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70th EAGE Conference and Exhibition incorporating SPE EUROPEC 2008
- Conference date: 09 Jun 2008 - 12 Jun 2008
- Location: Rome, Italy
- ISBN: 978-90-73781-53-5
- Published: 09 June 2008
101 - 200 of 556 results
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Effects of the Relative Humidity on the Crack Growth in Rocks
Authors Y. Nara, Y. Ichihara, M. Sawada, S. Tomita and K. KanekoIt is important to know the effect of the humidity on the fracturing in rock. However, none has reported the effect of only the relative humidity on the crack propagation under a constant temperature. In this study, the fracture toughness and subcritical crack growth in sandstone and granite were investigated in air. Especially, the effect of the relative humidity on the fracture toughness and the relation between the crack velocity and the stress intensity factor were investigated under a constant temperature. The fracture toughness decreased and the crack velocity remarkably increased in both rocks when the relative humidity was higher. The effect of the relative humidity was stronger than expected before. It is concluded that the change of the relative humidity in air has strong effects on the crack growth in rocks.
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Integrated Approach to 3D Near-surface Characterization
Authors A. Laake, C. Strobbia and A. CuttsAn integrated approach to 3D geologic and elastic characterization of the near surface is presented. The integration of high spatial resolution, relatively low-confidence remote sensing, and geologic data with sparse spatial resolution, high-confidence geophysical data in a GIS database allows for cross-calibration of both types of datasets, thus providing calibrated near-surface models. The method comprises the generation of a 3D near-surface geologic model, from which the input for a 3D elastic model is obtained. The elastic model is then calibrated with seismic data to provide the final 3D near-surface model. The method has been demonstrated at two case histories covering geomorphological features typical for desert areas in North Africa and the Middle East.
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Multi-layer Near-surface Attenuation Estimation Using Wave Propagation Modelling Including Topography
Authors N. El Yadari, F. Ernst and W. MulderWe estimated the near-surface attenuation in a multi-layered geological setting by comparing energy decay along reflection and refraction traveltime curves in observed and modelled seismic data. A 2D visco-acoustic finite-difference code provided the synthetic data. We applied the method to synthetic and to field data .The incorporation of topography and source and receiver elevations, both in the modelling and in the computed traveltimes, led to sharper estimates of the attenuation factors, especially for deeper layers.
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Can High Resolution Seismic Data Correct for the Near Surface Static's Issue in Saudi Arabia?
Authors T. A. Alkhalifah and M. AlMalkiThe Arabian Peninsula is plagued with a complex near-surface that has a destructive influence on the final seismic image. This is especially evident on a line acquired from the Delim region of central Saudi Arabia. We use high-resolution shallow seismic data to resolve the complex near surface and correct the conventional data. High-resolution shallow data allow us, if the conditions for preserving high frequencies are present, to estimate the velocity at the near surface at a higher resolution. We use such information as inputs in the near surface correction step and thus correct the deep data for the near surface. The application of this approach on the Delim 2-D seismic line provided superior results than what was obtained from previous applications. Though, our focused conventional approach provided similar results putting more emphasis on focused analysis of the infected area.
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A New Approach for Multimodal Inversion of Rayleigh and Scholte Waves
Authors M. Maraschini, F. Ernst, D. Boiero, S. Foti and L. V. SoccoThe knowledge of the near surface velocity can be of paramount importance for assessing near surface effects on exploration seismic data both on land and offshore surveying. Ground roll (Rayleigh waves) and mud roll (Scholte waves) present in seismic records can be processed and inverted to supply a shear velocity model of the first hundreds meters. In this paper an automatic multimodal inversion for Rayleigh and Scholte waves using a new approach is presented. The proposed algorithm minimizes the L1 norm of the modulus of the Haskell-Thomson matrix determinant of the synthetic profile evaluated on real data. This algorithm is very fast, and it allows considering simultaneously all modes. Tests on both synthetic and real data, relative to Rayleigh and Scholte waves, have shown the importance of accounting for higher modes and the capability of the proposed method to handle their inversion. Improvement on the reliability of the final model has been obtained with a fast and efficient technique.
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From 3D Seismic and Resistivity Logs to 3D Porosity - The Case Study of a Near Heterogeneous Aquifer
Authors J. L. O. Mari and G. PorelA high resolution 3D seismic survey has been recorded on an experimental hydrogeological site that has been developed for several years near Poitiers (France). The paper has shown how 3D seismic data can be processed to get a 3D seismic pseudo velocity block in depth. The results obtained are validated at 5 wells in which acoustic data have been recorded. The 3D seismic pseudo velocity block shows the large heterogeneity of the aquifer reservoir in the horizontal and vertical planes. In a second step, the 3D seismic pseudo velocity block is converted in a 3D resistivity block by using a Faust law obtained by adjustment of interval seismic velocities on resistivity logs recorded in 11 wells. After calibration, the 3D resistivity block is converted in porosity. The porosity distribution shows preferential connections between wells. The results obtained are confirmed by well pumping tests and pressure interference which show the hydrodynamic connection between wells.
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Electrical Resistivity and Seismic Refraction Tomography to Detect Buried Cavities
Authors E. Cardarelli, M. Cercato, A. Cerreto and G. Di FilippoCavities located in the near surface represent a major hazard especially in sites located in highly urbanized town centers. For this reason, the location, the determination of size and shape of the cavity and the knowledge of the elastic properties of the overlying subsoil are necessary to evaluate the risk of subsidence and to plan the intervention for safety restoration. In this study, electrical resistivity tomography and seismic refraction tomography were performed with the twofold purpose of detecting the cavities and of optimizing an experimental procedure to better identify and locate them. These methods were tested on both synthetic and real data and the integrated geophysical approach has proven successful to detect single cavities together with their physical and geometrical characteristics.
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Tunnel Seismic Investigations - 3D Imaging
Authors J. Tzavaras, S. Buske, K. Groß and S. A. ShapiroThe project OnSITE (Online Seismic Imaging for Tunnel Excavation in hard rock) aims at developing an integrated approach for seismic imaging around the tunnel and prediction ahead of the tunnel face during construction work. We present the recent results of our subproject which is concerned with the determination of the spatial locations of faults, fractures and heterogeneities by advanced seismic imaging techniques. So far we have extended existing 2D imaging techniques to 3D. We have implemented 3D versions of Kirchhoff prestack depth migration (KPSDM), Fresnel-Volume-Migration (FVM) and Reflection-Image-Spectroscopy (RIS). The application of the mentioned imaging techniques to data from the Gotthard base tunnel show significant improvements compared to standard (KPSDM) processing. The 3D RIS approach suppresses scattering effects in the low frequency band and increases resolution in the high frequency band so that some of the geological structures are much better visible. The 3D FVM technique uses slowness and polarization based estimates of the emergence angles at the receivers and restricts the imaging to the region around the actual reflection or diffraction point. We observe less spatial ambiguity and a higher resolution of most structures in the stacked section as well as in single receiver gathers.
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Applying Geostatistical Filtering to Near-surface Geophysics - Two Field Examples for Refraction and Gravity Surveying
Authors N. Jeannee and J. L. MariGeostatistical filtering techniques are commonly applied to improve the quality of seismic data such as velocity cubes or interpreted horizons. However, to our knowledge their application to near-surface geophysics is less common. The paper shows how geostatistical filtering can be used to decompose "geophysical anomalies" into long and short wavelength components. The present paper therefore aims at illustrating the benefit of these techniques through two examples. In the first one, dedicated to refraction imaging, geostatistics allow to filter out acquisition artifacts and to identify the main geological features. The second example presents how a global trend can be filtered out in a Bouguer anomaly dataset (gravity survey), letting appear interesting local anomalies.
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Introduction of OGP Survey and Positioning and the APSG Objectives and Resources
Authors J. Verouden and B. CarterThe two primary professional survey organizations in the energy sector have come together to raise E&P professionals’ awareness of the importance of spatial data management. Knowledgeable members will present from the prestigious Surveying and Positioning Committee of the International Association of Oil and Gas Producers Surveying and Positioning Committee (OGP S&P) with colleagues from the Houston based Americas Petroleum Survey Group (APSG). Eleven companies have permitted their subject matter experts to impart knowledge to the geosciences and engineering communities. Hundreds of years of education and experience are offered to EAGE attendees. Three of the presenters have been recognized by EAGE with 2008 Honorary Memberships for their work on the EPSG Geodetic Parameter dataset which has conferred “an incalculable benefit on the geoscience community at large”.
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Geodesy and Cartography in E&P – A Caution to Skilled Professionals
More LessA twenty-minute tutorial on the fundamentals of geodesy and cartography in E&P will capture the essence of a four-year university degree and tell the audience "the least you need to know", such as latitude and longitude are not unique. This education is core for all map creators and users, and an essential review for the spatial data management session. Geodetic Datums, Geographic and Projected Coordinate Reference Systems (CRS) are discussed as well as Datum Transformations and other geodetic topics and parameters. Examples are shown that clearly illustrate how incorrect assumptions about the underlying geodesy or cartography can lead to significant positioning errors for lease boundaries, seismic, wells (surface and subsurface), pipelines and facilities. These errors cost oil company operators millions of dollars in inefficiencies or lost opportunities. In the worst cases, simple geodetic errors can cause significant underperformance and remain undetected. You will benefit from decades of experience and be cautioned on some common errors.
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Concession and Lease Boundary Delineation
Authors R. Lott and L. Romeijn jr.This paper examines some practical aspects of geodesy and cartography applied to upstream operations. It reviews coordinate reference system issues associated with licence block descriptions, block boundary lines, well locations in proximity to boundaries and distance and area calculations. A study of leases in Texas has identified significant errors in the position of boundaries as recorded in oil industry databases and applications. This has resulted in mispositioning of existing wells in databases and of proposed development well locations when spudded.
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Geodetic Integrity in Seismic Acquisition and Processing
Authors M. Redford, J. Conner and D. SalvageGeodetic integrity in seismic acquisition and processing must be achieved and maintained throughout the E&P lifecycle. Positioning of sensors within geophysical surveys is of equal importance to the location of survey and the spatial relationship between sensors needs to be known with a high level of confidence. Geophysical surveys need to be shot in the correct, intended, location with the necessary level of positioning precision and accuracy. The resultant geospatial data requires correct field recording, processing and management so as to enable future interpretation of the subsurface imagery obtained. Importantly, in many cases, the resultant data will be interpreted in conjunction with other sources of data and it is important that the geodetic parameters from each data source are clearly understood so as to enable ‘merging’ of datasets. Not only is consistency within the data of importance but also the integrity of its ‘real world’ location. This paper will discuss historical and current geophysical survey types, their uses and the methods by which geodetic data is, and was, obtained. Sources of error will be highlighted with a discussion relating to how these errors can occur, methods to prevent their occurrence and the effects they may have on the E&P life cycle. Following the discussion on acquisition techniques the use of geospatial data in seismic processing will be described followed by a discussion relating to subsequent data management. In conclusion the paper will outline techniques recommended to maintain geodetic integrity; these will be of relevance not only to those tasked with the acquisition but also to those managing and using the data.
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Understanding Borehole Surveying
Authors A. Jamieson and M. MichellThere is a great need for cooperation across the disciplines when optimally positioning a well (and future completions) in the reservoir. Geoscience, surveying and drilling professionals gather, process and make use of multivariate data often with a limited understanding of the methods used, the accuracies achieved and the impact of quality. This paper sets out some of the key areas of wellbore surveying, the tools and methods available, the accuracies and the common human errors that can affect the success of the well. It also explains some key areas of cooperation particularly with regard to the propagation of wellbore position uncertainty as it affects target sizing. Geoscientists and drillers can take advantage of probability theory to better define targets with acceptable risk and design fit for purpose survey programmes to achieve shared objectives.
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The New EPSG Geodetic Parameter Registry
Authors R. Nicolai and G. SimensenGeodetic referencing of seismic navigation data is required for the correct interpretation of the coordinates. To assist in providing the right definition of the Coordinate Reference System of the data the EPSG Geodetic Parameter Dataset has been published since 1994. This dataset is the largest collection of geodetic parameters available globally. As such it has become the de-facto standard for geodetic referencing of spatial data in many communities. It is recommended by SEG for obtaining the relevant geodetic definitions. Previously only available as a MS-Access file, it is now accessible over the World Wide Web using standard Internet browser software. Users can now query the server-based dataset through the EPSG Geodetic Parameter Registry through an intuitive interface, as well as obtain the MS-Access database file. The EPSG Registry also permits software-to-software querying and retrieving of geodetic parameters. This permits new generation software to extract these parameters from the Registry at run time or as part of a periodic synchronization event. The Geodetic Parameter Registry was constructed under a Joint Industry Project facilitated by the International Association of Oil and Gas Producers (OGP).
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Truth or Consequences - Commercial Impact of Geospatial Data Management
Authors J. P. Stigant, M. H. B. Jensen and R. AhmadIndustry wide, the E&P business experiences many challenges in managing geospatial data associated with core datasets. Based on both four years of evaluation and significant anecdotal evidence gained through industry networking, combined with over 100 years of combined experience, the authors estimate that geospatial coordinates may be stored improperly (with significant errors) for: - 20-25% of all acquired seismic data - 35-50% of all surface and subsurface well data - 30% of all land boundary data Comparisons are made between historical surface and sub-surface well locations and results from either GPS survey or reprocessed location data (for subsurface locations). Further examples of well, seismic and boundary positions will be shown. One example shows that mis-positioning of a boundary can lead to an error in the order of 12% of the concession area. The impact of these statistics, on the preparation and presentation of maps can have catastrophic consequences. This paper examines some of the reasons for these errors and makes recommendations to industry and operators to manage the process of reducing this risk. This paper is the seventh of seven papers in a special session addressing various components of geospatial management in the Oil and Gas industry.
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Impact of Fault Rocks on Reservoir Characterisation - Multi-phase Flow Properties for Production Simulation Models
Authors Q. Fisher, S. M. S. Al-Hinai and C. GrattoniThe basic aim of this paper is to increase the understanding of the fluid flow properties of faults and hence increases our ability to model fluid flow in fault-compartmentalised reservoirs. The paper will assess the importance of accounting for the multiphase fluid flow properties of fault rocks in simulation models. We are then present some new experimental results on the relative permeability and capillary pressure measurements for fault rocks (cataclastic and faults in impure sandstone). Finally, the incorporation of these results into production simulation models suggest that it is vital in some cases to account for the multi-phase fluid flow properties to better achieve history matching and will for future field development plans.
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Estimation of Pressure and Saturation Changes from 4D Seismic AVO and Time-shift Analysis
Authors M. Trani, R. J. Arts, O. Leeuwenburgh and J. H. BrouwerAn inversion scheme that solves for reservoir pressure and fluid saturation changes from time-lapse pre-stack seismic attributes and post-stack seismic timeshifts is presented. It makes use of three equations expressing the production induced changes as a function of changes in zero-offset and gradient reflectivity and of compressional wave time-shifts. The equations related to the AVO (amplitude-versus-offset) response are a revised form of Landro's (2001) equations. The inversion scheme has been successfully tested on a synthetic reservoir where seismic data have been modeled before and after two years production. The results show that changes in fluid properties can be approximated with fairly high accuracy.
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Stochastic Inversion for Reservoir Properties Using Time-lapse Seismic and Well Production Data
More LessWe present a new method to create stochastic models which honor both time-lapse seismic and well production data. A two-stage inversion workflow is used. The stochastic modeling allows us to estimate the uncertainty in the reservoir model and predict the realizations and production data. We firstly design three experiments which show the advantage of using time-lapse seismic data in the reservoir modeling and prediction. From these experiments, the inversion result using time-lapse data gives a better match to the actual saturation distribution. Finally, we describe and then present possible ways to address the uncertainty of in the required functions (variogram model, the relationship between porosity and permeability) used in our inversion approach.
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Estimation of 4D Anisotropy above the Elgin Reservoir from 4D Seismic Time Shifts and Compaction Considerations
By K. HawkinsA method to estimate the in-situ 4D anisotropy changes generated around producing reservoirs is discussed and demonstrated using the depleting Elgin reservoir. The method compares the offset dependency of observed 4D seismic time shifts with synthetic anisotropic ray-traced time shifts. It requires sufficient well data, or other a priori information, to calibrate 4D vertical compaction in and above a producing reservoir. As well as demonstrating a technique to estimate the average fractional change in anisotropy throughout the overburden above the reservoir, a scheme is proposed in which the lateral variation of the anisotropy parameter delta, and possibly epsilon (Thomsen, 1986), may be determined in a qualitative manner.
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Crosswell Electromagnetic Tomography in Saudi Arabia - From Field Surveys to Resistivity Mapping
Authors M. Wilt, J. M. Donadille, S. AlRuwaili, S. M. Ma and A. MarsalaA series of crosswell Electromagnetic surveys were made at the super giant Ghawar oil field in Saudi Arabia as part of an ongoing project to apply deep EM technology to map oil and water saturated regions within the field. The technology applies the principle electromagnetic induction and tomographic imaging to obtain the interwell resistivity distribution. The surveys which utilized existing wells and one newly drilled well were able to successfully collect data at well spacings of more than 800m. Recovered images were consistent with logs and often revealed an interwell resistivity section much different than the interpolated logs suggest.
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Improved Reservoir Description of the Nelson Field Via Seismic History Matching
Authors A. Kazemi and K. D. StephenAccurate description of current reservoir conditions and an understanding of parameter uncertainty are important factors for forecasting and decision making in reservoir management. In the Nelson field we use time lapse (4D) seismic as part of history matching to improve the reservoir description with success in a number of regions of the reservoir. We change permeability, net:gross and fault transmissibilities as well as the representation of fault geometries via cell connections so that our model better matches six years of observed data. The result is improved confidence in predictions with better in-fill well targeting.
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History Matching with Time-lapse Seismic - Will Any Starting Model Do?
Authors N. R. Edris and K. D. StephenIn general our knowledge of the reservoir is insufficient for accurate representation. We generate models that honour well logs, core, 3D seismic etc. but there is inevitably some uncertainty which we try to include in simulatioms via geostatistical modelling. History matching is then based on updating a model to improve the fit to the observed dynamic data. The choice of starting model in the history matching process is therefore critical and we need to know that our parameterisation and updating scheme can convert it into a better representation resembling reality. In this paper we describe how we set up some synthetic test cases to validate the history matching and parameterisation scheme. We find that if we use a pilot point separation that is equivalent to the range of the variogram used in generation of permeability distributions, we can get a good representation of the model.
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History Matching and Flow Rate Optimization of Caratinga Field
Authors K. A. Novaes, F. S. Campozana and K. GomesReconciling high-resolution geologic models to dynamic data such as RFT data (re-formation test pressure), permanent pressure down-hole sensors, and TFR (formation test pressure) data is one of the most challenging and time-consuming aspects of the workflow for geoscientists and engineers. The large amount of data collected overwhelms traditional methods and calls for new solutions. A second problem is to optimize oil recovery and profit, given a configuration of wells. The conventional wisdom is to inject the most and open-up production wells the completely. However, as we show in this paper, that is not always the optimal production strategy
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Adding the Temporal Coherence Dimension to 4D Seismic Data - Assessing Connectivity in the Schiehallion Field
Authors M. Floricich, A. Evans, D. McCormick, G. Jenkins and J. StammeijerThe Schiehallion Field is located in the West of Shetlands area of the North Sea. In Schiehallion, connectivity has been recognized as a key factor for reservoir management; flow barriers are often subtle and connectivity cannot be guaranteed from a static 3D seismic image. Here, a new methodology is proposed and applied to the Schiehallion Field to assess reservoir connectivity using multiple 4D seismic surveys. Coherence analysis is used to assess the spatial variability of the time-lapse seismic. Flow barriers between compartments with different 4D signal result in sharp discontinuities in trace-to-trace coherence of the time-lapse difference seismic cube, producing delineation of low coherence values along the flow barriers. By comparing five seismic monitor surveys shot in the Schiehallion Field, flow barriers that are constant through production time can be identified. Comparisons with the actual barriers in the reservoir model suggest that many of the model barriers are too discontinuous hence cause better connectivity than the real data suggests, and the reservoir needs to be broken into several more compartments. The update of the reservoir model with the new set of barriers allows a better match between the reservoir simulation and the observed 4D seismic effects.
