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72nd EAGE Conference and Exhibition incorporating SPE EUROPEC 2010
- Conference date: 14 Jun 2010 - 17 Jun 2010
- Location: Barcelona, Spain
- ISBN: 978-90-73781-86-3
- Published: 14 June 2010
1 - 20 of 797 results
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Some Remarks on Gravity Reduction by Vertical Field Continuation
Authors P. L. Smilde and M. H. KriegerFully Bouguer reduced gravity data cannot be assumed to be located at the reduction level. After the effect of the topography has been removed by Bouguer and terrain reduction, performing the normal field reductions may be interpreted as a virtual movement inside the normal field down to the reduction level. But this “movement” does not take into account the effect of the mass anomaly. There are two ways to handle this issue: Either by modelling at original varying observation heights or by applying a vertical field continuation to a constant level. Advantages and disadvantages of different approaches are discussed.
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Geologically Constrained Full Waveform Inversion – Theory
Authors A. Guitton, F. Ortigosa and G. GonzalesThe waveform inversion problem is inherently ill-posed. Traditionally, regularization terms are used to address this issue. For waveform inversion, where the model is expected to have many details reflecting the physical properties of the Earth, regularization and data fitting can work in opposite directions, slowing down convergence. In this paper, we constrain the velocity model with a model-space preconditioning scheme based on directional Laplacian filters. This preconditioning strategy preserves the details of the velocity model while smoothing the solution along known geological dips. The Laplacian filters have the property to smooth or kill local events according to a local dip field. By construction, these filters can be inverted and used in a preconditioned waveform-inversion scheme to yield geologically meaningful models. We illustrate on a 2-D synthetic example how preconditioning with non-stationary directional Laplacian filters outperforms traditional waveform inversion when sparse data are inverted for. We think that preconditioning could benefit waveform inversion of real data where (for instance) irregular geometry, coherent noise and lack of low frequencies are present.
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Efficient Gauss-Newton Hessian for Full Waveform Inversion
More LessFull waveform inversion (FWI) has received an increasing amount of attention thanks to its ability to provide a high resolution velocity model of the subsurface. The computational cost still presents a challenge, however, and the convergence rate of the FWI problem is usually very slow without proper preconditioning on the gradient. While preconditioners based on the Gauss-Newton Hessian matrix can provide significant improvements in the convergence of FWI, computation of the Hessian matrix itself has been considered highly impractical due to its computational time and the storage requirements. In this paper, we design preconditioners based on an approximate Gauss-Newton Hessian matrix obtained using the phase-encoding method. The new method requires only 2Ns forward simulations compared to Ns(Nr+1)forward simulations required in conventional approaches, where Ns and Nr are the numbers of sources and receivers. We apply the diagonal of the phase-encoded Gauss-Newton Hessian to both sequential source FWI and encoded simultaneous source FWI. Numerical examples on Marmousi model demonstrate that phase-encoded Gauss-Newton Hessian improves the convergence of the FWI significantly.
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Acoustic Waveform Inversion Applicability on Elastic Data
Authors D. V. Vigh, E. W. Starr and P. ElapavuluriFull Waveform inversion is a computer intensive process, especially for 3D seismic data. After a tremendous number of synthetic examples, finally real 3D data sets have been undertaken by the industry. As field data is dominated with P waves, one feasible approach is to use the acoustic approximation. The Full Waveform Inversion described above is acoustic and real data is more accurately described by an elastic model. It is common practice to apply acoustic inversion, especially for 3D data sets because the elastic modeling is prohibitively expensive. Although the long offsets may suffer from elastic effects our experiment shows that the velocity field obtained using acoustic inversion vs. elastic data are reasonable in spite of the difference between the modeling and mother Earth. Although the elastic propagation would provide a better match to the acquired data the cost is still prohibitive compared to the acoustic propagation especially if large 3Ds are under consideration.
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Application of an Impedance-based Full-waveform Inversion Method for Dual-sensor, Single-streamer Field Recordings
Authors S. Kelly, J. Ramos-Martinez, B. Tsimelzon and S. CrawleyThusfar, waveform-based inversion methods have utilized the spatial distribution of back-projected reflectivities in order to refine velocity (and density) models. While these methods excel at refining a velocity model for high-wavenumber features, they face significant difficulties when attempting to invert for long-wavelength features several kilometres below the depth of acquisition. In this abstract, we develop the theory for a waveform-based inversion method that utilizes an impedance image, rather than a reflectivity image, in order to extract spatial variations in velocity and density. Results are presented for the first stage of an inversion using a 2-D line of dual-sensor, single-streamer, field recordings. It is shown that features with vertical scale sizes up to 0.5 km can be determined at depths up to 4 km, using data with a maximum offset of only 8 km and frequencies below 7.5 Hz.
