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80th EAGE Conference and Exhibition 2018
- Conference date: June 11-14, 2018
- Location: Copenhagen, Denmark
- Published: 11 June 2018
61 - 80 of 1073 results
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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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