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Integrating 4D Seismic, Geomechanics and Reservoir Simulations in the Elgin and Franklin Fields
Authors S. De Gennaro, A. Grandi, I. Escobar, A. Onaisi, L. Ben-Brahim, G. Joffroy, C. Tindle and V. NeilloSummary In recent years, the quest for better predictability of reservoir behaviour has led to the need to construct more complex and realistic models, where geosciences and reservoir engineering disciplines interact. Time-lapse (4D) seismic has been applied in both quantitative and qualitative manner to better understand and predict the dynamic behaviour of reservoirs. For stress sensitive reservoirs, hydrocarbon production induces changes in the effective stress field within and outside the reservoir, which result in predictable effects, including changes in seismic velocities and layer thickness. In this study, we present an integrated workflow that combines results from 4D seismic inversion, geomechanics modelling and reservoir simulations in a shared earth model. It has been applied to the Elgin and Franklin HPHT fields located in the North Sea Central Graben. These structurally complex fields are a challenge for a shared earth model approach. The 4D seismic have been inverted into interval time-shifts using an in-house warping method. In parallel, a coupled geomechanics and reservoir model has been used to determine the actual stresses within and around the reservoir, and to derive synthetic time-shifts with help of a rock physics model. Good match has been achieved between real and synthetic 4D time-shifts. This integrated study proved to be a useful tool for progressing understanding of Elgin and Franklin fields behaviour, with the ultimate goal being new drilling targets definition and casing integrity risk mitigation.
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Microgravity Surveys for Field Monitoring
Authors F. Italiano, M. Antonelli, D. Marzorati, I. Loretti, A. Cremonesi and I. GioriRecent improvements in instrumentation and surveying techniques have made time-lapse gravity and gravity gradient surveys a cost-effective and useful method for gas and hydrocarbon monitoring. The objective of this study was to investigate if time-varying gravity measurements can be correlated to the gas storage activities or not.
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An Integrated Methodology to Ensure Operations Readiness in a Complex Project Environment
Authors F. V. Verre and S. S. ScerraBe ready to operate it is one of the most critical steps in the development of an oil field. For complex projects characterised by technical risks and challenges a structured approach is mandatory to ensure operational excellence and readiness. The study proposed shows the methodology adopted by AgipKCO to achieve operational readiness for the Kashagan Field Development Project through a management system identified as Operations Readiness and Assurance (OR&A). The Operations Readiness and Assurance is the process to assure that the Field Production Facilities are designed, built, commissioned and started up taking into account lifecycle Operations requirements. This methodology to ensure operations readiness has been successfully applied to the development of Kashagan field. Kashagan is an extremely complex development in respect to the technical issues associated with developing a high pressure, sour hydrocarbon reservoir in a hostile environment. The management system applied through its structure and governance has enabled the Operations team to be fully integrated in the field development, to become a key decision maker for the operability of the facilities and to reduce
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3D Exploration for Remaining Oil Using Historical Production Data
Authors Y. Ling, X. R. Huang, D. S. Sun, J. Gao and J. X. Lin3D and 4D (time-lapse) seismics are techniques commonly used for exploration and reservoir exploitation. Increasingly, time-lapse seismic is becoming a more popular tool for reservoir development and management. However, its application is constrained by reservoir conditions, production mechanisms and seismic data repeatability. The technique of 3D exploration for remaining oil with historical production data uses high quality 3D seismic data, acquired after a certain period of reservoir production, and integrates it with historical production data to provide information for reservoir dynamics, such as the identification of additional resources and the delineation of remaining oil. In integrating the 3D seismic with historical production data, the 3D seismic data are time-stamped and then related to the reservoir dynamics. This enables 3D seismic data to represent reservoir dynamics in time. The method is applied to an onshore field in Western China. The result shows that the 3D seismic with historical production data(3.5D) can identify reservoir potentials and remaining oil.
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4D Seismic Modeling Integrated with the EnKF Method for History Matching of Reservoir Simulation Models
Authors M. C. Haverl and J. A. SkjervheimFrom a real Norwegian North Sea field case we show results from 4D seismic modeling using Eclipse simulated reservoir parameters in an Ensemble Kalman Filter framework, short EnKF. Seismic and geologic modeling is performed via the Compound model builder, our interface shared by geophysical and reservoir engineering data. The EnKF method is a Monte Carlo type sequential Bayesian inversion method suitable for history matching of reservoir simulation models and recently extended to incorporate seismic data.
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Modeling Reservoir Geomechanical Changes Caused by Fluid Injection - A Discrete Element Approach
Authors H. T. I. Alassi, R. M. Holt and M. LandrøA modified discrete element approach is introduced and used for modeling reservoir geomechanical response during fluid injection. The approach is based on the Discrete Element Method (DEM), where modifications are done to overcome some limitations of this method. The method is advantageous in modeling fracture developments in rocks. The approach is applied to two dimensional (2D) synthetic model based on the Gullfaks field in the North Sea as a case study. Two cases were modeled, one with low horizontal effective stress and the other with low vertical effective stress. Vertical fractures are developed in the first case, whereas horizontal fractures are developed in the second case. Finally these fractures are used with the help of a rock physical model to update the velocity model of the reservoir. The updating of the velocity model is important for interpretation of time-lapse seismics.
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The Value of Seismic Data in Production History Matching
Authors O. Leeuwenburgh, J. Brouwer, M. Trani and E. PetersSeveral approaches to the incorporation of seismic survey data into the history matching are explored. A 3D synthetic reservoir with known geo-statistical characteristics is used to simulate seismic data as well as flow rate and well pressure data under a given production scenario. An ensemble method is used to generate likely model realizations from the known geo-statistics. The Ensemble Kalman Filter is subsequently used to update the model porosity, permeability, pressure and saturation based on both the production and seismic data. Results of experiments that aim at quantifying the potential benefit of including seismic data are presented. A simple 2D reservoir is used to demonstrate the value of saturation information away from the wells. Two approaches are investigated with a 3D reservoir: In the first, each seismic survey is inverted to provide (depth-averaged) estimates of pressure and saturation, which are subsequently used during the model update. In the second case, two-way seismic travel times to the top and bottom of the reservoir are used directly by comparison with synthetic travel times. Both approaches can easily be extended to time-lapse mode.
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History Matching of Production Data and 4D Seismic Data on Girassol Field
Authors P. Berthet, L. M. Barens and P. D. PratIn order to match simultaneously the production data and time-lapse seismic data, two methodologies are explained and applied on an existing reservoir flow model for the deep offshore Girassol field, Angola. The workflow for both is based on 3D and 4D seismic facies characterisation. The next step is to upscale in the reservoir flow model, the petrophysical properties coming from a petrophysical inversion of the 3D seismic impedances. This reservoir flow model provides pressure and saturation for computing some simulated impedances to be compared to the real ones. During the update of the reservoir model, non-reservoir cells that proved to have shown production induced impedance changes are reassigned to reservoir facies. For the first method, only a few cells are concerned preserving active cells of the initial model. For the second one to improve the match of 4D seismic data, a selected area is completely updated using uncertainty on seismic facies. In a history matching optimisation loop, the breakthrough time of an injector/producer pair is significantly improved for both methods when compared to the initial reservoir model. These methods highlight the added-value of the 4D in the history matching phase.
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Efficient Model Selection Based on Connectivity for Geologically Complex Reservoirs
Authors G. de Jager, J. F. M. Van Doren and S. M. LuthiTo understand how hydrocarbon flow is influenced by the geological properties of channelized reservoirs an experimental design approach is employed. We examined the influence of geometrical properties (channel dimensions and sinuosity), petrophysical properties (permeability and net-to-gross), and derived properties (connectivity). It is found that the geometrical properties have a diffuse relationship with reservoir flow behavior, which is characterized by net present value (NPV). A strong correlation is found for net-to-gross and permeability. Connectivity shows the strongest correlation to production data. Calculating a quadratic response surface of connectivity versus NPV allows that an estimate can be made of the connectivity based solely on NPV and production data. Subsequently, based on the estimated connectivity an efficient selection from a large ensemble of reservoir model realizations can be made. It is demonstrated that there is a high similarity between the reservoir flow behavior of the selected reservoir models and the behavior of the synthetic truth. This suggests that connectivity can serve as a screening technique and can be used as a first step in history matching of model realizations which are created by any automated modeling technique.
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Associative Polymers for EOR - Towards a Better Understanding and Control of Their Adsorption in Porous Media
Authors D. Rousseau, R. Tabary, S. Paillet, Z. Xu, B. Grassl and J. DesbrièresAdsorption properties of associative polymers (AP) were investigated in porous media to evaluate the performances of these chemicals as EOR mobility control agents. Two different AP were studied. For AP Type 1, results indicated a strong adsorption as compared to equivalent non associative polymers and a poor in-depth propagation. AP Type 2 were easily injected, and their adsorption was found to depend on the ionic strength of the dilution brine. It is thus shown that: i) AP adsorption is a significant parameter for the success of polymer flooding operations since it can impair injectivity and reduce the effective viscosity of the polymer solution, and ii) a control of AP adsorption can be achieved through an adjustment of the injection conditions.
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Improved Waterflooding - Analytical Screening of Cyclic Parameters
Authors A. A. Shchipanov, L. M. Surguchev, G. A. Virnovsky and R. BerenblyumThe cyclic injection improves water-flooding sweep efficiency in heterogeneous reservoirs. The IOR-method was applied in many fields in Russia, USA, China. Cyclic injection potential to improve water-flooding efficiency was clearly demonstrated in a number of projects. The uncertainty which exists with the cyclic water-flooding is related to understanding the IOR mechanism, ability to accurately model and predict efficiency of the process, design a field application. In this work we discuss and evaluate parameters that affect cyclic water-flooding. The algorithms and analytical tool were developed to model cyclic injection and fluid cross-flow between stratified reservoir layers. The model accounts for compressibility effects, gravity and capillarity forces. The screening results of cyclic injection in the North Sea heterogeneous sandstone reservoir are presented. A wide-range sensitivity analysis was performed to estimate the process efficiency. The analysis with respect to rock-fluid parameters, heterogeneity, cycle length and pressure conditions allowed to rank the critical factors by their influence and importance for achieving efficient cyclic process. Cyclic injection has significant IOR potential in stratified sandstone reservoirs with high permeability contrast. The method can improve water-flooding sweep, accelerate oil production and increase oil recovery by up to 11%.
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Co-injection of Seawater and Produced Water to Improve Oil Recovery from Fractured North Sea Chalk Oil Reservoirs
Authors T. Puntervold, S. Strand and T. AustadCarbonate oil reservoirs are often fractured with moderate water-wet conditions, which prevent spontaneous imbibition of water into the matrix blocks. Average oil recovery from carbonates is usually less than 30%. Recent studies on chalk cores from the North Sea have shown that seawater is able to change the wettability towards a more water-wet condition at temperatures >100 °C. Seawater contains favorable concentrations of the potential determining ions Ca2+, Mg2+, and SO42- that are active in the displacement of strongly adsorbed carboxylic material from the chalk surface. During seawater injection the initial formation water mixes partly with seawater, and the amount and composition of the produced water varies with time. Due to environmental reasons, produced water should be re-injected together with seawater into the chalk formation. The question is: “Will mixtures of seawater and produced water displace oil in a similarly good manner as pure seawater?” This work showed that at T >100 °C the oil recovery by using PW:SSW-mixtures was significantly higher than by using pure PW in a spontaneous imbibition process. In a viscous flood, SSW appeared to be much more efficient than PW to displace the oil, and high recovery values were reached.
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Velocity Estimation by Waveform Tomography in the Canadian Foothill - A Synthetic Benchmark Study
Authors A. J. Brenders, S. Charles and R. G. PrattIn the structurally complex Canadian foothills, conventional seismic data processing is inadequate, given the effects of steep dips, rough topography, and near-surface weathering. Depth imaging is often necessary, and an accurate estimation of the macro-velocity model is essential. Waveform Tomography (i.e., traveltime tomography followed by frequency-domain waveform inversion) of long-offset data provides a possible solution. In order to investigate this, we generated synthetic data in a geologically realistic model exhibiting fold-thrust sheets, steeply dipping structures, and topographically elevated carbonate outcrops. The method shows promise for resolving the near-surface geological structures in this environment. Real data acquired with long-offsets and low-frequency sensors may provide the necessary input for Waveform Tomography to succeed in the Canadian foothills.
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Near Surface Void Detection Using Seismic Q-factor Waveform Tomography
Authors B. R. Smithyman, R. G. Pratt, J. G. Hayles and R. J. WittebolleSeveral shallow, engineering-scale seismic surveys were carried out in Seven Sisters, Manitoba, between 1999 and 2001. The surveys were designed to locate three buried riprap targets, located within a constructed clay embankment as a blind test of geophysical imaging methods. While previous studies used near-offset reflection methods to image the targets, in this paper we use Waveform Tomography of the long-offset, refracted arrivals to image the seismic velocity and the seismic attenuation. The refraction surveys were carried out with a 2 m spacing on both source and receiver positions, with 70 source positions and 48 receiver positions respectively. The targets were buried approximately 7 m below the surface of the constructed embankment. Using weight-drop data, with frequencies between 20 and 130 Hz, we were able to resolve the sub-wavelength targets using the seismic-Q images. A very good fit between model and observed data is an indication of the reliability of the results.
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Waveform Tomography Strategies for Imaging Attenuation Structure with Cross-hole Data
Authors R. Kamei and R. G. PrattWaveform Tomography, when implemented in the frequency-domain, potentially yields images of the instrinsic attenuation from seismic waveform data. The attenuation (or its inverse, the seismic Q value) is strongly related to useful geological variables such as rheology, fluid flow, pore fluid content and fractures. Since phase and amplitude anomalies in recorded data are also caused by velocity structure (due to geometrical and scattering effects), it is critical to assess inversion strategies as to their ability to resolve these effects. In this study, frequency domain Waveform Tomography was implemented with the subspace-search method, and evaluated with synthetic tests. We compared two sets of strategies: first, velocity and attenuation models were updated jointly at each iteration (“simultaneous inversion”). In a second test, (“sequential inversion”), a velocity model alone was first inverted followed by simultaneous inversion. While the predicted waveforms from both strategies agreed with the observed data, only the sequential inversion strategy imaged attenuation structure well in the presence of small-scale velocity heterogeneities. This highlights the strong dependence of attenuation imaging on the quality of the velocity model.
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Crosswell Waveform Tomography in Fine-layered Sediments - Meeting the Challenges of Anisotropy
Authors R. G. Pratt, L. Sirgue, B. Hornby and J. WolfeDuring June, 2004, BP carried out a crosswell seismic survey in the Rocky Mountains. The crosswell data were acquired in finely layered clastic sediments in the Mesozoic section. The reservoir sands are known to vary laterally in quality, and the crosswell survey was conducted in order to provide a high resolution, sub-seismic image of the target. High-resolution velocity images were obtained using Waveform Tomography. This was particularly challenging due to low signal-to-noise ratios, high intrinsic attenuation, and significant, variable, an-elliptic anisotropy with a vertical transverse isotropy (TIV) symmetry. The final Waveform Tomography results show we can resolve layers down to the order of 4 m in thickness, and that we can accurately track these layers across the target zone. A subtle change in dip in the geology is revealed below a known unconformity. The variability of the anisotropy played a critical role, and the development of fully anisotropic Waveform Tomography is a logical next step.
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3D Acoustic Frequency-domain Full Waveform Tomography (FWT) - An Application on SEG/EAGE Overthrust Velocity Model
Authors H. Ben-Hadj-Ali, S. Operto, J. Virieux and F. SourbierThree-dimensional quantitative seismic imaging in complex environments such those involving deepwater, thrust belts, sub-salt and sub-basalt structures is one of the main challenges of seismic exploration such that hydrocarbon exploitation. We present a 3D frequency-domain full waveform tomography (FWT) algorithm suited for wide-aperture seismic data. We aim to develop high-resolution P-wave velocity models at low frequencies. The inverse problem based on a classic gradient method consists of the successive inversion of few increasing frequencies. This defines a multiresolution imaging scheme and allows us to manage compact data volume. The code is fully parallelized and avoids disk swapping by keeping in core the foward-problem solutions in distributed format. We present two validations of our algorithm with the SEG/EAGE Overthrust model. The main structures were well imaged with a spatial resolution in accordance with the inverted frequencies. Further work is required to [i] perform more representative applications on larger computational platforms, [ii] assess the sensitivity of 3D FWT to the starting model and to the acquisition geometry and [iii] Investigate less demanding strategies to perform the forward problem in the frequency domain such that hybrid direct/iterative solver based on domain decomposition method.
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3D Frequency Domain Waveform Inversion Using Time Domain Finite Difference Methods
Authors L. Sirgue, J. T. Etgen and U. AlbertinFrequency-domain waveform inversion is typically perfomed using frequency-domain finite-difference modelling techniques. In 3D, these methods face significant computational challenges that limit any application to full-scale seismic applications. An alternative approach is to use a 3D time-domain finite-difference method and extract the frequency-domain wavefield by computing the terms of a discrete Fourier transform at each time step. This method combines the computational efficiency of 3D time-domain modelling while permitting casting the inverse problem in the frequency domain.
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Efficient and Effective 3D Wavefield Tomography
Authors M. Warner, I. Stekl and A. UmplebyWe demonstrate 3D wavefield tomography applied to surface-streamer seismic data and obtain a result which appears to remove the distoring effects of shallow high-velocity channels. The methodology is of wide general applicability.
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Asymptotically True-amplitude One-way Wave Equations in t
Authors N. Bleistein and Y. ZhangWe show that a simple one-way wave equation in time preserves amplitude just as asymptotic ray theory in frequency does.
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An Example of Integrated Use of Common Reflection Surface Stack and Pre Stack Depth Migration
Authors E. Gentile, M. Buia, F. Doniselli, S. Martinelli and D. RegazzoniThe 3D Common-Reflection-Surface (CRS) stack is a macro-model independent seismic reflection imaging method that has proved to accurately detect and characterize events in the pre-stack data domain. The great increase of fold that is peculiar of this technique, gives a clearer image compared to conventional time methods. At the same time PSDM is the most effective approach in imaging complex structures and it is a valid tool to properly evaluate the seismic velocity distribution in the subsurface. The present case history shows how in presence of complex environmental/geological issues, the integration between the two techniques previously mentioned leads to very good results in terms of horizon continuity and structure definition. The effectiveness of this approach is demonstrated by the reliable seismic interpretation of the main target performed on the seismic volumes and the successful understanding of the structural features. These interpretations helped the positioning of a successfully drilled well. The main aim of this paper is to demonstrate the importance of the combined use of these different technologies in such a very complex case.
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3D Beam Prestack Depth Migration with Examples from Around the World
Authors K. Schleicher, J. W. C. Sherwood, K. J. S. Sherwood and H. TiemanIn 1999 AGS started the commercial development of Beam Pre-Stack Depth Migration (BPSDM). The anticipated merits were simplicity, economy, flexibility and future development possibilities: the unknowns were the migration accuracy and quality. BPSDM consists of three important steps, decomposition, migration, and reconstruction. Decomposition is a multidimensional slant stack which decomposes the data into a basis of seismic wavelets. Each wavelet has a center location and determined dip components. These contribute to the properties of the wavelet. Migration computes a point to point mapping between the unmigrated and the migrated wavelet centers. Reconstruction composites each wavelet into its local region in migrated space and outputs seismic depth traces. Initial results, in 1999, were horrific but, with further development, BPSBM proved to have some superior aspects to Kirchhoff and wave equation migration. The quality and flexibility of BPSDM includes: capability for handling steep and overturned dip, demultiple options, the ability to reject coherent noise, incorporation of anisotropy, speed for migration iteration, calculation of residual 3D RNMO, extension to multi- and wide azimuth data acquisition, and the capacity to handle land and marine data. Data examples from around the world illustrate the quality and flexibility of the technique.
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Application of Shot-record Mirror Migration in Tilted Coordinates to Salt-model Building at Atlantis
Authors I. Ahmed, B. J. Nolte, P. Z. Mahob, D. Shepherd, R. Faerber and J. M. HowieWe apply shot-record wave-equation mirror migration in tilted coordinates to the problem of salt-model building at the Atlantis field in the Gulf of Mexico. This technique produces images on which steeply-dipping salt flanks can be picked with confidence, which is not possible on images computed with conventional one-way wave-equation migration because of its inherent steep-dip limitations.
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True-amplitude Angle Migration in Complex Media
Authors N. Ettrich, D. Merten and S. K. FossWe are presenting results from a true-amplitude prestack depth migration in the angle/azimuth domain. The implementation is based on the generalized Radon transform. Working directly in the angle/azimuth domain means that we do not need to calculate any Beylkin-determinant, and multi-valued traveltimes, in the presence of caustics, are naturally unfolded allowing for true-amplitude results even beneath a complex overburden. Such an implementation has until recently been considered unfeasible due to the computational complexity for large 3D volume migration. Our implementation is made possible through a massively parallel implementation with fast access to large dataset.
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Prestack Migration in Elliptic Coordinates
Authors J. C. Shragge and G. ShanRiemannian wavefield extrapolation is extended to prestack migration through the use of 2D elliptic coordinate systems. The corresponding 2D elliptic coordinate extrapolation wavenumber is demonstrated to introduce only a slowness model stretch to the single-square-root operator, enabling the use of existing Cartesian implicit finite-difference extrapolators to propagate wavefields. A post-stack migration example illustrates the advantages of elliptic coordinates in imaging overturning wavefields. Imaging tests of BP Velocity Benchmark data set illustrate that the RWE migration algorithm generates high-quality prestack migration images comparable to, or better than, the corresponding Cartesian coordinate systems.
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Direct Representation of Complex, High-contrast Velocity Features in Kirchhoff PreSDM Velocity Models
Authors J. W. D. Hobro, D. Nichols and R. FletcherWhen velocity models are built for Kirchhoff pre-stack depth migration, complex, high-contrast features such as salt boundaries are usually represented explicitly during the model-building process. However, the migration algorithm typically requires a smoothed version of this velocity model in order to allow ray-based methods (ray tracing or an eikonal solver) to produce a stable modelled wavefield. The method presented in this paper incorporates high-contrast structural features directly into velocity models during migration, accounting correctly for the physics of wave propagation across these features. This has the potential to improve the fidelity of ray-based modelling, and to bring consequential improvements to the migrated image at complex, high-contrast features and to regions imaged through these features. There are also significant practical benefits for the Kirchhoff depth migration workflow due to the reduced size of migration velocity models and removal of the requirement for model sampling between model building and migration.
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An Application of a Novel Frequency-domain Finite-difference Solver to Compute 3D Amplitude-preserving Migration Weights
Authors C. D. Riyanti, W. A. Mulder, G. Baeten and R. E. PlessixReverse-time migration provides a structural image by correlating forward and reverse-time wavefields. Finding amplitude-preserving migration weights can be a challenge if the Green functions used for imaging were not stored. As an alternative, we used a newly developed iterative solver for 3-D frequency-domain finite-difference wave-propagation modelling to estimate migration amplitude weights at just a single frequency. We applied this approach to two complex synthetic examples.
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Adaptive Limitation of the Migration Operator Aperture
Authors M. Alerini and B. UrsinIn Kirchhoff based migration, the relevant information is located at the specular reflection point along the migration operator. Limiting the migration operator around this specular point not only increases the speed of the process but also improves greatly the quality of the migration results by suppressing important aliasing artifacts. The aperture of the migration operator is connected to the notion of Fresnel zone and therefore depends on the position on the image domain. We present here an original way of determining this aperture for seismic prestack depth migration in angle domain assuming the geological dip is known. Our approach has several advantages compared to previously proposed ones: it is easy to implement in an already existing code, it is easy to use, it can take into account local errors in the estimated geological dips and it is possible to consider multi-oriented dips as in discontinuities.
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Seismic Velocity Estimation in the Curvelet Domain
Authors H. Chauris and T. NguyenCurvelets can be seen from the geophysical point of view as the representation of local plane waves. They are known to efficiently decompose any seismic gathers and possibly imaging operators. We study here how curvelets can be useful for velocity estimation. In that context, we first show that the Differential Semblance Optimization technique has a very simple expression in the curvelet domain. We then derive the gradient of the cost function, still in the curvelet domain. An application on a 2-D synthetic data set, generated in a smooth heterogeneous model and with a complex reflectivity, demonstrates the usefulness of curvelets to derive how the velocity model can be improved to better focalize energy in the sub-surface after migration.
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Direct Nonlinear Traveltime Inversion in Layered VTI Media
Authors P. J. Fowler, A. Jackson and J. GaffneyWe present a scheme for direct nonlinear inversion of picked moveout traveltimes in block-layered isotropic or VTI media. Using constant ray parameter traveltime differences between successive events allows inversion for velocity parameters of individual layers with no need for explicit layer stripping. We derive explicit closed-form inversion equations that allow this scheme to be implemented as a direct nonlinear method, rather than requiring iterative linearization. The method honors both anisotropy and ray bending effects more accurately than most conventional velocity analysis techniques.
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Wavepath Tomography for Model Building and Hazard Detection
Authors D. Bevc, M. M. Fliedner and B. BiondiIn complex velocity models, such as below rugose salt bodies, wavefield continuation migration is usually superior to Kirchhoff methods because of multi-pathing, sharp velocity contrasts and the bandlimited nature of seismic wave propagation. Wavepath tomography offers a way to build the velocity model in a way that is consistent with the migration operator: instead of tracing rays to backproject residual velocities, a "wavepath" is constructed using the actual wavefield continuation operator to represent the wave propagation between surface source/receiver pairs and subsurface reflection points by multiplication of impulse responses downgoing from the surface location and upgoing from the reflection point. The size of the inversion is kept manageable by restricting the wavepath to the first Fresnel zone. We demonstrate the applicability and superiority of this approach for building complex velocity models in areas of salt tectonics and for hazard detection.
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Applications of Wave-like Rays for Traveltime Tomography and Depth Imaging
Authors J. K. Washbourne, K. P. Bube, P. L. Carillo and C. M. AddingtonModeling seismic propagation is critically important to our work, and unhappily we often must trade simulation accuracy for reduced computational expense. We present a seismic modeling method that is as simple and computationally efficient as raytracing, but provides propagation paths and arrival times that are more consistent with finite bandwidth data. A significant benefit of finite frequency wave-like rays is the increased stability of propagation times and paths with respect to small changes in velocity. This leads to increased robustness of tomography and depth imaging, and can improve confidence in downstream interpretation. We refer to the modeling method as "wavetracing", and illustrate the advantages with field data examples for the crosswell seismic geometry.
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Poststack Velocity Analysis in the Dip-angle Domain Using Diffractions
More LessIn this study a method to analyze diffraction data for migration velocity analysis in the time or depth domain is presented. The method is based on the clear distinction between diffractions and reflections in the post-migration dip-angle domain. The attractive possibility to perform the analysis, using only stacked data as an input, is demonstrated on synthetic and real data examples.
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Wide-azimuth Tomography - Strategy and Application
Authors T. Huang, S. Xu, C. Ting, B. Liu and K. GiridharanOver the past three years, wide-azimuth towed-streamer acquisition (WATS) has become a major step improvement in marine acquisition technology for better imaging of complex subsalt structures in the Gulf of Mexico. However, wide-azimuth seismic acquisition, with shot and receiver locations covering a large portion of the two dimensional surface, presents a new challenge for tomography in deriving an accurate velocity model. A strategy for wide-azimuth tomography is proposed, and the benefit of vector-offset domain tomography over azimuth-angle domain tomography is discussed. Wide-azimuth tomography results from marine and land data are presented and are compared to narrow-azimuth tomography results. These case studies both demonstrate the advantage of wide-azimuth tomography over narrow-azimuth tomography.
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Revisiting PSDM Workflows Using Global Multi-scenario Traveltime Tomography
Authors A. Soudani, F. Adler, B. Duquet and P. JousselinPrestack depth migration (PSDM) has become a standard technique for imaging sedimentary basins in many oil provinces. The classical velocity model building workflow is based on linearized traveltime tomography in the depth migrated domain. It has the drawback of being time consuming, since each tomographic update involves a full volume PSDM, and therefore allows only little flexibility in the velocity model building process. In this paper we propose a depth migrated domain formulation of non linear traveltime tomography. It allows in principle to build a velocity model from a single PSDM and opens up the way to new velocity model building workflows, that we will illustrate on a North Sea data example.
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Interactive Imaging - An Effective Way to Build a Better Salt Model
Authors B. Wang, J. Ji, C. Mason, S. Gajawada and Y. KimIt is well understood that accurate salt interpretation is critical to subsalt imaging. When reflections from the salt-sediment boundaries are not discernible, it takes many iterations to test various scenarios based on the geologic consistency of reflectors underneath the salt. Multiple iterations with wave equation migration are extremely time consuming and its computational cost is unacceptably high. We have developed a new methodology by incorporating demigration, fast beam migration, and polygon-based salt model update. We first demigrate a prestack depth-migrated image to a subsurface datum, which provides well-focused reflections at their unmigrated locations on a regular grid. Next, we pick inline and crossline ray parameters by dip scan. We then carry out fast beam migration to test different salt boundaries to evaluate the geologic consistency of reflections under the salt. This methodology enables us to interactively test many scenarios to generate the final model much faster than iterations using other migration techniques. Application to a 3-D field data set from the Gulf of Mexico demonstrates the effectiveness our methodology.
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Two-step Explicit Marching Method for Reverse Time Migration
Authors R. Soubaras and Y. ZhangWe describe in this paper a new way of solving the two-way wave equation called the two-step Explicit Marching method. Compared to the conventional explicit finite-difference algorithms, which can be second or fourth order but are subject to stability conditions and dispersion problems that limit the magnitude of the time steps used to propagate the wavefieds, the proposed method is based on a high order differential operator and allows arbitrary large time steps with guaranteed numerical stability and minimized dispersion. Synthetic and real data examples show that it allows the reverse time migration to be performed with the Nyquist time step, based on the maximum frequency of the input data, which is the maximum time step that can be used for proper imaging.
