HR: 11:20h
AN: OS42A-05    [Abstracts]
TI: Changes in Pore Pressure due to Repeated Gas Hydrate Dissociation/Formation in Shallow Marine Sediments
AU: * Xu, W
EM: wenyue.xu@eas.gatech.edu
AF: Scholl of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332 United States
AU: Pecher, I
EM: i.pecher@gns.cri.nz
AF: Institute of Geological and Nuclear Sciences, P.O. Box 30-368, Lower Hutt, 12345 New Zealand
AU: Clennell, B
EM: Ben.Clennell@csiro.au
AF: CSIRO Petroleum, Kensington, Perth, WA 6101 Australia
AU: Germanovich, L
EM: leonid@ce.gatech.edu
AF: School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332 United States
AB: At water depths of hundreds of meters, conditions at seafloor are close to the stability boundary of gas hydrates depending on their composition. Known examples include Southern Ritchie Ridge on the Hikurangi margin offshore New Zealand, Bush Hill in the northern Gulf of Mexico, and the vicinity of ODP Leg 175 site on Congo continental slope. Data indicate a periodically or quasi-periodically varying seafloor temperature at all three sites. It has been suggested that temperature fluctuations in bottom waters (e.g. due to seasonal gyres) can lead to repeated formation and dissociation of gas hydrates in shallow sediment. Alternatively, periodic tidal loading/unloading may play a similar role. This repeated hydrate formation/dissociation may weaken the host sediment by a process similar to frost heave. On the other hand, if the sediment permeability is sufficiently low, the periodic tidal loading may significantly reduce the sediment friction angle (up to a few degrees or less) due to the accumulation of the residual shear strain. This study is to quantify the change in pore pressure during repeated formation/dissociation of gas hydrates in shallow marine sediments and discuss its potential effect on sediment deformation or failure. Volume expansion/contraction associated with dissociation/formation of natural gas hydrate at shallow water depths is relatively large compared to deeper environments. It leads to a considerable change in pore pressure. Models have been developed to estimate the change in pore pressure caused by hydrate dissociation/formation due to a varying temperature and/or pressure (tidal forcing) at the seafloor. The magnitude of excess pore pressure resulting from gas hydrate dissociation can be considerable and, when hydrate dissociation takes place close to the seafloor, may approach or even exceed local sediment loading. This leads to a layer of weakened or damaged sediment near the seafloor, which may contribute to slope instability and the occurrence of submarine landslides. Considerable reduction of the sediment friction angle increases the probability of the landslide even in the case of mild slopes (i.e., only of a few degrees). One of the effects of capillarity is a depressed gas hydrate stability temperature. Consequently, the repeated hydrate dissociation/formation is limited within a thinner layer and tends to take place more rapidly than it would without the capillary effect. This may profoundly affect the rate of gas hydrate dissociation and the amplitudes and rates of changes in pore pressure.
DE: 3002 Continental shelf and slope processes (4219)
DE: 3004 Gas and hydrate systems
DE: 3022 Marine sediments: processes and transport
DE: 3070 Submarine landslides
DE: 8010 Fractures and faults
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005