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80th EAGE Conference and Exhibition 2018
- Conference date: June 11-14, 2018
- Location: Copenhagen, Denmark
- Published: 11 June 2018
1 - 100 of 1073 results
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3D Seismic Forward Modeling from the Multi-Physical Reservoir Model at the Ketzin CO2 Storage Site
Authors Y. Zheng, B.U. Wiese, S. Lüth, R. Zeng and Y. WangSummaryAbout 67 kt of CO2 have been injected into the deep saline formation at the Ketzin pilot CO2 storage site from June 2008 to August 2013. During the injection, 3D seismic survey has been performed and repeated to monitor the migration of sequestered CO2. The results of seismic monitoring are limited by the acquisition and signal-to-noise ratio of the data. Reservoir simulation can provide information on the CO2 fluid behavior and the approximated model should be calibrated with the monitor results. In this work, the property models are delivered from the multi-physical model at the time of 3D repeated seismic survey. Seismic data modeled based on the models are compared with the real data and the results validate the effectiveness of the multi-physical inversion method. Time-lapse analysis shows the trend of the CO2 migration during and after the injection.
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Diffraction Imaging in 3D via Image Spectral Decomposition of Partial Images
Authors M. Protasov, K. Gadylshin, V. Tchevreda, A. Pravduhin and N. IsakovSummaryThe paper presents 3D diffraction imaging based on spectral decomposition of the different combination of selective or partial images. These images are got by the pre-stack asymmetric migration procedure which is weighted data summation. Spectral decomposition is done in Fourier domain with respect to spatial dip and azimuth angles. Numerical examples with application of different workflows for the synthetic and real data examples demonstrate detailed reliable reconstruction of the fractured zones and reliable reconstruction of fracture orientation on synthetic and real 3D data examples.
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Diffraction Modeling and Imaging of Sand Injectites
Authors M.A. Pelissier, D. Brethaut, R.L. Hartstra, M. Jaya and T.J. MoserSummaryIn this paper we use a conceptual model to investigate the diffraction response of sand injectites. Unlike conventional seismic attributes derived from a migrated image using a local averaging process, the diffraction image provides the full resolution of the wavefield. We model a dike representing a typical injectite wing. This is of particular interest due to the advantage in illumination provided by diffraction over reflection for the steep flanks of the dike. We show that the model produces three type of diffraction response. These are associated with the host rock reflector terminations, with discontinuities in reflectivity along the flanks of the dike due to layering of the host rock, and with the pinchout of the dike. In each case the diffraction response is the resultant of a pair of edge diffractors, and the interference of the imaged diffractors depends on the geometry of the injectite. These results illustrate the potential for diffraction imaging to provide additional resolution of injectite geometries.
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Application of Deep Learning Along Directional Image Gathers for High-Definition Classification of Subsurface Features
Authors Y. Serfaty, L. Itan, R. Levy and Z. KorenSummaryWe present a novel method for decomposing different geometrical characteristics of imaged seismic data. The automation of procedures for enhancing interpreted/classified image data is achieved by applying principle component analysis (PCA) to directional (dip/azimuth) gathers, followed by deep learning (convolutional neural network) classification. The subsurface geometrical objects to be classified are reflectors (continuous structural surfaces) and different types of diffractors (discontinuous objects such as small-scale fractures and faults). This approach shows great promise in identifying subsurface structural features with high accuracy, low cost (no processing preparation is needed) and simple yet scalable implementation. Our preliminary results show superiority over other methods involved in geometrical transformation (e.g., Radon) and specular/diffraction weighted stacks.
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An Outlook on Seismic Diffraction Imaging Using Pattern Recognition
Authors B. Lowney, I. Lokmer, C.J. Bean, G.S. O’Brien and M. IgoeSummaryA seismic image is formed by interactions of the seismic wavefield with geological interfaces, in the form of reflections, diffractions, and other coherent noise. While in conventional processing workflows reflections are favoured over diffractions, this is only beneficial in areas with uniform stratigraphy. Diffractions form as interactions of the wavefield with discontinuities and therefore can be used to image them. However, to image diffractions, they must first be separated from the seismic wavefield. Here we propose a pattern recognition approach for separation, employing image segmentation. We then compare this to two existing diffraction imaging methods, plane-wave destruction and f-k filtering.
Image segmentation can be used to divide the image into pixels which share certain criteria. Here, we have separated the image first by amplitude using a histogram-based segmentation method, followed by edge detection with a Sobel operator to locate the hyperbola.
The image segmentation method successfully locates diffraction hyperbola which can then be separated and migrated for diffraction imaging. When compared with plane-wave destruction and f-k filtering, the image segmentation method proves beneficial as it allows for identification of the hyperbolae without noise. However, the method can fail to identify hyperbolae in noisier environments and when hyperbolae overlap.
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Advanced PP and PS Model Building — An Offshore Mexico Example
Summary“Model building and depth imaging with PS converted-wave data is more complicated than with P-wave data alone. We must determine appropriate compressional and shear velocities such that PP and PS sections match, and accuracy of anisotropic parameters is crucial to minimize residual moveout on both data types. Fortunately, recent technological developments, notably joint PP-PS tomography, allow us to produce excellent PS depth-imaged results with reasonable turnaround time.
In this case study we describe the application of PP-PS model building and depth imaging to an OBC seismic survey. The area is characterized by complex structure combined with large velocity contrasts, both of which cause problems for PS model building. A state-of-the-art workflow was applied that included joint PP-PS tomography, maximum use of well data, and minimal interpretation. High-quality PP and PS results were produced in a reasonable timeframe.”
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Automated and Semi-Automated Depth Registration of PP and PS Images
Authors C. Bagaini, Y. Elmosharaf, A. Dhara, J. Mathewson and M. Acosta PerezSummaryThe registration of PP to PS images is an essential step of iterative model building of multicomponent data. Depending on the quality of the data and the accuracy of the initial model of the earth’s interior, this operation may or may not require identifying corresponding geological boundaries. We developed a method for PP to PS registration that operates directly on images defined in the depth domain. This method can use horizons (either manually or automatically interpreted) or perform this operation in a data driven (automated mode) without interpreter input using only PP and PS seismic images. The application to synthetic and real data demonstrates the value of this method.
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New Model-Based PS Deghosting Boost Data Quality at Grane PRM
Authors S. Ostmo, H. Westerdahl, H. Mehdi Zadeh, M.S. Guttormsen, B. King and L.J. TveitoSummaryIn permanent reservoir monitoring (PRM) PS data, acquired with buried receivers, we have observed a number of notches in the spectra. These notches were not present in the spectra of receivers on the seafloor. To restore the PRM PS data quality, a model-based PS deghosting method for buried receivers is proposed and tested on real data, assuming that the variation in spectra is created by strong contrasts in the shear wave velocity in the very near surface. Only predominantly vertically travelling SS-reflections and transmissions are considered, including important contribution from internal multiples. A shallow layered model is inverted for using picked notches on receiver spectra. Inverse filters, one per receiver, is constructed from the model response and applied to receiver gathers. The method was tested within a complete processing sequence and applied to PRM data from the Grane field in the North Sea. Results indicated very low S-velocities close to the seabed, in the order of 20–40 m/s. The deghosting clearly improved the continuity and resolution in the final image.
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An Elastic Reverse Time Migration Method Based on the S-Wave Quasi-Tensor
More LessSummaryIn this manuscript, an alternative converted-wave image with elastic reverse-time migration is proposed. In the proposed method, PS image is generated by applying the imaging condition to the P- and S-wave stress wavefields. In decoupled wave equation, which is used to separate wavefields, we can obtain the P- and S-wave particle-velocity (or displacement) wavefields and P-wave stress, however we cannot obtain the pure S-wave stress. To fully excavate potential of using stress to produce converted-wave image, we construct a quasi S-wave stress from S-wave particle-velocity wavefield using the so called ‘acoustic shear wave equation’. We illustrate such S-wave stress can provide polarity-consistent converted-wave image theoretically and numerically.
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PZ Processing of a Signal-Apparition Triple-Source Field Test in the North Sea
SummaryWe present results from a triple-source signal apparition-style field test carried out over a PRM array in the North Sea during the summer of 2017. In addition to the triple-source line, two reference lines were acquired firing the central source only.
Assessing the quality of acoustic and elastic P/Z combination confirms high fidelity data quality throughout the bandwidth of the data. Comparing prestack migrated images of elastic P/Z combination against Amundsen demultiple images shows that multiples are a significant challenge in the area and that removing all surface related multiples as done in the Amundsen demultiple method is critical for unambiguous interpretation.
Amundsen demultiple requires the deconvolution of the downgoing wavefield from the upgoing wavefield. Thus, the process relies on excellent data fidelity in separated wavefield constituents. We find that comparisons of reference line on reference line display a similar level of residual as the reference line compared to the isolated source result corresponding to the same source location. The reference line compared to the decoded source result corresponding to the other offset source locations display a larger residual. Our results thus confirm that triple-source signal apparition-style acquisition enables highly productive acquisition of seismic data without compromising on data quality.
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Time Delays in Simultaneous-Source Acquisition and Its Impact on Low Frequencies
Authors K.E. Haavik, Å.S. Pedersen and M. ThompsonSummarySimultaneous-source acquisition enables more efficient and/or denser seismic data acquisition. While acquiring simultaneous source data is a relatively simple extension of conventional operations, separating the signal from the different sources is a more involved process. Modern simultaneous source acquisition is enabled by applying time delays to one or more of the sources in order for the separation process to work. Here we present a theoretical study on the effect of this time delay on the emitted signal from the simultaneous sources. Our findings suggests that small time delays are preferable over larger time delays when considering the amplitude and radiation pattern for low-frequencies (<15 Hz).
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A Seismic Apparition Experiment on Towed Streamer Seismic Data
More LessSummaryThis paper discusses a field test of seismic apparition, a simultaneous source technique for marine airgun sources that uses modulation to facilitate source separation. The first part of the paper focuses on the design of the modulation codes. The second part introduces a field test where a single line of 3D data was acquired using three two-string sources. This field test provides real data for testing current and future implementations of the seismic apparition method. In this paper we present initial separation results obtained after minimal pre-processing of the recorded data. Conclusions are drawn on the challenges and opportunities of towed-streamer seismic apparition for multi-source surveys.
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High-Multiplicity Simultaneous Sources and Marine Vibrator Acquisition Effects
Authors D. Halliday, R. Laws, A. Özbek and J. HopperstadSummaryHigh-fidelity marine seismic vibrators allow control of the phase of the emitted seismic wavefield. Phase control of the seismic source can allow new simultaneous-source encoding techniques. These techniques may allow highly efficient seismic acquisition without sacrificing data quality. One approach that uses phase control is phase sequencing, where the emitted source wavefield phase is changed in a non-random way from shot-to-shot, allowing manipulation of data in the frequency-wavenumber domain. Combined with wavefield reconstruction, phase sequencing allows separation of simultaneous sources with minimal residual crosstalk. An important consideration when evaluating such techniques is that marine seismic vibrator deployment and utilization will be different from an air-gun source. For example, a marine vibrator source array is likely to move while emitting energy, it may have different array elements distributed at different depths, it will emit swept rather than impulsive waveforms, and it may behave differently in the presence of a rough sea. We demonstrate the separation of high-multiplicity simultaneous sources using phase sequencing, before discussing marine-vibrator specific acquisition effects. The impact of these acquisition effects will be demonstrated in future publications.
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Compressed Sensing Based Land Simultaneous Acquisition Using Encoded Sweeps
Authors R. Kumar, S. Sharan, N. Moldoveanu and F. J. HerrmannSummarySimultaneous shooting methods using encoded sweeps can enhance the productivity of land acquisition in situations where deployment of many vibrators and larger receiver spread is not possible in the field due to obstructions or permit limitations. However, the existing framework requires shooting the full sequence of encoded sweeps on each shot point to reconstruct the complete frequency bandwidth Green’s function. Although this simultaneous shooting method reduces the sweeping time vs conventional sequential shooting, the gain in efficiency is limited. To further reduce the sweeping time, we propose to acquire randomly selected subsets of the encoded sweeps sequences followed by a rank-minimization based joint source separation and spectral interpolation framework to reconstruct the full bandwidth deblended Green’s function. We demonstrate the advantages of proposed sampling and reconstruction framework using a synthetic seismic line simulated using SEG-SEAM Phase II land velocity and density model.
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Survey Designing for Blended Acquisition with Irregularly Sub-Sampled Geometries
Authors S. Nakayama, G. Blacquière, T. Ishiyama and S. IshikawaSummaryWe introduce a workflow to derive survey parameters responsible for source blending as well as spatial sampling of detectors and sources. The proposed workflow iteratively performs the following three steps. The first step is application of blending and sub-sampling to an unblended and well-sampled data. We then apply a closed-loop deblending and data reconstruction enabling a robust estimate of a deblended and reconstructed data. The residue for a given design from this step is evaluated, and subsequently used by genetic algorithms (GAs) to simultaneously update the survey parameters related to both blending and spatial sampling. The updated parameters are fed into a next iteration till they satisfy given stopping criteria. We also propose repeated encoding sequence (RES) used to form a parameter sequence in GAs, making the proposed designing workflow computationally affordable. We demonstrate the results of the workflow using numerically simulated examples that represent blended dispersed source array data. Difference attributable only to a way to design parameters is easily recognizable. The optimized parameters yield clear improvement of deblending and data reconstruction quality and subsequently provide optimal acquisition scenarios. Additionally, comparison among different optimization schemes illustrates ability of GAs along with RES to efficiently find better solutions.
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Blended Noise Suppression Using a Hybrid Median Filter, Normal Moveout and Complex Curvelet Transform Approach
Authors L.Q. Dong, M.G. Zhang, C.H. Wang, Y.M. Zhang and Y. ZhuSummaryThe high density acquisition way can uplift the subsurface imaging accuracy, whereas the high cost limits the widely application in practice. Blending acquisition way has emerged as a promising way of significantly increasing the efficiency of seismic acquisition. However, there will exist a large challenge of severe interference noise and decrease S/N ratio. Therefore, with recent processing practices, the success of blending acquisition relies heavily on the effectiveness of de-blending to separate signals from simultaneous sources. In the paper, we proposed a blended noise suppression approach using a hybrid median filter, normal moveout (NMO) and complex curvelet transform (CCT) approach. Firstly, the large step median filter is applied to the initial data after NMO correction. Next, we continue to extract the residual energy to get the de-blended result by the CCT-based threshold method. Then, re-iterate the difference data by subtracting the original pseudo de-blended data and the pseudo de-blended data of the de-blended result from each iteration as the above processing flow. Finally, the final de-blended data is derived by adding the remained energy of each iteration until the S/N ratio satisfies the desired one. We demonstrate through a simulated field data the effectiveness of the approach.
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Triple Source Isolation: Results from a North Sea Field Test
Authors J.O.A. Robertsson, F. Andersson, D.J. van Manen, K. Eggenberger, R. Walker, C.R. Berg, J. Gateman and I. GimseSummarySeabed seismic data have proven critical for addressing long-standing challenges in seismic imaging both for exploration and reservoir development and management. Significant advances have been made to bring down the cost and increase efficiency for deploying and retrieving ocean bottom nodes, such that the efficiency on the source side increasingly becomes the bottleneck for efficient operations.
We present results from a recent field test in the North Sea where three source lines were acquired simultaneously from a single vessel without compromising inline sampling such that the fold is doubled compared to conventional flip-flop acquisition. The acquisition and processing technology is partially based on the science of signal apparition that allows for an optimally large part of the data corresponding to the individual sources to be exactly recovered while accurately reconstructing the full bandwidth beyond the region in frequency-wavenumber space of exact reconstruction. For the triple source field test presented here, the productivity increases 50% compared to conventional flip-flop acquisition or three-fold compared to conventional single-source full-fold acquisition suitable for reservoir development and management applications.
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Incorporating Stationary-Phase Implementation into QPSTM and Application in Daqing Oilfield
More LessSummaryWe have improved the so-called QPSTM by introducing a dip-angle domain stationary-phase implementation. QPSTM compensates absorption and dispersion via an actual wave propagation path using effective Q parameters. However, this strongly degrades the resolution gained by the compensation and requires more computational effort than conventional PSTM. Our stationary-phase implementation improves QPSTM through the determination of an optimal migration aperture based on an estimate of the Fresnel zone. This significantly attenuates the noises and reduces the computational cost of 3D QPSTM. We have estimated the 2D Fresnel zone in terms of two dip angles through building a pair of 1D migrated dip angle gathers using PSTM. Our stationary-phase QPSTM was implemented as a two-stage process. First, we used conventional PSTM to obtain the Fresnel zones. Then, we performed QPSTM with the Fresnel-zone-based optimized migration aperture. We apply the QPSTM to the actual 3D seismic data, obtain higher resolution imaging results, and verify the practicability of the method.
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Visco-Acoustic Reverse-Time Migration based on the Stereo-Modeling Operator
More LessSummaryViscous effects causes seismic energy attenuation and waveform distortion. Conventional acoustic migration cannot account for this effect, which may produce images with poor illumination, reduced resolution, and wrong placement of reflectors. Reverse time migration (RTM) based on the visco-acoustic wave equation is an effective way to image subsurface media. Solving the wave equation accurately and efficiently affects the RTM results. Stereo-modeling methods have great ability in suppressing numerical dispersion, thus can improve computational efficiency with coarse grids. Based on the stereo-modeling operator, we derive a nearly-analytic central difference (NACD) method (4th-order accuracy in both time and space) to solve the visco-acoustic wave equation. The efficiency test shows it can suppress the numerical dispersion effectively and is more efficient compared with Lax-Wendroff correction (LWC) method with the same accuracy. The acoustic and visco-acoustic RTM based on NACD is performed on the fault and Marmousi models. The results show that the application of NACD in RTM improves the accuracy and the resolution of images. Moreover, the images obtained by visco-acoustic RTM are clearer and have higher resolution compared with images obtained by acoustic RTM because the viscous effects are considered and compensated.
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Angle Gathers from a Kirchhoff Pre-Stack Depth Migration using a Space-Lag Extended Imaging Condition
Authors G. O’Brien, S. Delaney and M. IgoeSummaryTraditional Kirchhoff pre-stack depth migration methods output surface offset common image gathers (CIG) but exploration workflows require subsurface reflection angle CIGs to extract accurate angle versus amplitude measurements. Providing any incremental uplift to the amplitude versus angle (AVA) attributes and the associated seismic inversions can significantly impact exploration and development success. Several high-end imaging techniques exist to extract subsurface angles directly, notably reverse time migration methods and common reflection angle migrations. Here, we explore the ability of the relatively cheap well-used Kirchhoff pre-stack depth migration to output subsurface reflection angle common image gathers via a local space-shift extended imaging condition. A layered synthetic model and a complex 2-D synthetic model are used to assess the space-shift image gathers output from such a migration scheme and to evaluate the seismic attributes relative to the traditional surface offset CIGs. The synthetic results show that the extended imaging condition clearly provides an uplift in the measured AVA over the surface offset migration. Finally, we show an example of a 3D space-lag gather from deep marine data and compare the resultant angle gathers with those generated from an offset migration and a time-shift imaging condition Kirchhoff migration.
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Image Domain Least-Squares Migration with Hessian Estimated by Non-Stationary Matching Filters
More LessSummaryThe key to image domain least-squares migration is the explicit calculation of the Hessian matrix. However, the full Hessian matrix is too big and expensive to compute and save. Guitton (2004) directly approximates the non-diagonal inverse of the Hessian with a bank of non-stationary matching filters, which can be seen as a low-rank approximation of the true inverse Hessian. The filters have the amplitude-balancing effect, but the ability to increase the resolution is missing. In this paper, to capture as much effect of least-squares migration as possible, we use non-stationary matching filters to approximate the non-diagonal Hessian first, and then solve a constrained optimization problem with the sparse and TV regularization for the result of the image domain least-squares migration. Numerical examples illustrate that the inverted images of the proposed method have both more balanced amplitudes and higher resolution than conventional migration images.
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Double Plane Wave Least Squares Reverse Time Migration with Sparse Frequency and Pane Wave Sampling
More LessSummaryLeast squares reverse time migration (LSRTM) is often formulated as an iterative updating process, where estimating the gradient of the misfit function is necessary. Traditional time domain shot profile LSRTM is computationally expensive because computing the gradient involves solving the two-way wave equation several times in every iteration. To reduce the computational effort of LSRTM, we propose to implement frequency domain LSRTM using double plane wave (DPW) data with sparse frequency sampling and sparse plane wave sampling. Theoretically, frequency sampling and plane wave sampling used in migration should obey the Nyqusit theorem. Otherwise, the well-known wrap-around artifacts and linear artifacts might contaminate images. In this research, we demonstrate that images with correct reflector amplitudes and reasonable resolution can be achieved even if frequency sampling and plane wave sampling are larger than that determined by the Nyquist theorem. The artifacts generated due to undersampling in frequency and plane wave can be effectively suppressed during iterations. Using sparse frequency and plane wave sampling for DPW LSRTM greatly reduces the number of wavefield computations. We show that the computational efficiency can be improved by an order of magnitude.
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Wavefield Transmission Imaging with Reverse Time Migration
Authors C. Willacy and M. KryvohuzSummarySteep salt boundaries can be poorly illuminated or completely absent in the migrated image. To provide a solution to this problem, we apply two reverse time migration imaging methods which use transmitted (refracted) wavefields. In the first technique, down-going waves, typically recorded in walkaway VSP surveys, are used to image the salt flank via the generation of aplanatic isochrones. It is demonstrated that this image can be generated in the absence of an explicit interpretation of the salt flank. In the second technique, we extend the basic theory to include imaging of up-going source wave fields which then refract at the base salt, as acquired by a surface acquisition geometry. This technique has similarities to the prism-imaging method, yet it uses transmitted instead of reflected waves at the salt boundary. We demonstrate that this is not only a viable technique for imaging of the salt boundary, but also to determine the correct salt velocity when used in conjunction with conventional reflection imaging. A combination of synthetic and field datasets is used to demonstrate the transmission imaging methodologies for imaging salt flanks with reverse time migration.
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A Separated Formulation of the Elastic Wave Equation in P- and S-potential Fields
More LessSummaryElastic wave imaging has been a significant challenge in the exploration industry due to the complexities in wave physics and in numerical implementation. In this paper, we derive the elastic wave equations without the assumptions of homogeneous Lamé parameters to capture the mode conversion between the P- and S-waves in an isotropic, constant-density medium. The resulting set of two coupled second-order equations for P- and S-potentials clearly demonstrates that mode conversion only occurs at the discontinuities of the shear modulus. Applying Born approximation to the new equations, we derive PP and PS imaging conditions as the first gradients of waveform matching objective functions. The resulting images are consistent with the physical perturbations of the elastic parameters, and hence are automatically free of the polarity reversal artifacts in the converted images. When implementing elastic reverse time migration (RTM), we show that scalar wave equations can be used to back propagate the recorded P-potential, as well as individual components in the vector field of the S-potential.
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A Wavefield-separation-based Elastic Least Squares Reverse Time Migration
More LessSummaryCompared with the elastic reverse time migration (ERTM), ELSRTM can produce images with higher spatial resolution, more balanced amplitudes and fewer artifacts. However, the crosstalk between compressional (P-) and shear (S-) waves can significantly degrade the imaging quality of ELSRTM. We developed an ELSRTM method to suppress the crosstalk artifacts. This method includes three crucial points. The first is that both the forward and backward wavefield are extrapolated based on the separated elastic velocity-stress equation of P- and S-waves. The second is that the separated vector P- and S-wave residuals are migrated to form reflectivity images of Lamé constants λ and μ independently. The third is that the reflectivity images of λ and μ are obtained by the vector P-wave wavefields achieved in the backward extrapolation of the separated vector P-wave residuals and the vector S-wave wavefields achieved in the backward extrapolation of the separated vector S-wave residuals respectively. Numerical tests with synthetic data demonstrate that our ELSRTM method can produce images free of the crosstalk artifacts.
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Channelized System Reservoir Characterization Using Diffraction Imaging - Practical Aspect of a Novel Technique
Authors J. Soldo, M. Koremblit, D. Lorenzo and M. SuarezSummaryThe seismic processing technique known as “Diffraction Imaging” has demonstrated to be an interesting method in order to map geological sub surface discontinuities, in special those high frequency features related to such as discontinuities. They have been study in the prestack time as well as in the prestack depth domain by several authors. Detection of geobodies (channel system) is important when developing continental type sandstone reservoirs. Understanding the location and orientation of such as features is important for optimal well placement and field delineation. Seismic diffraction can be used for imaging not only systems of natural fractures but also for mapping wedge limits caused by channels. Diffracted waves are created when an incident wavefield encounters small size objects or even discontinuities such as faults, sharp curvature geoforms or wedges. The diffractive portion of this wavefield can be extracted from its total in order to produce images that contain information regarding subsurface scattered energy due to discontinuities. This is the first time that this technique is applied in the company in order to characterise and improve the delineation of channel systems.
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A Holistic Approach to Model-Building in and around Injectites: A Case-Study Offshore Norway
Authors V. Valler, N. Payne, T. Hallett, M. Kobylarski, G. Venkatraman, J. Rappke and D. FaircloughSummaryThe presence of injectites is a common and often time consuming area to be addressed during velocity model building. In addition to their impact on deeper structures and prospects, re-worked injectites are increasingly being considered for hydrocarbon potential themselves. In order to handle the challenges above we should consider ways of producing an accurate velocity model of these structures within a framework that is efficient and commercially time-viable. Here we present a holistic approach and case study to model-building in and around injectites that utilizes robust broadband data pre-processing, a semi-automated identification and modeling of injectite bodies and subsequent high-resolution tomographic updating. Our results show that this method enables us to produce a highly accurate and detailed model of a complex injectite field and subsequent improvement on the deeper image within the timeframe of a conventional model building iteration.
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Separated Wavefield Imaging for Ocean Bottom Surveys - Feasibility Study of Receiver Decimation on a North Sea Dataset
Authors D. van der Burg, T. Martin, A. Asnaashari, S. Perrier, J. Fasterling and J. MusserSummaryWe present a method for imaging with separated wavefields from an ocean bottom cable acquisition to deliver images of the subsurface with enhanced illumination and angular diversity. Using this data, we investigate whether it is feasible to decimate the sources or receivers without degrading the structural image from separated wavefield imaging. We conclude by determining if this would be beneficial to reduce acquisition cost and to provide illumination below obstructions like a platform hole.
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Reservoir Imaging Using Fwi, Q-Tomography & Bayesian Classification - Black Sea Case Study from Acquisition to Drill Bit
Authors O. Costriiciuc, A. Fogg, R. Holden and J. MooreSummaryBlack Sea Oil and Gas’ concession XVa Midia is on the Romanian continental shelf of the Black Sea which the company operates on behalf of its partners Gas Plus International BV and Petro Ventures Resources SRL. This study demonstrates how a suite of seismic processing and QI (Quantitative Interpretation) technologies was optimised to fully resolve gas assets currently under development and highlight satellite exploration targets in Miocene to Pliocene age (late Pontian to Dacian) fluvio-deltaic sands and claystones. Near surface channel features and localised gas presence causes seismic imaging issues as amplitudes are attenuated and time structural push-downs are observed. A 3D survey was acquired during 2016 and processed together with three legacy 3D surveys. This data was used for exploration well planning for a drilling programme of two wells Q4 2017. During the seismic processing tests were run to investigate possible imaging improvements using Full Waveform Inversion (FWI) and Q-tomography. A subset of the 3D was reprocessed using these methods during Q4 2017. This presentation will demonstrate the imaging uplift achieved and the impact on the interpretation together with pre and post drill results guided by QI based on pre-stack seismic inversion and facies/fluid probability classification.
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Reviving a Mature Basin through High-End Imaging Technology
More LessSummaryThis paper highlights compelling imaging improvements achieved through modern high-end reprocessing in the Gippsland Basin. The area of study, largely represented by the Bass Canyon, has strong exploration potential as well as high risks. The major challenges in this region are related to geologic complexity and seismic imaging limitations, i.e.: a) extensive velocity anomalies leading to velocity uncertainties and false structural closures that increase drilling risk; b) strong noise interference and limited imaging clarity that affects AVO analysis. By integrating the modern techniques of 3D deghosting, full waveform inversion (FWI) and least-square Q pre-stack depth migration (LSQPSDM), the newly reprocessed data yields a substantial amount of added value over legacy datasets, resulting in an improved understanding of the subsurface geology and clearer prospect mapping. This reprocessing approach demonstrates, even in basins that are considered mature, that new ideas and technology can change long-held perceptions and rejuvenate exploration interest.
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Salt-Related Converted-Wave Attenuation – a Deep-Water Example
Authors M. Hegazy, A. Stewart, S. Hydal, K. Malave, O. Zdraveva and O. MataraciogluSummaryComplex shallow salt canopies, such as those found in offshore basins in the Gulf of Mexico, Brazil, and West Africa often suffer poor illumination, which results in poor subsalt imaging. The geometry of subsalt horizons truncating against the base of salt often determines if a trap exists and if it will be economically viable. It is, therefore, important to get an accurate image in these areas to reduce the seismic uncertainty. Unfortunately, the presence of converted mode energy can add spurious complexity to the subsalt image and degrade subsalt target interpretation. A novel C-wave attenuation workflow is introduced to address the C-wave energy. Synthetic and field data examples show a significant improvement in the image interpretability and demonstrates the importance of this application to earth model building and amplitude variation with offset AVO work
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Ultra-Large-Scale Wide-Azimuth Acquisition and Broadband Imaging in the Campeche Basin
Authors S.L. Yong, O. Zdraveva, E. Medina, S. Savoie and K. LyonsSummaryThe recent opening of Mexican acreage to foreign companies created a need for reliable regional interpretation over large prospective areas and lead to the wide-azimuth acquisition of new seismic data over ∼70,000 km2 in the Campeche basin. In this work, we discuss the overall acquisition and imaging strategy, allowing us to create high-quality, contiguous, broad-band images over large area of extreme geologic complexity in a short turnaround time. We describe all aspects of this strategy from the staged acquisition and fast-track products delivery, through broad-band processing and high-resolution anisotropic earth model building, to the use of complementary migration algorithms enabling supra-salt and subsalt target evaluation in this vast and prospective area located in the most southern part of the Gulf of Mexico.
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Unfolding Marine Multiazimuth Data from the Gulf of Mexico
Authors H. Roende, G. Hilburn and D. BateSummaryThe first evolution of WAZ seismic data acquired over 10 years ago provided a sizable uplift over previous NAZ data sets. The addition of orthogonal surveys, advances in data processing, and improved geological understanding have recently introduced another step change in the data quality. These improvements are now leading to a greater improvement in data analysis and interpretation than just structural imaging uplift. We can see 4 distinct behaviors in 6 azimuth M-WAZ marine data. When data is acquired over “simple” geology, we get common signal in all azimuths. We also notice that close to salt, there are major differences between the 6 azimuths over large depth ranges due to illumination. These two observations have been known and published for the last decade. Our findings show that there are untapped potential and knowledge in marine M-WAZ data if acquired and proccessed correctly. We believe that we can map azimuthal-amplitude variations, and therefore, map stress and fracture fields to facilitate the production of hydrocarbons. If the data is acquired with a significant time lapse between acquisition, there is also an opportunitiy to perform 4D studies.
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Seismic Re-processing and Q-FWI Model Building to Optimise AVO and Resolution in the Shallow Wisting Discovery
Authors G. Apeland, P. Smith, O. Lewis, S. Way, H. Veire, N. Stevens, L.M. Moskvil and J.R. GranliSummaryThe Wisting discovery in the south-west Barents Sea is a laboratory for geophysical studies due to its shallow reservoir and complex geological history. This discovery is constantly highlighting aspects of conventional seismic processing that require new approaches and technologies to allow AVO to be preserved and understood. Previous studies at the Wisting discovery have highlighted the importance of imaging in the depth domain with an accurate earth model for AVO preservation, including accounting for absorption within the imaging itself. In addition, AVO studies over the discovery have highlighted shortcomings with conventionally acquired and processed seismic datasets; The lack of near angles has made AVO studies over the discovery challenging and the AVO extracted from the seismic has not matched the modelled AVO at well locations. Following on from the recommendations of the previous AVO studies, OMV and partners initiated a reprocessing project that would focus on combining all previous experience with the Wisting seismic data. This paper describes the FWI model building and time reprocessing performed, and how we tailored the processing and QC to optimize the AVO response in the shallow section.
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Wave-Equation-Based AVO Inversion in the Presence of Coal Seams. A Synthetic Study from the Cooper Basin, Australia
Authors D. Gisolf, P. Haffinger, P. Doulgeris and S. SquireSummaryCoal seams, being very soft and light, cause strong internal multiple reflections and mode-conversions, as well as strong transmission effects. Conventional AVO inversion based on primary reflection coefficients only, will not be able to distiguish the primary from the multiple energy and consequently produce spurious results. Wave-Equation-Based AVO inversion, being based on the full elastic wave-equation, is able to account for all these effects and image the coal seams properly. Like all other AVO techniques, the WEB AVO is applied in the 1.5D data domain. In this paper we consider the logs from the well Moonanga-1 in the Cooper Basin in South Australia. This area is notorious for the presence of coal seams, hampering seismic interpretation. Full elastic synthetics are produced from the logs. The results from conventional Sparse-Spike-Inversion and from WEB inversion of the synthetic gather are compared, showing a clear improvement for the WEB inversion. This will lead to much better interpretability of the inverted data.