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Hybrid Finite Difference-pseudospectral Method for 3D RTM in TTI Media
Authors A. C. Lesage, H. Zhou, M. Araya-Polo, J. M. Cela and F. OrtigosaWe propose a new anisotropic wave equations system for 3D TTI media, which is an extension to 3D VTI media and 2D TTI media equations of Zhou et al. This system is based on two 3D rotations which permit us to deduce 3D TTI equations from 3D VTI equations. We propose to use a hybrid Finite Difference (FD) pseudospectral algorithm to solve it, this mainly consists of forward-backward 2D FFT in lateral dimensions (x-y plane) and 1D FD in the depth dimension. This algorithm allows us to get high order accuracy and simplifies the computation of the cross derivatives of the TTI equations. In this work, we develop the 3D TTI equation formalization, we also describe the implementation of the method to solve the proposed 3D TTI equations. To validate our proposal, we carry out impulse response experiments for modeling and migration.
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A Hybrid Approach for Efficient Reverse-time Migration Applications
More LessReverse-time migration (RTM) provides superior images in areas where there are steep salt flanks or other complex geologic structures. However, the high cost of running RTM often makes it difficult to use RTM for routine large volume production. By dividing the subsurface into two or three regions in depth according to the complexity of the velocity model and applying RTM only in the region where we can benefit from it, we are able to make RTM very cost effective for routine production usage. Since the top region is often structurally simple, we use one-way WEM to image the region and save the downward continued wavefields at the base. We then use the saved wavefields as an input to RTM to image the middle region where there are complex structures and save the wavefield at the base if there is a third region with a simpler structure. We switch back to one-way WEM to image the third region using the saved data. By combining the strength of different migration algorithms, this hybrid approach not only significantly reduces the computation time and memory requirement, but also generates images of higher qualities.
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A High-resolution Vector-imaging Condition for Elastic Reverse-time Migration
More LessWe introduce a new vector-imaging condition for elastic reverse-time migration to produce high-resolution migration images. This new imaging condition is developed by the use of polarization vectors of wave fields along with the conventional imaging condition. The conventional imaging condition as that used for scalar-wave migration, is established on the principle that incident and reflected waves coincide in phase at a reflection point. Our vector-imaging condition also makes use of directional coincidence of the forward and backward propagating elastic waves. The polarization vectors of elastic wave fields are obtained by spatial derivatives of the corresponding waves in the frequency domain. We demonstrate using synthetic examples that the use of polarization vectors in the imaging condition significantly improves migration images. Elastic-wave reverse-time migration with the new vector-imaging condition will enable us to reliably image complex subsurface structures, particularly for true-amplitude migration.
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Obliquity Correction for Reverse Time Migration
Authors J. C. Costa, F. A. Silva Neto, M. Rian, J. Schleicher and A. NovaisThe quality of seismic images obtained by reverse time migration strongly depend on the employed image condition. We propose a new imaging condition, which is motivated by stationary phase analysis of the classical cross-correlation imaging condition. Its implementation requires the Poynting vector of the source and receiver wavefields at the imaging point. An obliquity correction is added to compensate for the reflector dip effect on amplitudes of reverse time migration. Numerical experiments show that using an imaging condition with obliquity compensation improves reverse time migration by reducing backscattering artifacts and improving the illumination compensation.
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Construction and Analysis of an Optimized Compact Finite Difference Scheme for RTM
Authors H. Zhou, Q. Liao and F. OrtigosaWe present an optimized compact finite-difference scheme that has a fourth-order of accuracy but with higher resolving power over a larger range of wavenumbers compared to other traditional finite-difference schemes. In order to realize that, we borrow ideas from Lele’s compact scheme (Lele, 1992) and Tam’s DRP schemes (Tam and Webb, 1993). I.e., we design an optimized finite-difference scheme that uses short stencils but with an optimized coefficient. This way it may help us take advantage of the computer caches without losing higher resolving power. A detailed Fourier analysis on the proposed scheme has been analyzed. A migration impulse response has been tested using our optimized compact finite-difference scheme.
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Practical Issues of Reverse Time Migration - True-amplitude Gathers, Noise Removal and Harmonic-source Encoding
More LessWe analyze the amplitude behavior of reverse-time migration and show that modifying the initial-value problem into a boundary-value problem for the source wavefield, plus implementing an appropriate imaging condition, yields a true-amplitude version of RTM. We also discuss different ways to suppress the migration artifacts. Finally, we introduce a "harmonic-source" phase-encoding method to allow a relatively efficient delayed-shot or plane-wave RTM. Taken together, these yield a powerful true-amplitude migration method that uses the complete two-way acoustic wave equation to image complex structures.
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Reverse Time Migration with Gaussian Beams and Velocity Analysis Applications
Authors M. M. Popov, N. M. Semtchenok, P. M. Popov and A. R. VerdelIn this contribution we describe our method of Reverse Time Migration with Gaussian beams (Gaussian beam RTM) and introduce an improved imaging condition suitable for computation of velocity contrasts at reflectors in the framework of the true-amplitude concept. By combining ideas and techniques used in tomography and Gaussian beam RTM, an effective approach to iterative velocity model building is developed. We demonstrate our recent results in these directions along with the results of target-oriented migration for the BP-model to show the potential and practicality of Gaussian beam RTM in target-oriented seismic imaging.
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Accurate Detection of Reservoir Boundaries Using Electromagnetic Attributes and Inversion of Marine CSEM Data
Authors P. Dell‘Aversana and F. ZanolettiIn this paper we introduce a new electromagnetic attribute that can be helpful for detecting properly the borders of resistive layers, such as a reservoir, with an accuracy that depends only on the receiver spacing. After using that attribute for identifying the spatial distribution of the resistive layer(s), inversion is applied for a proper characterization in terms of resistivity inside the layer itself. 3-D Synthetic tests and applications to real data will be discussed.
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Characterisation of Anisotropic Resistivity from Marine CSEM Data
Authors C. J. Ramananjaona, D. L. Andréis and L. M. MacGregorWe present a method for interpreting marine controlled source electromagnetic data taking into account the vertical resistivity anisotropy which is commonly observed in earth structures based on an inversion approach to simultaneously derive both horizontal and vertical components of resistivity.
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Data Preprocessing and Starting Model Preparation for 3D Inversion of Marine CSEM Surveys
Authors J. J. Zach, F. Roth and H. YuanThe marine controlled-source electromagnetic (MCSEM) method has been evolving into a geophysical imaging tool for increasingly complex geological settings. At the same time, 3D inversion algorithms for arbitrary survey layouts demand improved data quality compared to standard processing. Using a state-of-the-art survey acquired in the fall of 2007, we present a processing sequence starting from time-domain electromagnetic data acquired by seabed receivers to providing frequency domain data input and data weights for advanced processing. This includes determination and/or quality control of receiver orientation and time synchronization, and we show the quality of azimuthal receiver data to be adequate to be included in future inversions. Further, navigation data are adapted to a discretized grid upon determining the seafloor bathymetry. For missing or inadequate coverage from seismic surveys, the bathymetry can be extrapolated from navigation data with a spline-based algorithm, which is also described. It has proven beneficial, both in computational time and for recovering a meaningful model, to obtain a starting model for a full 3D inversion scheme by inverting reference receivers using a global, simulated annealing scheme, the result of which is imprinted onto the seafloor.
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Inversion of CSEM Data with Model-based Inversion Algorithm
Authors A. Abubakar, M. Li, T. M. Habashy and A. ZerilliWe present the model-based inversion algorithm, which uses a priori information on the geometry to reduce the number of unknown parameters and improve the quality of the reconstructed conductivity image. This model-based inversion approach can be also used to refine the conductivity image that we obtained using the pixel-based inversion algorithm. The model-based inversion approach adopts the Gauss-Newton minimization method, with nonlinear constraints and regularization for the unknown parameters. It also employs a line search approach to guarantee the reduction of the cost function after each iteration. The forward modeling simulation is a two-and-half dimensional finite-difference solver, and the parameters that govern the location and the shape of a reservoir include the depth and the location of the user-defined nodes for the boundary of the region. The unknown parameter that describes the physical property of the region is the electrical conductivity. We will show some numerical examples to illustrate the advantageous of using this model-based inversion approach.
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Modeling and Migration of Marine CSEM Data in the Areas with the Rough Bathymetry
Authors M. S. Zhdanov and M. CumaIn this paper we present a new approach to interpretation of the MCSEM data in the areas with the rough bathymetry. This approach is based on a new formulation of the integral equation EM modeling method in the models with inhomogeneous background conductivity. The developed technique allows us to incorporate the known geological structures and bathymetry effects in the method of iterative EM migration/holographic imaging and inversion. This approach provides us with the ability to pre-compute only once the effect of the known geoelectrical structure (e.g., the bathymetry effect) and keep it unchanged during the entire modeling and migration process. The method is illustrated by the numerical examples of modeling and inversion of the marine CSEM data in the areas with the rough bathymetry.
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Inversion of Bathymetrically Distorted CSEM Data - A Case Study from the Baobab Field
Authors M. K. M. Hagdorn, L. MacGregor and B. RobertsonIn this paper we apply the 2.5D CSEM inversion to a real data set collected over the CNR operated Baobab field containing a large change in water depth. The effects of a varying seafloor are taken into account by building a mesh which includes the known seafloor topography. The resulting geo-electric model compares very well with the know outline of the reservoir.
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Marine Controlled Source Electromagnetics (mCSEM) 3D Test over a Known Target
Authors A. Price, P. Turpin, M. Erbetta and G. CairnsThis presentation details a test of newly acquired 3D mCSEM (marine Controlled Source ElectroMagnetic) data over a known field with the aim of demonstrating the technology and achieving an in-house validation of the technique to improve understanding on how best to use these data and integrate them into our interpretation methodology.
