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| Titel |
Estimating seismic velocity and thickness of a CO2 layer in the Sleipner plume |
| VerfasserIn |
James White, Gareth Williams, Andy Chadwick |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250048363
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| Zusammenfassung |
| Thin layers of CO2 in the Sleipner injection plume produce strong reflections on
seismic data but determining layer properties, such as thickness and velocity, remains
challenging. At Sleipner, the C02 is injected into the Utsira sand and the plume is
imaged as a series of sub-horizontal reflections representing thin layers of CO2
trapped beneath intra-reservoir mudstones. In this contribution we examine the
possibility of combining spectral decomposition and direct reflectivity measurements to
assess the thickness and velocity of a thin layer of dense-phase CO2 in a quantitative
manner.
Initial analysis focuses on detecting the leading edge of the advancing CO2. Synthetic
models, designed to represent thin layer spreading in the Sleipner plume, are utilised to
establish the accuracy with which the leading edge can be imaged. Once the limitations of the
imaging are determined, a spectral decomposition algorithm based on the Smoothed
Pseudo Wigner-Ville Distribution enables single frequency slices to be extracted
with sufficient frequency and temporal resolution to provide diagnostic spectral
information on individual CO2 layers. The method offers improved resolution over a short
time window in comparison to the windowed Fourier transform and the continuous
wavelet transform. The accuracy of the technique is assessed using a synthetic
wedge model prior to application of the algorithm to the topmost layer of CO2 in the
plume.
Useful energy above 100 Hz in the high resolution Sleipner data set allows
layers with thin temporal thicknesses to be studied and investigation reveals strong
evidence of thin layer tuning effects. Analysis of tuning frequencies suggests that
temporal thicknesses of individual layers can be derived with an accuracy of 1 to 2
ms.
Independent assessment of the thickness of the top layer of the Sleipner plume enables the
subsequent calculation of the layer velocity. This study has calculated layer velocities of
around 1550 ms-1 which are somewhat higher than the values expected from the rock
physics (of approximately 1430 ms-1). It is noted that the current range of uncertainty is
significant. An alternative approach to determine layer velocity is also undertaken in locations
where the top and base reflections from the upper CO2 layer are imaged explicitly. Here,
where the temporal thickness of the CO2 layer exceeds the tuning thickness, direct
measurements of the layers top and base reflectivity are made. Utilising previous work which
assessed the acoustic impedance of the virgin sand and caprock prior to injection,
calculation of a layer velocity of approximately 1470 ms-1 is possible. This value
provides closer agreement with the layer velocities expected from the rock physics. |
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