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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
201 - 300 of 556 results
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Fast Trace Interpolation and Statics Application by Least Squares Inversion
More LessA new prescription for rapid application of wave-equation statics by least-squares to seismic data is provided. As part of this process, trace regularization by interpolation occurs naturally. Real and synthetic examples are used to demonstrate the interpolation/statics aspects of the exact method and it's more useful approximation. It is expected that the approximation presented here will have significant application in conjugate gradient inversions where improved estimates of the preconditioning operator are needed to speed convergence.
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On the Estimation of Local Slopes
Authors J. Schleicher, J. C. Costa, L. T. Santos, A. Novais and M. TygelCurrent algorithms for the estimation of Common Reflection Surface (CRS) parameters are based on one- or multi-parameter coherency analysis (semblance) schemes applied to the data. Such procedures, besides computationally expensive, leads to significant uncertainties on the searched parameters. Conventional semblance methods can be avoided for a number imaging tasks if local slopes can be directly extracted from prestack data, for example, by filtering schemes. Although the idea is not at all new, recent literature shows its revival for various purposes, such as velocity analysis, tau-p imaging, migration to zero offset and time migration. Here, we discuss several different ways of extracting the desired slope information from the data. We propose a simple, straightforward correction to linear plane-wave destructors. The correction is based on the observation that additionally to the local slope, also its inverse can be extracted from the data in a fully analogous way. Combining the information of both extractions yields a simple but powerful correction to the local slopes. In our numerical examples, the naive application of simple linear plane-wave destructors with our simple, straightforward correction produced results of high quality, even in an example with a rather high noise level and interfering events.
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S-zero Stack, a Robust Way to Extract Density Information from Converted Waves
By K. ZouAnalyzing the Aki-Richards (1980) equation for converted waves (PS), I find that it is possible to decouple the effect of density contrast from that of shear velocity contrast. The two terms are mixed when the P-wave incident angle is less than 30°, but they start to separate at a middle angle range (approximately 40°). The term related to shear-wave velocity reaches zero at an incident angle around 60°. However, the other term which is related to the density contrast does not reverse polarity until 90°. Furthermore, this density term reaches almost the maximum magnitude around 60°. Based on those characteristics, a new method called "S-Zero Stack" has been designed to capture the density contrast reliably at the interface. In this paper, the theoretical decoupling is introduced and the method of S-Zero Stack is developed. A synthetic example is presented. Finally, the new method has been applied in a real 4C/3D PS data and the result is calibrated with density log. S-Zero Stack captures reliably the subsurface density anomalies without going to inversion. It is simple but robust, even with noise data. Combined with the traditional P-wave AVO technique, S-Zero Stack of PS waves may help discriminate commercial gas from fizz.
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Characterization of Subsurface Parameters with Combined Fluid-pressure and Particle-velocity Measurements
Authors K. N. van Dalen, G. G. Drijkoningen and D. M. J. SmeuldersThe idea of combined 3C-geophones and hydrophones (4C-receivers) often used in marine Ocean-Bottom Cables, can also be used for seismic surveying on land in saturated soils. The fluid-pressure data can be used to interpret the particle-velocity data. It can, however, also be used to extract more information about the subsurface, e.g., information about 2-phase behaviour. To gain insight in the behaviour of various wave modes in the different components, the three-dimensional point-force response of a homogeneous isotropic half-space has been calculated. Because the subsurface is considered as a poroelastic medium in 4C-experiments, Biot’s theory for poroelastic media needs to be used; this explicitly accounts for 2-phase behaviour. The responses show strong dependence on porosity and permeability. It is the combination of particle velocity and fluid pressure that reveals poroelastic characteristics of the subsurface, especially for the Rayleigh wave. Its dispersive behaviour, met in 4C-experiments, is not entirely understood yet. Furthermore, it is shown that it makes sense to use Biot’s theory to model seismic responses when dealing with 4C-measurements. The slow compressional mode may even contribute significantly to the Rayleigh wave.
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Adaptive Groundroll Filtering
Authors D. Le Meur, N. Benjamin, R. Cole and M. Al HarthyThe attenuation of Surface Waves while maintaining a "friendly" preservation of the Body Waves is a difficult goal to achieve in land data processing. However, Groundroll (GR) characteristics can be extracted from the input data themselves to feed an adaptive filtering in order to remove the Surface and Guided Waves. This approach is based on a cascaded Elastic Modeling of the signal and noise in the FX domain that uses the GR characteristics of several frequency bands. The part of the model corresponding to the noise is then subtracted from the data using a least squares approach. This method has several advantages over techniques such as FK filtering because it better preserves the signal and works even when the noise is aliased, dispersive and has irregular spatial sampling.
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Geophone Noise Attenuation and Wave-field Separation Using a Multi-dimensional Decomposition Technique
By K. L. CraftPerhaps the largest single obstacle to overcome in wave-field separation of P-Z data is the presence of noise energy (non P-wave specific) on the geophone that is not present on the hydrophone. In general, the noise manifests as coherent energy on common receiver gathers with moveout characteristics of converted wave velocity, and with relatively random phase and amplitude on shot gathers. Here we introduce a new method for processing dual sensor data, particularly in deep water, that includes intrinsic removal of V(z) noise as well as enhancing the wave-field separation. This method is based on a decomposition of data simultaneously into dip and frequency while retaining temporal locality. This energy has often been associated with acquisition integrity related issues such as coupling and phone orientation. Only recently, alternative explanations have been developed that describe a mechanism that will produce exactly the same type of energy (Paffenholz et al., 2006a, 2006b). This description explains all observed characteristics of the spurious energy on the geophones. This noise, hereafter referred to as "V(z) noise", often is so severe that without adequate tools for removal, the advantages of dual-component ocean bottom acquisition are practically negated.
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PZ Calibration in Shallow Waters - The Britannia OBS Example
More LessIn marine multi-component data processing, the vertical component of particle velocity Z is calibrated against the pressure component P to correct for coupling discrepancies and sensitivity differences between hydrophone and geophone. The PZ calibration processing step is crucial to the success of wavefield decomposition into up- and down-going waves, as well as into compressional and shear waves. In this paper, we present a new application of the conventional cross-ghosting technique. Our PZ calibration approach is implemented in the x-t domain and considers far offset reflected or refracted energy and their water layer reverberations. These events are commonly and easily identified in shallow water data as they arrive earlier than the direct arrival. Compared to other published techniques, our method is less sensitive to near-offset noise, source bubbles and irregular or sparse offset sampling. We successfully apply the proposed approach to an OBS data set acquired at 150 m water depth over the Britannia field in the North Sea.
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Time-lapse Measurements of Shallow Horizontal Wave Velocity over a Compacting Field
Authors P. B. Wills, P. J. Hatchell and S. J. BournePermanent ocean bottom cable (OBC) seismic systems represent a major emerging technology for the acquisition of time-lapse data over producing oil and gas fields. With permanent sensors on the seafloor, we can measure waves that are either not present or typically ignored on conventional (streamer) seismic surveys. Scholte waves and critically refracted compressional (CRC) waves, which propagate horizontally through the shallow subsurface, are two possibilities. These waves carry information on the shear and compressional properties of the shallow sediments and are therefore expected to respond to surface geomechanical changes that are driven by deep hydrocarbon production. Here, using existing active source data from the OBC array in the Valhall field located offshore Norway, we investigate time-lapse monitoring with shallow seismic waves. The large number of 4C surface receiver stations and a dense shot grid make the Valhall data an excellent laboratory for the investigation of time-lapse effects of Scholte and CRC waves. We find that these waves are observable field-wide and that time lapse changes in their velocities are measurable with high accuracy. Applications include reservoir-monitoring, studies of dependence of in situ rock velocity on strain, understanding of OBC noises and 4D shallow hazards.
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Shallow and Deep Time-lapse Effects on Valhall LoFS Converted Wave Data
Authors P. M. Zwartjes, P. Wills, J. W. De Maag and P. HatchellThe Valhall LoFS OBC system provides an excellent opportunity for 4D studies, both for P-wave and converted wave data. We have observed a very good correlation between the 4D effects in Scholte wave velocity, converted wave arrival times and shear wave splitting in the shallow overburden. These effects are indicative of shallow overburden compaction caused by production induced reservoir compaction. The strong shear velocity increase in the shallow overburden causes 4D converted wave time shifts that are up to 15ms large, as well as large changes in Scholte wave velocity, measured from the vertical geophone. Additionally we observe a reduction in shear wave splitting due to the shallow compaction. These strong shallow 4D effects obscure the top reservoir converted wave 4D effects, but by application of a receiver consistent 4D correction we are able to produce top reservoir converted wave 4D time-shifts that are consistent with the P-wave 4D signal.
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Two Pragmatic Approaches for Attenuation of Land Multiples
Authors P. G. Kelamis, Y. Luo, W. Zhu and K. O. Al-RufaiiThis paper presents two novel methodologies related to multiple elimination on land seismic data. The first one is based on a model-based algorithm that takes into account the unique characteristics of land seismic datasets. It operates entirely in the time-offset domain and thus avoids the artifacts associated with conventional multi-channel transforms. The second methodology is a full data-driven, automated approach firmly rooted to the principles of inverse data technology and targets internal multiples. Internal multiples are estimated by a simple muting process in the inverse data space. The performance of both methodologies is shown with a series of land datasets from the Arabian Peninsula.
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Surface Related Multiple Suppression in Dual-sensor Towed Streamer Data
Authors W. Sollner, E. Brox, M. Widmaier and S. VaageConventional surface related multiple prediction for towed streamer configuration is flawed by the sea surface level variation and sea surface reflection coefficient fluctuation. Knowledge of the sea surface and reflection coefficient allows approximate correction of the prediction errors. Based on a novel dual-sensor towed streamer acquisition we propose a multiple prediction approach from the down-going vertical velocity field and up-going pressure field. This approach handles the obliquity factor and sea surface variations implicitly and may reduce bad-weather-related acquisition downtime.
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General Surface Multiple Prediction (GSMP) - A Flexible 3D SRME Algorithm
Authors W. H. Dragoset and I. MooreIdeal 3D SRME has geometric requirements for its input data that are not met by marine streamer surveys. Consequently, one must either precondition the field data to meet the requirements, or modify the algorithm to match the data. In this paper, we present a new method in the latter category. Our algorithm has many desirable features: it is applicable for any survey geometry and geology, accounts properly for trace azimuth, does not require preconditioning of the field dataset, and predicts diffracted multiples more accurately than other approximations to the ideal 3D SRME algorithm. Analysis of multiples predicted by the algorithm shows that they closely match actual multiples.
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Multiple Attenuation on Deepwater Atlantis OBS Nodes Data - Beyond Wavefield Separation
By G. XiaMirror migration on the downgoing waves produced superior images than the upgoing waves, especially on the shallow section for OBS data. However, similar to upgoing waves, the downgoing waves contain source-generated multiples. These source-side multiples often interfere with the reservoir signals, making interpretation difficult. I extended the wavefield extrapolation demultiple technique designed for upgoing waves to handle the multiples on the downgoing waves. The new workflow was successfully applied to the Atlantis OBS nodes data.
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3D Multiple Attenuation and Depth Imaging of Ocean Bottom Seismic Data
Authors J. Mispel, B. Arntsen, A. Kritski, L. Amundsen, M. Thompson, O. Sandvin and L. JahrenIn 3D Ocean Bottom seismic surveys (3D-OBS) both pressure and vertical particle velocity is recorded. This presents the opportunity to decompose the recorded wavefields in up- and down-going components and apply to 3D common receivers gathers a designature/demultiple algorithm to attenuate free surface multiples. This single processing step replaces several steps in conventional processing usually encompassing τ-p predictive deconvolution and Radon demultiple. Using a 3D-OBS data set from the Gullfaks Sør field in the North Sea, the new 3D designature/demultiple approach, together with 3D common receiver depth migration, is shown to result in seismic images of better quality than by the traditional processing sequence.
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A 2.5D Method for Removal of Free-surface-related Seismic Events
More LessWe present a method for removal of free-surface related events in marine seismic data, specialized for cases where subsurface properties vary slowly in one direction (strike) and the survey is well sampled in the orthogonal (dip) direction. Under these assumptions of 2.5D subsurface and suitable acquisition geometry, it is possible to reduce the data sampling requirements and computational costs with respect to 3D multiple attenuation methods, and consequently, apply (2.5D) methods that are potentially more accurate than the standard (iterative) ones. We derive 2.5D versions of non-iterative multiple attenuation methods and illustrate their performance and data sampling requirements in an application to complex synthetic data, representative of a North Sea field.
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Deepwater Seismic Data Collection and Processing - A Study from the 2006 Turkish Black Sea Kozlu 3D Program
Authors J. E. Palmer and J. BaconModern deepwater marine 3D surveys require advanced infield systems in both data collection and data processing, if imaging and turnaround targets are to be met. In this paper we present the case history of a marine 3D reconnaissance survey, which presented challenges for both data collection and data processing. The challenges stemmed from the deep and complex waterbottom which caused issues with streamer geometry and also major multiple contamination within the seismic data.
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Adaptive Subtraction Using Complex Curvelet Transforms
Authors R. Neelamani, A. Baumstein and W. S. RossThis paper proposes a complex curvelet transform-based algorithm to adaptively subtract from seismic data those noise for which an approximate template is available. The complex curvelet transform decomposes a geophysical dataset in terms of small reflector pieces, with each piece having a different frequency, location, and direction. The properties of complex curvelet transforms enable us to precisely change the amplitude and shift the location of each seismic reflector piece in a template by controlling the amplitude and phase of its complex curvelet coefficient. Based on these insights, we can adapt a predicted noise template to the actual noise on an event-by-event basis by using the phase and amplitude of the data's and template's complex curvelet coefficients. Results illustrate that the proposed complex curvelet-based approach improves upon the conventional least squares subtraction approach and a more recent real curvelet-based approach.
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Velocity Dispersion and the Petrophysical Properties of Porous Media
Authors L. F. Sun and B. MilereitSeismic attenuation and velocity dispersion depend on the petrophysical properties of porous media. Using broadband uncorrelated vibrator VSP data, we measure velocity dispersion in the exploration seismic frequency band with satisfactory accuracy. Then, velocity dispersion data are fitted to petrophysical models. This approach provides new insights into the linkage between velocity dispersion and physical rock properties of porous or fractured media.
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Preconditioning for Linearised Inversion of Attenuation and Velocity Perturbations
Authors B. Hak and W. A. MulderWe consider the case of the linearised constant-density viscoacoustic wave equation, which involves simultaneous inversion both for velocity and attenuation contrasts or perturbations. The medium parameter can be characterised by a complex-valued velocity that includes wave speed as well as attenuation. The least-squares error measures the squared norm of the difference between modelled and observed data. Its gradient with respect to the medium parameter represents a migration image. We can use a gradient-based minimisation algorithm to invert for the model parameter. Convergence rates will improve by using a suitable preconditioner, which usually is some approximation of the Hessian. For the linearised, constant-density viscoacoustic wave-equation we derive an exact Hessian that differs from the more conventional Hessian by including the complex-valued part. For the inverse problem, we consider a single point-scatterer and investigate four different approximations of the Hessian as preconditioner. The method does not appear to improve the convergence rate. On the contrary, convergence rates are worse and the norm of the error is larger. We conclude that we have not been able to exploit the imaginary part of the Hessian with the preconditioners considered here, although there may be other options.
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Deriving 3D Q Models from Surface Seismic Data Using Attenuated Traveltime Tomography
Authors M. Cavalca and R. P. FletcherEstimation of the intrinsic attenuation of the Earth, conveniently described by the quality factor Q, is of great importance in seismic processing and rock physics interpretation. However, deriving a Q Earth model from seismic data is quite challenging, as anelastic and elastic attenuation effects on propagated waves can be difficult to dissociate. In this paper, we propose to derive 3D Q Earth models from prestack surface data by applying an inversion of the attenuated traveltimes of selected seismic events (first arrivals, reflections). Attenuated traveltime is defined as the traveltime of the propagated signal weighted by its effective-Q factor. This factor is obtained from a preliminary effective-Q mapping based on a suitable amplitude analysis and assumed to provide "intrinsic enough" effective-Q estimates. An estimate of the velocity is only required for a kinematic ray tracing during the inversion. This technique can be applied, for instance, to first-arrival events. A first-arrival tomography is implemented to derive both a near-surface velocity model (traveltime inversion) and a near-surface Q model (attenuated traveltime inversion). This approach is illustrated here on a synthetic dataset. Reflection attenuated traveltime tomography can be considered in a similar manner.
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3-D Tomographic Amplitude Inversion for Compensating Transmission Losses in the Overburden
More LessA tomographic inversion approach using prestack depth migrated common image gathers is utilized to compensate reflection data for amplitude loss caused by transmission anomalies in the overburden. The approach has the advantage of estimating transmission losses from anywhere within the overburden using the actual seismic raypaths. Examples show that the method can mitigate amplitude attenuation caused by transmission anomalies and should be considered as one of the processes for amplitude preserving processing that is important for AVO analysis when transmission anomalies are present.
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Modelling Near-field Effects in VSP-based Q-estimation
Authors A. B. Haase and R. R. StewartThe complete spherical wavefield emanating from a P-wave point source surrounded by a homogeneous isotropic medium is computed with the aid of Sommerfeld/Weyl integrals. In a resulting synthetic VSP, we observe a near-field, a far-field and a 90 degree phase rotation between the two. Depth dependence of magnitude spectra in these two depth regions is distinctly different. Log magnitude spectra show a linear dependence on frequency in the far-field but not in depth regions where the near-field becomes significant. Near-field effects are one possible explanation for large positive and even negative Q-factors in the shallow section that may be estimated from VSP data when applying the spectral ratio method.
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The Strategies for Attenuation Inversion with Waveform Tomography
Authors Y. Rao and Y. H. WangIn this paper, we compare two attenuation inversion strategies used in waveform tomography. The first strategy is a two-steps scheme: inverting velocity model and fixing attenuation model (1/Q) first, then with fixed velocity model to invert the attenuation model. Another strategy is to invert for these two models simultaneously. Although there are no big differences in synthetic data test, because the trade-off between update of the velocity and the attenuation factor is very sensitive to noise, there are big differences in real data tests. The first strategy can successfully avoid cross contamination of the velocity and attenuation fields. We also use curved layer perturbation tests to verify the high attenuation layer in the final attenuation model.
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Frequency Attenuation Tomography - A Tool for Imaging Carbonates Reef Reservoirs
Authors P. L. Carrillo, B. W. Bryans, M. Aldana and R. TurpeningAttenuation is an important measurement for our understanding of rock properties. The extraction of accurate values of attenuation could be useful for enhancing the interpretation of seismic data for reservoir definition. We present here a study using attenuation, velocity and reflectivity from crosswell seismic to optimize imaging of a carbonate reef reservoir in the Michigan Basin. Based on the analysis of traveltimes and average frequencies, we estimate velocity and attenuation coefficient tomograms; combining both tomograms we calculate a Q-value distribution image for wells spaced over 2000 feet. The attenuation coefficient tomogram validates the main features observed in the velocity tomogram, showing high attenuation rates inside the reef. Moreover, the attenuation tomogram shows a better image of the structure and relief of the pinnacle reef, contributing to an interpretation of its possible lateral and depth limits. In summary, the attenuation tomogram is independent information from the crosswell seismic data.
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A Seismic Characterization of Antarctic Glacial Sediments in the Ross Sea
Authors S. Picotti and G. RossiWe present a tomographic and modelling study of the Ross Sea glacigenic sediments (Eastern Basin) in order to reconstruct a 2-D multiparametric model of the geological structures involved in the erosion process of the WAIS (West Antarctic Ice Sheet). We analyzed a segment of the 2D seismic line IT-01, acquired in 1988, located in a zone very close to the continental margin. The combined use of traveltime and attenuation (quality factor Q) tomography provides a 2-D velocity-Q section, which can be used to map the spatial distribution of compacted sediments and occurrence of biogenic free-gas bearing sediments. The tomographic inversion revealed the presence of anomalous high velocity and Q values in the layer immediately above the RSU2 unconformity, which is an important erosion surface in the Lower Pliocene. This velocity-Q anomaly is correlated with sediments that were eroded and compacted by the load of the WAIS during its expansion on the continental shelf. These results are confirmed by a poroviscoelastic model of the analyzed sediments based on rock-physics theories that takes into account the partial saturation and the in situ pore pressure and temperature.
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3D Dual-sensor Data Processing with Proper Handling of Aliased Energy
By T. KlueverRecently, first processing results of 2D dual-sensor towed streamer data have been presented. A dual sensor streamer is capable of recording not only the pressure field but additionally the vertical component of the particle velocity field. A proper combination of both records yields the separation of up- and downgoing wavefields. In this paper, we review the theory behind the dual-sensor streamer specific processing steps for 2D and describe their extension to the 3D case. We highlight 3D specific problems related to typical marine acquisition geometries and propose a 3D data processing solution which addresses these problems. Part of the proposed processing scheme is a new method designed to deal with spatially aliased energy in a local way. We demonstrate this new technique using a field data example.