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Advancing Marine Controlled Source Electromagnetics in the Santos Basin, Brazil
Authors M. P. Buonora, A. Zerilli, T. Labruzzo and L. F. RodriguesThe Santos Basin marine Controlled Source Electromagnetic (mCSEM) data were acquired as part of a co-operation project between Petrobras and Schlumberger to assess the integration of deep reading Electromagnetic (EM) technologies into the full cycle of oil field exploration and development. Multi-component electric and magnetic fields data were recorded. All fields at each receiver location were processed and interpreted using an advanced integrated workflow. The main objectives of the survey were to calibrate mCSEM over known reservoirs, quantify the anomalies associated with those reservoirs with the expectation that new prospective location(s) could be found. We show that the mCSEM response of the known reservoirs yields signatures that can be imaged and accurately quantified by new processing and interpretation procedures. A further initiative was to advance the state of the art in integrated interpretation and establish guidelines toward the development of an industry standard workflow unavailable at present.
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Spatial Sampling and Multiple Suppression Properties of Wide-azimuth Towed Streamer Acquisition Geometries
Authors E. J. van Veldhuizen and G. HampsonWide azimuth towed streamer (WATS) surveys are gaining popularity and seem to result in a significantly better image quality than more conventional marine acquisition geometries. However, it still is not completely understood which properties of wide-azimuth acquisition cause the improvements. We examine WATS using conventional spatial sampling theory (Hampson, 1998) along with 2.5-D synthetic experiments. This shows (a) significant weaknesses in the offset sampling, and (b) how the variation of the multiple’s Green’s function with azimuth is a strong factor in multiple suppression. We quantitatively compare the multiple suppression performance of several towed streamer geometries. This illustrates the trade-off between larger cross-line offset and cross-line sampling.
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Crystal - A Wide Azimuth Towed Streamer Program - Acquisition Design Analysis
Authors E. Fromyr, C. James and P. E. DhelieHigh quality sub-salt imaging has remained a challenge for the seismic industry over the last two decades. In the marine environment depth migration of long offset (in-line) data has proven quite successful. Recent modeling has shown Regone (2006) that increased cross-line offset can improve the image quality significantly. It improves illumination and attenuates multiple energy in difficult sub-salt imaging environments, see Regone (2006) and Keggin (2002). It is well known that stationary receiver systems like vertical cables or OBC offer an acquisition solution adequate for solving the above mentioned challenges. However, due to the efficiency and cost effectiveness of the streamer method we can now achieve the same survey objectives with conventional streamer technology. This new Towed Streamer Wide Azimuth method represents however, new challenges in terms of survey design, acquisition methodology and operation. An incremental tile style acquisition offers a flexible, robust and well suited solution in terms of coverage and infill. In this paper we discuss these aspects in the context of a large scale wide azimuth exploration program in the Gulf of Mexico.
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Circular Geometry for Wide-azimuth Towed-streamer Acquisition
More LessWide-azimuth (WAZ) towed-streamer acquisition has been established as a successful method for exploration and development of the complex subsalt structures in the Gulf of Mexico. Wide-azimuth data acquired so far proved to have better illumination, higher signal-to-noise ratio, and improved seismic resolution. The acquisition geometry used for WAZ surveys is a parallel geometry that employs multiple vessels. In this paper, we propose to acquire WAZ surveys with a single streamer vessel using a circular geometry and we describe the features of this new WAZ method. We also present the results of a feasibility test performed with circular geometry acquisition.
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The Influence of Acquisition Geometry on Azimuthal Anisotropy Estimation
Authors P. M. Zwartjes, M. P. Boere and J. W. De MaagHorizontal transverse isotropy (HTI) is the simplest model for azimuthal anisotropy analysis and is widely used in parameter estimation. It implies a number of significant simplifications and assumptions. The reliability of parameter estimation depends to a large extent on the acquisition geometry. We have performed ellipse fitting on the normal move-out velocities of synthetic data for a simple HTI model. For a narrow-azimuth acquisition geometry the estimated anisotropy parameters are not representative of the subsurface even though they could be used to improve the stack response. For full-azimuth data the NMO ellipse can be reconstructed in a reliable way providing accurate estimates of the HTI parameters, even in the presence of considerable noise.
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Leveraging the Value of Multi-azimuth (MAZ) Seismic through MAZ-stack
Authors T. Manning, C. Page, S. A. Hall, J. Keggin, B. Barley, W. Rietveld, E. Fromyr and R. Van BorselenThe growing demand for energy requires that the search for hydrocarbons must extend into more challenging settings and become more reliant on technology, exposing the limitations of conventional 3D marine seismic. This decade has seen a major response to these challenges in the marine setting, in the form of Wide Azimuth seismic acquisition, like Multi-Azimuth, Wide Azimuth Towed Streamer, Nodes and OBC. These methods illuminate the sub-surface more completely, and sample problematic 3D noise across azimuth as well as offset for better attenuation during stack. These surveys are much more expensive to acquire and process, so it is thus important to lever all the value from the data. This paper discusses two seismic processing options to lever additional value from MAZ data. The first, MAZ-Stack, has been developed to weight up signal in areas of poor illumination, in other words, to favour strong signal over weak/absent signal. This approach has the effect of reducing fold and so decreases random noise suppression. The second technique, 3D warping, extends an existing method to align sub-surface seismic volumes, and so minimize registration errors between the azimuth stacks which result from imperfect knowledge of the sub-surface velocity field.
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Optimizing 3D SRME on Wide Azimuth Data
Authors D. Lin, C. Ting, W. Lin and M. GriffithsWe discuss how to optimize our 3D SRME method to take advantage of wide azimuth (WAZ) data, while overcoming challenges posed by the data. Compared to narrow azimuth data, the main advantage of WAZ data is the wider distribution of azimuth angles. The challenge is how to handle the larger source separation and the lack of near offsets. We use a real WAZ dataset to show that the optimized 3D SRME algorithm is effective in removing multiples.
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Lessons Learned from the First Exploration WATS Data Processing
Authors P. Wijnen, L. Comeaux, P. Aaron and R. van BorselenPrevious to now, Wide Azimuth marine surveys have generally been organized along prospect development scale. This paper concerns a 2007 Deep Water Gulf of Mexico Wide Azimuth Towed Streamer (WATS) project executed on much larger exploration scale. The size of the data volumes and the challenges introduced by WATS acquisition geometries demand a pragmatic approach to processing the data. In this paper, data processing insights and results are presented which have been achieved to date. Comparisons between seismic images from initial results to legacy Narrow Azimuth Towed Streamer (NATS) are shown, as well as imaging utilizing various classes of acquired traces. Furthermore, significant experiences in applying processing technologies such as tomography and 3-D SRME are also reviewed.
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Seismic Imaging Improvement Thanks to a True Wide-azimuth Pre-processing Sequence Applied to a 3D OBC Dataset
Authors J. -L. Boelle, S. Navion, F. Adler, S. Le Bégat, P. Hugonnet and K. KravikHigh density wide-azimuth Ocean Bottom Cable acquisitions have proven their efficiency to better image complex structures in the North Sea. Yet conventional pre-processing sequences have not taken into account the full potential offered by such acquisitions (Soudani et al. 2006). In this paper, we demonstrate the advantages of a true wide-azimuthal pre-processing regarding noise attenuation on data where the low signal to noise ratio is a main challenge. Following Soudani et al. (2006), a decomposition of the common receiver gathers into the 3D -px-py domain is used to filter out linear noise. A specific two-step sequence is proposed to attenuate the artefacts produced by the strongly aliased direct wave with acceptable run-time effort and data storage. Our results are compared, before and after pre-stack depth migration, with those derived from a more conventional processing approach.
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Elastic Interferometry for OBC Data - Theory and Examples
Authors J. E. Gaiser and I. VasconcelosElastic imaging from ocean bottom cable (OBC) data can be challenging because it requires the prior estimation of both P- and shear-wave (S-wave) velocity fields. Seismic interferometry is an attractive technique for processing OBC data because it performs model-independent redatuming; retrieving "pseudo-sources" at the positions of the re-ceivers. The purpose of this study is to investigate multicomponent applications of inter-ferometry for processing OBC data. This translates into using interferometry to retrieve pseudo-source data on the sea bed containing PP-, PS-, and possibly SS-waves. We discuss perturbation-based, elastic interferometry with synthetic and field OBC datasets. Conventional and perturbation-based interferometry integrands computed from a synthetic are compared to show that the latter yields little acausal response and fewer artifacts. A 4C (Xx, Xz, Zx and Zz) pseudo-source response retrieves pure-mode SS-reflections as well at PP- and PS-reflections. Pseudo-source responses observed in OBC data are related to P-wave conversions at the seabed rather than to true horizontal or ver-tical point forces. From a Gulf of Mexico OBC dataset, principal components of a 9C pseudo-source response demonstrate that Vp/Vs at the seabed is an important factor in the conversion of P to S for obtaining the pure-mode SS-wave reflections.
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Virtual Real Source
By J. BehuraEstimation of the seismic source signature is an important problem in reflection seismology, especially in seismic imaging problems. Existing methods of source signature estimation (statistical methods and well-log-based methods) suffer from several drawbacks. Here, I introduce a method of extracting the source signature based on the theory of seismic interferometry, also known as the virtual source method. The only requirement for this method is to have a receiver location lie at the shot position whose source signature we want to estimate (not necessarily a zero-offset receiver). Through modeling examples, I show that the Virtual Real Source method produces accurate source signatures even for complicated subsurface and source signatures. Source signature of each shot can be extracted reliably if they all have similar amplitude spectra even though their phase spectra might be completely different. This method of source signature estimation not only gives accurate traveltimes and amplitudes of reflection events, but also has the potential to solve other issues, such as finding source radiation patterns, measuring intrinsic attenuation, and estimating statics.
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Interferometric Seismic Imaging of Sparsely-sampled Data
By P. C. SavaMicro-seismicity induced by fluid migration can be used to monitor the migration of fluids during reservoir production and hydro-fracturing operations. The seismicity is usually monitored with sparse networks of seismic sensors. The sparsity of the sensor networks degrades the accuracy of the estimated event locations. This inaccuracy often makes it impossible to infer the fluid pathways at the desired accuracy. Micro-seismicity can be located using an imaging approach based on wave-equation imaging under the exploding reflector model. This idea in itself is not new. What is new, however, is the use of an interferometric imaging algorithm which suppresses artifacts in the location of targets (micro-seismic) and create crisper images, which translates into more accurate location estimates. The images produced with the interferometric imaging method can be used for robust automatic location of micro-earthquakes with application to 4D monitoring of fluid injection.
