William J. Winters1,
(1) U.S. Geological Survey, Woods Hole, MA
(2) University of Texas Institute for Geophysics, Austin, TX
Abstract: Physical Properties of Sediment Containing Natural and Laboratory-Formed Gas Hydrate
The presence of gas hydrate profoundly affects the physical properties and behavior of host sediment. Measurements of those properties in GHASTLI (the USGS's Gas Hydrate And Sediment Test Laboratory Instrument) provide input for models that predict in-situ behavior of gas hydrate.
We performed acoustic and triaxial strength measurements on four samples, which contained natural gas hydrate in sand collected at depths of 899 m and 913 m, from the Mackenzie Delta 2L-38 Mallik well. P-wave velocities (Vp) were related to hydrate saturation and were 2.73-3.9 km/s in ice- and gas-hydrate-bonded specimens, 2.4-2.9 km/s in samples containing gas hydrate and water, and 1.7-1.8 km/s in the same samples after dissociation was completed. Triaxial strengths were also greatly affected by the presence of gas hydrate in the pore space. Although the shape of the stress-strain curves and p'-q plots were similar, a sample containing gas hydrate during the shear phase was up to 9 times stronger than the other three samples that were sheared after dissociation was completed.
We performed similar tests using GHASTLI to grow gas hydrate in Ottawa sand samples. The Vp's of one recent sample, which was observed to contain nearly complete gas hydrate saturation, increased from 1.9 km/s to 3.75 km/s in 14 days. This sample also exhibited strong dilatant behavior with negative pore pressure response during shear and was similar in overall strength to the Mackenzie Delta specimen.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana