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80th EAGE Conference and Exhibition 2018
- Conference date: June 11-14, 2018
- Location: Copenhagen, Denmark
- Published: 11 June 2018
41 - 60 of 1073 results
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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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