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2D Acoustic Full Waveform Inversion of a Land Seismic Line
Authors W. A. Mulder, C. Perkins and M. J. van de RijzenThe application of waveform inversion to land data is even more challenging than the marine case because of strong elastic effects such as groundroll, near surface attenuation, scattering due to rapid geological variations and topography. In this paper, rather than considering full elastic waveform inversion which maybe too difficult for standard geophone data, we consider the application of acoustic 2D full waveform inversion with a frequency domain code. At lower frequencies, the data are very noisy so we carried out a fixed number of iterations with a small band of low frequencies and then repeated this for increasingly larger bandwidth. We present the results from this procedure, which show the waveform inversion improved the continuity of the main reflectors.
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3D Elastic Wavefield Inversion in the Time Domain
Authors L. Guasch, M. R. Warner, I. Stekl and A. P. UmplebyWe have developed a 3D tomographic wavefield inversion code that solves the fully elastic wave equation in the time domain using finite differences. We show results of applying this elastic code to different synthetic 3D problems.
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Time-lapse Elastic Full Waveform Inversion using Injected Grid Method
Authors S. C. Singh and G. T. RoyleFull waveform inversion is increasing being used in oil industry to quantify P and S-wave velocities of the sub-surface. So far, waveform inversion methods have been either 3D acoustic or 2D elastic. Furthermore, only low frequencies have been used because of the high computation cost. The application of elastic full waveform for 3D reservoir monitoring is still beyond the reach of present day computing technology. Here, we demonstrate the application an injected grid method, where the forward modelling for iterative elastic waveform inversion is performed in a small volume surrounding the reservoir, reducing the cost significant for full waveform. The application of algorithm is demonstrated on Marmousi II data set in a time-lapse mode.
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Waveform Tomography by Correlation Optimisation
Authors T. van Leeuwen and W. A. MulderAn important ingredient of any tomography-based velocity inversion method is the determination of traveltime differences. When a ray-based method is used, the modelled traveltimes are directly available and the traveltimes in the observed data need only to be picked once. When using wave-equation-based methods, however, the traveltime difference between two complex waveforms needs to be determined at each iteration. A straight-forward approach automatically picks the onset of relevant arrivals, either directly or via a correlation of the two waveforms. If the waveforms are not very similar, however, this approach is problematic. We propose to measure the traveltime difference via a weighted norm of the correlation of the two waveforms. The weighted norm can be used directly as an optimisation criterion for waveform tomography. We illustrate this with a synthetic and real data example.
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Elastic Inversion of 3D Seimic Data – A Market Benchmark
Authors I. Escobar and H. Peder HansenBecause of an increasing use of seismic inversion products within Maersk Oil, it was needed to perform a benchmarking exercise to identify seismic inversion/reservoir characterization contractor(s) of choice. To ensure robustness of the benchmark a highquality dataset was used (high SNR, good well coverage and well established rock physics model) from a clastic Palaecene reservoir showing a strong class II AVO response. The exercise included four contractors and Maersk’s internal inversion group, each asked to provide relative and absolute acoustic impedance, Poisson’s ratio volumes along with Bayesian seismo-facies prediction. The comparison was carried out by measuring correlations between inverted and measured relative acoustic impedance and relative Poisson’s ratio, combined with visual inspection of different sections, maps and along well locations in terms of predictability and continuity of events. We have found that the reliability in estimating acoustic impedance was similar for all the participants, and it was in fact the ability to estimate Poisson’s ratio that was the main differentiator. Other important aspects were significant differences in software runtimes, efficiency of workflows and processing/pre-conditioning capabilities. This benchmark has given us a sound overview of the abilities of the participating contractors.
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Sensitivity of Time-lapse Changes in Pressure and Saturation to Seismic AVO and Time-shifts
Authors M. Trani, R. Arts, O. Leeuwenburgh and J. BrouwerAn inversion scheme that solves for reservoir pressure and fluid saturation changes from time-lapse pre-stack seismic attributes and post-stack seismic time-shifts is presented. It makes use of four equations expressing the changes in zero-offset and gradient reflectivities, compressional and shear waves time-shifts as functions of production induced changes in fluid properties. The method has been successfully tested on a realistic, synthetic reservoir, where seismic data have been modeled before and after 30 years production and water injection. Results show very accurate estimations if information about the vertically averaged reservoir porosity is available. The use of the gradient reflectivity equation causes biased estimations of real changes in saturation and strong leakage between the two different parameters. However, if the equation related to the S-wave time-shift can replace the gradient reflectivity equation, the inversion results may be very accurate. In cases where shear wave data might not be acquired, the approximation of the exact changes in this seismic attribute becomes more accurate if quadratic terms in relative changes of seismic properties are not neglected. The improved forward approximation in this attribute leads to inversion results characterized by weaker leakage and sharper discrimination between different fluid effects.
