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73rd EAGE Conference and Exhibition incorporating SPE EUROPEC 2011
- Conference date: 23 May 2011 - 27 May 2011
- Location: Vienna, Austria
- ISBN: 978-90-73834-12-5
- Published: 23 May 2011
41 - 60 of 799 results
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Migration-velocity Analysis Using Image-space Generalized Wavefields
Authors C. Cardoso and B. BiondiIn areas of complex geology, migration-velocity analysis (MVA) should use methods that describe the complexity of wavefield propagation, such as focusing and defocusing, multipathing, and frequency-dependent velocity sensitivity. MVA by wavefield extrapolation has the ability to address these issues because it uses wavefields as carriers of information. However, its high cost and lack of flexibility with respect to model parametrization have prevented its routine industrial use. We overcome those limitations by using new wavefields: the image-space generalized wavefields. These wavefields are synthesized from a prestack image computed with wavefield-extrapolation methods, using the pre-stack exploding-reflector model. Cost of MVA by wavefield extrapolation is decreased because only a small number of image-space generalized wavefields are necessary to accurately describe the kinematics of velocity errors and because these wavefields can be easily used in a target-oriented way. Flexibility is naturally incorporated because modeling these wavefields have selected reflectors as the initial conditions, allowing the use of a horizon-based parametrization of the model space. In a 3D real data example, we show that using wavefields synthesized by the prestack exploding-reflector model greatly improves efficiency of migration-velocity analysis by wavefield-extrapolation, while yielding a final accurate migration-velocity model evidenced by good quality and reliable depth images.
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Well-based Interval Velocity Analysis Using Multi-parameter CIG
More LessThis study presents a unique approach of performing interval velocity analysis using multi-parameter CIGs for the 2D case. During the analysis, summation-free CIGs are generated and are analyzed as constant depth slices at every image location. When formation tops information is available, an efficient procedure for updating the velocity can be utilized. A specific pattern which can be used to identify correct and wrong velocity within each of the depth slices is used as the core of the analysis. The ability to analyze the structural parameters of each image point improves the accuracy of the velocity and at the same time provides valuable structural information at the analysis location. The procedure and the potential accuracy of proposed method are demonstrated for the 2D case.
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Anisotropic Tomography Using Rock Physics Contraints
Authors Y. Li, D. Nichols, K. Osypov and R. BachrachAnisotropic model building is a well-known challenging problem for its non-linear, ambiguous nature. To reduce the null-space and stabilize the inversion, we propose a new preconditioning scheme in the linearized tomography to include the rock physics prior information. The rock physics information in terms of covariance among P-wave vertical velocity (v0), epsilon and delta is generated by stochastic realizations of a compacting shale model. We design a VSP synthetic survey with the common industry geometry on two different examples to study the effect of applying prior rock physics prior information on proper and improper media, respectively. The results show that by utilizing the proper prior rock physics information, tomography can better resolve the anisotropy parameters, especially in the area where inversion is poorly constrained by the data. However, cautions should be taken when the lithology of the subsurface is unknown. Finally, a posterior uncertainty analysis is performed to evaluate the contribution of the rock physics prior information. The results show that the null-space is greatly reduced by introducing the prior information.
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Building Anisotropic Velocity Model Using Rock Physics and Local Tomography – A North Sea Case Study
Authors R. Bachrach and J. MathewsonAnisotropic velocity models are poorly constrained by surface seismic data alone. Rock physics anisotropic modelling of shale compaction and digenesis provides information which can be used to constrain anisotropic velocity model building. We demonstrate this new anisotropic model building methodology which incorporates shale compaction model, check shot data and tomography to build a 3D spatially varying anisotropic velocity model in the North Sea. The initial rock model is built by analysing well log data, temperature gradient and additional geologic information. Tomography with checkshots (local tomography) which takes into account both residual moveout of surface seismic data and vertical traveltime from checkshot data was used to calibrate rock model to predict a similar anisotropy profile. After calibration a 3D anisotropic model is built to produce spatially varying anisotropic parameters consistent with both rock model and checkshot data.
