HR: 11:35h
AN: OS22A-06    [Abstracts]
TI: Molecular and dissociation studies of natural gas hydrates collected from different oceanic environments
AU: * Bourry, C
EM: cbourry@ifremer.fr
AF: Departement Geosciences Marines, IFREMER C/Brest, Plouzane, 29280, France
AU: charlou, J
AF: Departement Geosciences Marines, IFREMER C/Brest, Plouzane, 29280, France
AU: Donval, J
AF: Departement Geosciences Marines, IFREMER C/Brest, Plouzane, 29280, France
AU: Focsa, C
AF: Lab. Physique des Lasers, Atomes, Molecules(PhLAM), UNiversite de Lille 1 UMR CNRS 8523 - CERLA, Villeneuve d'Ascq, 59655, France
AU: Chazallon, B
AF: Lab. Physique des Lasers, Atomes, Molecules(PhLAM), UNiversite de Lille 1 UMR CNRS 8523 - CERLA, Villeneuve d'Ascq, 59655, France
AB: Natural gas hydrates occur globally in marine sediments or in permafrost regions when specific conditions of high pressure, low temperature and sufficiently methane concentration are combined to initiate their formation and stabilize their structure. As well as they appear attractive for gas industry, natural gas hydrates can have an important impact in continental slope stability or climate change. Therefore, it is important to focus our attention on structural evolution and thermodynamical stability of these natural minerals. For this, high-resolution powder X-ray synchrotron diffraction and Raman spectroscopy techniques are efficient and powerful tools to determine the hydrate structures. We performed a first physical characterization of two intact natural gas hydrates from the Congo-Angola and the Nigerian margin by X-ray synchrotron diffraction. The collected samples exhibit a preponderance of structure I (sI) (cubic lattice with space group Pm n). The Rietveld refinement of lattice parameters for the type I structure gives values intermediate between lattice constant of less pure methane specimens and pure artificial methane hydrates. This indicates that lattice constant can be affected by the presence of encaged CO2, H2S and other gas molecules, even in small amount. Thermal expansion is also presented for Congo-Angola hydrate in the temperature range 90-200 K and coefficients are comparable with values reported for synthetic hydrates at low temperature, whereas they tend to approach ice thermal expansion coefficient at higher temperature. In a second step, we performed a physical characterization by Raman spectroscopy of natural gas hydrates recovered from Haakon Mosby Mud Volcano (Norwegian Margin) during the Vicking cruise (HERMES project, 2006). These samples exhibit as well a preponderance of structure I (sI) embedded in ice originating from frozen pore water and hydrate dissociation during recovery. The dissociation temperature (Td) of these hydrates is investigated by Raman spectroscopy at atmospheric pressure from 77 K to 260 K. Td shows to depend on the size of the hydrate particles. It is found to increase as the particle size increases. These results are consistent with previous data reported by Takeya et al. (2005). A "multi-layer" dissociation mechanism can be established for large hydrate particles thanks to the high spatial resolution of the micro-Raman technique. This effect will be discussed in the context of the stability of hydrates in natural environments. Takeya et al. (2005), Particle size effect of CH4 hydrate for self-preservation, Chem. Eng. Sci., 60, 1383-1387.
DE: 0400 BIOGEOSCIENCES
DE: 0714 Clathrate
DE: 1051 Sedimentary geochemistry
DE: 3004 Gas and hydrate systems
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting