Triaxial strength and acoustic properties of repressurized samples from the 2006 National Gas Hydrate Program of India Expedition

Authors: Winters, W.J. (U.S. Geological Survey, speaker), Waite, W.F. Mason, D.H., and Kumar, P.

Venue: India’s Directorate General of Hydrocarbons’ International Conference on Gas Hydrates in Nodia (New Delhi), India, February 6–8, 2008 (http://www.dghindia.org/site/pdfattachments/upcomingevents/Updated_Programme_gAS[1].pdf [PDF-external site]).

Abstract: Pressure cores were recovered and preserved at near in situ conditions as part of an international program to study gas hydrates offshore India in 2006. Three chilled, whole-round, pressurized core sections were shipped to the USGS Gas Hydrate and Sediment Test Laboratory Instrument (GHASTLI) facility at Woods Hole, MA. The samples were continuously pressurized until their transfer into GHASTLI at atmospheric pressure. Although complex, sub-vertical planar gas-hydrate structures were observed in the silty clay samples prior to entering GHASTLI, the three tested samples yielded little gas upon dissociation following the shear phase. This suggests that most, if not all, gas hydrate dissociated during sample transfer, which was conducted partly at ambient room temperature. Self-preservation of gas hydrate in trimmings from two tests suggests that sample transfers conducted completely at subfreezing temperature preserves hydrate more efficiently. The samples, recovered from Site 21A, core 2Y, were disturbed differently during the transfer process. Two (GH115 and GH117) had clean breaks through the entire sediment cross-section caused by gas expansion. The other (GH116) remained intact but swelled and required significant manipulation to complete the transfer. After consolidation to 400 kilopascals (kPa), the strength behavior was similar between samples GH115 and GH116. Maximum shear strength of about 175 kPa developed between 6 and 10 percent strain. Peak friction angles were 30° to 31°, assuming no cohesion intercept. P-wave velocities (Vp) measured axially were related to effective stress (0 to 400 kPa), initial water content, and/or disturbance. GH115, having a water content of 55 percent, yielded Vp of 1.78 to 1.90 kilometers per second (km/s). Vp for GH116, which had a water content of 58 percent, increased with effective stress from 1.56 to 1.63 km/s. Similarities in mechanical properties suggest that effective stress exerts a primary control on behavior despite significant differences in initial sample integrity. Vp for GH117 was between the other samples, but the maximum shear strength of 119 kPa and maximum friction angle of 25° were both lower than the other samples. Extensive cracks, higher initial water content (61 percent), and the presence of a thin (a few grains thick) sub-vertical plane of very fine sand may have caused the sample’s relative weakness. The plane of sand grains, in addition to facilitating shear, may have been the location of previously observed hydrate veins. These tests, and a previously conducted study using Ottawa sand containing laboratory-formed gas hydrate, suggests that mechanical properties of hydrate-bearing marine sediment are best measured by avoiding sample depressurization. Mechanical properties of hydrate-free sediment, shipped and stored at atmospheric pressure, however, can be approximated by consolidating core material to in situ effective stress.

Related NETL Project
The USGS conducts scientific studies of natural gas hydrates to support DOE efforts to evaluate and understand methane hydrates, their potential as an energy resource, and the hazard they may pose to ongoing drilling efforts. This project, DE-AI26-05NT42496, extends USGS support to the DOE Methane Hydrate Research Program previously supported under DE-AT26-97FT34342 and DE-AT26-97FT34343.

NETL Project Contacts
NETL – Robert Vagnetti (robert.vagnetti@netl.doe.gov or 304-285-1334)
USGS – Deborah R. Hutchinson (dhutchinson@usgs.gov or 508-457-2263)