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Quantifying the Effect of Overburden Multiples on Miocene Reservoirs in the Levantine Basin
Authors P. Doulgeris, D. Anestoudis, P. Haffinger, A. Droujinina and D. GisolfSummaryThe Karish Main field was discovered in 2013 in the Levantive Basin, 75km offshore the northern coast of Israel. The Oligocene-Miocene submarine fan systems form the primary reservoirs of most commercial hydrocarbon discoveries to date in the region. At the end of the Miocene, the Mediterranean Sea became isolated from the Atlantic Ocean, which led to the deposition of up to 1,500 m of evaporites. The prospect was supported by seismic amplitudes and was proven by drilling well Karish-1. The aim of the current study is to model and understand the impact of the overburden evaporite on the seismic response of the reservoir. The wave-equation-based interbed multiples investigation technique deployed was able to identify the major multiple generators while quantifying the level of constructive interference expected on amplitudes at reservoir level from overburden multiples.
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AVAZ Quantitative Interpretation Based on Orthorhombic Medium-Part Two: AVAZ Inversion
More LessSummaryIn this paper, we mainly talk about orthotropic AVAZ inversion technique, it has proven to be important and effective for shale gas evaluation and development. We use pre-stack AVAZ inversion to predict elastic properties, it is carried on pre-stack azimuthal gathers with constrained wells. A slight modification was made on the basis of Bachrach’s (2004) equation in order to obtain elastic parameters such as P-wave impedance, S-wave impedance, combinations of anisotropic parameters and fracture orientation from inversion. This work could offer multiple elastic parameters that characterize the quality of shale reservoirs. This method is successfully applied in real data of Sichuan Basin, it shows that the results of inversion are effective and reasonable.
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Regional Porosity Estimation in Danish North Sea Chalk
Authors H. Wagner, H. Klemm and P. GeltingSummaryWhen performing seismic inversion over a large area we often face lateral changes in the seismic signature not related to the geology of the target. Therefore a methodology to equalize seismic data over large areas was developed. The methodology was used to prepare the data for absolute seismic acoustic inversion. The P-wave impedance result was subsequently transformed into porosity via a relationship from well data. The methodology builds on the ability to assess differences in data using an un-biased wavelet and well tie estimation method. The method is utilized to correct for data issues without removing geologic information. We did this by constructing a matching filter from a wavelet parametrization that varied with depth. The methodology was successfully applied for regional porosity estimation over an area of 6400 km2, or more than 40 million seismic traces, in the Danish North Sea. The regional nature of the results is beneficial for the large scale geological understanding of the area especially for exploration purposes. Furthermore, it provides insights into relations between porosity and geomorphology, and helps identifying new exploration opportunities.
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Seismic Inversion for Statfjord Well Maturation
Authors J. Lippard and E.T. DelaneySummaryWe present a case study where a new seismic dataset’s improved data quality has enabled a geologically-constrained Bayesian pre-stack inversion algorithm to produce reliable and usable inversion results to uncover significant pockets of oil. The chosen inversion methodology was PCube+: A Bayesian algorithm that uses a local neighborhood approach to perform a one-step inversion to lithology-fluid classes. By using the resulting sand map together with production and well data, areas with remaining reserves can be identified for infill drilling. One such area that was identified led to the drilling of an oil producer that is one of the best performing wells on the Statfjord field in years.
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Amplitude Variation with Angle Inversion Based on Propagator Matrix Modelling
More LessSummaryMost existing amplitude variation with angle (AVA) inversions are based on the exact Zoeppritz equation and its linearized approximations. However, such modellings consider the primary (P-wave) waves only and ignore transmission loss, multiples and many other converted wave modes. It means that strict pre-processing is required to satisfy the requirement of the input data to inversions, which is difficult to accomplish. To overcome this problem, a new AVA inversion based on the propagator matrix method of Carcione (PMC) is proposed in this abstract. PMC is derived from the 1D wave equation and can accurately simulate full wave responses within horizontally multi-layered models. For computational accuracy, we use a Bayesian framework to define the augmented function and choose the L-BFGS optimization method to solve it. Through a model test and real data application, it is found that the proposed AVA inversion based on PMC can effectively eliminate transmission loss and internal multiples and helps to provide better S-wave velocity and density estimations.
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Anisotropic Inversion for VTI Media Based on a Modified Approximation of the PP-Wave Reflection Coefficient: Theory And
More LessSummaryAnisotropy has significant influence on the seismic amplitude variation with offset (AVO) response. Although analytical solutions and approximations of the PP-wave reflection coefficient for tranversely isotropic media have been explicitly studied, it is diffcult to apply these equations to anisotropic AVO inversion, because five parameters need to be estimated, and such an inverse problem is highly ill-posed. A modified approximation of the PP-wave reflection coefficient is proposed in this paper based on Rüger’s formula for VTI (tranverse isotropy with a vertical symmetry axis) media. The derived equation is formed using the attributes of acoustic impedance, anisotropic shear modulus, and an anisotropic P-wave velocity, which is proportional to the vertical P-wave velocity with the natural exponential function of Thomsens’s ε. Numerical tests show that the approximation has sufficient accuracy over a wide range of angles. The field-data application in a lower Silurian-age shale formation reveals that the inverted attributes are very useful for the characterization of a shale-gas reservoir. The Thomsen’s anisotropy parameter ε can be recovered subsequently. Inverted ε not only provides a reliable model for the anisotropic imaging and full waveform inversion, but also acts as an indicator of gas-saturated shale, which shows lower anisotropy than surrounding shales.
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Integrating Petrophysical Evaluations and Rock Physics
By I. EscobarSummaryWe have developed an optimization methodology to consistently integrate petrophysical evaluations and the construction of rock physics models using multiple wells. This approach allows exploring the different parameters/coefficients used both for the evaluation and rock physics model, while ensuring complete repeatability and traceability of results. It has been used to build a rock physics model for the Chalk member in the Danish North Sea area, allowing us to cover the transition from clean limestone in the upper-Cretaceous to the shale-rich lower-Cretaceous limestone. During this process, some of the initial petrophysical evaluations were also updated.
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Porosity and Pore-Type Estimation from Seismic Data Using the Xu-Payne Multi-Porosity Model for Carbonate Reservoir
Authors H.B. Li, J.J. Zhang, H.J. Pan and S.J. CaiSummaryCarbonate reservoir has complex pore structure. The popularXu-Payne multi-porosity model has been widely used to quantitatively characterize pore-type distribution from logging data and experimental data. But it remains a great challenge to perform the pore-type inversion from seismic data while using the Xu-Payne model because accurate porosity and saturation can not be pre-given like well observations. In this paper, we present a two-step method to estimate the porosity and saturation and quantitatively characterize pore-type distribution from seismic data for carbonate reservoirs. Firstly, the pore systems of carbonate reservoir are treated as single porosity system with an effective pore aspect ratio. The porosity, fluid saturation and effective pore aspect ratio are simultaneously inverted from the inverted elastic properties by integrating Gassmann equations and the differential effective medium analytical model. Secondly, the pore systems of carbonate reservoir are treated as triple-porosity system that the Xu-Payne multi-porosity model defines. The porosities of three pore types can be inverted using the inverted elastic and physical properties as input. The real application shows that the proposed rock physics modelling and inversion method can yield a good quantitative estimation of porosity and characterization of pore-type from well log data and seismic data.
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Digital Rock Physics Based Pore Type Effect Analysis on Acoustic Properties of Carbonate Rocks
More LessSummaryIn this paper, we first subsample numerous datasets from digital rock model, and then the subvolumes were used to simulate the elastic properties with FEM solver. We verified the FEM results through ultrasonic measurements, and presented the cross-plot of acoustic velocity versus porosity. Regardless of the pore shape, Vp-φ crosspiot showed chaotic and scatter. Based on 3D porous structure of each subvolume, we classified such data points into several groups by different pore type. We found that for a certain porosity, pore type greatly affects the acoustic velocities of carbonate rocks. For an instance, we can observe that the acoustic velocities of carbonate rocks with moldic pores are much higher than the ones of carbonate rocks with dissolved intercrystalline pores. Carbonate rocks with compliant pores, let’s say fracture or microcrack, exhibit the lowest acoustic velocity compared with the ones with stiffer pores like moldic pores. Through the classification approach in terms of pore type, one can obtain better correlations between acoustic velocity and porosity concerning with different pore-type carbonate. This probably enables us to gain profound insights into carbonate reservoir prediction based on reservoir inversion with digital rock physics knowledge.
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Quantification of Pore Type System in Carbonate Rocks for Rock Physics Modelling
Authors J. Sharifi, M. Mirzakhanian, M.R. Saberi, M. Moradi and M. SharifiSummarySeismic responses of carbonate reservoirs are not well understood due to complex nature of pore system in these rocks. Therefore, it has been a challenge for researchers to develop an accurate rock physics model for carbonate rocks. In this study, a method was proposed for quantification of pore system in carbonate rocks for rock physics modeling purposes. Accordingly, Wyllie time-average equation (1956) and the methods introduced by Schlumberger (1974) and Lucia and Conti (1987) were used to obtain pore type in a given reservoir. With the aim of proposing a method for determining pore system, parameters of the rock physics model proposed by Xu and Payne (2009) were evaluated. Next, the results were verified based on core studies and thin section and SEM analyses, indicating a good agreement between estimated pore type from well log data and laboratory analysis results. The proposed methodology can help develop rock physics models for estimating continuous pore type logs in professional software packages.
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Predicting Seal Brittleness of Conventional Hydrocarbon Reservoirs Using LMR - a Case Study from the Norwegian Barents S
By N.H. MondolSummaryAmong other causes (e.g. pressure release and gas expulsion, tilting and spillage from pre-uplift hydrocarbon accumulation, cooling of source rock etc.) failure of seals linked to the uplift have been proposed the main reason for the lack of success in finding commercial petroleum accumulation in the Norwegian Barents Sea. This study predicts seal quality of Upper Jurassic Fuglen Formation from well log data of three exploration wells Skrugard (7220/8-1), Drivis (7220/7-3S) and Havis (7220/7-1) in the tectonically complexed, uplifted Norwegian Barents Sea using the LMR (LamdaRho-MuRho) rock physics templet. A simple rock physics model which allowed to compute theoretical values of dynamic elastic parameters for common constituents of shales and sandstones are utalized to constructe the LMR templet. The template shows the variation in LMR for a combination of mineralogical mixtures versus porosity. Results show that overall the Fuglen Formation in all three wells is a good seal though the caprocks/top seals have significant variations of brittleness values where a small upper sandy unit of Havis well shows high brittleness compared to the shale dominated sections. Brittleness indices estimates based on elastic parameters are easy to use but require calibration of lab observation.
