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  • 1
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    Seismic scattering attenuation estimates for the German KTB area derived from well-log statistics (2001)
    In:  Geophys. Res. Lett., Dordrecht, Netherlands, Dr. W. Junk, vol. 28, no. 19, pp. 3761-3764, pp. L08304, (ISSN: 1340-4202)
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    Publication Date: 2001
    Keywords: Seismology ; Scattering ; Attenuation ; Borehole geophys. ; Physical properties of rocks ; 0935 ; Exploration ; geophysics ; Seismic ; methods ; (3025) ; 5100 ; Physical ; properties ; of ; rocks ; 5144 ; Wave ; attenuation ; 7203 ; Seismology ; Seismics (controlled source seismology) ; scientific drilling ; Statistical investigations ; Body ; wave ; propagation ; GRL ; Mueller ; Muller
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    BIBLIOGRAPHY SEISMOLOGY
  • 2
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    Numerical upscaling of frequency-dependent P- and S-wave moduli in fractured porous media (2016)
    Wiley
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    Publication Date: 2016-06-29
    Description: ABSTRACT Relating seismic attributes to the characteristics of mesoscopic fractures is inherently challenging, yet these heterogeneities tend to dominate the mechanical and hydraulic properties of the medium. Analytical approaches linking the effects of material properties on seismic attributes, such as attenuation and velocity dispersion, tend to be limited to simple geometries, low fracture densities, and/or non-interacting fractures. Furthermore, the influence of fluid flow within interconnected fractures on P-wave and S-wave attenuation is difficult to accommodate in analytical models. One way to overcome these limitations is through numerical upscaling. In this paper, we apply a numerical upscaling approach based on the theory of quasi-static poroelasticity to fluid-saturated porous media containing randomly distributed horizontal and vertical fractures. The inferred frequency-dependent elastic moduli represent the effective behaviour of the underlying fractured medium if the considered sub-volume has at least the size of a representative elementary volume. We adapt a combined statistical and numerical approach originally proposed for elastic composites to explore wether the overall statistical properties of simple fracture networks can be captured by computationally feasible representative-elementary-volume sizes. Our results indicate that, for the considered scenarios, this is indeed possible and thus represent an important first step towards the estimation of frequency-dependent effective moduli of realistic fracture networks.
    Print ISSN: 0016-8025
    Electronic ISSN: 1365-2478
    Topics: Geosciences , Physics
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  • 3
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    Saturation scale effect on time-lapse seismic signatures (2016)
    Wiley
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    Publication Date: 2016-06-29
    Description: ABSTRACT Quantitative interpretation of time-lapse seismic signatures aims at assisting reservoir engineering and management operations. Time-lapse signatures are thought to be primarily induced by saturation and pressure changes. Core-flooding and reservoir flow simulations indicate that a change of the driving forces during dynamic fluid injection gives rise to a varying saturation scale. This saturation scale is yet another variable controlling the time-lapse seismic signal. In this work, we investigate the saturation scale effect on time-lapse seismic signatures by analysing simple modelling scenarios. We consider three characteristic saturation scales, ranging from few millimetres to metres, which may form during gas injection in an unconsolidated water-saturated reservoir. Using the random patchy saturation model, we compare the corresponding acoustic signatures, i.e., attenuation, reflectivity, and seismic gather associated with each saturation scale. The results show that the millimetre saturation scale produces minimum attenuation and the same seismic signatures with those obtained from the elastic modelling. The centimetre saturation scale produces maximum attenuation, whereas the metre saturation scale causes highest velocity dispersion. The analyses of the time shift and amplitude change indicate that ignoring a time-dependent saturation scale can result in biased estimation/discrimination of the saturation and the fluid pressure. In particular, the 4D signal can be strongly affected by the saturation-scale change when the reservoir gas saturation is low and the effective pressure is high. In the presence of an increasing (decreasing) saturation scale during injection, interpreting an observed time shift and amplitude change using the Gassmann model will lead to underestimation (overestimation) of the change in gas saturation and fluid pressure. We show that including the effects of capillarity and residual saturation into the rock physics modelling can potentially reduce the interpretation uncertainty due to the saturation-scale change.
