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| Titel |
Natural and synthetic gas hydrates studied by Raman spectroscopy |
| VerfasserIn |
Jean-Philippe Savy, Nikolaus Bigalke, Giovanni Aloisi, Elke Kossel, Moritz Pansegrau, Matthias Haeckel |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250036287
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| Zusammenfassung |
| Over the past decade, the interest in using CH4-hydrates as an energy resource and
CO2-hydrates as a storage option for anthropogenic CO2 has grown in the scientific
community as well as in the oil and gas industry. Among all the techniques used to
characterize gas hydrates, the non-destructive, non-invasive Raman spectroscopy provides
significant insights into the structure and composition of hydrates.
In this study, we compare gas hydrates synthetically produced in the laboratory with
natural hydrate samples collected from marine sediments. CO2 and CH4 gas hydrates were
investigated with a high-resolution Raman microscope at in-situ p-T conditions. A
water-filled glass capillary (inner diameter: 1.7 mm) was placed in a stainless steel cell,
which was sealed, cooled down to 3.6 Ë C and pressurized to 60 bar with liquid CO2. Video
images taken after 1 h revealed droplets (~10 μm in diameter) trapped in the ice-like solid.
The two Fermi dyads of CO2 in the liquid and hydrate phase at 1274 & 1381 cm1 and 1280 &
1384 cm-1, respectively, confirm the presence of liquid CO2 droplets trapped in a
CO2-hydrate matrix.
Equivalent experiments were conducted with CH4 gas at 1 Ë C and 90 bar.
The nucleation of CH4-hydrate was followed in the Raman spectral region of the
C-H stretching mode. At the early stage of the nucleation, the peak at 2915 cm-1
(CH4 in small cages) was stronger than the one at 2904 cm-1 (CH4 in large cages)
indicating that methane starts to populate the small 512 cages of the s-I hydrate structure
first and then, as nucleation continues, the large cages are stabilized leading to
a quickly growing peak at 2904 cm-1 until a final peak intensity ratio of 3.7 is
established. In contrast to other studies, intermediate stabilization of the s-II structure was
not observed. Video images confirmed the absence of gas inclusions. The hydrate
density, 1.1 & 0.9 for CO2-hydrate and CH4-hydrate respectively, compared to the
one of water may explain the formation of inclusions during the crystallization of
hydrates.
Finally, we investigated a natural CH4 hydrate sample collected from Hikurangi Margin,
New Zealand. Areal mapping of the sample revealed a constant peak area ratio of 3.7
between large and small cages, identical to our synthetic CH4-hydrate. We also used the
relative Raman signal intensities between CH4 and H2O to quantify the spatial variation in
cage occupancy and methane concentration in the natural hydrate sample. Compared to
synthetic CH4-hydrate, the natural sample shows an inhomogeneous overall distribution of
methane content. |
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