Field and laboratory data indicate anomalously high reflectivity from fluid-saturated reservoirs at the low-frequency seismic range. It also has been observed that the reflected signal is frequency-dependent and is strongly related to the reservoir flow properties. We have obtained an asymptotic representation of the seismic reflection from a fluid-saturated porous medium in the low-frequency domain. The frequency-dependent component of the reflection coefficient is proportional to the square root of the product of frequency of the signal and the mobility of the fluid in the reservoir. This provides an opportunity for locating the most productive zones of the field before drilling. In the presented example we quantify low-frequency imaging amplitude attribute in terms of reservoir fluid mobility and reservoir production rate.
Identification of reservoir zones associated with increased cavernosity in massive unstratified rocks from seismic data is a challenge. The difficulty of id entifying is explained by both having to pick weak signals against the noise background and the fact that the entire combination of methods and technologies of modern seismic is unsuitable for such media, because those methods and technologies primarily aim to determine fracturing characteristics in sedimentary rocks that display appreciably high coherent energy of reflections. Such identification is gaining importance as more and more deep-seated massive sedimentary and metamorphic rocks and crystalline basement are increasingly involved in hydrocarbon exploration. This article discusses how the character of seismic waves scattered by heterogeneities of cavernous reservoirs has been determined using numeric modeling. A suite of techniques is proposed permitting delineation of anomalous zones of higher scattered energy. This technology was tested on real hydrocarbon deposits located in the granite basement and was able to help identify cavernous-fractured zones as illustrated with specific case studies in this article.
We provide a framework for preconditioning nonlinear 3D electromagnetic inverse scattering problems using nonlinear conjugate gradient (NLCG) and limited memory (LM) quasi-Newton methods. Key to our approach is the use of an approximate adjoint method that allows for an economical approximation of the Hessian that is updated at each inversion iteration. Using this approximate Hessian as a preconditoner, we show that the preconditioned NLCG iteration converges significantly faster than the non-preconditioned iteration, as well as converging to a data misfit level below that observed for the non-preconditioned problem. Similar conclusions are also observed for the LM iteration; preconditioned with the approximate Hessian, the LM iteration converges faster than the non-preconditioned version. At this time, however, we see little difference between the convergence performance of the preconditioned LM scheme and the preconditioned NLCG scheme.
The technique of correcting finite-difference models is applied in one and two dimensions. The means of deriving relatively small spatial correction filters is explained and demonstrated. A quantification of uncorrected numerical dispersion is also developed and illustrated.
High-resolution geophysical data can bridge the gap in terms of resolution and coverage between traditional hydrological methods, such as core analyses and tracer/pumping tests. Although there is ample evidence of links between geophysical and hydrological parameters, such relationships are often weak, ambiguous and site-specific. Therefore, alternative approaches must be developed to combine geophysical and hydrological data measured at different scales. The quantitative integration of such a diverse database represents one of the major challenges in the field of hydrogeophysics. In this study, we explore the usefulness of conditional geostatistical simulations for this purpose. Using a realistic porosity model of a heterogeneous alluvial aquifer, we simulate corresponding neutron porosity logs and crosshole georadar tomographic surveys. The reason for the choice of these geophysical methods is that they provide the most direct estimates of the porosity structure. We then use a conditional simulation approach based on simulated annealing to integrate this database. The effectiveness of this approach is assessed by comparing the results for a variety of modeled porosity fields that differ fundamentally in terms of their conditioning data. Our results indicate that this approach allows for a realistic characterization in the sub-meter range of the porosity distribution in heterogeneous alluvial aquifers.
We develop a simplified theoretical treatment of asymptotic ray-based beam inversion for common-offset. We apply the theory to the development of a true-amplitude imaging algorithm using Gaussian beams. The theoretical treatment is valid for arbitrary velocity variations, and we find that the amplitude behavior of our algorithm is reasonably close to true-amplitude common-offset Kirchhoff inversion. We give a further example of the application of our algorithm to imaging the Atlantis structure in the Gulf of Mexico (GOM). In areas where illumination is poor, asymptotic beam and Kirchhoff inversion do not apply, and migration deconvolution or dip equalization is required. We derive the form of the Hessian for common-offset Gaussian beam migration, and use it to produce inverse migration deconvolution operators for the same Atlantis GOM structure.
Erlangga, Yogi A. (Delft University of Technology) | Vuik, Kees (Delft University of Technology) | Oosterlee, Kees (Delft University of Technology) | Plessix, Rene-Edouard (Shell International Exploration and Production B.V.) | Mulder, Wim A. (Shell International Exploration and Production B.V.)
An iterative numerical method for solving the wave equation in an inhomogeneous medium with constant density is presented. The method is based on a Krylov iterative method and enhanced by a powerful preconditioner. For the preconditioner, a complex Shifted-Laplace operator is proposed, designed specifically for the wave equation. A multigrid method is used to approximately compute the inverse of the preconditioner. Numerical examples on 2D problems show that the combined method is robust and applicable for a wide range of frequencies. Extension to 3D is straightforward.
In this paper the approximate linearized solution for PS-wave reflection coefficient has been obtained for the first time using assumption that elastic parameters contrast at separating interface and anisotropy are weak. The concrete formulas have been obtained for the case of transversally isotropic medium with horizontal axis of symmetry, but solution has been considered for arbitrary anisotropic half-space. The numerical tests have showed good results and allow us to hope for successful solution of an inverse AVO problem.
With the continuous development of petroleum exploration, seismic prospecting on land is mainly operated in the more and more complicated mountain, desert, Gobi, loess plateau and swamp areas. The difficulties for seismic exploration include the very complicated near surface and subsurface structures, the difficult data acquisition, too much coherent and random noise, and the seismic data with poor S/N ratio and resolution. However the exploratory development in these areas has a high requirement on the precise of seismic data. Under these challenges, the geophysicists of China have undertaken large amount of technical research on optimization of acquisition geometry design, selection of shooting parameters, static correction, noise suppression and pre-stack depth migration etc. in the recent years. All these have further improved the imaging precision of seismic data in complex areas and greatly increased our exploration capability in complex areas on land.
Fluid flow and compressional wave propagation measurements were made on fractured samples prior to and after the chemical deposition of calcium carbonate. We observed that the initial void geometry (aperture and contact area) controlled the amount and spatial distribution of mineral deposition within the fracture. The most reliable seismic indicator that the fracture had been altered was a reduction in the variance of the frequency distribution of the received signal. The reduced variance indicates that the fracture is homogenized by mineral deposition in the fracture voids. Homogenization occurs because the mixing predominantly takes place in the dominant flow paths within the fracture, which tend to have lower fracture stiffness. The results indicate that acoustic