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68th EAGE Conference and Exhibition incorporating SPE EUROPEC 2006
- Conference date: 12 Jun 2006 - 15 Jun 2006
- Location: Vienna, Austria
- ISBN: 978-90-73781-00-9
- Published: 12 June 2006
401 - 462 of 462 results
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Building 4D Constraint for History Matching from Stratigraphic Pre-Stack inversion - Application to the Girassol Field
Authors P. Nivlet, T. Tonellot, P. Sexton, J. L. Piazza, F. Lefeuvre and O. DuplantierAdequate history matching is mandatory to understand properly the physical changes occurring in an hydrocarbon field that are related with production. However, this process has no unique solution. 4D seismic, which consists in repeating a 3D seismic survey at different calendar times is an additional source of data, which provides information about production effects. Its integration in the history matching process can therefore help in better identifying permeability barriers, by-passed oil zones, and therefore influence the field management.
In this paper, we focus on the definition of this 4D seismic constraint on Girassol, a giant deep-offshore field located offshore Angola. The methodology we have used is based on a sequential stratigraphic inversion of pre-stack data from the different seismic vintages. The methodology to invert the monitor survey is divided into 4 parts:
- time realignment of seismic events from monitor survey on the base survey;
- verification of the invariability of wavelets with calendar time;
- updating of a priori model;
- pre-stack stratigraphic inversion of the monitor survey;
Finally, we interpret the variations of inverted impedances between the different vintages in terms of production effects.
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Integration of 4D Seismic Data and the Dynamic Reservoir Model - Revealing New Targets in Gannet C
Authors P. S. Rowbotham, R. Staples, A. Cook, J. Braisby and A. MabillardTwo sets of 4D seismic data gave major new insights into the structure and dynamic behaviour of the Gannet C oil and gas reservoir in the UK Central North Sea. The 4D data revealed major extensions of reservoir units previously presumed to be absent or thin over much of the reservoir. Furthermore, in a subsea field with significant uncertainty of production allocation, 4D also proved an invaluable history matching parameter for the dynamic model. Together, the dynamic model and the 4D data gave rise to the identification of one recompletion opportunity and two infill well opportunities, to produce oil volumes in existing and newly identified reservoir sands.
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Integrated 3D/4D Structural and Stratigraphic Interpretation on Nelson Accounts for Variable Fluid Contact Levels
Authors A. MacLellan, P. S. Rowbotham, R. Rogers and J. MillingtonTo optimize the value of time-lapse seismic data, it is essential to understand the reservoir architecture and structure in 3D. Only then can we attribute 4D signal to specific reservoir flow units, and 4D signal boundaries to flow barriers such as stratigraphic margins or faults. We present a case study on the UK Nelson oil field, which, with its multiple seismic monitor surveys and strong 4D water sweep signal, is a world-class example of 4D data. We show how 3D and 4D interpretation can explain variability in sweep and thus identify infill drilling requirements.
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Impact of Time Lapse Processing on 4D Simultaneous Inversion - The Marlim Field Case Study
Authors A. Castoro, C. Reiser, E. Thedy Ambrosini, P. Johann Schroeder and P. LeeThis paper presents a 4D simultaneous inversion case study from the Marlim field, offshore Brazil. The paper aims at illustrating the influence of processing for time-lapse AVO inversion.
An AVO inversion of the Marlim 4D seismic dataset has been performed on an original dataset, not acquired or processed for time-lapse purposes. A seismic pre-conditioning processing sequence was applied to the post-stack migrated angle stacks to improve the seismic repeatability. The sequence included the creation of a 'common' seismic dataset generated from the coherent signal between base and monitor data. The common dataset is characterized by higher signal-to-noise ratio than both base and monitor vintages. The pre-conditioning efficiently improves the seismic repeatability of the vintages. However, the inversion results show a significant amount of noise contaminating the 4D signal.
For this reason, on behalf on Petrobras, the 4D dataset has been subsequently reprocessed in parallel by CGG. The reprocessing involved the application of the same processing sequence to the two vintages. The AVO inversion is being repeated on the new dataset. The 4D parallel reprocessing dramatically increases the repeatability of the two vintages allowing an efficient reduction of the 4D noise for the inversion.
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Plan, Acquire, Process and Interpret - How to Turn Around a Time Lapse Survey in Three Weeks
Authors C. R. Schiott and A. KingThe turnaround of seismic surveys and time lapse data in particular can be greatly reduced if the objectives are well defined at an early project stage. Through the definition of these objectives by the integrated reservoir management team consisting of all subsurface and production specialists, the geophysicists can design the survey geometry, processing workflow and prepare an interpretation program that delivers initial answers on sweep geometry and compartmentalization. It is demonstrated how a towed streamer monitor survey shot in September-October 2005 was planned and prepared enabling the availability of amplitude and time change maps for analysis within three weeks of the last shot fired whilst still including full processing and prestack migration of the monitor survey.
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Enhanced Streamer Positioning for 3D and 4D Seismic
Authors J. A. Musser, M. Burnham and D. RidyardStreamer positioning technology has evolved from streamer shape computations based on magnetic compasses to hybrid systems combining compasses with limited acoustic sensors to improve positioning errors. Correlated positioning errors can have a detrimental impact on the resolution of 3D surveys. The potential impact on 4D seismic is even more serious. These problems can be addressed by the use of more rigorous fully cross-braced acoustic positioning networks. Such networks can improve positioning accuracy by 60% and facilitate the use of steerable streamers for minimizing streamer separation, streamer offset from obstructions and errors in repeatability for 4D seismic monitoring surveys.
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Understanding 4D Repeatability Variograms
Authors H. C. Hoeber, S. Butt, C. Lacombe, S. Campbell and D. N. WhitcombeRecent discussions of 4D repeatability have focused primarily on the effect of acquisition differences prior to processing. We show that acquisition differences in areas with target reflector dip will always result in non-repeatability prior to regularization and that this is caused by differing reflection points. Processing elements such as 4d binning and regularization attempt to move traces to a common subsurface position (in a 4D sense) and to their bin center. They are designed to overcome differences in acquisition and cannot be overlooked in a full repeatability analysis. Since regularization and imaging destroy the association of traces and navigation attributes, a variogram analysis based on differences in source and receiver positions cannot be performed beyond these steps. Consequently it is difficult to predict final repeatability from a variogram analysis prior to regularization.
