Abstract
SUMMARYKnowledge of the effect of methane hydrate saturation and morphology on elastic wave attenuation could help reduce ambiguity in seafloor hydrate content estimates. These are needed for seafloor resource and geohazard assessment, as well as to improve predictions of greenhouse gas fluxes into the water column. At low hydrate saturations, measuring attenuation can be particularly useful as the seismic velocity of hydrate-bearing sediments is relatively insensitive to hydrate content. Here, we present laboratory ultrasonic (448–782 kHz) measurements of P-wave velocity and attenuation for successive cycles of methane hydrate formation (maximum hydrate saturation of 26 per cent) in Berea sandstone. We observed systematic and repeatable changes in the velocity and attenuation frequency spectra with hydrate saturation. Attenuation generally increases with hydrate saturation, and with measurement frequency at hydrate saturations below 6 per cent. For hydrate saturations greater than 6 per cent, attenuation decreases with frequency. The results support earlier experimental observations of frequency-dependent attenuation peaks at specific hydrate saturations. We used an effective medium rock-physics model which considers attenuation from gas bubble resonance, inertial fluid flow and squirt flow from both fluid inclusions in hydrate and different aspect ratio pores created during hydrate formation. Using this model, we linked the measured attenuation spectral changes to a decrease in coexisting methane gas bubble radius, and creation of different aspect ratio pores during hydrate formation.
Highlights
IntroductionGas hydrates are naturally occurring, clathrate compounds of gas (predominantly methane) found in marine and permafrost environments (Sloan & Koh 2007)
Gas hydrates are naturally occurring, clathrate compounds of gas found in marine and permafrost environments (Sloan & Koh 2007)
We interpret two separate trends for hydrate saturations below and above about 6 per cent; attenuation increases with frequency for hydrate saturations below 6 per cent and decreases with frequency for higher saturations
Summary
Gas hydrates are naturally occurring, clathrate compounds of gas (predominantly methane) found in marine and permafrost environments (Sloan & Koh 2007). Natural hydrates commonly exist in several morphologies within host sediments: (i) hydrate forming cement between mineral grains, known as cementing hydrate; (ii) disseminated hydrate growing freely in the pore space away from grain contacts, known as porefloating or pore-filling hydrate; (iii) hydrate contacting neighbouring mineral grains, known as pore-bridging or load bearing or frame supporting hydrate Sahoo et al (2018a) recently provided evidence for (iv) an ‘interpore hydrate framework’ morphology that is created when hydrate from adjacent pores coalesces to interlock the host sediments. The non-cementing morphologies (ii–iv) are thought to dominate natural hydrate systems, and (ii) & (iii) have been sampled and/or inferred at locations such as Mallik, Mackenzie Delta (Uchida et al 2000), the Nankai Trough (e.g. Fujii et al 2015), Alaminos Canyon, Gulf of Mexico (Boswell et al 2009) and Mount Elbert, Alaska North Slope (Stern et al 2011). Morphology (iv) probably occurs in situ, but has only just recently been identified in the laboratory (Sahoo et al 2018a)
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