Konstantin Osypov

Building tilted transversely isotropic depth models using localized anisotropic tomography with well information

Tilted transverse isotropyTTI is increasingly recognized as a more geologically plausible description of anisotropy in sedimentary formations than vertical transverse isotropy VTI .A lthough model-building approaches for VTI media are well understood, similar approaches for TTI media are in their infancy, even when the symmetry-axis direction is assumed known. We describe a tomographic approach that builds localized anisotropic models by jointly inverting surface-seismic and well data. We present a synthetic data example of anisotropic tomography applied to a layered TTI model with a symmetry-axis tilt of 45 degrees. We demonstrate three scenarios for constraining the solution. In the first scenario, velocity along the symmetry axis is known and tomography inverts for Thomsen’s and parameters. In the second scenario, tomography inverts for, , and velocity, using surface-seismic data and vertical check-shot traveltimes. In contrast to the VTI case, both these inversions are nonunique. To combat nonuniqueness, in the third scenario, we supplement check-shot and seismic data with the profile from an offset well. This allows recovery of the correct profiles for velocity along the symmetry axis and. We conclude that TTI is more ambiguous than VTI for model building. Additional well data or rock-physics assumptions may be required to constrain the tomography and arrive at geologically plausible TTI models. Furthermore, we demonstrate that VTI models with atypical Thomsen parameters can also fit the same joint seismic and check-shot data set. In this case, although imaging with VTI models can focus the TTI data and match vertical event depths, it leads to substantial lateral mispositioning of the reflections.

Modeling elastic wave velocities and attenuation in rocks saturated with heavy oil

Although properties of bulk heavy oil can be approximated by an appropriate viscoelastic model, only a few attempts to model properties of rocks saturated with heavy oil have been reported. Rock-physics models used for rocks saturated with conventional fluids are inapplicable to those saturated with heavy oil because its viscoelastic rheology invalidates the main assumptions of the Gassmann and Biot theories. We estimate viscoelastic properties of mixtures of rock and heavy oil by considering (1) a system of layers of a solid and a viscoelastic medium and (2) by computing Hashin-Shtrikman (HS) bounds for this system. These two methods give approximate bounds for the frequency- and temperature-dependent velocities and attenuation coefficients in rocks saturated with heavy oil. We also propose a method to compute a realistic estimate of these properties that lie between those bounds. This estimate is based on a self-consistent equivalent-medium approach known as coherent-potential approximation. In a more general form, this approximation can be used for approximate fluid substitution for heavy oil. This approach gives frequency-dependent velocities and attenuation values that are qualitatively consistent with experimental observations.

Uncertainty And Resolution Analysis For Anisotropic Tomography Using Iterative Eigendecomposition

Tomographic velocity model building has become an industry standard for depth migration. Anisotropy of the Earth challenges tomography because the inverse problem becomes severely ill-posed. Singular value decomposition (SVD) of tomographic operators or, similarly, eigendecomposition of the corresponding normal equations, are well known as a useful framework for analysis of the most significant dependencies between model and data. However, application of this approach in velocity model building has been limited, primarily because of the perception that it is computationally prohibitively expensive, especially for the anisotropic case. In this paper, we extend our prior work (Osypov et al., 2008) to VTI tomography, modify the process of regularization optimization, and propose an updated way for uncertainty and resolution quantification using the apparatus of eigendecomposition. We demonstrate the simultaneous tomographic estimation of VTI parameters on a real dataset. Our approach provides extra capabilities for regularization optimization and uncertainty analysis in anisotropic model parameter space which can be further translated into the structural uncertainty within the image.

Application of Steering Filters to Localized Anisotropic Tomography With Well Data

Andrey Bakulin, Marta Woodward*, Yangjun (Kevin) Liu, Olga Zdraveva, Dave Nichols, Konstantin Osypov WesternGeco Summary Estimation of anisotropic parameters for depth models requires some type of joint inversion of seismic and borehole data. We demonstrate that conventional grid reflection tomography can be adapted to simultaneously invert for all parameters of a local 3D anisotropic model. Success requires three key ingredients: jointly invert seismic and well data, localize tomography to a small volume around the borehole, and steer the updates along seismic horizons with steering filters. We describe steering filters and demonstrate 3D anisotropic tomography regularized with steering-filter preconditioners on a synthetic data set.