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Interpretation of the Estimated Green Function Extracted from Seismic Energy Generated by the North Sea Surf
Authors X. H. Campman, D. Draganov and G. G. DrijkoningenBy crosscorrelating and summing seismic energy measured at two receivers, one obtains an estimate of the Green function between those receivers. However, the reliability of the estimated Green function depends strongly on the characteristics and distribution of the seismic energy, used for the correlations. We show results from estimating the Green function using seismic energy recorded at the beach in Hoek van Holland, The Netherlands. Using passive data recorded with an areal array, we image the origin of the noise in the direction of the North Sea. In turn, this allows us to reliably interpret the crosscorrelation results in the direction perpendicular to the seashore as surface waves. This interpretation is confirmed by a comparison of active and passive data recorded with the same linear array perpendicular to the seashore. Both the main surface wave arrival and the dispersion show an excellent match. On the other hand, the correlation result in other directions cannot be interpreted as a surface wave. This is demonstrated using active data and correlation results of passive recorded with the areal array.
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Imaging Internal Multiples through Seismic Interferometry - Subsalt VSP Examples
Authors I. Vasconcelos, R. Snieder and B. HornbySeismic interferometry has become a technology of growing interest for imaging from borehole seismic data. We demonstrate that interferometry of internal multiples can be used to image targets above a borehole receiver array. We use an interferometry technique that targets the reconstruction of specific primary reflections from multiply reflected waves. In this target-oriented interferometry approach, we rely on shot-domain wavenumber separation to select the directions of waves arriving at a given receiver. We provide a description of this method along with two conceptual applications, and compare it to other approaches to seismic interferometry. Using a numerical walkaway VSP experiment recorded by a subsalt borehole receiver array in the Sigsbee salt model, we use the interference of internal multiples to image the salt structure from below. In this numerical example, the interferometric image that targets internal multiples reconstructs the bottom and top salt reflectors above the receiver array, as well as subsalt sediment structure between the array and the salt.
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Seismic Interferometry-by-deconvolution for Controlled-source and Passive Data
By K. WapenaarWe discuss seismic interferometry by multi-dimensional deconvolution for controlled-source data as well as for passive data and compare both approaches with the corresponding correlation-based interferometric methods. For the controlled-source situation we derive the virtual source method as an approximation of the multi-dimensional deconvolution method. For the passive data situation we show that the deconvolution method and the cross-correlation method are essentially different, and we discuss the merits and drawbacks of both approaches.
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Perturbation-based Interferometry - Applications and Connection to Seismic Imaging
More LessWith the objective of extracting only the scattered wavefield that propagates between two receivers as if one of them acts as a source, I present a seismic interferometry method that relies on perturbation theory. This approach follows from an exact correlation-type repre-sentation theorem in perturbed media. When the receivers lie between the physical sources and the imaging target, this perturbation-based theorem can be adapted for inter-ferometry of both dual- (i.e., pressure and particle velocity) and single-field measure-ments. In the case of ocean-bottom cable (OBC) data, dual-field perturbation-based inter-ferometry retrieves the response between the receivers without the free-surface multiples. Here the reference field is taken as the direct arrival along with water bottom multiples. In the context of seismic imaging, I show that the representation theorem in perturbed media can lead to the commonly used cross-correlation imaging condition in migration by wavefield extrapolation. Thus the exact form of the perturbed representation theorem can in principle be used for the migration of multiply-scattered waves, as well as for cor-rections for migration amplitudes and artifacts.
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Effect of Intrinsic Losses on Seismic Interferometry
Authors D. Draganov, R. Ghose, E. Ruigrok, J. Thorbecke and K. WapenaarSeismic interferometry (SI) is the process of generating new seismic traces from the cross-correlation of existing traces. One of the starting assumptions for deriving the SI representation equations is that of a lossless medium. In practice, this condition is not always met. Here, we show what the effect is of intrinsic losses on the SI result with the help of a laboratory experiment in a homogeneous sand chamber. Using numerical modelling results, we further show that, in the case of a dissipative inhomogeneous medium with internal multiple scattering, ghost reflections will appear in the cross-correlation results.
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Interpolation of Irregularly Sampled Data by Matching Pursuit
Authors A. Ozbek, A. K. Özdemir and M. VassalloWhile several methods exist that aim to interpolate between regularly sampled traces or reconstruct traces missing from a dataset that is sampled on a regular grid, there are few techniques designed to handle truly irregular sampling. We introduce a new iterative method (IMAP) to regularize irregularly sampled seismic data, based on the matching pursuit technique. In this technique, the data are modeled as a sum of basis functions characterized by a set of parameters. At each iteration, the optimal new basis function is found to reduce the residual at the measurement points. If sinusoids are used as basis functions, it turns out that the optimal wavenumber to use is given by the maximum of the Lomb periodogram, which is considered the state-of-the-art method for computing the spectrum of unevenly sample data. As the IMAP method is iterative, it improves on the original Lomb spectrum result. The new technique is demonstrated using a finite-difference synthetic data example with severe sampling irregularity.
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Bandwidth Optimization for Compact Fourier Interpolation
Authors I. Moore and R. -G. FerberIn this paper, we present a bandwidth-optimization technique for Compact Fourier Interpolation (COMFI). COMFI is a minimum mean-square-error interpolation technique for data sampled at irregular locations. A particular application of interpolation is the estimation of data on a global, Cartesian grid (regularization). The sampling space can be of arbitrary dimension. The interpolated data are computed as a weighted sum of the actual data in a neighbourhood of the selected interpolation locations. The interpolation operator depends on the actual sampling locations and the interpolation location, but not on the data themselves. The operator is designed to have the minimum mean-square interpolation error among all linear operators over a suitable class of spatially band-limited basis functions. We consider, however, the spatial bandwidth as a parameter to be selected in an optimum way. Typically, we compute the COMFI operator to have the largest bandwidth that the actual sampling regime supports, in the sense that the mean-square interpolation error is below an acceptable threshold. We show that the quality of the operator can be strongly dependent on the bandwidth used for its design, and that the optimum bandwidth can vary significantly with spatial location.
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Sampling Functions and Sparse Reconstruction Methods
Authors M. Naghizadeh and M. D. SacchiIn this paper we investigate the effects of different sampling operators on the performance of sparse reconstruction methods. The common paradigm in seismic data processing is to favor regular sampling. We will show, however, that regular sampling often hampers our data recovery efforts. Random sampling, on the other hand, can lead to algorithms where the reconstruction is almost perfect when the underlying spectrum of the signal can be assumed sparse. Also, simple 1D, 2D and 3D synthetic examples are provided to test the sparse reconstruction of signals sampled by various sampling functions.
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Seismic Data Regularization in the Presence of Aliased Energy
Authors S. Tegtmeier-Last, R. A. Ergas, K. P. Bube and T. NemethMany different techniques have been developed to produce a regularly sampled, unaliased data set from irregularly sampled, aliased data. Almost all techniques to correct for the aliased energy rely on the assumption that it wraps around in the frequency-wavenumber spectrum of the data, and that it can be easily removed or unwrapped. However, different sampling geometries have an effect on the way aliased energy is distributed in the spectrum of the data. In this paper we present a case study where synthetic data are produced for a number of different sampling geometries varying from regular to almost random sampling. For every data set the spatial appearance of aliased energy is analyzed. The results of this study clearly show that the sampling influences that appearance. The assumption that the aliased energy wraps around is only correct for regular or nearly regular sampling, but it does not hold for more irregular sampling geometries. Based on these findings we propose to carefully revise the de-aliasing techniques applied in current regularization methods and, where appropriate, replace them with an approach that handles the aliased energy in a sampling-dependent way.
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5D Interpolation, PSTM and AVO Inversion for Land Seismic Data
Authors J. E. Downton, B. Durrani, L. Hunt, S. Hadley and M. HadleyTo address the issue of inadequate sampling, typical of land seismic data, an AVO processing flow should include interpolation and prestack migration prior to the AVO inversion. It is well established that seismic data should be prestack time migrated prior to AVO yet the irregular sampling inherent in land data can introduce migration artifacts which distort the estimates of the AVO inversion. By performing 5D minimum weighted norm interpolation prior to the PSTM, the wavefield is better sampled leading to better migration and AVO results. By working in five dimensions, the algorithm can interpolate through gaps that are problematic for lower dimensional interpolators. The 5D interpolation is amplitude preserving and appears to improve the signal-to-noise ratio with minimal evidence of smearing. In order to support these assertions, a series of parallel processing test flows were performed and compared on a 3D seismic survey from Alberta, Canada with extensive well control. For each of these flows, Ostrander gathers at key wells, AVO attributes, and their ties to 29 wells were examined. The interpolation PSTM flow prior to AVO inversion produced the best correlation to the well control.
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Seismic Data Reconstruction Using Multidimensional Prediction Filters
Authors M. Naghizadeh and M. D. SacchiThe Multi-Step Auto-Regressive (MSAR) reconstruction method is extended to reconstruct multidimensional seismic data. The validity of MSAR prediction filtering estimation for more than one spatial coordinate is proved. In addition, simple graphical interpretations are provided in order to explain the details pertaining the practical implementation of our algorithm. Finally, the performance of MSAR on synthetic 2D and 3D data set and a real 3D data example are examined.
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On the Use of Multicomponent Streamer Recordings for Reconstruction of Pressure Wavefields in the Crossline Direction
Authors J. O. A. Robertsson, I. Moore, M. Vassallo, A. K. Özdemir, D. J. van Manen and A. ÖzbekMeasuring three components of particle velocity along with the pressure brings significant benefits to towed marine seismic data. In addition to well-known deghosting benefits, we show that the particle velocity components add important constraints that enable reconstruction of the wavefield between the multicomponent streamers. Finally, we outline several cases of particular interest that will make towed marine seismic data more "3D like".
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