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4D Pre-stack Inversion Workflow Integrating Reservoir Model Control and Lithology Supervised Classification
Authors S. Toinet, S. Maultzsch, V. Souvannavong and O. Colnard4D pre-stack inversion is used in the industry to image reservoir changes due to production and injection, and to make reservoir management decisions in order to optimize hydrocarbon recovery. We present an innovative workflow to prepare, constrain and compute 4D pre-stack inversion attributes. Specific properties of the studied field (huge time-shifts due to gas coming out of solution, various turbiditic contexts) implied building a composite warping result, filtered using a 4D mask to build the initial monitor model for 4D inversion. The pre-stack 4D inversion workflow not only integrates seismic information, but also well information, used to discriminate sand from shale during the 4D mask building, and a 4D rock-physics model. Applied to simulated reservoir properties, the rock-physics model defines a range of relative density and velocity variations in which the inversion results can vary. Moreover, because water-bearing sands are hard to discriminate from shales in some of the field reservoirs using a cross-plot of P and S impedances, information from the reservoir grid was also introduced to help locating water-bearing sands in the 4D mask. Preliminary analyses of 4D inversion attributes show an improved image compared to previous 4D attributes.
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Ray Impedance Inversion on the Tight-sand Gas Reservoir
More LessIn a tight sand gas reservoir, due to its complicated geology and extremely low porosity and permeability, conventional inversion methods cannot always accomplish the task of characterizing reservoir distribution. In this paper, we apply the ray-impedance inversion on a tight-sand gas reservoir. We perform inversion on constant ray parameter profiles, with robust estimate of wavelet and stable result of reflectivity series to start with. Comparing the inverted ray impedance with the acoustic impedance, the elastic impedance and shear impedance from the commercial software, as well as these elastic parameters extracted from well logs, it shows that the frequency-dependent ray impedance is superior in identifying fracture zone and the characterization of reservoir distribution for this tight-sand gas reservoir.
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Statistical Rock Physics and Bayesian Classification – Are Marginal Distributions Important?
Authors I. Escobar, H. Peder Hansen and M. BellerThe importance and impact of the way marginal (unconditional) distributions are considered in statistical rock physics and Bayesian classification of inverted seismic data was analyzed. Two scenarios were compared, one assuming perfect knowledge of the probability density functions and perfect and unbiased sampling; and another one where an unclassified group is included, with a given distribution in order to account for imperfections in the data, models, and estimation techniques. Using a dataset from a clastic Palaeocen field in the North Sea, it was shown that assuming the first scenario (perfectly known distributions) leads to probabilistic volumetric estimations of hydrocarbon saturated sands at least 3 times bigger than in the second case where an unclassified group is included. For the sake of further downstream engineering and facilities analyses, it is straightforward to realize the impact of such over-predictions in the estimation of P10, P50 and P90 volumes of hydrocarbons in place.
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Applicability of AVO Inversion Based on Effective Reflection Coefficients to Long-offset Data from Curved Interfaces
Authors L. V. Skopintseva, A. M. Aizenberg, M. A. Ayzenberg, M. Landrø and T. V. NefedkinaAVO inversion is an essential and powerful interpretational tool. However, conventional AVO-inversion workflow in application to long-offset data will unlikely succeed, because it is limited to relatively plane interfaces, weak parameter contrasts and moderate incidence angles, where the offset dependence of the reflection amplitude can be described by linearized plane-wave reflection coefficients (PWRCs). It is also known that the PWRC is insensitive to the interface curvatures and breaks down at the near-critical offsets, where the head wave is generated, as well as at the post-critical offsets, where the reflected and head waves interfere. To avoid these inconsistencies, we exploit effective reflection coefficients (ERCs) that generalize PWRCs for curved interfaces and nonplane waves. Based on our previous results for plane interfaces, we generalize the improved approach to AVO inversion for the curved interfaces. Using a synthetic data example, we also show that the theoretical description of the actual reflection from a curved interface is directly applicable in AVO inversion.