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Nonlinear, Full Waveform Modeling in Terms of Forward Multiple Scattering
Authors A. J. Berkhout and D. J. VerschuurThe WRW-model shows how to decompose seismic measurements in terms of responses of individual subsurface grid points that include multiple scattering and, optionally, wave conversion. These Grid Point Responses (GPRs) can be used in full waveform seismic modeling algorithms, allowing a better understanding of the complex (blended) codas of seismic reflections and, above all, utilizing these codas for imaging purposes.
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Seismic Diffraction Modeling with the Tip-wave Superposition Method
Authors A. M. Aizenberg, M. A. Ayzenberg and K. D. Klem-MusatovThe transmission-propagation operator theory provides mathematical tools for describing wave propagation in laterally inhomogeneous layered media. The theory is a hybrid of the integral representation (integral-equation) method and the spatial-frequency decomposition (slowness) method. The theory introduces a new wave-motion statement of the forward problem using two operators; the convolutional transmission operator and the feasible propagation operator. In this theory, the exact solution of the initial-boundary value problem is written as the sum of sequentially reflected and transmitted wave events. We further develop the tip-wave superposition method based on the approximations of the two operators in the seismic-frequency range. The feasible propagation operators inside the layers are approximated in the form of layer matrices. The transmission operators at the interfaces are approximated with effective reflection/transmission coefficients, which generalize plane-wave reflection/transmission coefficients to curved interfaces, non-planar wavefronts and finite frequencies. Each element of the layer matrix contains a feasible beam of the tip waves diverging from a small interface element with diffraction at concave parts of the layer boundary and reflecting/transmitting at the interface. We illustrate the potential of the tip-wave superposition method through diffraction modeling of the Green’s function in a layered medium.
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3D CRS-attributes Based Diffraction Imaging
Authors S. Dell and D. GajewskiImaging of diffractions is a challenge in seismic processing. Standard seismic processing is tuned to enhance reflections. Separation of diffracted from reflected events is frequently used to achieve an optimized image of diffractions. We present a method to separate diffracted events in the time domain based on the Common-Reflection-Surface approach. The Common-Reflection-Surface method provides a simulated zero offset section as well as kinematic wavefield attributes. Using synthetic data we show, how these attributes can be used to effectively separate reflected and diffracted energy in the 3D case. The objectives of this work are seismic event separation in 3D case and time migration velocity analysis for diffractions implemented in the poststack domain.
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Edge and Tip Diffraction Imaging in Three Dimensions
By T. J. MoserThe dimensionality of the diffracted wave is related to the dimensionality of the diffractor: a surface reflected ray is uniquely determined, edge diffracted rays strike out a one-parameter pencil, tip diffracted rays a two-parameter pencil of rays. These topological notions are important to analyse the effect of diffraction imaging by specularity suppression. In diffraction imaging reflectivity is fully suppressed, edge diffractivity partly suppressed, while tip diffractivity stands out unsuppressed. These observations have a significant impact on the structural interpretation of three-dimensional diffraction images. Edges constitute the skeleton of a structural model and tips indicate the joints of the skeleton.
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Diffraction Imaging Based on the Diffraction Operator
Authors J. J. S. de Figueiredo, F. Oliveira, E. Esmi, L. Freitas, A. Novais and J. SchleicherSeismic diffractions carry detailed structure information. Some of these structures are well-known as common hydrocarbon traps, such as faults, pinch-outs, unconformities and structures the size of which is in the order of the wavelength. Therefore, the development of computational resource capable of detecting diffractor points with a good resolution is desirable, but has been a challenge in the area of seismic processing. In this work, we present an approach to seismic diffraction imaging based on the diffraction operator, which can be used in both the time and depth domains, in accordance with the complexity of the area. This method that does not require any knowledge apart from the migration velocity field, i.e., rms velocities or interval velocities consists in application of pattern recognition to the amplitudes along the diffraction operator.