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A New Rock Brittleness Index Based On Energy Dissipation in the Process of Uniaxial or Triaxial Compression Test
More LessSummaryThe previous rock brittleness evaluation methods based on stress- strain curve generally simplify the complex curve into several lines, and establish the evlauation equation by the characteristics of simplified curve. These methods have such problems as the loss of effective informaion, the difficulty of curve simplification and the multi solution of the calculation results. In order to solve these problems, a new brittleness index based on the conservation of energy transformation of the pre-peak and post-peak stages in compression test is presented in this paper, which has a clear physical implication. The new method does not simplify the stress- strain curve, uses the full information of curve, and then increases the accuracy of the calculation results. In addition, this paper further analyzes the relationship between the new brittleness index and seismic elastic parameters and establishes their correlation expression by using the experimental data, which is of great significance to the practical application of the new index in the oilfield.
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Effects of Cementation and Compaction on Elastic Wave Velocities of Sandstone
More LessSummaryCompaction and cementation are important properties that influence the elastic wave velocities of sandstone. In this work, we propose a theoretical model to quantify their effects. By relating the compaction rate to the grain coordination number and the cementation rate to the cement layer radius respectively, their effects on the elastic wave velocities can be studied using the Contact Cement Theory (CCT). To validate the proposed model, we measure the P- and S- wave velocities on the synthetic sandstone samples through the ultrasonic pulse transmission method. The theoretical predictions by the proposed model using the sample parameters are then compared to the experimental data, which shows good agreement between them. Furthermore, it also shows that without considering the compaction effects, the velocities can be obviously underestimated.
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Evolution in Physical Properties of Quartz-bearing Rocks with Evolving Degree of Microfissuration
Authors L. Pimienta and M. ViolaySummaryRocks are complex media that may contain very different geometries of pores. The two extreme families are the spherical pores, making up most of the total porosity, and the thin elongated micro cracks, controlling most of the rock elastic response. While the two families may exist in crustal rocks, they were found to be present in most sandstones. Using a technique to induce a set amount of micro cracks (thermal cracking) in a target rock, the purpose of this work is to investigate the relative effects of the porosity populations on the elastic and transport properties. We will show that, while elastic properties are similarly affected for all studied rocks, a strong effect of the rock initial porosity is observed on the resulting transport property. Moreover, consistently with existing theories, transport properties are additionally affected by the opening of micro cracks, which depends on the temperature of treatment.
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Sparse Frequencies Data Inversion: an Application to a Near Surface Experiment
Authors T. Alkhalifah, B. Sun, Y. Choi, F. Alonaizi and M. AlMalkiSummary“With an objective to invert for the subsurface velocity in the near surface rather than developing an image, we substitute the commonly used broadband acquisition scenario with a novel narrow band acquisition at coarse shot locations. We conduct the acquisition of narrow band seismic data, with an effect of 3 simultaneous sources vibrating at different bands (14–15 Hz, 24–25 Hz, and 49–50 Hz) of the frequency spectrum. The separation of the shot gathers corresponding to the simultaneous sources becomes natural as the shots fall in different bands of the frequency spectrum. The narrow band acquisition allows us to inject more energy of these frequencies using the same conventional vibrator sweep time (6 seconds). We mute regions of low signal-to-noise ratio, and then insert the data into a frequency domain waveform inversion algorithm. The inverted model down to 250 meters depth showed structure corresponding to a low velocity zone at around 80 meter depth. For comparison a conventional full sweep acquisition (30–170 Hz) at a dense shot spacing we recorded. We migrated this conventional dataset using the inverted model. The agreement between the inverted model and the image, extracted from the two independent datasets, supports the accuracy of the inverted model.”
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Application of Anisotropic Wave Form Inversion on Data from the Niger Delta
Authors N. Mitchell, A. Oni, A. Stopin, C. Pérez Solano and R. PlessixSummaryIn transition regions or river deltas, surveys often combine onshore and offshore types of acquisition. We discuss our approach for inverting a Niger river delta survey in which dynamite and geophones are used on land, with airguns and hydrophones in the rivers and creeks. This context presents certain challenges, such as differences in the source characteristics between dynamite and airguns. The airgun data lacks the low 4–6.5Hz frequencies that the dynamite data possesses. We therefore split the survey into two separate dynamite and airgun surveys, processing them independently. This allows for conventional marine and land workflows but introduces acquisition gaps in the separated surveys. We perform a multi-stage multiparameter inversion, first using only dynamite data to improve the starting model so it’s suitable for higher frequencies. Acquisition gaps in the dynamite only inversion are overcome using spatially adaptive smoothing which increases in areas of low illumination. This prevents formation of spurious oscillations. We then run a combined airgun-dynamite inversion. The results of our inversions lead to better stack and gather quality across the field compared to the initial model.
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Inverting Near-Surface Absorption Bodies with Full-Waveform Inversion: a Case Study from the North Viking Graben in the
Authors B. Xiao, A. Ratcliffe, T. Latter, Y. Xie and M. WangSummaryShallow absorptive bodies are an ongoing challenge in velocity model building due to the dispersion and attenuation they cause to seismic data: ignoring absorption in model building can lead to erroneous velocities and poor imaging. Ray-tracing-based tomographic inversions for attenuation can perform well, but typically provide lower resolution than a full waveform approach. Also, the method carries inherent drawbacks in the near surface, where absorptive bodies are often at their most influential, due to acquisition limitations. This work highlights visco-acoustic full-waveform inversion (Q-FWI) as a method for estimating high-resolution velocity and attenuation models. We present a very large, real data, case study where Q-FWI has been applied to ~36,000 km2 of 3D, narrow azimuth, variable-depth streamer data over the North Viking Graben region of the Norwegian North Sea. The results delineate both known and previously unknown absorptive bodies of varying size and strength. Our results show that Q-FWI can invert for high-resolution velocity and attenuation models, providing superior imaging using an attenuation compensating pre-stack depth migration.
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Efficient Depth Model Building Using FWI Early in the Workflow: a Case Study from the Norwegian Sea
Authors O. Lewis, G. Apeland, A. Osen, E.B. Raknes and R. MilneSummaryThe Skarv field in the Norwegian Sea lies in the structurally complex area of the Nordland Ridge. Velocity complexity in the near surface has proved challenging for model building in previous studies and this complexity is targeted in the current study via full-waveform inversion (FWI). FWI is a valuable depth model building tool that can provide accurate, high-resolution velocity models of shallow to intermediate depths. In this study, FWI was employed early in the model building workflow, providing rapid updates of a simple starting model. This case study demonstrates the efficiencies that can be gained by utilizing FWI at these early stages, by leveraging the strengths of FWI in updating shallow structure which can prove challenging for reflection tomography. In addition, further efficiency was gained by early FWI work in parallel to the initial model building stage of the workflow which can quickly highlight areas of the initial model that require further investigation. The results of this study provide an accurate, robust, and high-resolution model of the shallow to intermediate model depths that give a solid foundation for further model building via reflection tomography.
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Full Waveform Inversion over the Valemon and Kvitebjørn Area in the Norwegian North Sea
Authors O.K. Øye, J. Synnevåg, T.E. Rabben, F.A. Maaø and J.P. FjellangerSummaryWe present a case study on Full Waveform Inversion (FWI) velocity model building from the Valemon and Kvitebj0rn fields in the Norwegian part of the North Sea. The area has a strong shallow seismic anomaly, causing pull-down at deeper reflectors. Previous attempts at including the anomaly in velocity models have created a smooth velocity reduction in the area around the anomaly, not conforming to geology. We show that FWI produces a very localized low velocity anomaly that corrects for the observed imaging problems, conforms to geology and match predictions from rock physics for sediment velocities in the anomaly. We also show how we optimize well ties with FWI and anisotropy, and finally we discuss how we can use reflections to both increase the detail in the velocity model and estimate background velocity trends.
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Extending Model Depth and Resolution Using Diving Waves and Reflections with Full-Waveform Inversion for a Dataset
Authors Y. Cobo and C. Calderón-MacíasSummaryExploration and prospect identification in complex geology requires seismic data with long offsets and wide signal bandwidth in combination with high-resolution model building and imaging methods. Full waveform inversion (FWI) is being used in model building flows for improving the resolution of the near surface velocity with a potential impact on the imaging of deeper structures. This type of inversion makes use mostly of first arrivals from the longest offsets. Signal from smaller offsets and later times extend the depth range and resolution of FWI. In this work we invert first for first arrivals and then for reflections using a standard FWI method in combination with reconstructed wavefield method. We apply this flow to a deep water GOM dataset in an area characterized by relative shallow folding, sand channels and faulting. The resulting velocity model has a resolution good enough to identify these structural features, and furthermore the inverted velocity produces a good match with available well information.
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Joint Surface and Borehole Seismic Tomography with Simultaneous Model Parameter Updates at Thunder Horse South
Authors O. Zdraveva, G.A. Solano, S.O. Zaman, E. Saragoussi, A.R. Castelan, S. Chakraborty, Q. Li, C. Joy, K. Hartman and A. ReitzSummaryWe present the most-recent effort in fine-tuning the earth model at the Thunder Horse production field. This update was initiated because of and in parallel with the new ocean-bottom node survey acquisition in 2015. To minimize uncertainty in the resulting earth model, the chosen approach relies on using all available information. For this imaging exercise, this entails incorporating six surface seismic surveys with differing geometry, five 3D vertical seismic profile (VSP) surveys, three walk-away VSPs, five zero-offset VSPs, multiple sonic logs and well-depth markers in the earth model building process to derive a single anisotropic model that adequately minimizes misfit for all inputs. To achieve this, we use joint tomography of surface and borehole seismic data with simultaneous updates of model parameters, including the VSP transit times and well-depth markers as a data constraint. This strategy resulted in improved surface and borehole seismic images that better resolve structural complexity and stratigraphic details, yielding a more accurate interpretation and more reliable future well placement.
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Acoustic Vs. Elastic 3-D Full-Waveform Inversion at the East Pacific Rise 9°50’n’
Authors M. Marjanovic, R.E. Plessix, A. Stopin and S. SinghSummaryOver the last decade the acoustic 3-D full-waveform inversion technique has become almost a common tool for imaging geologically complex structures in marine, as well as in land settings. However, it has been speculated that excluding the elastic effect from the waveform modeling in some cases could have a big impact and potentially result in an erroneous image of the subsurface. To examine the contribution of the elastic effect, we conduct acoustic and elastic 3-D FWI on the 3-D seismic data, collected at the East Pacific Rise (EPR) 9°50’N, deep-water environment (minimum depth ~2500 m) with high compression velocity contrast at the seafloor. We first establish a strategy for inversion within the EPR setting, which suggest simultaneous multi-parameter inversion for the frequencies <7 Hz. Comparison of the results shows that the total misfit for the elastic case is reduced for ~10% more when compared to the acoustic, suggesting that the elastic effect is not negligible. Furthermore, the images of the upper-crust obtained using the two approaches differ significantly, not only in velocity amplitude, but also structurally, leading to different implications for crustal accretion processes, with the elastic approach leading to geologically more plausible solution.
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Elastic Model Low-To-Intermediate Wavenumber Inversion Using Reflection Traveltime and Waveform of Multicomponent Data
More LessSummaryIf the starting velocity model does not appropriately contain low-to-intermediate components to avoid cycle skipping, full-waveform inversion (FWI) is likely to converge to a local minimum. Generally, these components could be retrieved from diving or/and refracted waves. However, long offsets are required to record these transmitted waves. For better depth penetration, reflected waves should be used to recover the long-wavelength velocity structures. Retrieving background P- and S-wave velocities with multicomponent data is more difficult. In this abstract, we propose a three-stage inversion of the background P- and S-wave velocities using reflectioa traveltime and waveform of the multicomponent seismograms, respectively.