Model‐uncertainty quantification in seismic tomography: method and applications

Uncertainty is inherent in every stage of the oil and gas exploration and production (E&P) business and understanding uncertainty enables mitigation of E&P risks. Therefore, quantification of uncertainty is beneficial for decision making and uncertainty should be managed along with other aspects of business. For example, decisions on well positioning should take into account the structural uncertainty related to the non-uniqueness of a velocity model used to create a seismic depth image. Moreover, recent advances in seismic acquisition technology, such as full-azimuth, long-offset techniques, combined with high-accuracy migration algorithms such as reverse-time migration, can greatly enhance images even in highly complex structural settings, provided that an Earth velocity model with sufficient resolution is available. Modern practices often use non-seismic observation to better constrain velocity model building. However, even with additional information, there is still ambiguity in our velocity models caused by the inherent non-uniqueness of the seismic experiment. Many different Earth velocity models exist that match the observed seismic (and well) data and this ambiguity grows rapidly away from well controls. The result is uncertainty in the seismic velocity model and the true positions of events in our images. Tracking these uncertainties can lead to significant improvement in the quantification of exploration risk (e.g., trap failure when well-logging data are not representative), drilling risk (e.g., dry wells and abnormal pore pressure) and volumetric uncertainties. Whilst the underlying ambiguity can never be fully eradicated, a quantified measure of these uncertainties provides a valuable tool for understanding and evaluating the risks and for development of better risk-mitigation plans and decision-making strategies

Anisotropic model building with uncertainty analysis

Velocity estimation is usually an ill-posed problem even for isotropic media. Widespread use of anisotropic imaging has been shown to aid better focusing and positioning. However, it greatly escalates the complexity of the model building and makes the velocity estimation much more illposed. Conventional techniques continue to rely on gradient-based methods that deliver a single solution (or realization) of the model to the user. Here we demonstrate an alternative approach that acknowledges the nonuniqueness of the problem. It delivers an entire suite of models that fit the data equally well, allowing the user to select the most geologically plausible solution.

Robust refraction tomography

Summary Refraction tomography is a popular tool for near-surface velocity model construction and statics estimation. However, tomographic results are usually more sensitive to the initial model and pick quality than solutions of delaytime methods. An alternative refraction tomography approach, presented in this paper, is robust since it does not require an initial model or explicit ray tracing. The technique is based on a local form of the Herglotz-Wiechert transformation. A field data example demonstrates the robustness of the new technique.

Anisotropic Velocity Model Building Using Rock Physics: Comparison of Compaction Trends And Check-shot-derived Anisotropy In the Gulf of Mexico

Anisotropic velocity models are poorly constrained by surface seismic data. Rock-physics anisotropic modelling of shale compaction and digenesis provides information that can constrain anisotropic velocity model building. In this study we compare rock-physics-based anisotropic velocity models to an extensive data set where 18 Gulf of Mexico (GoM) wells were analyzed using checkshot data. 3D anisotropy estimates derived by interpolation and extrapolation of the checkshot data set are compared to 3D anisotropy estimates derived from rock-physics modelling. We show that regional anisotropy trends are consistent with compaction driven anisotropy as predicted by the rockphysics model. We also show that not all anisotropy observed in the region can be explained by compaction model and that check-shot data carries additional information.

From Quantifying Seismic Uncertainty to Assessing E&P Risks And the Value of Information

Accurate well placement in oil and gas exploration and production (E&P) requires accurate positioning of interpreted geological structure in the depth domain. In the past decade, with the advancement of sensor and computer hardware technology, the seismic industry has made great improvements in the data acquisition designs as well as depth imaging and model building algorithms. However, the cost/benefit justification for the value of information (VOI) obtained by utilization of these technologies remains mainly to be qualitative. This paper discusses how seismic uncertainty analysis can lead to quantifying the VOI and technical risks associated with E&P projects.

Building TTI depth models using anisotropic tomography with well information

Tilted transverse isotropy (TTI) is becoming recognized as a more realistic description of anisotropy in sedimentary formations than vertical transverse isotropy (VTI). This is especially true in complex geological settings. While model building approaches for VTI are well understood, similar approaches for TTI media are in their infancy, even when symmetry-axis direction is known. We present an approach that allows building localized anisotropic models utilizing joint inversion of seismic and well data. We present a synthetic data example of anisotropic tomography applied to a layered TTI model with a symmetry-axis tilt of 45 degrees. We demonstrate three cases of introducing additional information. In the first case velocity along the symmetry axis is known and tomography inverts for Thomsen’s e and δ. In the second case, tomography inverts two Thomsen parameters and velocity from a joint dataset that consists of seismic data and vertical checkshot traveltimes. In contrast to the VTI case, such inversion is non-unique. To combat non-uniqueness in the third case we supplement checkshot and seismic data with the Thomsen’s δ profile from an offset well. This allows recovery of correct profiles for velocity along the symmetry axis and Thomsen’s e. We conclude that TTI model building may remain non-unique even in the presence of well information. Therefore additional assumptions need to be added or uncertainty analysis has to be conducted to pick a geologically plausible model from a range of equivalent models.