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Seismic Diffraction Separation in 2D and 3D Space
More LessSeismic diffractions contain valuable information of small objects underground. The main porosities in carbonate reservoirs with secondary storage space are dissolution caves and fractures, which generate diffraction waves. Therefore, it is valuable to research on diffraction separately. There exists a significant distinction between seismic diffractions and reflections in the post-migration dip-angle domain CRP. The difference response in dip-angle gather between the two types of waves is analyzed, and a flexible approach, which can separate the diffraction from refection efficiently, is described and illustrated in this abstract. The corresponding responses of diffraction and reflection in dip-angle gather for 2D and 3D cases are derived. This approach is proven to be effective and stable by applying it to two physical models data.
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Point and Edge Diffractions in Three Dimensions
More LessSmall geological objects – faults, fractures, karst, etc. – are presented by the diffracted component in the wavefield registered by seismic survey. Diffraction events analysis allows getting information about these objects. It this study we demonstrate how diffraction events behave in the migrated dip-angle domain. Using synthetic and real data we show that the event shape analysis makes it possible to detect a single diffraction point position, fractures direction and fault orientation.
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Fracture and Cavernous Reservoirs Prospecting by the CSP Prestack Migration Method
Authors A. N. Kremlev, G. N. Erokhin, L. E. Starikov and S. V. RodinResults of fracture and cavernous reservoirs forecasting using scattered waves extracted from multichannel seismic data are presented. The forecast is based on an original prestack migration method – the Common Scattering Point (CSP) method, which enables constructing two independent seismic cubes: a conventional cube with reflectors and a new cube with diffractors – the image of space distribution of acoustic impedance inhomogeneities. The last are joined with fracture or cavernous zones – strong sources of scattered waves. The method was tested on synthetic data and in areas with oil deposits in carbonate and shale rocks.
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Diffraction Imaging as an Interpretation Tool
Authors C. Tsingas, B. El Marhfoul and S. SattiThe main objective of imaging diffracted arrivals is to produce high resolution seismic sections in time or depth, which in turn will enhance the interpretation of fault edges, pinchouts, reef edges, fracture zones and other geological discontinuities. The accurate identification of these geological discontinuities plays an important role in the final interpretation of these features and their associated contribution in the formation of potential hydrocarbon traps. The seismic responses of these structural features can be observed in the diffracted part of the recorded wavefield. In the following we present, using real data examples the ability of the methodology to map fracture corridors and subtle geological discontinuities by focusing the associated scattering energy in the prestack domain. Finally, we conclude by highlighting the benefits of the application on land seismic data (i.e., sparse acquisition and low signal to noise ratio). The obtained diffraction image sections are interpreted in conjunction with the conventional reflection time migrated results and coherence type attributes.
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Diffraction Imaging Using 3D Azimuthally-anisotropic Velocity Continuation
Authors W. A. Burnett and S. FomelWe demonstrate 3D time-domain diffraction imaging using azimuthal velocity continuation on a field data set from the Piceance Basin, Northwest Colorado, USA. Azimuthal velocity continuation describes how a seismic image changes given a change in azimuthally anisotropic migration velocity. By using a spectral implementation of velocity continuation, we generate a suite of diffraction images corresponding to a range of azimuthally anisotropic velocity models. We use kurtosis to measure where diffractions become well focused among these images, and thus indicate the optimal anisotropic velocity model. Example timeslices from image and parameter volumes indicate subtle lateral heterogeneity in azimuthal anisotropy throughout the study area.
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Diffraction Imaging with Cross-well Seismic Data
Authors A. Nikitchenko, D. Kiyashchenko, B. Kashtan and V. TroyanScattering objects (diffractors), such as faults or salt inclusions, are of great interest for exploration. But the scattered wave amplitude may be so weak that it is difficult to see those objects in usual reflection images. The location of scattering objects requires special data pre-processing and/or imaging. We have developed the reflection and diffraction imaging methodology for cross-well data. It is based on the vector Kirchhoff migration with special weighting functions that allow us to discriminate reflecting interfaces and diffractors. We apply the noise imaging diagnostics to discriminate between signal and noise in the cross-well reflection and diffraction images. We verify the validity of reflectors and diffractors by building the Common Image Gathers (CIGs).