For traveltime inversion, we use a scalar acoustic propagator to extrapolate the normal and adjoint wavefields in the first two stages. This can avoid artefacts in the calculated gradients due to the non-phyiscal mode conversion and save lots of computational resources. To honor the waveforms of the multi-component data, we use an elastic propagator to extrapolate the normal and adjoint vector wavefields. The gradients are preconditioned through P/S mode decomposition to mitigate the artefacts and to improve the two-parameter inversion in the last stage. Numerical example will demonstrate the validaty of this hierarchical inversion approach.
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A Modified Approach for Tomographic Full Waveform Inversion Using Variable Projection
Authors G. Barnier, E. Biondi and B. BiondiSummaryWe tackle the problem of non-global convergence in seismic velocity model building. We develop a modified approach for tomographic full waveform inversion (TFWI) that allows us to avoid the initially proposed nested-loop scheme and reduce the number of inversion parameters. We use the variable projection method to ensure accurate matching between predicted and observed data. By doing so, we control the cycle-skipping behavior of the data fitting term by letting the regularization term (on which we have better control) guide our objective function. We compare our method to conventional full waveform inversion (FWI) on two examples. We show convergence on a reflection problem in which FWI also converges to the true solution, and global convergence on a transmission test for which FWI cycle-skips. Our proposed algorithm inverts all model scales simultaneously and does not require any frequency-continuation approach.
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Separation of Migration and Tomography Modes of Full-Waveform Inversion in the Plane-Wave Domain
Authors G. Yao, N. da Silva, M. Warner and T. KalinichevaSummaryFull-waveform inversion (FWI) includes both migration and tomography modes. The migration mode acts like a non-linear least-squares migration, mapping model interfaces with reflections, while the tomography mode builds the background velocity model. The migration mode is the main response of inverting reflections while the tomography mode exists in response to inverting both the reflections and refractions. To emphasize one of the two modes in FWI, especially for inverting reflections, the separation of the two modes in the gradient of FWI is required. Here, we present a new method to achieve this separation with an angle-dependent filtering technique in the plane-wave domain. We first transform the source and residual wavefields into the plane-wave domain with the Fourier transform and then decompose them into the migration and tomography components using the scattering angles between the transformed source and residual plane waves. Scattering angles close to 180° contribute to the tomography component, while the others correspond to the migration component. We found that this approach is very effective and robust even when the medium is relatively complicated with strong lateral heterogeneities, steeply dipping reflectors, and strong seismic anisotropy. This is well demonstrated by theoretical analysis, and numerical tests with synthetic and field datasets.
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Seismic Inversion for a Smooth Velocity Model Using a Wave to Diffusion Transform
By R. MittetSummaryNon-linear seismic inversion faces difficulties with multiple local minima due to cycle skipping. It is possible to demonstrate that cycle skipping is unlikely to appear in the diffusive domain with a very common definition of the error.
Seismic data can be transformed to the diffusive domain and inverted to obtain intermediate estimates of the velocity model. This intermediate velocity model may serve as initial models for inversion processing in the wave domain or may serve as a velocity model for reverse time migration.
One of the properties of the wave to diffusion transform is that it effectively extracts refractions from a common shot gather or common receiver gather containing reflections, diffractions and refractions.
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Velocity Model Building without Tomography: Results from a 2D FWI Benchmark Involving 3 Contractors
Authors C. Hidalgo and B. BruunSummaryFull Waveform Inversion (FWI) is now a well-established technology widely used to derive migration velocities in complex settings where conventional travel time tomography is inadequate. Most seismic contractors have developed their own flavours of FWI workflows which differ in several ways; they utilize different codes with their own strengths and weaknesses, utilize different parts of the recorded wavefield and follow different update-strategies. When running a production FWI project several complex choices must be made and their combined impact is difficult to predict. This paper investigates some of the impacts of different FWI approaches by comparing the results from a benchmark study where 3 seismic processing contractors used the same 2D data set to run FWI free of tomography. It was found that each of them have a distinctive piece of technology that helps to overcome specific challenges in a FWI production flow. Here we show and discuss their results, as well as their route map to get there.
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Robust FWI for Reflection Data
Authors C. Fortini, J. Panizzardi and N. BienatiSummaryThe conventional L2 misfit function for Full Waveform Inversion (FWI) is known to suffer from the cycle-skipping problem that makes it strongly dependent on the initial solution. Moreover, its application is often limited to the use of the refracted energy only. In the last years several alternative misfit functions have been proposed with the aim of making FWI more robust. In this work, we propose to exploit the Normalized Integration Method (NIM), which consists in applying an integral transform to the data before computing the residuals, in order to compare monotonic signals instead of oscillating signals. The NIM cost function has been shown to provide full convexity with respect to signal shifts. However, its effectiveness has been only shown on transmission experiments. Here we show a method that allows exploiting such cost function also for reflected events, without the need of a starting model containing sharp contrasts. We show the effectiveness of the proposed methodology on synthetic data and on an offshore field dataset.
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High-Resolution Reflection FWI
Authors M. Warner, T. Nangoo, A. Umpleby, N. Shah and G. YaoSummaryWe demonstrate reflection FWI on a less-than-ideal 3D narrow-azimuth towed-streamer dataset that contains little refracted energy and that is deficit in low frequencies. We begin from a very simple starting model built rapidly from stacking velocities. We fist use an FWI scheme that alternates between a migration-like and a tomography-like stage, showing that this can both recover the background velocity model and generate high vertical resolution. We follow this by using global inversion to build the long-wavelength anisotropy model. Finally, we use more-conventional reflection-based FWI to introduce the full range of wavelengths into the recovered velocity model, and show that this both migrates the reflection data and is structurally conformable with the reflections.
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How Shall We Use the Reflections Beyond the Diving Wave in Full-Waveform Inversion?
Authors D. Vigh, K. Jiao, X. Cheng, D. Sun and L.X. ZhangSummaryFull-waveform inversion (FWI) is a high-resolution model building technique that uses the entire seismic record content to build the earth model. Conventional FWI usually utilizes diving and refracted waves to update the low-wavenumber in other words the background components of the model; however, the update is often depth-limited due to the limited offset range acquired. To extend conventional FWI beyond the limits of the transmitted energy, we must use reflection data as well. Field data examples demonstrate that, even in a complex subsalt Gulf of Mexico setting, the background velocity model can be updated from shallow to deep water using conventional FWI followed by reflection-based FWI.
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4D Broadband Towed-Streamer Assessment, West Africa Deep Water Case Study
Authors D. Lecerf, D. Raistrick, B. Caselitz, M. Wingham, J. Bradley and B. MoseleySummaryA 4D broadband assessment has been performed in a deep offshore West Africa environment using repeated dual-sensor streamer acquisitions. Three sail lines have been re-acquired only a few weeks after the original acquisition with standard 4D acquisition consideration. The study has compared the data repeatability metric NRMS along the processing sequences using, on one side, a reconstructed band limited 4D data corresponding to a single hydrophone and on the other side an extended bandwidth 4D data using the up-going wavefield only. Because the datasets have been recorded using the same acquisition, the main differences come from the bandwidth discrepancy and the variable sea-state. The 4D “Up-going on Up-going” preserves, for all processing steps, around 1% NRMS benefit against the 4D hydrophone-only. Because the NRMS comparison is biased by the dominant frequency discrepancy, the 1% gain does not fully reflect the detectability advantage of the 4D broadband. The NRMS analysis on the low frequency part has demonstrated the clear improvement on 4D broadband and a qualitative evaluation on the 4D differences has highlighted the negative effect of the sea-state on the hydrophone-only 4D results.
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Improved Reservoir Imaging Using Specular Dip-angle Migration
Authors A. JafarGandomi, J. Richardson, H. Hoeber, M. Galyga, P. Smith and A. IrvingSummaryIn this paper we show that specular imaging in the dip-angle domain significantly reduces 4D imaging noise. Specular imaging limits the migration aperture to rays obeying Snell’s law. Since it is based on improved image formation, specular imaging is superior to signal processing methods applied to a noisy image. In the context of 4D, the improved imaging can prove useful in simplifying, and therefore speeding up, the 4D processing. By better imaging the data, based on a physical principle, we can avoid extraneous signal processing, such as dip-filtering, and thereby avoid signal damage and additional processing time. Separating the seismic energy at the imaging stage into specular and non-specular components in the dip-angle domain additionally provides the opportunity to identify diffraction energy. The particular behaviour of diffracted energy in the dip-angle domain, i.e. creating flat events across the whole dip-angle range, makes them separable from both specular energy and migration artefacts. We present the performance of the proposed specular imaging approach on a 4D dataset from the Caspian Sea. Significant uplift is obtained not only on the 4D image, but also on the 3D pre-stack gathers leading to improved AVO.
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Surface-Wave Analysis and Inversion of Ambient Noise Recordings
Authors C. Leone, D. Boiero and S. MitchellSummaryA new approach is proposed to estimate the S-wave velocity of the near surface (and its changes over time) from passive data. The method consists of 5D interpolation followed by surface-wave analysis and inversion. This allowed us to overcome the issue of velocity estimation in between detector cables avoiding the phase velocity tomography step. The method was tested on virtual shots generated by interferometry techniques applied on ambient noise recorded at the Valhall Life of Field Seismic permanent monitoring network. Maps of the phase velocities allowed identifying several geological features in the near surface. A time-lapse analysis was also performed and the changes in S-wave velocity shows correlation to the 4D time shifts from converted-wave data.
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New Time Lapse Seismic Attribute Linking 4D and Geomechanics
Authors M.A. Calvert, A.J. Cherrett, U. Micksch, F.G. Bourgeois and A.S. CalvertSummaryA new time lapse attribute called the “Lateral Gradient of the Overburden Timeshift” (LGOT) has been created to help identify areas potentially impacted by depletion-induced overburden deformation in the Danish North Sea chalk fields. We consider this attribute to be a proxy for shear strain in the overburden. Maximum LGOT values highlight a donut-shaped zone representing the overburden region with higher shear strains and higher likelihood of wellbore deformation. Currently, in the Tyra field there are six wells with known overburden well deformation, all occurring at the inner edge of the LGOT donut. In addition, the delivery of new wells traversing through the donut has also proved challenging. To highlight wells potentially at risk due to overburden deformation in other fields, time strain inversions were run on 4D data from different vintages over several of the Danish chalk fields. Examples of the lateral gradient attribute from Tyra and other Danish chalk fields are presented here.
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Evaluating 4D Overburden Velocity Perturbation for the Shearwater Field via Pre-Stack Time-Shift Inversion.
