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Laboratory observations of frequency-dependent ultrasonic P-wave velocity and attenuation during methane hydrate formation in Berea sandstone

Laboratory observations of frequency-dependent ultrasonic P-wave velocity and attenuation during methane hydrate formation in Berea sandstone
Laboratory observations of frequency-dependent ultrasonic P-wave velocity and attenuation during methane hydrate formation in Berea sandstone
Knowledge 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.
0956-540X
713-723
Sahoo, Sourav K
6dab0376-36df-44c5-9f36-cb4a29d9b03b
North, Laurence J.
65837b6b-40f1-4a1c-ba66-ec6ff2d7f84b
Marín-Moreno, Hector
e3eb9576-bca1-4d35-9512-e1bb88b0e4a6
Minshull, Tim A.
bf413fb5-849e-4389-acd7-0cb0d644e6b8
Best, Angus I.
cad03726-10f8-4f90-a3ba-5031665234c9
Sahoo, Sourav K
6dab0376-36df-44c5-9f36-cb4a29d9b03b
North, Laurence J.
65837b6b-40f1-4a1c-ba66-ec6ff2d7f84b
Marín-Moreno, Hector
e3eb9576-bca1-4d35-9512-e1bb88b0e4a6
Minshull, Tim A.
bf413fb5-849e-4389-acd7-0cb0d644e6b8
Best, Angus I.
cad03726-10f8-4f90-a3ba-5031665234c9

Sahoo, Sourav K, North, Laurence J., Marín-Moreno, Hector, Minshull, Tim A. and Best, Angus I. (2019) Laboratory observations of frequency-dependent ultrasonic P-wave velocity and attenuation during methane hydrate formation in Berea sandstone. Geophysical Journal International, 219 (1), 713-723. (doi:10.1093/gji/ggz311).

Record type: Article

Abstract

Knowledge 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.

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Accepted/In Press date: 10 July 2019
e-pub ahead of print date: 17 July 2019
Published date: October 2019

Identifiers

Local EPrints ID: 433628
URI: https://eprints.soton.ac.uk/id/eprint/433628
ISSN: 0956-540X
PURE UUID: 77f7c2ba-1798-432e-b10d-fa6711e5978d
ORCID for Tim A. Minshull: ORCID iD orcid.org/0000-0002-8202-1379

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Date deposited: 28 Aug 2019 16:30
Last modified: 02 Oct 2019 00:36

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Contributors

Author: Sourav K Sahoo
Author: Laurence J. North
Author: Hector Marín-Moreno
Author: Tim A. Minshull ORCID iD
Author: Angus I. Best

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