Low Temperature X-ray Diffraction Study of Natural Gas Hydrate Samples from the Gulf of Mexico

Authors: C.J. Rawn, R. Sassen, S.M. Ulrich, E.A. Payzant, B.C. Chakoumakos, and T.J. Phelps

Venue: 6th International Conference on Gas Hydrates, Fairmont Hotel, Vancouver, Canada July 6-10, 2008. http://www.icgh.org/ [external site].

Abstract: Clathrate hydrates of methane and other small alkanes occur widespread as terrestrial components in marine sediments of the continental margins and in permafrost sediments of the arctic. Quantitative study of natural clathrate hydrates is hampered by the difficulty in obtaining pristine samples, particularly from submarine environments. Bringing samples of clathrate hydrate from the seafloor at depths without compromising their integrity is not trivial. Most physical property measurements are based on studies of laboratory-synthesized samples. Here we report x-ray powder diffraction measurements of a natural gas hydrate sample from the Green Canyon, Gulf of Mexico (GOM). The first data were collected in 2002 and revealed ice and structure II gas hydrate. In the subsequent time the sample has been stored in liquid nitrogen. Recently new x-ray powder diffraction data have been collected as a function of temperature. Rietveld refinements on this new data show that there is approximately 50 wt % gas hydrate with structure type II and 50% ice at -140, -130, -115, -100, and -85^oC. The Rietveld refinements on the data sets collected at -70 and -55^oC show the amount of structure type II hydrate decreasing to approximately 40% and 37%, respectively. The Rietveld refinement of the data set collected at -40^oC shows a sharp decrease in the amount of structure type II hydrate to approximately 9%. Rietveld refinements on the data sets collected at -25 and -10^oC indicated that the structure type II hydrate is still present at 7 and 3%, respectively.

Related NETL Project
This presentation is related to the NETL project FEAB111, “Hydration Formation and Dissociation via Depressurization in Simulated and Field Samples”. The results of physical property measurements will be made available to other researchers for integration with studies related to resource evaluation and mechanical modeling of seafloor sites relative to drilling in the offshore. It is anticipated that natural gas hydrate cores from ODP, and DOE sponsored cruises will provide core samples for analyses to compare with the synthetic samples used to develop the technology and instrumentation to understand the behavior of these unconventional gas reservoirs. The plan was to obtain and analyze samples from the Gulf of Mexico in FY2005.

Project Contacts
NETL – Robert Vagnetti (robert.vagnetti@netl.doe.gov or 304-285-13341)
Oak Ridge National Laboratory – Tommy Joe Phelps (phelpstj@ornl.gov or (865) 574-7290)