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Optimal Deghosting Algorithm - A Robust Deghosting Approach for Over/Under Towed-streamer Data
Authors A. K. Özdemir, A. Ozbek, P. Caprioli, J. Robertsson and E. KraghOver/under acquisition provides broadband data at all frequencies by combining the advantages of shallow and deep cable depths. These benefits are achieved by using data processing algorithms that combine data from the over/under cables into a single dataset. However, the conventional algorithms used for this purpose do not consider the presence of noise. The optimal deghosting (ODG) algorithm is proposed for processing the noisy over/under towed-streamer data. The new method estimates the statistics of the noise on over and under streamers and minimizes the residual noise on the deghosted data in a least-squares sense. We show on a real data example that the ODG method may provide significantly quieter deghosting results, especially in very noisy environments. Hence, by optimally combining the data from both streamers, the ODG method results in a broad bandwidth with enhanced low-frequency response and optimal signal-to-noise ratio.
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Over-under Deghosting - 1D, 2D or 3D Algorithms in the F, FK or FXY Domains
By B. GratacosFor streamer acquisition, the reflection of the up-going wave field at the sea surface (or ghost) contaminates the recordings, and in particular, significantly attenuates the lower frequencies. In order to eliminate the ghost, we may choose to advocate the recording of the wave field at two different depths: the so-called 'Over-Under' technique. In this paper, we propose different strategies to combine these hydrophones recordings, taking into account the characteristic of the swell noise which is sometimes a severe issue. We also show that the relative positioning of the cables is not a strict constraint with a proper 3D algorithm. The different algorithms are demonstrated on synthetic and real datasets.
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Target Oriented Directive Designature
Authors C. Lacombe, H. Hoeber, S. Campbell and S. ButtThe traditional marine designature process does not correct for far-field variations with the direction of propagation. We propose a simple and robust method to take this directivity into account. It is a trace-by-trace target-oriented technology which has the advantage of simultaneously respecting take-off and emergent angles, azimuth and receiver depth. A time variant application of this approach gives comparable results to a tau-p based correction valid at small azimuth only.
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Pre-processing Requirements for Reverse Time Migration
By I. F. JonesAlmost all conventional pre-processing is conceived of with one-way wave propagation in-mind. If we take into account the existence of two-way wave propagation arrival events, then many of the underlying assumptions of moveout behaviour implicit in some pre-processing techniques must be re-evaluated. Using 2D synthetic data, we demonstrate that the moveout behaviour of double bounce arrivals (a class of two-way propagating events) can be compromised by pre-processing designed to remove events exhibiting ‘anomalous’ moveout behaviour. These observations are of interest to us, as we are now beginning to employ two-way migration schemes to image complex structures. However, if we continue to use conventional pre-processing techniques, we run the risk of removing the very events we are trying to image. The observations made on the basis of synthetic modelled data, are extended in this work to real data examples, all from the North Sea, where in the central graben, we commonly have steep piercement salt diapir structures, which are good candidates for producing useful double bounce arrivals, which can be imaged using RTM.
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Time-varying Wavelet Estimation and Deconvolution for Surface Seismics
More LessPhase mismatches sometimes occur between final processed sections and zero-phase synthetics based on well logs - despite best efforts for controlled-phase acquisition and processing. The latter are often based on deterministic corrections derived from field measurements and physical laws. A statistical analysis of the data can reveal whether a time-varying non-zero phase is present. This assumes that the data should be white with respect to all statistical orders after proper deterministic corrections have been applied. Kurtosis maximisation by constant phase rotation is a statistical method that can reveal the phase of a seismic wavelet. It is sufficiently robust that it can even detect time-varying phase changes. Phase-only corrections can then be applied by means of a time-varying phase rotation. Alternatively amplitude and phase deconvolution can be achieved using time-varying Wiener filtering. Time-varying wavelet extraction and deconvolution can also be employed as a data-driven alternative to amplitude-only inverse-Q deconvolution.
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Seismic Noise Attenuation Using 2D Complex Wavelet Transform
Authors Z. Yu and D. Whitcombe1D wavelet transform based filtering is increasingly being used in seismic data noise attenuation. However, it doesn’t take into account the spatial continuity of reflections, and is less effective when the dip is the main distinguishable feature between signal and noise. We have explored the characteristics of multi-scale and multi-orientation higher dimensional wavelet transforms. We discuss a wide range of desirable criteria, from practical applicability to general signal analysis. We have selected the complex wavelet transform, mainly from a practicality perspective and developed an adaptive noise attenuation approach to define a multi-dimensional threshold filter function in the time-space-scale-orientation space. The effectiveness of this complex wavelet transform based multi-dimensional filter is discussed together with a field data example.
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Curvelets - A Versatile Tool for Denoising Seismic Data
By R. NeelamaniThis paper demonstrates that the curvelet transform is a simple yet powerful and flexible tool for denoising both stacked and prestack seismic data. The ability of curvelet-based denoising to provide superior noise attenuation with minimal impact on the desirable signal is illustrated using stacked data. The versatility offered by a curvelet-based approach is displayed by attenuating undesirable wave modes from prestack data.
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A New Pseudo-acoustic Wave Equation for VTI Media
Authors X. Du, R. P. Fletcher and P. J. FowlerWe propose a new approximate partial differential equation for qP-waves in transverse isotropy with a vertical symmetry axis (VTI) media. We analyse its relationship to two other published "pseudo-acoustic" VTI equations. All three pseudo-acoustic VTI wave equations are coupled systems of second-order PDEs in time, derived from the same dispersion relation for qP waves by introducing different auxiliary functions. The new method combines efficient implementation and low artifacts. Modeling and reverse-time migration are shown to validate the wave equation.
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Parsimonious Finite-volume Frequency-domain Method for 2D P-SV-wave Modelling
Authors R. Brossier, J. Virieux and S. OpertoA new numerical technique for solving the 2D elastodynamic equations in the frequency domain based on a finite-volume P0 approach is proposed for application to full waveform inversion. The associated discretisation is through triangles and the free surface is described along the edges of the triangles which may have different slopes. By applying a parsimonious strategy, only velocity fields are left as unknowns in triangles, minimizing the core memory requirement of the simulation. Efficient PML absorbing conditions have been designed for damping waves around the grid. The method is validated against analytical solutions of several canonical problems and with numerical solutions computed with a well-established finite-difference time-domain method in heterogeneous media. In presence of a free surface, the finite-volume method requires ten triangles per wavelength for a flat topography and fifteen triangles per wavelength for more complex shapes, well below criteria required by the staircase approximation of finite-difference methods. Comparison between the frequency-domain finite-volume and the second-order rotated finite-difference methods also shows that the former is faster and less-memory demanding for a given accuracy level, an encouraging point for application of full waveform inversion in realistic configurations.