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Modeling and Observation of the 3D Diffraction Response of Faults and Fractures
Authors M. A. Pelissier, T. J. Moser, J. Pajchel and M. GrasmueckDiffractions not only mark the location of a local heterogeneity, their amplitude distributions carry important structural and stratigraphic information. We address the 3D diffraction response associated with faults and fractures and illustrate this by synthetic modeling, along with seismic and GPR data. In each of these cases, we observe similar phenomena. On vertical sections, edge diffractions are characterized by a phase polarity reversal across the reflection tangent point, and an amplitude asymmetry is often visible between the two legs of the diffraction hyperbola. On time slices, often diffractions appear in the form of concentric half-circles with different radii. The objective of our paper is to investigate these phenomena and provide a theoretical basis for them.
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Diffraction Multifocusing Stack
Authors A. Berkovitch, I. Belfer and N. ScharffDiffraction energy is an important part of the total seismic wavefield. It indicates the present of small scale geomogical objects such as faults, karsts, fractures in the subsurface. Multifocusing has a specific feature to adequately charachterize diffraction traveltime surfaces. This feature can be used to detect and to identify duffraction energy in the total wavefield. Diffraction Multifocusing Stack represents an imaging where diffraction events are enhances and specular reflections are attenuated. These type of images can be used for reliable detection and characterization of important subsurface elements.
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Fractured Basement Reservoir – Limitations of Seismic Evaluation
Authors S. Sengupta and N. C. NandaBasement fractures are cracks, fissures and joints present in the igneous and metamorphic rocks and are usually of sub-seismic scale to be imaged directly by seismic. However, indications of fractures, their potential distribution, density and orientations are possible from high-density-high-resolution (HDHR) seismic attribute studies of the basement reflection. But the extension of the fractures in depth, an important parameter for determining the reservoir thickness, is difficult to infer from seismic due to lack of reflections from below, and within the basement. With severe seismic limitations in delineation and characterization of basement fractures, large orders of uncertainties creep into the estimate of hydrocarbon reserves and the production profile. This makes the exploration for fracture basement reservoirs, as a primary target, a highly risky and expensive venture. However, in favorable geologic-specific situations, such reservoir may be explored as secondary/alternate targets.
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Deriving Microstructure and Fluid State within a Geothermal Reservoir, Reykjanes Peninsula (Iceland)
Authors M. Le Ravalec, M. Adelinet, C. Dorbath, J. Fortin and Y. GuéguenIn this presentation, we refer to a mechanical framework and develop an effective medium model for estimating velocities in porous media. It makes it possible to derive the distribution of crack density beneath the Reykjanes Peninsula from accurate tomography data. Outside the active hydrothermal areas, crack density is shown to decrease with depth. This can be explained mainly by two reasons: the closure of cracks because of the increasing overburden and the secondary filling of cracks because of hydrothermal flows. However, locally beneath the southwestern part of the Kleifarvatn lake, crack density increases with depth. This is consistent with the presence of a deep reservoir with supercritical fluids under pressure, which would trigger off hydrofracturing processes. This study stresses that capturing the link between seismic data and the physical properties of crust is very difficult. The combination of mechanical concepts and effective medium theory contributes to improve our understanding of the phenomena occurring within the Icelandic crust.
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Application of Spectral Decomposition and Ant Tracking to Fractured Carbonate Reservoirs
More LessThis paper uses the integrated application of ant tracking and spectral decomposition to detect minor faults and fractures based on the VSP-driven processing seismic data in fractured carbonate reservoirs. A new method is developed to improve imaging of faults and fractures using “discrete frequency ant tracking data”. Discrete frequency ant tracking data is computed using discrete frequency phase data obtained from spectral decomposition. Discrete frequency ant tracking data is more effective at detecting faults and fractures than full spectrum data. The different discrete frequency ant tracking attribute maps show that the high-frequency ant tracking data is more effective at detecting faults and fractures. When compared with the full spectrum data, the reconstructed ant tracking data produces more detailed images of faults and fractures. The results of our case study shows, this has the potential to be a highly effective and very valuable seismic method.
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