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Hydrate-Bearing Sample Alteration from Core Retrieval, Handling, and Preservation

Kneafsey, Timothy 1; Waite, William 2; Lu, Hailong 3; Moridis, George J.1; Winters, William 2; Hunter, R. 4; Walker, Michael 5
1 Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA.
2 U.S. Geological Survey, Woods Hole, MA.
3 Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa, ON, Canada.
4 ASRC Energy Services, BP Exploration (Alaska), Inc., Anchorage, AK. (5) OMNI Laboratories, Houston, TX.

The recovery, storage, transport, and laboratory analysis of cores of natural hydrate-bearing sediments (HBS) effect changes in the temperature, pressure, and stress conditions of samples. Core handling after retrieval at the surface prior to sample preservation (either by chilling in liquid nitrogen (LN) or repressurization using methane) provides additional time for sample changes to occur. The extent to which these disturbances alter the properties of the HBS depends on the specific sample handling techniques, as well as on the sample itself. HBS cores recovered during the February 2007 BP-DOE Mount Elbert gas hydrate drilling project on the Alaskan North Slope provide a basis for comparing the sample alterations induced by multiple handling techniques. These cores were recovered in fine-grained sands from near 600 m depth. Using a chilled oil-based drilling mud helped preserve the samples by eliminating the adverse (dissociating) effects of salts and/or alcohols (common ingredients in drilling muds used in the arctic). The core samples were shaped under cold ambient conditions, and then stored in LN or in vessels pressurized with methane gas.

HBS samples from the Mount Elbert project were visually inspected and examined using x-ray computed tomography (CT), powder x-ray diffraction, Raman spectroscopy, and nuclear magnetic resonance techniques. CT results show that samples initially preserved in LN have non-uniform short “crack-like” low-density zones in the center that probably do not extend to the outside perimeter. Samples initially preserved by repressurization show fewer “crack-like” features and higher densities. Two samples analyzed by Lu et al (AAPG - this meeting) show reduced hydrate saturations near the outer surface, while substantial hydrate remained in the central region. Comparison of predicted and observed core conditions will be presented.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009