4 of − 64.1‰ PDB). UV-Raman spectroscopy reveals structure II for gas hydrates, with CH 4 trapped in the small (5 12) and large (5 12 6 4) cages, and with C 2 H 6 , C 3 H 8 and i-C 4 H 10 trapped in the large cages. Hydrate composition is in good agreement with equilibrium calculations, which confirm the genetic link between the gas hydrate and gas bubbles at Western High and the K.Marmara-af offshore gas field located north of the Western High. We calculate the characteristics of the hydrate stability zone at Western High and in the Çinarcik Basin using the CSM-GEM computer program. The base of the structure II hydrate stability field is at about 100 m depth below the seafloor at the Western High site, whereas in the Çinarcik Basin, P-T conditions at the seafloor correspond to the uppermost range for structure I hydrate formation from microbial gas.

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X‐ray synchrotron diffraction study of natural gas hydrates from African margin

Cited by 17 publications

References 36 publications

Lattice constants and expansivities of gas hydrates from 10 K up to the stability limit

Lattice constants and expansivities of gas hydrates from 10 K up to the stability limit

Can gas hydrate structures be described using classical simulations?

Free gas and gas hydrates from the Sea of Marmara, Turkey

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