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dc.contributor.authorBakk, Audun
dc.contributor.authorHolt, Rune Martin
dc.contributor.authorBauer, Andreas
dc.contributor.authorDupuy, Bastien
dc.contributor.authorRomdhane, Mohamed Anouar
dc.description.abstractDepletion or injection into a reservoir implies stress changes and strains in the reservoir and its surroundings. This may lead to measurable time-shifts for seismic waves propagating in the subsurface. To better understand the offset dependence of time-shifts in the overburden, we have systematically quantified the time-shifts of three different overburden shales in controlled laboratory tests. These experiments may be viewed as an analogue to the time-shifts recorded from seismic field surveys. For a range of different stress paths, i.e. the ratio between the horizontal and the vertical stress changes, the changes of the P-wave velocities in different directions were measured such that the offset dependence of time-shifts for different stress paths could be studied. The time-shifts are stress path dependent, which is particularly pronounced at large offsets. For all stress paths the time-shifts exhibit a linearly decreasing trend with increasing offset, i.e. a negative offset-gradient. At zero offset, for which the ray path is normal to the bedding, the time-shifts are similar for all investigated stress paths. The isotropic stress path is associated with the smallest offset-gradient of the time-shifts. Contrary, the constant-mean-stress path shows the largest gradient with a flip in the polarity of the time-shifts for the largest offsets. The separate contributions from the strain and velocity changes to the time-shift were also quantified. The time-shifts for the isotropic stress path are dominated by the contribution from velocity changes at all offsets. Contrary, the strain contributes significantly to the time-shifts at small offsets for the constant-mean-stress path. This shows that the offset dependence in pre-stack seismic data may be a key to understand the changes of subsurface stresses, pore pressure and strain upon depletion or injection. To utilize this knowledge from laboratory experiments, calibrated rock physics models and correlations are needed to constrain the seismic time-shifts and to obtain an adequately updated geological model reflecting the true anisotropic nature of the subsurface. This may have important implications for improved recovery and safety, particularly in mature fields.en_US
dc.rightsAttribution-NonCommercial-NoDerivatives 4.0 Internasjonal*
dc.titleOffset dependence of overburden time-shifts from ultrasonic dataen_US
dc.typeJournal articleen_US
dc.typePeer revieweden_US
dc.source.journalGeophysical Prospectingen_US
dc.relation.projectNorges forskningsråd: 294369en_US
dc.description.localcode© 2020 The Authors. Geophysical Prospecting published by John Wiley & Sons Ltd on behalf of European Association ofGeoscientists & EngineersThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use anddistribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations aremade.en_US

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Attribution-NonCommercial-NoDerivatives 4.0 Internasjonal
Except where otherwise noted, this item's license is described as Attribution-NonCommercial-NoDerivatives 4.0 Internasjonal