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A 3D Ray-based Pulse Estimation for Seismic Inversion of PSDM Data
Authors F. Georgsen, O. Kolbjørnsen and I. LecomteWe develop a methodology for estimation of the point-spread function of prestack depth migrated data (PSDM). The parametrization of the point-spread function is given by a ray-based approach which incorporates the effects of wave propagation through the use of illumination vectors. The only unknown factor in the expression for the point-spread function is a 1D pulse. This pulse is estimated from reflection coefficients in wells, and co-located PSDM data using a least squares approach. The estimate of the point-spread function is the first step towards seismic inversion of PSDM data. In comparison to traditional wavelet estimation, the model using PSDM data has a larger range of validity, i.e. we are able to remove the assumption of a horizontally layered earth. In examples we show that our method is identical to the 1D convolution when the earth has a constant dip, but gives an improvement when this assumption is violated.
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Pilot Fracture Characterization Study Using Seismic Attributes Derived from Singular Value Decomposition of AVOAz Data
Authors G. Chao and S. MaultzschThis work presents a pilot study of the application of a seismic inversion technique for fracture density and fracture orientation in an area where available image log data revealed the presence of open fractures at a given well location. The inversion method is based on a singular value decomposition of azimuthal AVO data. This decomposition allows us to calculate seismic attributes which are linked to the fracture density of the fracture network through anisotropic rock physics modeling. The results reveal an area of high fracture density around the well where open fracture had been observed. The fracture orientations obtained are consistent with the interpreted image logs and outcrop studies in the area. The outcome of this pilot study is promising since the obtained results are consistent with existing image log data. Further testing and research on a larger area will be valuable to assess if the fracture characterization results are consistent with regional geological interpretations and other analysises of fracture networks in the reservoir.
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Simultaneous Sources – Processing and Applications
Authors I. MooreThere has been considerable interest recently in data acquisition using simultaneous sources because of the enormous improvements in acquisition efficiency and source sampling that the method proffers. Realizing these improvements in practice requires an appropriate combination of survey design, acquisition technology and data processing capabilities. Given a suitably-designed survey, I show that simultaneous-source data can be separated effectively into equivalent datasets for each source. These datasets may then be processed using conventional techniques, which benefit naturally from any improvements in sampling. Several field datasets, employing a variety of acquisition geometries, illustrate the viability and limitations of this approach. The main limitation comes from ambiguities in the separation process, which can be resolved to a large degree by using source dithering techniques in combination with a separation algorithm that includes an effective sparseness constraint. The use of more than two sources simultaneously adds to the potential of the method and is shown to be viable provided the survey is designed appropriately.
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Iterative Method for the Seperation of Blended Encoded Shot Records
Authors A. Mahdad and G. BlacquièreSeismic acquisition is a trade-off between economical and quality considerations. Generally seismic data is recorded with large time delays between illuminating sources in order to avoid interference in time. The consequence is a poorly sampled shot domain. However, in the concept of blended acquisition, the time delay between sources is reduced significantly. Moreover, the sources may transmit encoded signals. Depending on the acquisition objectives, blended acquisition significantly improves the economics or quality or both by adding additional degrees of freedom in the acquisition design. By a deblending procedure, the individual source responses are retrieved. However, the deblended result contains residual noise due to the interference from other sources. The level of this noise depends on the choice of blending parameters. E.g., in the case of a simple code like linear phase encoding (i.e., applying time delays), it is larger than in the case of a more sophisticated code like transmitting sweeps as in vibroseis technology. In this paper an iterative approach is proposed for deblending, based on the estimation and subsequent adaptive subtraction of the interference noise. The type of coding that is applied is one of the factors that determines the required number of iterations.
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High Quality Separation of Simultaneous Sources by Sparse Inversion
Authors R. L. Abma, T. Manning, M. Tanis, J. Yu and M. FosterThis paper demonstrates a method of producing pre-stack gathers from blended acquisition data that are as noise-free as gathers from conventional acquisition. Filtering out the interference from the overlapping shots and stacking the data are fairly effective in attenuating the interference from blended shots. However high-quality separation of the interference from the pre-stack data would make the data more suitable for amplitude dependent analysis, such as that for amplitude-versus-offset, time-lapse measurements, and fracture detection. The method presented here produces seismic records in which the interference is attenuated to the point that it is well below the background noise. This high quality attenuation is achieved by using a sparse inversion process that solves a modified version of Berkhout's matrix system, allowing the source responses to be separated for high quality amplitude measurements. The success of this source separation step in the data processing of blended acquisition means we can produce results with a quality that is comparable to that of conventional acquisition. Furthermore, this method improves the data quality while retaining the lower cost and higher productivity of ISSTM acquisition when compared with conventional acquisition.
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