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Accuracy of qP Wave Modeling in Anisotropic Acoustic Media by a Finite-difference Frequency-domain Method
Authors A. Ribodetti, S. Operto and J. VirieuxWe assess the kinematic and dynamic accuracies of a finite-difference frequency-domain method for qP wave modelling in transversally isotropic acoustic media with tilted symmetry axis. This method was developed as a tool for frequency-domain full-waveform inversion which requires accurate traveltime and amplitude modelling. The modelling method is based on the parsimonious mixed-grid method which requires 5 grid points per wavelength in homogeneous media to mitigate numerical dispersion. We compare seismograms computed with the acoustic frequency-domain method with that provided by the complete solution of the transversally isotropic elastic wave equation. As expected we observed strong traveltime and amplitude mismatches in the case of strongly anisotropic materials such as zinc crystals. For weak anisotropy, we obtain a reasonable agreement although slight delay of the acoustic wide-angle reflections was observed in the case of a two-layer medium. The footprint of these inaccuracies in full-waveform inversion will need to be assessed before considering application to real data.
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Frequency-domain Acoustic Wave Modeling Using a Hybrid Direct-iterative Solver Based on a Domain Decomposition Method
Authors F. Sourbier, A. Haidar, L. Giraud, S. Operto and J. VirieuxDesigning an efficient modeling tool is a key point for large 3D frequency-domain full-waveform inversion problems. We present a frequency-domain acoustic wave modeling using a hybrid direct-iterative solver based on a parallel domain decomposition method and Schur complement approach. The main interest of mixing solvers is to overcome the huge memory complexity of direct solvers while partially preserving efficient multi-RHS simulations and mitigating the iteration count in iterative solvers. To improve the convergence rate of the iterative solver, a preconditioning based on an additive Schwartz approach is used. Discretization of the Helmholtz equation is based on a parsimonious finite-difference method but the domain decomposition method could apply to any numerical scheme such as finite-element or finite-volume methods and to any media such as elastic, anisotropic ones ... To asses the efficiency of the hybrid approach, we computed simulations in the 2D Marmousi II and 3D SEG/EAGE Overthrust model, and compared results with that of a direct solver.
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Automatic 3-C 3-D VSP Wavefield Separation
By E. BliasThe purpose of this paper is to present a new method for automatic 3-C 3-D VSP wavefield separation. The method uses an iterative global non-linear optimization scheme, which includes two major steps: automatic 3-C velocity analysis and wave-by-wave extraction/ subtraction. Regular waves (P down and up, P-S down and up, etc.) are extracted from the 3-C wavefield one wave at a time. Automatic velocity analysis predicts the strongest wave, which is extracted and subtracted. The next strongest wave now becomes the dominant wave and the process is repeated. As each new event is extracted, all previous removed events are re-examined and updated.
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Velocity Calibration and Wavefield Decomposition for Walkover VSP Data
Authors J. Mann and M. von StehtGeneralized stacking velocity analysis tools like the Common-Reflection-Surface stack method provide stacking parameters beyond the conventional stacking velocity. These parameters can be expressed in terms of useful geometrical wavefront properties. However, this requires a good estimation of the tuned velocities which are valid for imaging in the vicinity of source and/or receiver of the recorded data. Using the downgoing first arrivals, we introduce a simple and efficient method to determine the velocities at every receiver level in a walkover VSP experiment. The calibrated wavefront properties are subsequently used for the decomposition into upgoing waves arriving either as P- or S-waves at the receivers.
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Integrated Crosswell Seismic Inversion - Test on Belayim Land Data
Authors C. D‘Agosto, M. Antonelli, F. Miranda, R. Micheli and M. SalahThis paper describes the application of elastic inversion on real crosswell data, with the objective of discriminating sand and shale with high resolution at the reservoir level. For this work we used crosswell seismic profiles acquired at Belayim Land field, Sinai, Egypt. The results showed that elastic inversion of crosswell seismic data is able to discriminate between sand and shales with a resolution of 5m. Knowledge of Vs information at the reservoir level is important in order to obtain absolute results from the elastic inversion.
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Imaging Beneath Igneous Sills Using Reverse Time Depth Migration
Authors R. Hardy, J. B. Bednar, C. Bednar, K. Fernandes and S. M. JonesA variety of imaging problems are associated with igneous sill complexes. Existing processing solutions provide good images of the shallowest sills, but are unsuitable for the imaging of deeper sills, for the steep dips associated with the sill feeder mechanisms and for resolving the sedimentary sequences underlying the intrusions. In this paper we construct a 2D model incorporating a variety of sill geometries. We use the model data to demonstrate that existing one-way depth imaging technology cannot provide adequate images. Internal multiples are also an issue for consideration. Reverse Time Depth Migration is used for the first time to provide accurate images of the sill feeder structures and sedimentary sequences beneath the sills. This powerful imaging approach has the potential to dramatically improve image quality in the North Atlantic continental margins regions such as the NW of Ireland, Britain and Norway. Results will benefit those studying sill emplacement and those exploring beneath igneous intrusions.
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A Generalised Derivative Operator for Potential Field Data
More LessThe enhancement of potential field data using filters based on horizontal and vertical derivatives is common. As well as the direct use of the gradients themselves they are used in filters such as sunshading, total horizontal derivative, analytic signal, horizontal and vertical Tilt angles, the Theta map, etc. These techniques are highpass filters of different types and so enhance noise as well as detail in the data. A generalised derivative operator is introduced in this paper, which combines the effects of some of the previously mentioned filters in a user-controllable manner. The filter is demonstrated on aeromagnetic data from South Africa.
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Mapping the Structure and Depth of Sedimentary Basins Using the Magnetic Tilt-depth Method
Authors J. D. Fairhead, A. Salem, S. E. Williams, A. J. Bourne, C. M. Green and E. M. SamsonThe aim of this contribution is to show that it is possible to transform a magnetic anomaly map using the recently introduced 'Tilt-Depth' method, which only uses first order derivatives, into a form that allows an initial and rapid means of evaluating the depth and structure of sedimentary basins. We do this by introducing a new colour mapping method. This method assumes a simple buried vertical contact model such that the 0 degree contour of the Tilt derivative closely following the edge of the vertical contact whilst the distance between the 0 degree and the +/-45 degree contours providing the depth estimate to the top of the buried contact. The method has been tested on simple 2D and 3D models with success and is used here to illustrate its value at regional (continental) and local (exploration) scales data sets.
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Inversion of Potential Field Data Using the Structural Index as Weighting Function Rate Decay
More LessNonparametric inverse methods provide a general framework for solving potential-field problems. The use of weighted norms leads to a general regularization problem of Tikhonov form. We present an inversion method to estimate the source density distribution from potential field measurements using a flexible depth weighting function in the Tikhonov formulation. Our approach is close to the formulation proposed by Li and Oldenburg (1996), but differs significantly in the definition of the weighting function, which in our formalism is not given with a fixed exponent but with the structural index of the Euler Deconvolution theory as exponent. Hence the allowed values for the weighting function exponent depend on the range of structural index for magnetic sources, being 0≤N≤3. Simple tests on 2D sources such as dipoles, dipole lines, dykes or contacts, validate our hypothesis. The main aspect of the proposed inversion scheme is that it brings a new link between two widely used types of inverse methods, namely the Euler deconvolution and the block model solution of a Tikhonov-form regularization problem with additional a priori information.
