Effects of Methane Hydrate on the Physical Properties of Sediments
William J. Winters, William F. Waite, David H. Mason, 2009. "Effects of Methane Hydrate on the Physical Properties of Sediments", Natural Gas Hydrates—Energy Resource Potential and Associated Geologic Hazards, T. Collett, A. Johnson, C. Knapp, R. Boswell
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Grain size, pore content, and arrangement of pore constituents have a profound effect on acoustic and strength properties of sediments. We tested specimens containing gas hydrate, methane, and water in the pore space of coarse- and fine-grained sediments to simulate the marine environment and of frozen coarse-grained sediment to simulate permafrost conditions.
The measured compressional wave velocity (Vp) changes with the extent to which the pore material cements sediment grains. Hence, for equal effective stresses, Vp is lowest in gas-charged sediments, increases for water-saturated sediments, then increases significantly for hydrate-bearing sediments because of sediment cementation provided by hydrate. Frozen sediment, effectively fully saturated and fully cemented sediment, exhibits the highest Vp.
Sediment strength follows the same pattern but also shows a strong dependence on sediment grain size. For consolidation stresses associated with the upper several hundred meters of subbottom depth, pore pressures decreased during shear in coarse-grained sediments containing gas hydrate, thereby increasing strength, whereas pore pressure in fine-grained sediments typically increased during shear, which decreased strength. The presence of free gas in pore space damped the pore-pressure response during shear and reduced the strengthening effect of gas hydrate in sands.
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In September 2004, the American Association of Petroleum Geologists (AAPG) convened a Hedberg Research Conference in Vancouver, British Columbia, Canada titled "Natural Gas Hydrates: Energy Resource Potential and Associated Geologic Hazards." As a continuation of the Hedberg Research Conference in Vancouver, the conveners of the conference and the editors of this Memoir have worked with more than 150 authors and coauthors to prepare this Memoir on gas hydrates. This publication follows the goals of the Hedberg conference; however, the contents of this Memoir were expanded to include all aspects of gas hydrates in nature. This Memoir contains 39 individual contributions, ranging from long topical summaries to shorter focused research papers. This Memoir has been published in two parts, with digital versions of all the complete research papers included on the enclosed CD. The hardcopy portion of the Memoir includes abstracts and several key figures for each of the contributions along with a complete copy of a gas hydrate technical review. The digital portion of this Memoir has been organized into a series of topical sections consisting of review articles, marine gas hydrate papers, and gas hydrate laboratory and modeling studies. Because of the rapidly emerging worldwide interest in gas hydrates, this comprehensive treatise on the geology of gas hydrates will be valuable to both the gas hydrate research community and exploration/development geologists working in arctic and deep marine environments.