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    Electronic ISSN: 1365-2478
    Topics: Geosciences , Physics
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  • 4
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    Generalized poroelasticity framework for micro-inhomogeneous rocks (2016)
    Wiley
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    Publication Date: 2016-06-15
    Description: ABSTRACT Poroelastic modelling of micro-inhomogeneous rocks is of interest for applications in rock physics and geomechanics. Laboratory measurements from both communities indicate that the Biot poroelasticity framework is not adequate. For the case of a macroscopically homogeneous and isotropic rock, we present the most general poroelasticity framework within the scope of equilibrium thermodynamics that is able to capture the effects of micro-inhomogeneities in a natural way. Within this generalized poroelasticity framework, the concept of micro-inhomogeneity is generically related to partial localization of the deformational potential energy either in the solid phase, including the interfacial region or in the fluid phase. The former case can occur in the presence of surface roughness or multi-mineralic frame and the latter case can be related to suspended particles residing in the fluid phase. A measure for micro-inhomogeneity is the coefficient that governs the effective pressure dependence of porosity changes as described by the porosity perturbation equation of this framework. It can be therefore equivalently interpreted as porosity effective pressure coefficient or as micro-inhomogeneity parameter. We show how this parameter and the other poroelastic constants embedded in this framework can be expressed in terms of experimentally accessible poroelastic constants.
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    Topics: Geosciences , Physics
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  • 5
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    Forced imbibition into a limestone: measuring P-wave velocity and water saturation dependence on injection rate (2014)
    Wiley
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    Publication Date: 2014-04-03
    Description: ABSTRACT Quantitative interpretation of time-lapse seismic data requires knowledge of the relationship between elastic wave velocities and fluid saturation. This relationship is not unique but depends on the spatial distribution of the fluid in the pore-space of the rock. In turn, the fluid distribution depends on the injection rate. To study this dependency, forced imbibition experiments with variable injection rates have been performed on an air-dry limestone sample. Water was injected into a cylindrical sample and was monitored by X-Ray Computed Tomography and ultrasonic time-of-flight measurements across the sample. The measurements show that the P-wave velocity decreases well before the saturation front approaches the ultrasonic raypath. This decrease is followed by an increase as the saturation front crosses the raypath. The observed patterns of the acoustic response and water saturation as functions of the injection rate are consistent with previous observations on sandstone. The results confirm that the injection rate has significant influence on fluid distribution and the corresponding acoustic response. The complexity of the acoustic response —- that is not monotonic with changes in saturation, and which at the same saturation varies between hydrostatic conditions and states of dynamic fluid flow – may have implications for the interpretation of time-lapse seismic responses.
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    Topics: Geosciences , Physics
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  • 6
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    Seismic attenuation and velocity dispersion in fractured rocks: The role played by fracture contact areas (2014)
    Wiley
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    Publication Date: 2014-09-11
    Description: ABSTRACT The presence of fractures in fluid-saturated porous rocks is usually associated with strong seismic P-wave attenuation and velocity dispersion. This energy dissipation can be caused by oscillatory wave-induced fluid pressure diffusion between the fractures and the host rock, an intrinsic attenuation mechanism generally referred to as wave-induced fluid flow. Geological observations suggest that fracture surfaces are highly irregular at the millimetre and sub-millimetre scale, which finds its expression in geometrical and mechanical complexities of the contact area between the fracture faces. It is well known that contact areas strongly affect the overall mechanical fracture properties. However, existing models for seismic attenuation and velocity dispersion in fractured rocks neglect this complexity. In this work, we explore the effects of fracture contact areas on seismic P-wave attenuation and velocity dispersion using oscillatory relaxation simulations based on quasi-static poroelastic equations. We verify that the geometrical and mechanical details of fracture contact areas have a strong impact on seismic signatures. In addition, our numerical approach allows us to quantify the vertical solid displacement jump across fractures, the key quantity in the linear slip theory. We find that the displacement jump is strongly affected by the geometrical details of the fracture contact area and, due to the oscillatory fluid pressure diffusion process, is complex-valued and frequency-dependent. By using laboratory measurements of stress-induced changes in the fracture contact area, we relate seismic attenuation and dispersion to the effective stress. The corresponding results do indeed indicate that seismic attenuation and phase velocity may constitute useful attributes to constrain the effective stress. Alternatively, knowledge of the effective stress may help to identify the regions in which wave induced fluid flow is expected to be the dominant attenuation mechanism.