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Target Delineation Using Full Tensor Gravity Gradiometry Data
Authors C. A. Murphy and J. BrewsterFTG Gravity data acquired on airborne and marine platforms measure 5 independent Tensor components that describe a total gravity field. The components capture unique signature patterns related to attributes of target geology that when collectively interpreted enable detailed imagery of the target itself in terms of geometry, composition and depth of burial. The horizontal tensor components are commonly used to identify and map lineaments associated with structural and/or stratigraphic changes or target geometry in a survey area. The vertical tensor component is used to estimate depth and predict compositional information related to target geology. However, these components have traditionally been interpreted separately from one another and run the risk of missing out on key information. This paper describes application of a semi-automated approach that combines the individual components into singular representations to best extract the signature pattern common to all components as revealed by the underlying geology. The examples presented are taken from an Air-FTG® survey onshore Brazil to image the structural framework and a Marine-FTG® survey offshore Norway to resolve salt body geometries. The resultant interpretation enables the end-user to fast-track the exploration initiative by quickly evaluating target geology for detailed follow-up.
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Marine Time Domain CSEM - The First Two Years of Experience
Authors N. A. Allegar, K. M. Strack, R. Mittet and A. PetrovThe CSEM method can measure and understand the fluid content and its changes in a pore space. The method is based on detecting the resistivity contrast of the resistive reservoir fluid to its conductive surrounding media. Recent advances in electronics now make it possible to reliably collect marine time domain electromagnetic data, or tCSEM™. As the tCSEM™ method records only the Earth’s response and is collected in a style similar to seismic, it can be robustly integrated with seismic data. By recording in the absence of the active source, the airwave phenomena also can potentially be isolated from the subsurface response in shallow water. Commercially available nodal systems have successfully collected marine tCSEM™ data and examples are discussed. New cable based systems are under development which contain densely spaced sensors for recording both the electric and magnetic fields. Combining cable and nodal systems will provide for more robust integration with seismic and borehole measurements.
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On the Removal of MT Signals from SBL Data
Authors S. de la Kethulle de Ryhove and F. MaaøWhen performing a seabed logging (SBL) survey during a period of high magnetotelluric (MT) activity, it is not uncommon for MT signals - due to their highly variable and nonstationary nature - to significantly reduce the quality of SBL data. In the most dramatic instances, this can create the need for retowing some SBL lines at a high additional cost. We propose and describe a simple method to partially remove MT signals that may be superimposed to SBL data acquired during periods of high MT activity, and subsequently illustrate the method's performance with a real data example.
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Modeling Results of On- and Off-time B and dB/dt for Time-domain Airborne EM Systems
Authors C. Yin, R. S. Smith, G. Hodges and P. AnnanAirborne time-domain EM modeling generally uses the convolution of the transmitting current with the system responses. However, if no special attention is paid to the singularity of the system responses, serious stability problem may occur in the modeling of on-time and off-time magnetic induction B and time-derivative of magnetic induction dB/dt. In this paper, after a through investigation of different convolution modes, we choose a stable algorithm for modeling the responses of time-domain airborne EM systems, where the step response of the airborne EM system is calculated and convolved with the transmitting current by Gaussian quadratures. While the algorithm can be used for any arbitrary transmitting waveforms, we choose a half-sine and trapezoid wave as an example. The preliminary results for a half-space model show a clearly consistent integral-derivative relationship between the B and dB/dt responses.
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Investigation of Key Controls on Estimating Fracture Orientations from AVOA Data
Authors M. J. Harrison, E. Liu and R. E. WhiteWe test how well fracture orientations can be estimated from fitting amplitude variations with offset and azimuth (AVOA) under varying signal-to-noise (S/N) ratios. We perform these tests for the range of AVO classes, with different azimuth sampling. We also show that modifying the fracture properties can have a large impact on the reliability of estimating the AVOA parameters. We summarise levels of S/N required to reliably estimate fracture orientations under different AVO class and varying azimuth sampling. We conclude that estimating fracture orientations is most sensitive to S/N ratio, followed by the properties of the fracture network, the AVO class, and the sampling in offset-azimuth space.
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Thin-bed Reservoir Characterization Using Relative Impedance Data, Joanne Field, U.K.
Authors L. T. Brown, J. Schlaf and J. ScorerThe Joanne Field, in the central North Sea, produces oil from Cretaceous and Tertiary age chalk at approximately 3 km depth. The porosity characterization of the reworked chalk beds in the reservoir units is one of the main geologic challenges in redevelopment. The best reservoir zone, T1, is approximately 1/12th of a seismic wavelength thick, and has porosity variation of 15% to 37%. 3-D seismic data show high amplitudes associated with areas of high T1 porosity, but interpreting them quantitatively has been difficult. A rock physics and synthetic modeling study was undertaken to determine the contribution of porosity, thickness, and fluid changes on the seismic response of the T1 and to determine what seismic attributes are most useful for reservoir characterization. The seismic response at the T1 is highly sensitive to T1 porosity and relative impedance amplitudes are superior to reflectivity amplitudes for porosity characterization. Since the T1 is a thin-bed, the seismic response is a function of thickness as well as rock properties, but the limited T1 thickness and lithology variation allows robust porosity estimation. Modeling shows that the thickness limit for rock property characterization for the T1 is around 1/20th wavelength.
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Detailed Geological Model of Carbonate Reservoir Based on Geostatistical AVA-inversion - A Case Study
Authors O. Rodina, A. Rabey, L. Makarova, D. Neudachin and J. CorballoThe goal of reservoir characterization is to draw inferences about the petrophysical properties of the reservoir (porosity, permeability, saturation, etc.) on the bases of geological assumptions about the character of the subsurface, together with the evidence from seismic and well log data. The geostatistical inversion is a modern and one of the most effective tools for such purposes. This study demonstrates how geostatistical AVA-inversion workflow was used to obtain reservoir properties and risk assessment of the Lekharyaga prospect area. The main target of this region was Lower Permian siliciclastic-carbonate oil reservoir. Recent results of geostatistical inversion of prestack seismic data now indicate spatial distributions of effective porosity and permeability within the target interval. Based on our investigation an adjustment of the production drilling scheme was performed.
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Integrated Seismic Characterization and Reservoir Modelling in a Structurally Complex Carbonate Field, Libyan Offshore
Authors C. Monesi, F. Cappanera, A. Cappelletti, I. El-Ageli, M. Fervari, C. Magistroni, F. Porrera and C. RizzettoThis paper discusses a well-driven seismically constrained interpretation and characterization workflow optimized to quantitatively describe the "field scale" heterogeneities of a structurally complex carbonate reservoir and how the new structural framework and seismically predicted 3D reservoir properties can be used to drive a geologically robust updating of the available reservoir model.