Authors I. Dvorak, C. MacBeth and H. AminiSummaryWe present the results of a pre-stack time-shift analysis and inversion study for evaluating the influence of dynamic overburden-related anomalies on 4D reservoir monitoring. This study consists of the application of line fitting techniques to analyse pre-stack time shift fields to generate time shift intercept and gradient maps. A new inversion technique is then designed to invert pre-stack time-shift variation with offset (TVO) directly to 4D velocity perturbations. This method is based on the application of a V-scanning operator in the distance-offset domain and the direct derivation of a velocity perturbation section utilising a geometrical relationship between an overburden anomaly and its pre-stack time shift signature. The results of the study are presented for synthetic analogues and real data examples from the Shearwater field. We show the use of pre-stack time-shift analysis offers improved description of time-shift maps in relation to post-stack derived equivalents and that time-shift inversion via V-scanning inversion determines the velocity perturbation fields. These approaches offer the ability to delineate the influence of dynamic overburden effects on 4D reservoir models and incorporate the associated 4D velocity perturbation into imaging scheme during the processing stage.
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High Resolution Time Lapse Time Strains
By C. SchiøttSummaryHigh resolution, low noise time strain estimates are derived through a deliberate choice of penalty function in the time shift inversion. In seismic time lapse analysis, time shifts and their derivative time strain has the potential to directly pin-point transit time changes inside the producing reservoir. But time strains are noise prone, and it has proved challenging to produce high resolution, low noise time strain estimates. The high resolution character of the method presented here is demonstrated at a single trace extracted from a base line survey and compared against two monitor survey traces. Widely used smoothness penalty functions typically succeed in estimating absolute time shifts, but fail to deliver high resolution time strains; this paper shows that this can be achieved by adapting an appropriate penalty function. The resolution of the method presented here is comparable to the resolution achieved through 4D AVO inversion, as demonstrated on a 2D line. This gives promise that a joint inversion of time shifts and amplitude changes leading to improved constraints in time lapse inversion can be established.
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Integrated 4D Processing and Inversion Workflow Using Multi-well Wavelet Extraction
Authors U. Micksch, H. Klemm, A.J. Cherrett and P.J. ChristianSummaryWe present an integrated processing and inversion workflow, applied to a major 4D project in the Danish North Sea, covering 9 fields with up to 4 seismic vintages. Intermediate processing QC volumes are used for multi-well wavelet extraction and 4D inversion. The results of these intermediate inversions enable us to feed observations back into the processing loop, and can also be used at early stages by the reservoir management teams to influence ongoing workover and well planning activities.
The key to this workflow is being able to perform the wavelet extractions and 4D inversions in a timely manner, which requires dedicated proprietary software. For example, one of the larger 4D inversions, covering 4 fields and using 19 wells for wavelet extraction, took 5 days to complete.
We will present results from wavelet extraction studies and show examples where up to 3 intermediate QC outputs were inverted before the final processing. We emphasise the value of an integrated project team working closely with assets and external contractors.
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A Successful 4D Seismic Pilot in Middle-East Carbonate Reservoir Context
Authors F. Cailly, C. Hubans, A. Lafram, T. Al-Romani and A.S. AL-KaabiSummaryThis paper describes a successful 4D interpretation in a very challenging Middle East carbonate context. 4D monitoring is clearly justified by the fact that a lot of expensive wells are drilled per year and their positioning is not straight forward. Particularly the crucial fluid movement prediction (gas and water) is difficult and so is considered as the main monitoring objective.
The operator decides to acquire in 2013 a 4D pilot over approximately 25km2 repeating at best the 1990’s OBC vintage acquisition design. After a cautious 4D processing step accounting for strong coherent noise and high contamination of short period multiples, relevant 4D attributes are estimated from warping and inversion, and 4D anomalies are validated and interpreted with different degrees of confidence. In particular the interpretation of the water front movement during the 4D time lapse is calibrated to well data and so can be judged as robust. This is described in the paper and proves the concept that a reliable 4D signal can be extracted over such carbonate reservoir.
A second phase of the project considering a 4D processing of different acquisition design is also alluded.
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Global Exploration Discoveries and Industry Outlook
By A. LathamSummaryThis paper reviews global discovery trends in conventional oil and gas exploration and considers the outlook for the industry. Key changes to the scale and nature of industry exploration programmes are summarised. These include the characteristics of companies involved, likely scale of investments expected, play types in favour and implications for the sector’s long-overdue return to profitability.
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Guyana’s First Deepwater Discovery
By M. CousinsSummaryThe Liza discovery in the Guyana Basin marked the opening of a new and significant deepwater oil provenance.
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The SNE Discovery Offshore Senegal - Moving a Frontier Basin to Emergent
By E. HathonSummary“In 2014 Cairn Energy operated the first deep water wells offshore Senegal in the MSGBC basin. Both the FAN-1 and SNE-1 wells were basin-opening discoveries, encountering light oil and gas in both basinal and platform settings. SNE-1 was the largest global oil discovery in 2014, and is currently estimated to hold 2C reserves of over 550 million barrels of recoverable oil and 1.3 TCF gas. These frontier discoveries demonstrate how careful analysis of available data pre-drill can illuminate opportunity, mitigate risk and lead to success in moving a basin from frontier to emergent.
The SNE field has been defined by eight exploration and appraisal wells. Hydrocarbons are contained within a combination structural and stratigraphic trap. Two Albian-age reservoir zones have been designated: the shallower S400 Series and the deeper S500 Series. Both zones are turbidites deposited in a slope to toe-of-slope setting. The maximum extent of the field is 350 square kilometers, with a 100 m column of 31o API oil. The SNE Field has now been fully appraised and is moving towards final investment decision (FID). A phased development is planned, with a target plateau of 75,000–125,000 bopd with subsea infrastructure tied back to an FPSO.”
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Delivering Success in the Mauritania Senegal Basin
By T. HendersonSummaryThe recent success in the Mauritania Senegal Basin by Kosmos and BP has demonstrated a world-class hydrocarbon province with a significant discovered gas resource and substantial follow-on potential. It is the largest new petroleum system opened in the last 15 years along the Atlantic Margin outside of Brazil, with 25 TCF of gas discovered and 50 TCF de-risked (pmean). With an acreage position of roughly 50,000 square kilometres and only 7 wells drilled to date, much of the acreage position remains underexplored. Two drilling campaigns have been conducted. The first campaign was completed in 2016 resulting in the Tortue, Teranga and Marsouin discoveries. The second campaign, which began in early 2017, yielded further success with the world’s largest discovery of 2017, the Yakaar discovery. The second drilling campaign will be completed in early 2018 with the Requin Tigre well in northern Senegal.
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Giant Regional Play Fairways from Modern Seismic Data
Authors K. Rodriguez and N. HodgsonSummaryA new deep water exploration trend has emerged as a number of key wells drilled recently in North West Africa have started to open the door to a low risk play that has achieved stunning success to date and looks set to draw the industry into exploring a new play type down on the basin floor. Previous to this Petrobras’s discoveries in Brazil’s Sergipe basin have shown that not only can the deep water be incredibly rewarding, with fast development opportunities, but it is also rich in oil. Recently, Exxon’s discoveries in Guyana have highlighted the great potential in stratigraphic traps, while the Ranger discovery has highlighted the positive role of magmatism in the development of petroleum systems.
Regional petroleum systems evaluation of an extensive 2D seismic database with global coverage has been carried out in order to identify future exploration frontiers. Modern seismic, smarter geoscience models and enhanced drilling techniques, will open frontier areas to explore new play fairways never accessible before: the base-of-slope and basin floor fans of the South Atlantic, where prospect resource potential is created and determined by plate scale geometries, Guyana stratigraphic analogues and the multiple play types offshore Somalia to name but a few.
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Reactivation of Fault Zones due to Thermo-Poroelastic Stress Changes
Authors T. Meier and T. BackersSummaryThis work investigates numerically by means of the Finite Element Method (FEM) the influence of a) homogeneous poroelastic medium, b) a heterogeneous poroelastic medium containing small-sized defects, and c) a hydraulic fracture within a poroelastic medium on the stability of a fault system during simultaneous depletion and injection.
Since the mechanism of fault slip are a matter of research, this modelling campaign aims at investigating the size of a possible reactivated area on the fault system. The size of the reactivation is related to Amonton’s law that governs fault reactivation as soon as the ratio of the shear (⊤) to normal (σn) stress acting on a fault exceeds the friction coefficient (µ) of the fault (see for example Moeck and Backers 2011 ). The size of the reactivated area can then be related to moment magnitudes through empirical correlation (e.g., Wells and Coppersmith 1994 ). This approach assures a conservative measure of possible earthquake magnitudes by employing numerical methods linked with statistics.
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Reservoir Strain Changes from 4D Time-Strains
By H. KlemmSummary“Time-lapse seismic surveys have proven extremely valuable to guide reservoir management decisions. Changes in seismic reflection amplitudes can provide information on fluid contact movements or pressure changes, whereas variations in travel-times of seismic signals can provide information on geomechanical effects such as reservoir compaction, overburden strain and subsidence. Here, we present a new methodology to obtain reservoir strain changes via geomechanical inversion of seismic time-lapse time-strains. If a linear pressure-strain relationship is assumed, the volumetric strains can be translated into reservoir pressure changes. We demonstrate the method on an example from the Danish North Sea and discuss some observations.”
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Stress Prediction and Evaluation of Heterogeneous Reservoir
More LessSummaryWe proposed a new approach for stress prediction and evaluation of heterogeneous reservoir. New azimuthal P wave equation in form of Young’s modulus, Poisson’s ratio and crack density and Differential Horizontal Stress Ratio (DHSR) formula expressed by Poisson’s ratio and crack density which reflects the reservoir stress property were derived. Then, pre-stack angle gathers of different azimuth angle were applied to execute AVAZ inversion for elastic parameters and crack density. We utilized the inverted Poisson’s ratio and crack density to estimate DHSR. The real data application showed that stress predicted result agreed with geology deposition property and the new drilled well interpretation. The novel approach can offer reliable and convenient stress evaluation evidence for heterogeneous reservoir.
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Dynamics of Stick-Slip Sliding Induced by Fluid Injection in Large Sandstone Block
Authors V. Oye, S. Stanchits, N. Seprodi, P. Cerasi, A. Stroisz and R. BauerSummarySafe geologic sequestration of CO2 is important to decrease the concentration of greenhouse gases in the atmosphere. However, the injection could increase the underground pore pressure and potentially induce sliding of critically stressed faults. We report results from a laboratory test where fluid injections close to an artificial interface of ∼1m length were observed to induce sliding. During the injection, the pore pressure at the injection point reached up to 6.2 MPa and after shut-in, it dropped down to almost zero. However, about 10 minutes later, a sudden sliding of the interface (stick-slip motion) was recorded. Two types of acoustic emission (AE) signals were detected: short bursts and long-lasting oscillations (tremors). The analysis of the spatial distribution of the AE energy was applied to monitor the dynamics of stick-slip, indicating a nucleation phase of the sliding, then the rupture propagated through the whole interface with an average rupture velocity of a few m/s. The speed and the energy radiated during this event were approximately 6 orders of magnitude larger than observed during quasi-static sliding preceding the stick-slip. This observed stick-slip motion can be considered a laboratory analogue to earthquakes, and its occurrence can be related to the injection of fluids.