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Time-Spectral Analysis for 4D Data Q-Controlled Calibration
Authors D. Lecerf, M. Rogers and F. LefeuvreConventional approaches in time-lapse studies often ignore seismic transmission effects such as attenuation. For example, gas injection may produce changes in the factor Q. Consequently, amplitude and phase variations in time-lapse seismic data may be wrongly interpreted To correct such spectral distortion, we present a cross-equalisation technique based on differential Q-controlled calibration. The methodology should be applied in a 4D context when frequency attenuation variation occurs inside a reservoir or in the overburden.
The spectral characterisation of the data is tested for two different techniques: Multi-taper and Wavelet decomposition. We present the advantages and the disadvantages specifically for time-lapse studies.
The cross-equalisation is defined as "attribute-driven-processing". Using the constant Q definition, an attribute, called 4DQ, is computed simultaneously on the base and monitor with linear fitting of the logarithmic spectral ratio.
The calibration, controlled by the 4DQ attribute, removes the effect of the absorption variation.
The methodology is tested on real data. The measured 4DQ attribute shows a clear spatial correlation with the reservoir in production, but its direct interpretation seems to be critical. Furthermore, the calibration process and a single-Q compensation are applied providing a high-resolution 4D signature.
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Use of Spectral Decomposition to Detect Dispersion Anomalies Associated with Gas Saturation
Authors M. Chapman, J. Zhang, E. Odebeatu, E. Liu and X. Y. LiSignificant evidence suggests that hydrocarbon deposits are associated with abnormally high values of seismic attenuation, and the ability to detect such zones would aid seismic exploration. Unfortunately, attenuation is difficult to measure and it is not clear how to interpret observed frequency responses. Based on forward modelling, we believe that the effect of the frequency dependent reflection coefficient which results from high dispersion in the hydrocarbon saturated zone can often be the dominant observable effect. We show how the theory of spectral decomposition can be used to detect such effects and validate the technique with synthetic data. We show examples of spectral anomalies associated with gas reservoirs in field data, and demonstrate how these anomalies can be modelled in terms of gas-induced dispersion.
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Seismic Wave Attenuation and Dispersion in Patchy-Saturated Rocks - Numerical Experiments
Authors F. Krzikalla, T. M. Müller, B. Hardy and B. GurevichHydrocarbon bearing reservoir rocks often contain a mixture of several fluids in their porespace (e.g. oil, water and gas). If the pore fluids are immiscible and form pockets on a mesoscopic length scale (exceeding the typical pore size but still small compared to seismic wavelength), the fluid saturation is referred to as patchy saturation. Elastic waves travelling through such a rock will exhibit a characteristic frequency-dependent attenuation and velocity dispersion. These dynamic effects are believed to contribute significantly to the overall characteristics of the seismic wavefield. We numerically simulate wave propagation in a partially saturated rock model containing water with gas inclusions and extract attenuation and dispersion from synthetic seismograms. The results are compared to a theory of frequency-dependent attenuation and velocity dispersion in partially saturated media. Our numerical results are in reasonable agreement with those predicted theoretically. Furthermore, we are able to accurately infer information about the size of the patches from the extracted frequency-dependent attenuation. This underlines the possibility that the size of the fluid patches can be estimated from seismic data.
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Attenuation and Dispersion in Partially Saturated Porous Rock - Random vs Periodic Models
Authors J. Toms, T. M. Müller, B. Gurevich and D. L. JohnsonMesoscopic heterogeneities often occur on a mesoscopic scale, that is scale which is greater than pore-scale but less than wavelength scale. Presence of mesoscopic fluid patches in a porous rock may cause a substantial phase velocity dispersion and attenuation. This is a result of wave induced fluid flow, which arises when a passing wave induces spatial gradients in fluid pressure. Attenuation and dispersion arising from mesoscopic heterogeneities is affected by the spatial distribution of saturating fluids. Here we compare theoretical models for attenuation and dispersion which utilize a 3D random and periodic distribution of fluid heterogeneities. In particular, the periodic model proposed by Johnson (2001) is reinterpreted within the context of the random model. Good agreement between estimates of attenuation and phase velocity is obtained showing that with the right choice of parameters Johnson’s model can describe random as well as periodic distribution of fluid patches.
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Relationships Among Ultrasonic Velocities and Attenuations of Carbonate Reservoir Rocks
Authors A. I. Best, J. Sothcott, T. A. Johansen, P. Avseth and C. McCannUltrasonic P- and S-wave velocity (Vp, Vs) and attenuation (Qp-1, Qs-1), including azimuthal variations in Vs (wave propagation parallel to bedding), were measured on 12 core samples from a Russian carbonate reservoir at 40 MPa effective pressure using the laboratory pulse echo-system. While relationships between velocities, attenuations and porosity and permeability do not reveal any significant features, cross-plots of the ratios Vp/Vs and Qp-1/Qs-1 allow some categorisation of these carbonate rocks in terms of porosity and permeability ranges. In particular, low porosity (< 5%), low permeability (< 0.1 mD) carbonate rocks plot in a separate group to intermediate porosity and permeability carbonate rocks. The results show the value of combined velocity and attenuation datasets to reservoir characterisation.
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Complex Fault Network Generation Using a Fused Hierarchy
Authors K. S. Hoffman, J. W. Neave and E. H. NilsenFault network modeling of complexly faulted structures - those containing hundreds, if not thousands, of faults - can be an extremely difficult and time-consuming process. A variety of methods exist to create the appropriate relationships between the faults, such as truncations, crossing, or offsets due to younger episodes of faulting, but each of these has limitations which make working in the multi-hundred fault areas unwieldy. The fused fault block technique removes the limitations of previous fault modeling techniques and provides a robust, repeatable fault network builder. This new technique expands on the concept of a binary fault tree, but allows compound truncations that are impossible with a strict binary tree approach. The fault network is the framework for subsequent modeling - stratigraphic, facies, or petrophysical - and can be used to generate a grid suitable for reservoir simulation.
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Discrete Element Modelling for Coupling Geomechanics and Seismics in Reservoir Monitoring
Authors H. T. I. Alassi and R. M. HoltDiscrete element method (DEM) is used as a tool for modeling seismic wave propagation as well as stresses and strains associated with reservoir depletion. The advantage of DEM lies in its ability to model strain localization and evolution of fractures, which can be directly linked to time-lapse seismics and micro-seismic events. For this purpose, DEM is tested to verify its ability to model wave propagation. Then a simplified 2D synthetic model for typical North Sea field is constructed. The simulated geomechanical response to reservoir depletion is compared to that obtained by use of the Finite element method. Finally a simple 4D seismic profile is created for the reservoir top.
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Improving Seismic Interpretation Integration in Reservoir Model
Authors C. Brunel and T. ModianoBuilding a reservoir model from seismic data interpretation can be very time consuming and cause incorrect or poor quality results without any specific methodology. Limitation of the seismic scale regarding to the reservoir detail needs, incomplete horizon or fault picking and structural 3D incoherencies can prevent from getting structural surfaces adapted to structural reservoir modeling. In addition, geomodel software limitations must also be taken into account in order to ensure that critical structural heterogeneities described from seismic interpretation will be ranked and adapted for an appropriate modeling . The following paper proposes a methodological workflow that enables the delivery, of representative and appropriate structural surfaces, ready for reservoir modeling. The workflow underlined how artifacts, incoherencies and software limitations can be corrected. All specialists, geophysicist, geologist and reservoir engineer must work together early in the process in order to define structural modeling strategy and choices in agreement with structural, sedimentology and dynamic criteria. In any case such workflow will ensure a better description of essential heterogeneities. In addition, as the reservoir grid remains consistent with seismic data then lithoseismic cubes can be considered to constrain the facies and petrophysical modeling.
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Integrating Geology and Depth Imaging in a Mature Overthrust Area - A Case History
Authors W. Ritchie, M. Popovici and M. FleidnerWe present a depth-domain processing flow-chart based on iterative ray+Born migration/inversion and Very Fast Simulated Annealing (VSFA) inversion. The quantitative migrated image computed by ray+Born inversion is used as an input in the VSFA inversion to build a structural velocity model. Aim of the VSFA inversion is to remove limited-bandwidth effects in the migrated image resulting from the limited bandwidth of the source and the limited aperture coverage. The input data of the VSFA inversion are velocity profiles of the migrated images and the output models are the corresponding structural velocity profiles. The forward problem associated with the VSFA inversion is approximated by a time-domain convolution with the source wavelet, which makes a global exploration of the model space to be possible.
The flow-chart is assessed thanks to the Marmousi model. We computed 9 iterations of single-arrival ray+Born modelling/inversion, which allowed us to derive an accurate true-amplitude migrated image. Second, we applied the VSFA inversion using as input all the vertical profiles of both the true perturbation model (obtained by band-pass filtering the Marmousi model) and the ray+Born migrated image. The output is a structural velocity model, which mimics the true Marmousi model.
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Integrated Prestack Depth Migration/Inversion and Simulated Annealing Optimization for Structural Model Building
Authors S. Operto, A. Ribodeti, W. Agudelo and J. VirieuxWhether refining seismic images to evaluate opportunities in mature areas and exploit the maximum resource, or exploring in frontier areas, determining an accurate velocity model within the turnaround time constraints of reservoir management and exploration timeframes is critical. Speed, robustness, and accuracy are equally important. Seismic imaging has made great strides in recent years with the advent of so-called wave-equation migration imaging methods. Given the correct acoustic propagation velocity for seismic waves in the Earth’s subsurface, these wave-equation methods yield the highest resolution and most accurate images of the earth. However, the process of determining the correct acoustic propagation velocity can be an elusive, time consuming, and costly procedure. We describe an approach both to shorten the process and to make the process less biased and more accurate. The process is shortened by automating the labor-intensive portion of the workflow, and made less biased and more robust and accurate by using much more data than is commonly used in manual picking approaches.
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Automating the Velocity Building Process
Authors D. Bevc, M. M. Fliedner and J. VanderKwaakIn seismic reflection tomography, the velocity model of the subsurface is updated by back-projecting travel-time residuals along ray-paths. The travel-time residuals are picked from the seismic data itself and the methodology used to gather these picks is a fundamental part of any velocity inversion workflow. In particular, the density at which the residuals are represented in the four-dimensional data space (inline, crossline, offset and depth/time) appears to have a significant effect on the precision of the velocity updates that are output from the tomographic inversion. Utilising dense, hyperbolic- (or parabolic-) fitting means that the residuals are finely sampled in the data space but does not necessarily represent their true values with great accuracy. Dense, non-hyperbolic fitting offers a similarly fine sampling but with greater adherence to the true residual value. These two methodologies have been compared and contrasted on a complex synthetic dataset. It can be seen from this comparison that the dense, non-hyperbolic tomography offers greater potential for resolving small-scale velocity heterogeneities in the Earth.
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Resolving Small-Scale Heterogeneities with Dense Non-Hyperbolic Seismic Tomography
Authors J. Brittan and J. YuanThe main goal of 3D CRS (Common Reflection Surface) stack is the improvement of the image with respect to NMO/DMO processing route. However, also the set of parameters defining the CRS stacking surface are a precious source of information on subsurface properties. In 3D, the CRS second order traveltime trajectory is defined by eight parameters, that can be divided into three groups: KN, KNIP (each one is a 2x2 matrix with three independent parameters) and angles (azimuth and emergence). KN are mainly related to the subsurface reflectors geometry, KNIP to velocity; angles describe the normal ray emergence. The NIP-wave tomography method described in Duvenek, 2004 uses KNIP and angles to reconstruct a smooth velocity model in depth, suited for a PSDM and/or for petrophysical studies . In this paper, the first application to a real 3D data-set (from West Africa) of the method is presented. The comparison among common image gathers obtained from a reference velocity model and the ones coming out using NIP-wave tomography shows that a reliable velocity model can be reconstructed in a very cost-effective way directly from CRS processing.
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3D CRS-Based Velocity Model Building - An Accurate and Cost-Effective Approach
Authors D. Della Moretta, T. Kluever and P. MarchettiThe resolution of first arrival traveltimes tomography is limited by the size of the first fresnel zone for each ray, the experimental device and the structure studied itself. This leads to uneven ray coverage and locally varying resolution. Besides, the sensitivity kernel of each ray induce high frequency information even where theoretical resolution is poor.Smoothing constraints are often added to the tomographic system to deal with these problems. We propose an alternative adaptive parametrization based on second generation wavelet transform, as introduced by Wim Sweldens.
We explain the principles and interesting properties of wavelet transform and show how we insert it in the tomography process. Then we present a synthetic example showing difficulties generated by rays in velocity model recontruction. We present the effects of classical gaussian smoothing and of our adaptive parametrization using wavelets.We show that wavelet transform allows a better control of the resolution power depending on the parameter location and that it is more adaptive than classical methods.
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Adaptive Traveltime Tomography Using Wavelets
More LessWe have implemented a finite-difference algorithm for image-wave time-remigration in FORTRAN~90, and studied its theoretical properties in detail. For a numberof synthetic models, numerical experiments have been realized. For these examples, we obtained perfect agreement between the theoretical predictions and numerical results. The examples also prove the computational efficiency of the algorithm. An example using ground-penetrating-radar (GPR) data demonstrates how image-wave remigration can be used to estimate a model of the medium velocity.
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Velocity Determination by Image-Wave Remigration
Authors A. Novais, J. Costa and J. SchleicherFrom large-offset PP seismic reflections one can estimate three traveltime parameters: the zero-offset two-way traveltime, the NMO velocity and a heterogeneity coefficient, using the shifted hyperbola approximation or a fractional approximation. From large-offset PS seismic reflections one can estimate two traveltime parameters: the zero-offset two-way traveltime and the NMO velocity.
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Parameter Estimation for Linear Velocity Functions
By A. M. StovasThe porosity-preserving and permeability-reducing Fe-chlorite coatings present in three sandstone reservoirs coming from the Lower Cretaceous of Pakistan and the Silurian of Tunisia, were characterised in order to understand their evolution in the 90-160 °C temperature range. The two chlorite resulted to be respectively a thuringite and a chamosite and showed a difference in total thickness. This difference is tentatively attributed to thermal evolution, as the Cretaceous samples underwent a more severe heating (present temperature around 160 °C) than the Silurian ones (present temperature around 90 °C). The original clay minerals (precursors) were completely transformed to chlorite in this temperature range. This might imply that prediction of reservoir quality in Fe-chlorite bearing sandstones has to take into account temperature as a key-factor.
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The Impact of Low Frequencies in Optimization and Inverse Scattering Based Wavefield Inversion
Authors J. B. Bednar, C. Shin and C. BednarFull wavefield inversion of seismic data has a less than stering reputation as a process for direct Earth model estimation. One reason for this reputation is the absence of low frequencies in observed seismic data. In this presentation we discuss the theoretial aspects of this issue and provide empirical evidence of its validity. We further discuss this dearth of low frequencies and its relation to inverse scattering based inversion.
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An Integrated Regional Framework for Seismic Depth Imaging in the Deepwater Gulf of Mexico
Authors A. M. Popovici, M. Davidson, M. Leander, K. Mohn, S. Crawley, W. Ritchie and A. PieprzakRegional geologic context is an important factor in the effective assessment of exploration risk in frontier areas such as the ultra-deep waters of the Gulf of Mexico. The use of regional frameworks has been reported during the prospect evaluation and discovery phases of several large deepwater fields in the Gulf of Mexico. This paper describes a new, fully integrated, region-wide seismic framework that addresses this need. The approach incorporates:
- Long offset, long record length, high resolution data acquisition.
- Integrated, area-wide subsurface model-building incorporating salt interpretive experience, seismic velocity estimation, and guidance from 3-D gravity control.
- Pre-stack depth imaging techniques.
A robust, integrated workflow with associated analytical tools has been designed to create a high quality seismic database that provides both regional scale and image detail in depth to support exploration understanding across this complex Gulf of Mexico geologic regime. We discuss the issues related to providing accurate seismic images in depth across this regionally consistent grid.
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Linearized Seismic Waveform Inversion Using Banded Triangular Preconditioners
Authors S. P. Oliveira and M. J. PorsaniIn linearized inversion methods, the computational cost is dominated by the generation of the sensitivity matrix and by the least-squares solution of linear systems. The multiple re-weighted least square (MRLS) is more robust than to the Gauss-Newton method and explores the model space in an extensive and effective way, since many models can be generated from the same sensitivity matrix. In the present paper we investigate the use of preconditioners associated to triangular matrices formed by L-bands in order to decrease the computational cost of the MRLS method. The preconditioners are generated from a partial orthogonalization of the sensitivity matrix. The original system of linear equations is modified in order that the coefficient matrix, in the normal equation, has a band of L co-diagonals with null elements. We apply the new approach to the inversion of 2D seismic waveform inversion. Numeric examples illustrate the performance of the MRLS approach and the decrease in the total computational cost as a function of the number L of bands in the preconditioners.
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Some Practical Aspects of Migration Velocity Analysis - (1) Modified Differential Semblance Objective Function
Authors V. P. Singh, B. Duquet, M. Léger and M. SchoenauerWe have developed a robust objective function that measures the discrepancy to flatness of the coherency panels. This objective function is a modified depth-offset differential semblance function. This function is nicely convex for a quite large range of high and low velocities, and it is stable with respect to the frequency content and to the depth of the events.
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Frequency-Domain Full-Waveform Inversion of OBS Wide-Angle Seismic Data
Authors S. Operto, J. Virieux and J. X. DessaIn deep offshore, crustal-scale seismic imaging is classically performed by traveltime tomography applied to wide-angle seismic data recorded by a network of Ocean Bottom Seismometers (OBS). The resulting velocity model is of limited resolution. If the wide-angle experiment is multifold thanks to densely sampled OBSs, this large-scale velocity model can be used as a starting model for full-waveform inversion. Considering the full wavefield recorded over a broad range of apertures is expected to provide a significant resolution improvement in the velocity models.
We applied 2D frequency-domain full-waveform inversion to wide-angle data recorded by 100 OBSs deployed perpendicular to the trench axis of a subduction zone, offshore Japan. The full-waveform modelling/inversion are entirely implemented in the frequency domain. Thirteen frequencies ranging from 3 to 15 Hz were inverted sequentially with a weighted least-square method, the velocity model obtained for each frequency being used to start the inversion of the next one. The resolution of the velocity models inferred from traveltime and full waveform tomographies were estimated to be ~10 km and 500 m respectively at 10 km depth suggesting a resolution improvement by an over order of magnitude. The relevance of the structures was assessed by ray tracing and full-waveform modelling.
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3D CRS Velocity Depth Model Building and Depth Processing in Overthrust Geology
Authors G. Gierse, J. Pruessmann, B. Heise, R. Coman, E. Laggiard, H. G. Linzer and M. SchachingerThe Molasse Basin in Upper Austria is a very mature exploration area. The main problem is the low fold 3D seismic database with a generally good imaging in the northern Molasse Foreland basin and fair to poor imaging in the imbricated Molasse and alpine Flysch areas to the South. As the exploration targets are moving south the image quality in the southern part becomes more and more important. Failure of prestack time migration methods due to the low fold led to the application of Common Reflection Surface (CRS) stacking methods to improve imaging in the Imbricates and in and under the Flysch as well as derive a more reliable interval velocity model in depth.
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Velocity Model Update Using Migration to Residual Time
By T. KlueverThe Common-Reflection-Surface stack extracts kinematic wavefield attributes from pre-stack data which can be used for a tomographic inversion scheme to determine smooth velocity models for depth migration. These kinematic wavefield attributes give a second-order approximation of the diffraction traveltimes. Thus, the obtained velocity models can only explain the pre-stack data up to second order. In this paper I present a technique to update these models. The method makes use of residual traveltime information picked in CIG gathers and is, therefore, beyond second order. Migration is performed only for selected depth points and directly to residual time. The picking of residual moveout does not need to follow a specific trend. Moreover, there is no pulse stretch phenomenon in migration to residual time. The inversion algorithm is demonstrated on a synthetic data example.
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Tracking of Seismic Events Using a Combined Global and Local Optimization Strategy
More LessSome seismic processing algorithms require a smooth and continuous track of a seismic event in a multi-dimensional domain. Often tracking algorithms are based on trace-to-trace correlations and have a local search character. They show a discontinuous behaviour, are sensitive to cycle-skipping and cannot bridge bad data zones. Therefore, it is proposed to design a multi-dimensional tracking algorithm that is based on a global solution that can avoid local minima, produces smooth and laterally continuous tracks and can be optimally guided by the user. In order to keep the algorithm efficient a hybrid optimization strategy is developed, in which a global optimization method - being a Genetic Algorithm - is combined with a local optimization method - the Simplex algorithm. For a 2D section the track is parameterized by spline functions and for a 3D volume the parameterization is extended with a polynomial description. Results on field data show the effectiveness of the tracking procedure.
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Velocity Model Building by Semblance Maximization of Modulated-Shot Migration Gathers
Authors R. Soubaras and B. GratacosThis paper presents a full wave-equation methodology for velocity model building based on the non-linear inversion of a semblance criterium with respect to the velocity field. A new type of migration, called the modulated-shot migration, is used to obtain the necessary gathers. The semblance of these gathers after spatial averaging is used as the cost function. This methodology is shown to successfully image the Marmousi model and the sub-salt part of the Sigsbee model.
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Image Differencing and Focusing in Wave-Equation Velocity Analysis
Authors U. K. Albertin, P. Sava, J. Etgen and M. MaharramovWe describe a methodology for updating velocity models for depth imaging based on one-way wavefield extrapolation. The method uses an objective function that optimizes focusing in the offset domain, or equivalently, gather flatness in the angle domain. We illustrate various aspects of the method with 2D synthetic and 3D data examples from an area exhibiting strong velocity contrasts due to thin carbonate layering in sediment.
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Nesterov's Method and L-BFGS Minimisation Applied to Acoustic Migration
Authors W. A. Mulder, R. H. Steenweg and C. RoosSeismic full waveform inversion based on the acoustic wave equation attempts to find the acoustic parameters of the subsurface from seismic data. Because the least-squares error between observed and modelled data has local minima, a good initial velocity model is
required. We therefore prefer the name nonlinear migration. We compared three gradient-based minimisation methods on a two-dimensional acoustic test problem: the limited-memory BFGS method with or without bounds on the model parameters, and Nesterov's method.
We found that the L-BFGS method without bound constraints performs best on the test problem, followed by Nesterov's method. However, the cost of decreasing the error by a given factor increases dramatically when the error becomes smaller.
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Subsalt Scans Using Wave Equation based Focusing Analysis and Poststack Modeling
Authors B. Wang, D. Wheaton, F. Audebert and V. DirksWe have developed a new methodology to produce a best focused zero-offset image, even though the migration velocity model has errors. In this methodology, first we generate Common Focusing Error (CFE) images using wave equation based prestack migration and applying zero-time as well as non-zero-time imaging conditions. Then a semi-automatic procedure is applied to pick the focusing error (DT) field, and the picked DT field is used to select the best focused image from the CFE images. To produce an accurately positioned and well focused zero-offset image, we separate the prestack focusing effect from the poststack focusing effect, by performing poststack demigration followed by DT compensated poststack remigration.
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Subsalt Velocity Analysis Using One-Way Wave Equation Based Poststack Modeling
Authors B. Wang, F. Qin, F. Audebert and V. DirksSubsalt velocity analysis using prestack wave equation migration scans through perturbed velocity model is an accurate but expensive approach. To reduce turnaround time and computation cost, we investigated an alternative approach for subsalt velocity analysis. In this approach, we separate the prestack focusing step from the poststack modeling step. We first use the current best velocity model to perform one prestack migration to produce a subsalt image. We then use the subsalt image as a reflectivity model to perform upgoing one-way exploding reflector modeling to produce demigrated zero-offset seismic data. Using this demigrated data as input, we perform a scan of poststack wave equation migrations through perturbed subsalt velocity model. We demonstrate the necessity of demigrating only to the base of salt in order to avoid significant image degradation and we show the feasibility of this methodology using the 2D Sigsbee data set.
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Fat Ray Tomography
More LessWe propose a fat ray tomography algorithm for 3D velocity model building. As compared to conventional rays, the resulting tomography matrix is far less sparse. Thus, while the new matrix may still be ill-conditioned, less model regularization is required to produce a well-posed system. In turn this should lead to a higher resolution tomography update. A subsalt velocity analysis test on the Sigsbee 2a model shows promising results.
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Benefits of Low Frequencies for Subsalt Imaging
Authors S. J. Kapoor, C. Stork and M. S. EganOur analysis shows that frequencies less than 6-7 Hz, if they are recorded in marine surveys, are often very noisy and of little use. Here we analyze the potential benefits of these lower frequencies to justify our extensive acquisition efforts to record them with less noise.
We find that the recording and use of low frequencies for subsalt imaging offers the following advantages:
1) A sharper wavelet,
2) Ability to produce interpretable images where there were none before,
3) Ability to improve velocities so that higher frequencies can be imaged.
4) Low frequencies penetrate into shadow zones
5) Provide stability for wave equation inversion algorithms
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Application of Crosswell Seismic Reflection Imaging for Interpretation of Connectivity
More LessHow to interpret the connectivity of thin reservoirs in A-wellsite in DB oil field has been an important topic for geophysicists, geologists and petroleum engineers to study. In the area around the wellsite, surface seismic data is of limited help because of low S/N ratio and low dominant frequency. A crosswell seismic survey was conducted in A1/A2 wellsite in order to make an attempt for analysis of some problems arising in production. The strategy for better using crosswell seismic data was focused on the application of reflection wave instead of direct wave that was generally used for crosswell velocity tomography. The reflection imaging from data processing indicates its dominant frequency is around 5 times higher than that of surface seismic imaging and can be used for interpretation of thin beds with a thickness of several meters. An interpretation incorporated with logging, production and surface seismic data was done. The conclusions based on the interpretation are that the connectivity between the two wells is controlled by faults and lateral lithological variations.
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Data Driven Surface-Related Multiple Elimination on Walkaway VSP Data
Authors E. Otnes, K. Hokstad, G. Rønholt and S. K. FossSurface-related multiple elimination (SRME) can predict and attenuate all multiples generated by the air-water interface. Earlier SRME schemes for walkaway VSP (WVSP) data involve a model in order to generate primaries recorded at the air-water surface which are not recorded in a WVSP experiment. In this paper, we generate surface data from the WVSP using interferometry. Using this approach we obtain a data driven SRME scheme for WVSP data. The SRME algorithms developed for marine seismic can be used with minor revisions.
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Application of TRA to the Pre- and Post-Stack Imaging of a Salt Dome Flank
Authors R. Lu, X. H. Campman, M. E. Willis, M. N. Toksoz and M. V. de HoopWe present results of applying Time-Reversed Acoustics (TRA) to seismic imaging of a salt dome flank. We create a set of synthetic traces representing a multi-level, walk away VSP for a model composed of a simplified Gulf of Mexico vertical-velocity gradient and an embedded overhanging salt dome. To process these data, we first apply source-receiver reciprocity and the concepts of TRA to the synthetic traces. This creates a set of redatummed traces without having to perform any velocity analysis, moveout corrections or other processing. Each of these redatummed traces is equivalent to the output of a down-hole source and down-hole receiver pair. For quick turn around analyses, we can select from these redatummed traces only the zero-offset, or collocated, down-hole source/receiver pairs. For a more thorough analysis we utilize all the redatummed traces. We then apply conventional post- or pre-stack depth migration, respectively, to these traces to produce the final image of the salt dome flank. The migrated results show that the redatummed turning ray energy reflected from the salt dome flank is handled correctly by this method and produces effective images of the salt.
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Vertical Seismic Profile First-Break Amplitude Study
Authors Y. K. O. Kupelikilic and A. T. El RefaeThere are several published papers on Vertical Seismic Profile (VSP) First-Break Amplitude (FBA) studies that attempt to relate seismic attenuation to depth. In this study, the basic principle is to normalize FBAs and then to transform them into the decibel (dB) domain. Thereafter, dB values are plotted in the depth-decibel domain rather than in the conventional linear depth-amplitude domain. The depth-decibel curve shows very good correlation with sonic logs. The similarity between a sonic log and a dB curve depends on the type of geophone used for VSP recording. A correlation can be achieved only when the accelerometer is sensitive to particle velocity acceleration. Acceleration and amplitude are high in unconsolidated lithology. Conversely, acceleration and amplitude are low in consolidated layers. Thus, the dB plot mimics a sonic log. Using this method, individual and cumulative attenuation gradients for varying lithological units can be defined and formation boundaries identified. Formation densities and interval velocities can also be computed using the FBA technique. It would appear that this methodology is not widely employed because most amplitude studies do not take into consideration the type of geophone used for VSP recording.
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2D CO CRS Imaging for Multicomponent Data Recorded by the VSP Geometry
By M. Von StehtIn this paper a new sophisticated design for a processing sequence to deal with prestack data recorded with the vertical seismic profile (VSP) acquisition geometry will be introduced.
The approach is based on assumptions and approximations valid for the Common-Reflection-Surface (CRS) stack theory, in particular on the extension to common-offset (CO) imaging firstly derived by Zhang et al. (2001). CRS-stack-based imaging bears the significant advantages of an increased signal-to-noise ratio, a high degree of automation throughout the processing and convenient quality control by the user. Additionally to the high-quality stacked sections the processing by the means of CRS technology yields traveltime parameters related to so-called kinematic wavefield attributes. These CRS attributes can serve as an input to imaging steps beyond stacking as, i.e., velocity modeling and migration. Recently, there have been great efforts by Boelsen (2005) to even extract PP- and PS-conversions immanent in multicomponent data, in order to separate both types of wavefield. The presented approach can also be applied to these issues.
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Walkaway VSP TI Anisotropy Estimation and Migration
Authors G. Rønholt, S. K. Foss and K. HogstadStatoil and Hydro, with READ Well Services as contractor, have jointly acquired 3 walkaway VSP lines from offshore Norway. With a total of 24 geophones settings and around 200 shot points we were able to image below, and estimate anisotropy parameters from, the dipping beds in the overburden. Utilizing migration algorithms that take into account a transversely isotropic medium with a tilted symmetry axis, had a significant impact on the interpretation of the target structures.
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Prestack Phase-Screen Migration for VSP Data in Gas Hydrate Zone
Authors J. Byun, Y. G. Choi and S. ShinPrestack phase-screen migration for surface seismic data was adjusted to apply to VSP data. In addition, it was improved to be able to compensate the absorption, which may occur significantly in gas hydrate formation, if Q-factors of the layers are provided. To verify the algorithm, we applied it to synthetic seismic data obtained from the velocity model simulating gas-hydrate bearing zone, and compared the results of migration without and with compensating for absorption. The compensation for absorption during migration significantly improved the resolution of the migrate image. With spatial spectral analysis, we confirmed that the migration with compensation for absorption recovered the amplitudes of higher frequencies, which are affected more by absorption. Results of the test with real VSP data from gas hydrate zone showed that the base of gas hydrate zone, which is weakly depicted by conventional migration, was clearly identified by the phase-screen migration with compensating for absorption.
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Outlook for Irregular System Implementation in Seismic Survey
More LessThe present-day most efficient acquisition method in geophysics is 3D seismic survey. However, it usually requires application of regular dense network of seismic source and receiver points. If the area under survey comprises living facilities, forests, agricultural lands, it results in significant economic and environmental losses. In Western Siberia application of 3D survey is complicated by difficult natural and climatic conditions (non-frozen swamps, lakes, rivers, deep snow, heavily forested areas). In this case the problem cannot be completely solved even with application of modern equipment. In respect to HSE requirements irregular systems are more acceptable. They enable to lay the lines where it is possible notwithstanding the template designed.
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Enhanced Imaging with Seafloor Seismic Compared to Towed Streamer
Authors C. Walker, D. Ridyard, J. Musser and P. StewartElevated energy costs have caused reserves in increasingly challenging environments to be deemed commercial. This has led to rising expectations for seismic image quality: better illumination, better resolution, better well ties, more reliable amplitudes etc. In the marine environment, towed streamer technology remains the most common acquisition method, and techniques such as "single sensor recording"(1), "over-under"(2) and "wide azimuth"(3) have delivered impressive results. These techniques have added cost and complexity to traditionally "simple" towed streamer operations. Recent developments in seafloor seismic have delivered significant improvements in image quality. Furthermore, the cost differential between towed streamer and seafloor seismic has narrowed considerably.
In this paper, we will show the results of an investigation into the relative value of four inherent differences between towed streamer and seafloor seismic. The differences considered here are (1) sensor type and performance (2) towed array vs. static point receiver (3) PZ summation and (4) geometry.
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A Practical Workflow of PP/PS Event Registration in Multicomponent Data Interpretation
Authors K. Zou, J. Dai, H. Xu, A. Hannan and A. KoesoemadinataPP/PS event registration (also called "event matching" or "event pairing") is an essential element in multicomponent data analysis. It aligns the related events in P-wave and C-wave (converted wave, also called "PS-wave") sections. Among other things, this enables the Vp/Vs ratio (also called "Gamma") to be calculated. It can be used for updating the migration velocity field, and under favorable conditions, it can be applied directly in lithology/fluid property estimation. In this paper, we discuss the problems of PP/PS event registration, and describe a new target-based, rock physics-guided, iterative workflow for implementing it. A Gulf of Mexico example using the methodology is presented.
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Wellbore Measurement Simulations for the Seismo-Electric Effect
Authors A. Ranada Shaw, E. C. Slob and C. P. A. WapenaarIn this paper we derive global reflection and transmission coefficients from the one-way operators to in a similar way as Kennett does, later we use them to simulate a vertical seismo-electric profile (VSEP) and an electro-kinetic well bore to well bore survey.
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Effects of Shale Content and Porosity on Elastic Parameters of Reservoir Rocks
By M. BalaThe paper presents effects of shale content and porosity on elastic parameters of rocks. The analysis was based on theoretical relationships for porous media, known as the Biot-Gassmann’s and Kuster and Toksöz’s models, and on Raymer-Hunt-Gardner formulas. Well-logging data and results of the quantitative interpretation of well logs were also analysed. Data were acquired by the Geofizyka Krakow SA under the project no 6T12 2003 C/06006 realized in 2003 (Pietsch et al. 2003). The relationships between P-wave and S-wave velocities and reservoir parameters may contribute to solving some problems associated with seismic interpretation of wave forms in autochthonous Miocene deposits.
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GASLOG - Numerical Modelling of Wireline Logs from Borehole Gas and Drilling Data
Authors R. Arkalgud, A. Hurst, S. Cuddy and G. TischlerMeasurements of gas contained in drilling mud are derived routinely during the drilling of boreholes. Gas data are obtained to identify changes in saturation, particularly when free hydrocarbon gases may be present, and to support safety control measures. Hence, the gas data are a subsurface signature and may be used for predicting lithology, permeability and fluid composition. Because gas data are acquired continuously during drilling, they can potentially be used to derive real-time estimates of subsurface characteristics. Our research investigates this potential and the potential for incorporating real-time drilling data, in the prediction of formation characteristics while drilling.
The conventional method of wireline logging in formation evaluation is an expensive, time consuming process in oil and gas exploration. Current research proposes an alternate inexpensive way of formation evaluation, which can be used alongside/instead of conventional methods. The current research focuses on predicting the fundamental elements of formation evaluation such as porosity and permeability using the available gas data and mass spectrometer data. The research has developed synthetic wireline logs in real-time, using advanced mathematical techniques, which can be interpreted analogous to conventional logs for formation evaluation. The validation of current methodology is demonstrated using North Sea test case wells.
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Measured Radiogenic Heat Production Based on Borehole Logs in Sediments from Husów, Carpathian Foredeep
By J. KrawiecThis paper contains description of calculation of a radiogenic heat production rate in the sedimentary rocks: the sandstones and the mudstones. The radiogenic heat is one of the elements of the surface heat flow. This heat represents the important component in modelling of the oil basins, especially in thermal calculating connected with the source rocks. It is claimed that the production of the radiogenic heat causes increasing of the temperature. That process leads to quick maturation of a kerogen, so the oils production is faster. Radiogenic heat production rate maight be computed on the samples or in the boreholes with using SGR and RHOB logs, according to the suitable equations.
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Analysis of Acoustic Full Waveforms Based on Matching Pursuit Parameterization
More LessMatching Pursuit (MP) algorithm is high-resolution signal analysis in time-frequency space. The method relies on an adaptive approximation of a signal by means of Gabor atoms chosen from a very large and redundant dictionary of functions. This paper presents application of MP algorithm to analyses of Acoustic Full Waveforms (AFW). Methodology of extraction of fluid wave and decomposition of AFW into separate P, S and Stoneley waves is shown. Gabor atoms that fit best to the signal structure were analyzed in order to select the ones that represent the acoustic waves. On the basis of the parameters of these atoms (position, width in time and frequency) time-frequency properties of waves are presented.
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Lithologic Determination of Devonian Dolomitic Carbonate-Siliciclastic Rocks from Estonia by Physical Parameters
Authors A. Shogenova, A. Kleesment and K. ShogenovDevonian fully dolomitized sedimentary succession occurring in the southern Estonia is represented by carbonate, siliciclastic and mixed-carbonate siliciclastic rock types. These three lithologic rock types were subdivided using 50 and 70% borders of insoluble residue (IR). Carbonate rock type includes dolostones (IR<25%) and dolomitic marlstones (IR>25% and IR<50%). Mixed rock type is represented by primary dolomitic marlstones and siltstones and diagenetic sandstones and sandy siltstones. Siliciclstic rock type is represented mainly by sandstones, siltstones and silty claystones. Porosity-density crossplots together with gamma-ray logs permit determination of clean lithologies, while discrimination of mixed rocks usually causes difficulties. The aim of the work is lithologic determination of mixed rocks. Density, porosity and magnetic susceptibility measured on 150 core samples from 8 boreholes accompanied with gamma-ray logs were analysed together with thin-sections and geochemical data. As a result mixed rocks were distinguished from carbonate and siliciclastic. Primary mixed rocks were discriminated from diagenetic. Density, porosity, magnetic susceptibility and gamma-ray spectrometry logging tools are recommended for lithologic determination of the rocks.
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On the Use of Global Optimization Methods in the Interval Inversion of Borehole Geophysical Data
Authors M. Dobroka and N. P. SzaboIn the paper two global optimization methods called FGA (Float-Encoded Genetic Algorithm) and Very Fast Simulated Annealing (VFSA) are presented in solving the nonlinear geophysical well logging inverse problem. Applying global optimization for the interpretation of well logging data has got a strong advantage against conventional linear inversion methods. Furthermore, they can even improve the performance of modern inversion methods like the so-called interval inversion method developed by us, which estimates petrophysical parameters more accurately and more reliable than point by point inversion techniques owing to an effective joint inversion procedure. Moreover, it gives an estimate for the layer-thicknesses that can not be treated as unknowns in the conventional point by point well logging inversion. In the paper, we make a comparison between global interval inversion techniques using synthetic and measured well logging data.
