HR: 1340h
AN: OS43A-0607    [Abstracts]
TI: Effects of Methane Gas Hydrate Formation on Pore Pressure Response, Shear Strength, and Acoustic Properties of Sediment
AU: * Winters, W J
EM: bwinters@usgs.gov
AF: U.S. Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543 United States
AU: Waite, W F
EM: wwaite@usgs.gov
AF: U.S. Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543 United States
AU: Mason, D H
EM: dmason@usgs.gov
AF: U.S. Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543 United States
AB: The Gas Hydrate And Sediment Test Laboratory Instrument (GHASTLI) has been used to study the effect of hydrate formation on mechanical properties and behavior of reconstituted sieved Ottawa sand (1-2 phi size) and silt-sized quartz. Natural sediment samples from terrestrial and marine locations have also been tested. Regardless of the degree of initial water saturation, which influenced the P-wave velocity (Vp) at the beginning of testing, maximum Vp in sand reached 4.0 km/s for hydrate saturations greater than about 70 percent. The Vp increase was significantly lower in fine-grained sediment. We have performed shear strength tests on samples containing various degrees of pore filling by gas hydrate, water, gas, and ice. Strength properties varied considerably depending on the material(s) filling the pore space. For example, Ottawa sand with gas hydrate almost completely filling the pore space had a shear strength of 10.4 MPa at a consolidation stress of 0.3 MPa. However, a dry specimen containing air in the pore space had a strength of only 0.4 MPa, a 26-fold decrease. A water-saturated sample had a maximum strength of 7.7 MPa. Grain size also affected pore pressure response and strength of samples containing hydrate. Coarse-grained sands exhibit a pore pressure decrease during shear, increasing strength. Fine-grained sediment, however, typically demonstrates contractive behavior (pore pressure increase during shear) and an increased potential for slope instability. Hydrate formation technique also influences sediment properties. Formation from an interconnected gas phase, as is commonly done in the laboratory, can lead to cementation of sediment grains by hydrate. Formation from methane in the dissolved phase, as is thought to occur in many natural hydrate deposits and is currently being investigated in the laboratory, can lead to hydrate growth in the pore space. Although pore-filling hydrate contributes less to acoustic velocities than does an equal volume of cementing hydrate, it still augments the pore pressure and strength effects described above. Testing of sediment samples that contain natural gas hydrate which has been partially dissociated during recovery is being experimentally modeled in the laboratory. However, we have recently recovered natural hydrate that has not been depressurized during recovery. This sets the stage for future testing of samples containing natural hydrate that has been only minimally disturbed during the recovery process.
DE: 3004 Gas and hydrate systems
DE: 3025 Marine seismics (0935, 7294)
DE: 5102 Acoustic properties
DE: 5114 Permeability and porosity
DE: 5194 Instruments and techniques
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005