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Robust Non-Linear Inversion for Reservoir Deformation from Surface Displacement Data
Authors K. Bisdom, M. Cid Alfaro, K. Hindriks and A. KudarovaSummaryCompacting and expanding reservoirs cause respectively subsidence and heave at the surface which at present can be measured using different geodetic methods. Knowing the source of compaction and expansion is of large value to our subsurface operations as it aids in assessment of reservoir conformance and possible containment issues. However, measured subsidence cannot be related one-on-one to actual reservoir compaction: An inversion process is required. Due to the spatial convolution and filtering nature of the subsurface on the actual reservoir compaction or expansion problem, inversion is a complicated process.
This paper describes an iterative inversion workflow, taking full advantage of the speed of semi-analytical forward models while adhering to the actual complexity of the subsurface captured in a detailed finite element model. The iterative scheme allows for handling of the ill-posed nature of the problem. The workflow presented here shows the utility of integrating geodetic data sources to estimate reservoir volume changes. This technique has proven to add value when it comes to monitoring areal and temporal changes in the reservoir. It is relatively easy to implement as it does not require large amounts of input data and can be applied to any field where surface deformation data is available.
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How the Presence of Oil and Water Affects Chalk Mechanics at Isotropic Stresses
Authors J.S. Sachdeva, A. Nermoen, R.I. Korsnes and M.V. MadlandSummaryThis study focuses on chalk mechanics for oil and water saturated cores. Tests were carried out on four Kansas chalk cores initially saturated by 1.1 M NaCl-brine. Two cores were wettability altered by a mixture of Heidrun oil - heptane (60/40 volume ratio) and then aged for three weeks at 90°C. The wettability index was estimated from test series of equally treated cores. All four cores were hydrostatically loaded to 1.5 times above yield at 130°C and left to creep. During the first 15 days of creep the fluids in the pores were stagnant. It was followed by flooding of 0.219 M MgCl2-brine at two flow rates (0.01 ml/min and 0.04 ml/min) for different durations. It is observed that during loading with stagnant fluids inside the pores, the stiffness and strength are affected by wettability. The axial creep strain curves for different wettability cores overlap during the stagnant fluid creep period and during flooding of MgCl2-brine at both flow rates. We see that the creep curves, given that the creep stress is 1.5 times the yield stress, are insensitive to the initial wettability and oil/water saturation. Hence yield stress is the determining factor for these experiments.
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Handling Wellbore Instability in Overburden Tertiary Shales
Authors F. Ditlevsen, F. Bourgeois and M. CalvertSummaryFrom November 2015 to April 2017 nine infill wells were drilled in the Danish Tyra South East field. The geomechanical challenges experienced in the Tertiary shales while delivering these wells were much larger than anticipated. Limited reservoir compaction had induced stress changes in the overburden, not fully captured in the pre-drill models, which caused the delivery to be more challenging and the requirement of more comprehensive models apparent. A more detailed 3D geomechanical model was developed and associated to new 4D seismic observations after drilling more than half of the wells in the campaign. Also, real-time geomechanical evaluations and near wellbore 3D models were coupled to the 3D geomechanical model to further investigate the stress changes in the overburden. Two main failure mechanisms were identified to have contributed to the instability experienced; shear failure and bedding plane instability. The investigations and new modelling results showed that the minor stress changes in the overburden caused the MW strategy used to be connected to both shear failure and bedding plane instability. Minor changes to the MW strategy and better control of the mud pressure invasion into the shale would improve stability and the delivery of the sections through the Tertiary shales.
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Localization and Characterization of Hydraulically Conductive Fractured Zones at Seismic Scale with the Help of Geomecha
Authors N. Dubinya, I. Bayuk, S. Tikhotskiy and O. RusinaSummaryIn the paper an approach to develop a fracture network model is proposed. The key feature of this approach is the ability to distinguish hydraulically conductive and non-hydraulically conductive fractures. This ability is granted by modeling the fracture network with the use of rock physics methods using data based on both seismic survey data and geomechanical modeling results. Following the concept of hydraulically conductive and critically stressed fractures we carried out an analysis on fractures present in the reservoir to determine which of them are hydraulically conductive in the actual stress state. Then the fracture network was developed for these fractures applying the rock physics modeling methods. The integration of geomechanical properties of fractures into rock physics modeling provided an opportunity to predict how the conductive fracture network would alter with dynamically changing stress field throughout the reservoir development. A fracture network model was developed for an oil field in Russia and the results of well productivity analysis provided a solid basis for establishing the relationship between fracture network model parameters and well productivity. The obtained results strongly enhance the reliability of fracture network model based on geomechanical modeling results.
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Improving Quantitative Interpretation beneath Sand Injectites: A North Sea Case Study
Authors C. Leone, A. Osen, M. Cavalca, R. Fletcher and M. FerridaySummaryOverburden geological complexities such as cemented sand injectites can degrade the image quality at reservoir level, introducing errors and uncertainties for quantitative interpretation.
In this study, we employ depth domain inversion to improve amplitude fidelity and resolution beneath the injectites, and to obtain a more reliable estimation of acoustic rock properties.
The dip-dependent illumination effects caused by overburden heterogeneities are modelled by 3D deterministic operators, that are calibrated to well information to perform seismic amplitude inversion directly in the depth domain. This migration-inversion approach is an image-domain Least-Squares Kirchhoff Depth Migration.
The result is a higher-resolution image, showing better-balanced amplitudes and improved event continuity. The depth domain inversion to acoustic impedance demonstrated an increased reliability of seismic amplitudes, leading to an improved quantitative interpretation.
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Comprehensive Prestack Seismic-to-Well Tie Using Data Combination for Better Seismic Reservoir Characterization
More LessSummaryAll quantitative Seismic Reservoir Characterization workflows rely on careful seismic-to-well ties. It turns out that many Quantitative Interpretation (QI) workflows consist in linear combinations of prestack seismic data such as rotations / projections of Near / Far angle stacks or AVO intercept / gradient in their respective cross-plot domains. More sophisticated techniques such as prestack seismic inversion designed to estimate acoustic impedance and Poisson’s ratio to be further used for facies classification have a quasi-linear behavior relative to their input. Some of these combinations may reveal quite unstable so that good seismic-to-well ties of their input may not be sufficient to ensure quality results.
It is proposed here to assess the quality of seismic input not only within its measurement range but also across a comprehensive range of linear combinations of the same input.
Examples will illustrate that:
-It may be hazardous to plug real data into a linear combination derived from synthetic data analysis only.
-When scanning real data for an appropriate combination that matches a given elastic well log or even when using constrained seismic inversion results, the outcome might not genuinely represent the said elastic parameter.
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Quantitative Seismic Interpretation of a Class-1 AVO Turbiditic System Located Offshore Cote d’Ivoire, West Africa
Authors P. Alvarez, W. Marin, J. Berrizbeitia, P. Newton, F. Bolivar, M. Barrett, H. Wood and L. MacGregorSummaryThis paper presents a case study where a Class-1 AVO turbiditic system located offshore Cote d’Ivoire, West Africa, was characterized in terms of rock properties (lithology, porosity and fluid content) and stratigraphic elements using well log and prestack seismic data. The methodology applied involves, 1) the conditioning and modeling of well log data to several plausible geological scenarios at the prospect location, 2) the conditioning and inversion of prestack seismic data for P- and S-wave impedance estimation, and 3) the quantitative estimation of rock property volumes and their geological interpretation. The approaches used for the quantitative interpretation of these rock properties were the multi-attribute rotation scheme (MARS) for lithology and porosity characterization, and a Bayesian litho-fluid facies classification (SRP – statistical rock physics) for a probabilistic evaluation of fluid content. The result shows how the application and integration of these different AVO- and rock physics-based reservoir characterization workflows help to understand key geological stratigraphic elements of the architecture of the turbidite system and its static petrophysical characteristics (e.g. lithology, porosity, and net sand thickness).
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Reservoir Elastic Parameters Estimation from Surface Seismic Data Using JMI-res: A Full-Wavefield Approach
Authors A. Garg, S. Sharma and D.J. VerschuurSummaryAny target-oriented localised inversion scheme for reservoir elastic parameters is as good as the input dataset. Thus, the accuracy of the input dataset i.e. local reflection response or impulse response (virtual source-receiver response) is of utmost importance, especially when the target area is below a complex overburden. In these subsurface settings, the overburden internal multiples and associated transmission imprint obscure the local response, which in turn affects the estimated elastic parameters resolution. Here, we demonstrate a novel process called JMI-res, based on Joint Migration Inversion (JMI), to estimate the reservoir elastic parameters from the surface seismic elastic data for a complex subsurface scenario. In JMI-res, we first obtain the accurate local impulse responses at the target depth level, while correctly accounting for overburden internal multiples and then we apply a localized inversion scheme on the estimated impulse responses to get the reservoir elastic parameters. Moreover, the propagation velocity estimation is an integral part of JMI-res. In this paper, we show that JMI-res provides much more reliable local target impulse responses, thus yielding high-resolution elastic parameters, compared to standard redatuming based on time reversal of recorded data, courtesy of proper handling of internal multiples in the redatuming step.
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Bringing Broadband Data, Wavelet Extraction Methods and Inversion Together: With Surprising Low-Frequency Phase Results
Authors N. Ahmad and H. PelletierSummaryMany papers over the past few years have delved into the challenges, and subsequent rewards, of better determining the frequency content of our broadband seismic data. In this paper we will use a broadband dataset -acquired and processed - from the Northern Danish Central Graben to highlight a practical workflow that optimally extracts usable broadband wavelets for inversion purposes. Furthermore, low-frequency phase uncertainty was addressed during inversion testing yielding a surprisingly significant effect on the inversion result.
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A Depth Variant Seismic Wavelets Extraction Method for Inversion of Post-Stack Depth Domain Seismic Data
More LessSummaryDepth domain seismic data could suffer from stretch effects compared with time domain seismic data, which could cause strong spectral variation and non-stationarity. This could result in the invalidation of stationary assumption for conventional convolution model, which also causes difficulty for directly inverting the depth domain datasets for reservoir characterization. In this paper, we propose a depth variant wavelets extraction method by using the S-transformation with incorporation of a non-stationary convolution model to accommodate the spectral variation on the depth domain seismic data. The technique has been successfully applied on a field dataset for inversion for subsurface reflectivity and acoustic impedance. The inversion results show good fit with the well-log data, which have demonstrated its effectiveness on post-stack seismic data.
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Integration of Results from Two Seismic Inversion Methods – A Case Study
Authors R. Srivastava and S. MaultzschSummary“Seismic inversion results are non-unique and can suffer from various pitfalls related to prior model assumptions or seismic data quality issues. Combining results from two entirely different inversion algorithms can substantially improve the understanding of these results. In this study, we use an inversion for probabilities of litho-facies, referred to as Pcube+, together with a classical elastic inversion. Pcube+ is a Bayesian method of updating prior probability models of lithology-fluid classes defined in the elastic parameter space. Outputs are posterior probability volumes for each pre-defined lithology-fluid class. A standard elastic inversion yields elastic parameter volumes, which can subsequently be transformed into probabilities of litho-facies using classification.
Comparing results from the two methods in areas away from well control has proven valuable in this case study. The main data source for pre-defining the lithology-fluid classes in PCube+ are well logs. However, wells tend to be biased and may not represent the full elastic parameter space that is required to explain the seismic data away from wells. In these situations, a cross-check with an elastic inversion gives crucial insight into which elastic parameter distributions are required to match the seismic AVO response, so that prior models for PCube+ can be fine-tuned.”
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