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A Fracture-only Reservoir Simulator with Physically-based Transfer Functions
Authors E. Unsal, H. Lu, S. K. Matthai and M. J. BluntWe propose a simulation methodology that combines the strengths of discrete fracture models with conventional dual porosity simulation. Constructing a grid and solving for flow in both fracture and matrix in a discrete fracture model is frequently so computationally demanding that only small systems can be studied. In contrast, while dual porosity models are more computationally efficient and can be applied at the field scale, they average the fracture properties and the transfer of fluids between fracture and matrix. In our approach we capture the complex geometry and connectivity of the fractures through explicit gridding of the fracture network. However, to avoid the prohibitive computational cost associated with gridding both the fracture and the matrix, we apply transfer functions to accommodate the flow of fluids between these two domains. We use a physically-based approach to modeling the transfer that overcomes many of the limitations of current formulations. The model is based on CSMP, an object-oriented discrete fracture simulator. We validate the method through comparison with one-dimensional analytical solutions and comparison with experiments and simulations where both fracture and matrix are represented. We then present three-dimensional simulations of multiphase flow in a geologically realistic fracture network.
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Improved Characterization of Naturally Fractured Reservoirs by Joint Inversion of Production and Seismic AVOZ Data
Authors A. Shahraini and M. JakobsenThis paper proposes a method for characterization of natural fractured reservoirs by integration of production and seismic data. The method is based on a unified model for the effective hydraulic and elastic properties of fractured porous media (which takes into account the effects of fracture geometry and fracture-fracture interaction in a consistent manner) and a Bayesian method of inversion which provides information about uncertainties as well as mean values. Our method can deal with complex models of fractured reservoirs .However, the initial inversion results presented here are based on a simplified model (involving a single set of vertical fractures). At the same time, the simplified model is both heterogeneous and anisotropic, since the reservoir is assumed to consist of an unfractured zone in addition to a fracture zone (characterized by a fracture density and azimuthal orientation). An application to synthetic data suggests that one may obtain a significantly better estimate of the parameters of the fractures within a fractured reservoir by using seismic AVOZ data in addition to reservoir production (bottomhole and oil production) data in the history matching process. Seismic and production data are complementary in the sense that they have good resolution in space and time, respectively.
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Fractured Reservoir Prediction from 3D Seismic Volumetric Curvature
Authors A. Alam, D. M. Buck and J. D. TaylorNaturally fractured reservoirs are important targets for exploration and production of hydrocarbons. Past methods to detect fractures included the processing of pre-stack seismic data for velocity and amplitude variation with azimuth, or bi-refringence analysis of multi-component P- and S-wave data. Both methods are expensive. Recent less expensive methods from post-stack 3D seismic data include horizon-based or volumetric curvature indicators. Current volumetric curvature methods compute lagged cross-correlations followed by eigenstructure analysis. These methods are computer-intensive and slow. We present an alternative efficient method that derives curvature from spatial derivatives of phase spectra of laterally separated time-windowed traces. The method detects faults, fractures, ridges and valleys. We used high volumetric curvature as an indicator to detect fractures from 3D post-stack seismic data. A map of high indicator values overlaid on target horizon structure matched with the map of independently picked faults. More significantly the azimuth of the lineament on horizon indicator map at a well location matched with the natural fracture orientation observed from borehole image analysis. Further, the lineament trajectory followed a tight doubly folded ridge. With these supporting observations we used high curvature as a fracture predictor away from the well where hydrocarbons were found in a fractured zone.
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An Innovative and Multi-disciplinary Methodology for Modelling Naturally Fractured Reservoirs
Authors F. Daniau, C. Aug, T. Lemaux, R. Lalou and O. LemaireIn this paper, a complete workflow to analyse and model natural fractures in hydrocarbon bearing reservoirs is described using fracture description from cores, borehole images and wireline logs. There are indices testifying the impact of fractures on the dynamic performances of a fractured reservoir: the difference between core permeability and well test permeability and/or heterogeneous production and/or difficult history match and/or early water breakthrough. The workflow described here is a four steps process: (1) data analysis that leads to the definition of fracture sets and finding a driver for the fracture density between wells, (2) building of a 3D Discrete Fracture Network model (DFN), (3) dynamic calibration and (4) computation of equivalent fracture permeability, porosity, matrix block sizes and shape factor (upscaling). An iterative process allows updating the fracture model parameters according to the history matching results. The 3D final fracture porosity/permeability model can be used as input for the dynamic reservoir simulation model using single or dual porosity formulation.
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Benchmarking Prestack AVO Inversion
By J. P. NeepIn 2005, AVO inversion of prestack seismic data from a BP gas field was performed by three different companies. The results were highly variable and inconclusive at identifying the known zones of fizz or residual gas saturation within the field. In 2006, BP decided to perform a benchmark of the available AVO inversion algorithms using a very simple synthetic AVO dataset, generated from a known model containing variations in gas saturation at all depths. Six companies took part and delivered predictions of variations in acoustic impedance (AI), shear impedance (SI) and density away from a calibration well. Results were analysed by cross-plotting attributes from band-limited inversions versus attributes from the true band-limited model. Comparative results are presented here and show that three companies were able to recover variations in AI, SI and density with a high degree of confidence. The remaining three companies struggled with either excessive scatter or non-linearity in the inversion results. It is suggested that AVO inversion codes should be tested using appropriate synthetic AVO gathers before commencing AVO inversion of real prestack data.
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AVO Is Not an Achilles Heel But a Valuable Tool for Successful Exploration West of Shetland
Authors N. Loizou, E. Liu and M. ChapmanApproximately 40 wells have been positioned on an amplitude or AVO anomaly located within the Palaeocene West of Shetland, but only nine encountered notable hydrocarbons. Many of these failed wells were positioned on interpreted AVO anomalies. To try to understand the reasons for this failure we performed an in-depth study of four wells. We argue that, despite the failures, AVO forms a valuable tool for exploration.
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High-quality AVO Products from High-resolution Seismic
Authors D. Ellis, D. N. Hood, E. C. Rushmere and S. G. DavisRubbish in, rubbish out - interpreters need to have confidence in seismic data quality when working with inverted seismic and amplitude-versus-offset (AVO) volumes, or projections or attributes generated from these data. In this paper we present results of noise attenuation applied to stabilise the low-frequency response, and we also show an “AVO-quality” indicator that allows the interpreter to gauge the quality of AVO products on the workstation.
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Seismic Evaluation of Reservoir Quality and Gas Reserves of DHI Supported Deep-water Systems, Offshore Nile Delta
Authors M. Galbiati, M. Fervari and G. CavannaWe present a seismically-consistent methodology predicting reservoir quality (porosity and thickness) and gas reserves in deep-water clastic reservoirs in the first appraisal stages of a deep-water Nile Delta amplitude-supported gas play, where reservoir display a wide range of thickness, often below the vertical seismic resolution. Accurate evaluations of seismically thin reservoirs must take into consideration tuning and resolution issues, as failure to explicitly address those problems in the study area can lead to prediction errors of OGIP in excess of 30%. Our approach is based on a reference petro-elastic model and subsequent seismic calibration through forward modelling or acoustic inversion, depending upon the physical scale and inner architectural complexity of the gas-bearing objects.