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    Topics: Geosciences , Physics
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  • 7
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    Stochastic theory of dynamic permeability in poroelastic media (2011)
    American Physical Society (APS)
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    Publication Date: 2011-08-30
    Description: Author(s): Tobias M. Müller and Pratap N. Sahay A theory for the dynamic permeability in deformable porous media is developed. The analysis is based on the momentum flux transfer from the slow compressional into the slow shear wave (a proxy for the viscous wave in a Newtonian fluid) in the presence of random pore-scale heterogeneities. A first-or... [Phys. Rev. E 84, 026329] Published Mon Aug 29, 2011
    Keywords: Fluid dynamics
    Print ISSN: 1539-3755
    Electronic ISSN: 1550-2376
    Topics: Physics
    Published by American Physical Society (APS)
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  • 8
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    Comparison of P-wave attenuation models of wave-induced flow (2014)
    Wiley
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    Publication Date: 2014-11-15
    Description: ABSTRACT Wave-induced oscillatory fluid flow in the vicinity of inclusions embedded in porous rocks is one of the main causes for P -wave dispersion and attenuation at seismic frequencies. Hence, the P -wave velocity depends on wave frequency, porosity, saturation, and other rock parameters. Several analytical models quantify this wave-induced flow attenuation and result in characteristic velocity–saturation relations. Here, we compare some of these models by analyzing their low- and high-frequency asymptotic behaviours and by applying them to measured velocity–saturation relations. Specifically, the Biot–Rayleigh model considering spherical inclusions embedded in an isotropic rock matrix is compared with White's and Johnson's models of patchy saturation. The modeling of laboratory data for tight sandstone and limestone indicates that, by selecting appropriate inclusion size, the Biot-Rayleigh predictions are close to the measured values, particularly for intermediate and high water saturations.
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    Topics: Geosciences , Physics
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  • 9
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    Compensating elastic transmission losses for P‐wave attenuation estimation from sonic logs (2019)
    Wiley
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    Publication Date: 2019
    Description: ABSTRACT The decay of seismic amplitude is caused by a variety of physical phenomena that can be divided broadly into elastic transmission losses (including geometrical spreading, interface transmission losses and scattering attenuation) and intrinsic attenuation, where wave energy is converted into heat due to viscous friction. The so‐called statistical averaging method is currently considered as the most advanced attenuation estimation method, and there exist various implementations of this method. But the way elastic transmission losses – that mask the true intrinsic attenuation – are compensated for appears to be an issue and in some cases this correction has been overlooked. In this paper, we revisit the statistical averaging method for intrinsic attenuation estimation with particular focus on the role of elastic transmission losses. Through synthetic examples we demonstrate the importance of compensating for elastic transmission losses even if the variation of velocity and density with depth is not notable. Our implementation of the method makes use of finite‐difference simulations thereby providing a versatile and accurate way to generate synthetic seismograms. We use a combination of elastic and viscoelastic finite‐difference simulations to demonstrate the significant error without accurate compensation of the elastic transmission losses. We apply our implementation of the method to sonic waveforms acquired in an exploration well from Browse basin, Australia. The resulting intrinsic attenuation estimates are indeed indicative of gas‐saturated zones identified from petrophysical analysis in which viscous friction are thought to be of importance. This article is protected by copyright. All rights reserved
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    Topics: Geosciences , Physics
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  • 10
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    Velocity-saturation relation in partially saturated rocks: Modelling the effect of injection rate changes (2016)
    Wiley
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    Publication Date: 2016-04-29
    Description: ABSTRACT The relationship between P -wave velocity and fluid saturation in a porous medium is of importance for reservoir rock characterization. Forced imbibition experiments in the laboratory reveal rather complicated velocity–saturation relations, including rollover-like patterns induced by injection rate changes. Poroelasticity theory-based patchy saturation models using a constant fluid patch size are not able to describe these velocity–saturation relations. Therefore, we incorporate a saturation-dependent patch size function into two models for patchy saturation. This recipe allows us to model observed velocity–saturation relations obtained for different and variable injection rates. The results reveal an increase in patch size with fluid saturation and show a reduction in the patch size for decreasing injection rate. This indicates that there can exist a distinct relation between patch size and injection rate. We assess the relative importance of capillarity on velocity–saturation relations and find that capillarity stiffening impairs the impact of patch size changes. Capillarity stiffening appears to be a plausible explanation when a decrease in the injection rate is expected to boost the importance of capillarity.
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    Topics: Geosciences , Physics
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