6 research outputs found
Methane gas hydrate effect on sediment acoustic and strength properties
Author Posting. © The Author(s), 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Journal of Petroleum Science and Engineering 56 (2007): 127-135, doi:10.1016/j.petrol.2006.02.003.To improve our understanding of the interaction of methane gas hydrate with host
sediment, we studied: (1) the effects of gas hydrate and ice on acoustic velocity in
different sediment types, (2) effect of different hydrate formation mechanisms on
measured acoustic properties (3) dependence of shear strength on pore space contents,
and (4) pore-pressure effects during undrained shear.
A wide range in acoustic p-wave velocities (Vp) were measured in coarse-grained
sediment for different pore space occupants. Vp ranged from less than 1 km/s for gascharged
sediment to 1.77 - 1.94 km/s for water-saturated sediment, 2.91 - 4.00 km/s for
sediment with varying degrees of hydrate saturation, and 3.88 - 4.33 km/s for frozen
sediment. Vp measured in fine-grained sediment containing gas hydrate was substantially
lower (1.97 km/s). Acoustic models based on measured Vp indicate that hydrate which
formed in high gas flux environments can cement coarse-grained sediment, whereas
hydrate formed from methane dissolved in the pore fluid may not.
The presence of gas hydrate and other solid pore-filling material, such as ice,
increased the sediment shear strength. The magnitude of that increase is related to the
amount of hydrate in the pore space and cementation characteristics between the hydrate
and sediment grains. We have found, that for consolidation stresses associated with the
upper several hundred meters of subbottom depth, pore pressures decreased during shear
in coarse-grained sediment containing gas hydrate, whereas pore pressure in fine-grained
sediment typically increased during shear. The presence of free gas in pore spaces
damped pore pressure response during shear and reduced the strengthening effect of gas
hydrate in sands.This
work was supported by the Coastal and Marine Geology, and Energy Programs of the
U.S. Geological Survey and funding was provided by the Gas Hydrate Program of the
U.S. Department of Energy
Geological controls on focused fluid flow through the gas hydrate stability zone on the southern Hikurangi Margin of New Zealand, evidenced from multi-channel seismic data
Highly concentrated gas hydrate deposits are likely to be associated with geological features that
promote increased fluid flux through the gas hydrate stability zone (GHSZ). We conduct conventional
seismic processing techniques and full-waveform inversion methods on a multi-channel seismic line that
was acquired over a 125 km transect of the southern Hikurangi Margin off the eastern coast of New
Zealandâs North Island. Initial processing, employed with an emphasis on preservation of true amplitude
information, was used to identify three sites where structures and stratal fabrics likely encourage focused
fluid flow into and through the GHSZ. At two of the sites, Western Porangahau Trough and Eastern
Porangahau Ridge, sub-vertical blanking zones occur in regions of intensely deformed sedimentary
layering. It is interpreted that increased fluid flow occurs in these regions and that fluids may dissipate
upwards and away from the deformed zone along layers that trend towards the seafloor. At Eastern
Porangahau Ridge we also observe a coherent bottom simulating reflection (BSR) that increases markedly
in intensity with proximity to the centre of the anticlinal ridge. 1D full-waveform inversions conducted at
eight points along the BSR reveal much more pronounced low-velocity zones near the centre of the ridge,
indicating a local increase in the flux of gas-charged fluids into the anticline. At another anticline,
Western Porangahau Ridge, a dipping high-amplitude feature extends from the BSR upwards towards the
seafloor within the regional GHSZ. 1D full-waveform inversions at this site reveal that the dipping feature
is characterised by a high-velocity zone overlying a low-velocity zone, which we interpret as gas hydrates
overlying free gas. These results support a previous interpretation that this high-amplitude feature
represents a local âup-warpingâ of the base of hydrate stability in response to advective heat flow from
upward migrating fluids. These three sites provide examples of geological frameworks that encourage
prolific localised fluid flow into the hydrate system where it is likely that gas-charged fluids are converting
to highly concentrated hydrate deposits
Methane release at the top of the gas hydrate stability zone of the Hikurangi margin, New Zealand
Dissolved methane and high resolution bathymetry surveys were conducted over the Rock Garden region of Ritchie Ridge, along the Hikurangi margin, eastern New Zealand. Multibeam bathymetry reveals two prominent, northeast trending ridges, parallel to subduction along the margin, that are steep sided and extensively slumped. Elevated concentrations of methane (up to 10 nM, 10Ă background) within the water column are associated with a slump structure at the southern end of Eastern Rock Garden. The anomalous methane concentrations were detected by a methane sensor (METS) attached to a conductivity-temperature-depth-optical backscatter device (CTDO) and are associated with elevated light scattering and flare-shaped backscatter signals revealed by the shipâs echo sounder. Increased particulate matter in the water column, possibly related to the seepage and/or higher rates of erosion near slump structures, is considered to be the cause of the increased light scattering, rather than bubbles in the water column. Methane concentrations calculated from the METS are in good agreement with concentrations measured by gas chromatography in water samples collected at the same time. However, there is a c. 20?min (c. 900?m) delay in the METS signal reaching maximum CH4 concentrations. The maximum methane concentration occurs near the plateau of Eastern Rock Garden close to the edge of a slump, at 610?m below sea level (mbsl). This is close to the depth (c. 630?mbsl) where a bottom simulating reflector (BSR) pinches out at the seafloor. Fluctuating water temperatures observed in previous studies indicate that the stability zone for pure methane hydrate in the ocean varies between 630 and 710?mbsl. However, based on calculations of the geothermal gradients from BSRs, we suggest gas hydrate in the study area to be more stable than hydrate from pure methane in sea water, moving the phase boundary in the ocean upward. Small fractions of additional higher order hydrocarbon gases are the most likely cause for increased hydrate stability. Relatively high methane concentrations have been measured down to c. 1000?mbsl, most likely in response to sediment slumping caused by gas hydrate destabilisation of the sediments and/or marking seepage through the gas hydrate zone