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Automatic Rock Classification Based on Texture Analysis in Electrical Borehole Wall Images
Authors M. Linek, M. Jungmann, R. Pechnig, T. Berlage and C. ClauserElectrical borehole wall images are widely used for borehole inspection and reservoir characterization. So far, this data is mostly qualitatively used to investigate structure and lithology mapping. We present a method for image characterization which is based on the application of texture analysis in order to transform image data into quantitative log curves. We derive so-called Haralick texture features from borehole wall images. Based on a supervised classification technique, we train texture features within assigned rock classes, determine their classifiers, and apply classification function on the entire data set. This enable automatic rock determination based on quantitative image data.
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Dispersion Analysis - An Essential Consideration for All Sonic Tools
By J. MarketMany studies have been published showing dispersion characteristics for borehole sonic data, particularly for monopole, dipole, and quadrupole modes for both wireline and LWD tools. However, with the exception of LWD dipole tools, the dispersion corrections are often considered "negligible" and not, in practice, applied systematically to the data. Instead, the measured flexural velocity is computed with time domain methods and often presented as the shear velocity. Whereas it is true that the dispersion correction for wireline flexural and LWD quadrupole modes are often minimal, (less than 2%) there are many cases where the dispersion amount is significant and must be accounted for to provide an accurate shear velocity.
Modelling of dispersion curves and excitation functions are presented for wireline and LWD borehole modes. Data examples showing the application of dispersion curves in a variety of formations environments are discussed. In addition, the advantages of frequency domain processing for analysing dispersive modes are illustrated.
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New approach towards processing of general gamma-ray activity and adsorptive diffusive activity data and their implementation in interpretation of well log methods
Authors V. V. Pestov and S. M. ShevchenkoNovel approach is based on the method of reduction of gamma-ray activity and adsorptive diffusive activity core data towards relative parameters of well log methods and plotting relationships on their basis.
On the basis of core and geophysical data, relationships of dual gamma-ray index of gamma log with median diameter of grain fraction; gamma-ray activity; sum of close-grained fractions; sand fraction content; dominated fraction content; median pore diameter; correlation of Asp with content of clay minerals were plotted.
According to combination of comprehensive geological and physical information, definite number of reservoir types present in section is singled out (sandstones - medium-grained, fine-grained, close-grained, and siltstones). Reservoir type is defined by variation range of dominated filtrating fraction diameter and its proportion in the rock volume.
High concentration of siltstone fraction leads to significant distortion of parameters defined via well log.
Suggested approach allows evaluating content of sand and siltstone fractions, their proportion, average diameter of dominated fraction, and its volume. Together with clay fraction evaluation (volume of clay minerals) it is possible to perform a "simplified" lithologic analysis using well log, i.e. divide rocks into groups and perform a more sound evaluation of permeability and other volumetric data.
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New Ways in Understanding Borehole Breakouts and Wellbore Stability by Fracture Mechanics Based Numerical Modelling
Authors I. Moeck and T. BackersThe knowledge of stress magnitudes and orientation of principle stress axes are essential for calculating mechanical models. Our case study aims to understand the petromechanical conditions of observed borehole breakouts in 4100 m depth by developing a numerical fracture model applying the newly developed software FRACOD2D. We present the determination of the in situ stress state in 4100 m depth by using results from hydraulic stimulation, the Anderson faulting theory and Mohr-Coulomb criterion. Additionally, laboratory tests on well cores encompass the determination of fracture toughness Mode I und Mode II, the latter is based on an innovative experimental approach. The combined use of predicted stress state and geomechanical parameters including the tested parameters from core material, enabled us to calculate the initiation and propagation of analysed borehole breakouts in a testing well. The results represent a key element for future models, that will be processed in order to assess risk zones in a nearby planned well.
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Lithologic Determination of Ordovician Carbonate Rocks from South Estonian Boreholes by Physical Parameters
Authors K. Shogenov and A. ShogenovaOrdovician carbonate succession from southern Estonia is mainly represented by primary limestones and calcitic marlstones and by dolomitized carbonate rocks (dolostones and dolomitic marlstones). For case study 110 core samples from two boreholes were analysed by geochemical and petrophysical methods. Rocks were initially subdivided into groups using data of carbonate chemical analysis. It is shown that primary and dolomitized rocks could be lithologically discriminated using porosity-density cross-plot. Gamma-ray log permits to determine thin reference layers of metabentonites and claystones, and to determine clayey rocks (marlstones) from more pure carbonates. Ordovician dolostones are characterized by increase in magnetic susceptibility the most significant in the late diagenetic dolostones. Their magnetic susceptibility may be higher than in marlstones. Magnetic susceptibility is offered to apply for dolostone genesis determination together with density, porosity and gamma-ray logs, and for express determination of dolostones in drill cores and outcrops.
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Prediction of Petrophysical Characteristics from Borehole Gas and Drilling Data
Authors G. Tischler, R. Arkalgud and A. HurstSeveral sources of data are available at the well site during drilling operations that have potential for prediction of petrophysical characteristics. Data of particular interest include gas analyses (borehole gas), which conventionally comprises low molecular-weight hydrocarbons (C1 to C3 ± C4 and C5) but more recently may include a wide range of non-organic gases from well-site mass spectrometry, and rate of (bit) penetration (ROP) and the associated drilling exponent. (DXC, defined as R/N = A (W/B)2 where R = rate of penetration, N = revolutions per minute, A = matrix strength, W = weight on bit and B = bit diameter). Because borehole gas, ROP and DXC are available in real time, and at no extra operational expense, their amenability to prediction of petrophysical characteristics is important to establish as by using them it may be possible to produce synthetic wireline logs and/or direct estimates of porosity, saturation and permeability during drilling at the well site.
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True-Amplitude Wide-Angle One-Way Operators and High-Gradient Zones
Authors C. J. Thomson and C. H. ChapmanAccurate one-way wave equations should simulate waveforms with a completeness that is intermediate between ray-based methods and full wave equation finite differences, for an intermediate computational effort. There is a hierarchy of one-way options, including exact coupled forward and backward operators which are mathematically equivalent to the omni-directional parent wave equation. We show numerical results obtained by using an approximate one-way equation with high-accuracy full wave equation finite-difference results, for both reflected and transmitted waves. The effects of lateral gradients in particular are discussed.
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