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- Volume 39, Issue 2, 2021
First Break - Volume 39, Issue 2, 2021
Volume 39, Issue 2, 2021
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A robust approach to estimate Q from surface seismic data and inverse Q filtering for resolution enhancement
Authors Pardeep Sangwan and Dinesh KumarAbstractAccurate estimation of seismic quality factor (Q) is important in seismic data processing to correct for the velocity dispersion effects and to compensate for absorption losses through inverse Q filtering. To estimate it, often logarithmic spectral ratios of the non-stationary seismic signal between two depth levels are linearly inverted. As these ratios are usually derived from the standard Fourier transform (FT) which has a poor time-frequency resolution, this can lead to biased Q estimation. We have calculated spectral ratios from the high-resolution time-frequency spectrum using the Stockwell transform (ST). We then non-linearly inverted these improved spectral ratios to estimate Q using the Levenberg-Marquardt (LM) method. For a synthetic wedge model, results demonstrate a reasonable improvement in the accuracy of Q estimation with the combined ST-LM approach. Application on a real P-wave reflection seismic dataset revealed an anomalous attenuation zone of very low Q (29–75) values. Inverse Q filtering using the estimated Q profile has enhanced the seismic resolution below it and minimized the differential viscous losses.
Frequency slice filtering on Q compensated data has improved the S/N ratio and hence the amplitude fidelity of reflectors.
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New diffraction directivity prestack attributes for seismic fracture prediction
Authors Konstantin Smirnov, Evgeny Landa and Igor FedyaevAbstractLocating small-scale faults and fractures is an important task in reservoir characterization. These linear subsurface features generate azimuthally oriented diffractions. A diffraction directivity can be utilized for seismic fracture prediction. A numerical procedure was developed to calculate two new prestack diffraction attributes: the fracture azimuth and the fracture detection reliability. The new attributes can be used to refine structural and stratigraphic interpretation and to guide the fracture properties modelling in 3D geostatistical applications. The evaluation of these attributes on a field dataset containing a fractured reservoir showed promising results for seismic fracture characterization.
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Using onset times from frequent geophysical surveys to understand reservoir fluid flow and to estimate flow properties
Authors Donald W. Vasco, Gil Hetz and Akhil Datta-GuptaAbstractSeismic characterization of flow properties is a difficult prospect, due to the indirect relationship between permeability and seismic velocity and attenuation. At best, seismic time-lapse changes can detect the effects of saturation and pressure changes in a reservoir due to fluid flow. Even in this case, the interpretation of the seismic observation in terms of the state of the reservoir depends intimately on the properties of the rock physics model, which are usually poorly known. The onset time, the calendar time of geophysical changes, provides an alternative datum for characterizing properties such as reservoir permeability. The main advantage of an onset time is that it is sensitive to the flow properties of the reservoir yet insensitive to the details of the rock physics model. Two examples of the utility of onset times are discussed: the use of travel time shifts induced by the injection of carbon dioxide between two wells, and the time-lapse time shifts for elastic waves propagating through a reservoir undergoing enhanced oil recovery. In both examples, the onset times are mapped into permeability estimates using a trajectory-based approach akin to seismic tomography.
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Application of spectral decomposition as a direct hydrocarbon indicator within the challenging rock physics environment
Authors Mirza Naseer Ahmad and Sabrina SultanaAbstractA study was conducted to use low-frequency amplitudes obtained through spectral decomposition as a direct hydrocarbon indicator within the geologically and seismically challenging area. An offshore data set from the Gulf of Thailand with well-known geology and good well control for both water and hydrocarbon bearing zones were used in this study. The rocks physics reveals similar P-impedance for gas and water-wet sands. Moreover, organic shales have similar Poisson’s ratio as of gas saturated zones. This makes differentiation of fluids and lithologies through post-stack seismic inversion and AVO analysis difficult.
Reservoir wedge modelling, spectral decomposition of the synthetic and field data and analyses were conducted to understand the spectral decomposition response to thickness variation and reservoir fluids. Spectral decomposition was also applied on partial angle stacks (near, mid and far).
Gas sands show bright amplitudes at low frequencies (from 10 to 15 Hz). However, spectral decomposition of the synthetic seismogram of the organic shales indicates that it consists of only relatively higher frequencies (greater than 25 Hz). Therefore, low-frequency amplitudes of spectral decomposition may help to identify hydrocarbon saturated zones.
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Reprocessed time-lapse seismic data provides new reservoir information on the Njord field
Authors Patrick Smith, Paul Glenister, Marianne Yanez, Kristoffer Sundøy and Øyvind KvamAbstractImprovements in time-lapse (4D) seismic processing methods provide opportunities to extract additional information from older time-lapse seismic data sets. This new information can help to optimize reservoir production. Four marine streamer time-lapse seismic data sets on the Njord field, offshore mid-Norway, were previously processed in 2007. Reprocessing in 2020 concentrated on better noise and multiple attenuation, improved imaging, and a focused step-by-step workflow to estimate and correct timing variations within and between surveys. The results dramatically improved on the 2007 processing. Gas injection could be tracked in greater detail and previously unidentified 4D signal was interpreted. The new data sets are expected to enhance reservoir understanding and improve future field performance.
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Volumes & issues
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Volume 42 (2024)
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Volume 41 (2023)
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Volume 40 (2022)
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Volume 39 (2021)
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Volume 38 (2020)
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Volume 37 (2019)
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Volume 36 (2018)
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Volume 35 (2017)
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Volume 34 (2016)
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Volume 33 (2015)
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Volume 32 (2014)
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Volume 31 (2013)
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Volume 30 (2012)
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Volume 29 (2011)
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Volume 28 (2010)
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Volume 27 (2009)
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Volume 26 (2008)
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Volume 25 (2007)
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Volume 24 (2006)
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Volume 23 (2005)
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Volume 22 (2004)
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Volume 21 (2003)
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Volume 20 (2002)
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Volume 19 (2001)
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Volume 18 (2000)
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Volume 17 (1999)
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Volume 16 (1998)
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Volume 15 (1997)
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Volume 14 (1996)
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Volume 13 (1995)
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Volume 12 (1994)
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Volume 11 (1993)
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Volume 10 (1992)
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Volume 9 (1991)
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Volume 8 (1990)
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Volume 7 (1989)
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Volume 6 (1988)
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Volume 5 (1987)
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Volume 4 (1986)
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Volume 3 (1985)
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Volume 2 (1984)
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Volume 1 (1983)