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High-Resolution Dielectric Estimation of Gas Hydrate Amount in the Mount Elbert-01 Gas Hydrate Stratigraphic Test Well, North Slope, Alaska

Sun, Yuefeng 1; Goldberg, D. 2; Collett, T. 3; Hunter, R. 4
1 Geology and Geophysics, Texas A&M University, College Station, TX.
2 Lamont-Doherty Earth Observatory, Palisades, NY.
3 US Geological Survey, Denver, CO.
4 ASRC Energy Services, BP Exploration (Alaska), Inc., Anchorage, AK.

A dielectric logging tool, electromagnetic propagation tool (EPT), was deployed in 2007 in the Mount Elbert-01 gas hydrate stratigraphic test well, North Slope, Alaska. The dielectric measurements together with density logs result in high-resolution (cm-scale) estimates of gas hydrate saturation. In the two massive hydrate zones of more than 20m thick, Zone D (upper) and Zone C (lower), the average in-situ hydrate saturation based on the EPT and density logs is about 65%, ranging from 45% to 90%. In the hydrate zone D and the upper part of the Zone C, which mostly consist of silty sands and are relatively homogeneous and thick, the dielectric estimates are in good agreement with lower resolution estimates from the combinable magnetic resonance log (CMR). In the lower part of the Zone C, a 1.2-m-thick layer of gas hydrates in clean sands has a hydrate saturation of over 75%. This thin hydrate layer is sandwiched above and below by alternating layers of clay and thin hydrate beds with hydrate saturation over 90%. These thin-bedded hydrate layers are usually less than 15-cm thick whereas the clay layers are about 5-cm thick. In these zones, CMR log underestimates hydrate saturation by three to five times because of the spatial averaging effect of the magnetic resonance tool.

The dielectric measurements reveal many thin-bedded layers of hydrates at various depths above and below massive hydrate zones C and D. These layers range from 30-cm to about 1-m thick and indicate hydrate saturations of 50% to 90%. Comparison of these thin hydrate layers identified using dielectric logs to borehole images from the oil-base microimager (OBMI) allow many of them to be observed. Thin layer hydrates are not indicated in other logs or core data, however.

Numerous questions remain regarding the occurrence of high hydrate saturation within thin layers, their variation and interbedding with clays, the associated processes of hydrodynamic formation, and future strategies for assessment of potential reserves. In order to address these questions, as well as assess any influence on future production strategies, the deployment of high-resolution logs such as dielectric, density and imaging tools is strongly recommended for other future gas hydrate wells.


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