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Methods of Multicomponent Seismic Data Interpretation
More LessAnalysing both the compressional (PP) and converted (PS) wavefields from a multicomponent seismic survey can provide more information about subsurface structures, lithologies and their fluid saturants. This paper discusses the techniques of jointly interpreting P-wave data in association with converted-wave seismic data. The methods include log analysis, generation of synthetic seismograms, VSP correlation, along with registering, picking, and calculating ratios of the PP and PS sections. Two cases of oilfields in clastic (sand-shale) environments are discussed - the Cambay Basin, India and the Williston Basin (Ross Lake), Saskatchewan. In addition, a carbonate case is considered from the Cantarell oilfield in Mexico. Vp/Vs values from traveltime thickness (isochron) ratios and amplitude inversion are especially useful for characterizing lithologies.
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Stretch Compensation in Angle Domain - Implication on Wide Angle AVO/AVA Studies
Authors D. Lecerf and J. L. BoelleFor wide angle datasets there are difficulties in calculating the correct reflection angle values. In addition, wide angle data suffer from imaging stretch effects. In this paper we describe a new approach to compute accurate angle CMP gathers from time migrated offset CMP gathers. The dependencies offset, angle and time are computed through a kinematic pre-stack time de-migration process, taking into account local dips, the effective velocity and anelipticity model. After the kinematic de-migration, we have access to the relationships between the reflection angle and the differential stretch in time of the wavelet for different offsets. A direct (and reverse) angle transform is defined in order to provide accurate seismic traces regularly distributed in angle. Because the amount of stretch is nearly stationary as a function of time in a common angle time-migrated trace, a spectral harmonization along angles can be easily implemented using single spectral reshaping operators. We suggest computing an optimal zero angle wavelet from different angle stacks which minimises errors for all angle spectral shaping operators. The presented stretch compensation impacts on the AVO attribute quality and resolution minimising the low frequency contamination.
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Uncertainty Analysis in Prestack Stratigraphic Inversion - A First Insight
Authors F. Delbos, K. Sghiouer and D. SinoquetStratigraphic inversion of prestack seismic data allows the determination of subsurface elastic parameters (density, P and S-impedances). Based on a Bayesian approach, the problem is formulated as a non-linear least-squares local optimization problem. The objective function to be minimized is composed of two terms, the first one measures the mismatch between the synthetic seismic data (computed via a forward operator) and the observed seismic data, the second one models geological a priori information on the subsurface model. It is crucial to estimate the a posteriori uncertainties because the solution model of the inversion is only one solution among the range of admissible models that fit the data and the a priori information . The goal of this paper is to propose an optimized deterministic method to estimate a posteriori uncertainties in stratigraphic inversion. The proposed method is based on the hypothesis that the covariance matrices describing the uncertainties on the data and on the model are laterally uncorrelated (no cross correlation among parameters of different traces). Moreover, the covariance matrix on the data is also supposed laterally stationary. Application on 2D synthetic PP data illustrates the performances of the method. Extensions and limitations of the method are discussed.
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Fast Bayesian Seismic Prestack Inversion with Consistent Coupling to Rock Physics
Authors A. Buland, O. Kolbjørnsen and R. HaugeBoth categorical lithology and fluid classes and continuous rock properties can be predicted from seismic amplitude data given a rock physics model. However, the uncertainty of such predictions may be high, and the uncertainty should therefore always be assessed. We here present a fast Bayesian inversion method for lithology, fluid and rock property prediction from prestack seismic data. The main objective of the inversion is to find the probabilities for different lithology-fluid classes from seismic data and geological knowledge. The method combines stochastic rock physics relations between the elastic parameters and the different lithology-fluid classes with the results from a fast Bayesian seismic simultaneous inversion from seismic data to elastic parameters. The method is illustrated on a prospect offshore Norway.
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Monitoring of Geological Carbon Sequestration Using the Coda-wave Interferometry Method and Time-lapse VSP Data
More LessInjection and movement/saturation of CO2 in a geological formation can cause changes in seismic velocities and attenuation, resulting in changes in seismic-wave scattering and propagation. Accurately estimating seismic-velocity changes from time-lapse seismograms can provide valuable information about where CO2 moves. We investigate the capability of the coda-wave interferometry method for monitoring geological carbon sequestration using synthetic and field time-lapse VSP data. The coda-wave interferometry method can estimate relative temporal changes in seismic velocities. Synthetic time-lapse VSP data sets were generated using a finite-difference elastic-wave equation scheme for a CO2 injection scenario and a possible leakage scenario. Pre-injection and post-injection field VSP data sets were acquired for monitoring of injected CO2 in a brine aquifer. We estimate the temporal velocity changes at the centers of a moving time window using the coda-wave interferometry method, and then obtain the mean velocity change by averaging the temporal velocity changes over time. The estimated mean velocity changes, from both synthetic and field VSP data, increase significantly for receiver positions approaching the top of a CO2 reservoir. This demonstrates that the coda-wave interferometry method can be used for reliable monitoring of CO2 injection using time-lapse VSP data.
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Estimation of CO2 Saturation Considering Patchy Saturation at Nagaoka
Authors C. Konishi, H. Azuma, D. Nobuoka, Z. Xue and J. WatanabeWe have to monitor underground CO2 distribution and saturation in CO2 geological sequestration. In monitoring techniques, geophysical surveys play an important role, however appropriate interpretation is required. When we interpret seismic velocity data and estimate property of partially saturated rocks using Gassmann fluid substitution equation, consideration about different CO2 saturation state is very important. We have studied sonic log and neutron log obtained at OB-2 well in Nagaoka CO2 geological sequestration project and the CO2 saturation state is considered as patchy saturation. Furthermore existence of critical saturation is expected from the logging data. We estimate CO2 saturation for each saturation state, uniform, patchy, and based on Brie’s equation. Although uniform saturation gives much smaller value than actual saturation, patchy saturation and Brie’s equation gives reasonable value and the result shows good agreement with CO2 saturation calculated from neutron log. Therefore we conclude that CO2 saturation state at Nagaoka is expressed as patchy saturation.
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Modelling of CO2 Injection in Faulted Reservoirs - Importance of Multiphase Properties of Fault and Stress Path Evolution
Authors S. Skachkov, J. M. Segura and Q. J. FisherNumerical modelling is used to investigate CO2 injection into geological formations. We begin by using a production simulation model to investigate how faults might impact CO2 movement in the subsurface. We highlight the importance of considering relative permeability and capillary pressure of the fault rocks when modelling CO2-brine flow across the faults. We then investigate the stress path evolution within a reservoir during CO2 injection using a coupled production simulation - geomechanical model
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Time Lapse VSP Program for Otway Basin CO2 Sequestration Pilot Project
Authors M. Urosevic, D. Sherlock, A. Kepic and S. NakanishiThe Australian Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) is currently undertaking the Otway Basin Pilot Project, which involves the injection and storage of 100,000 tonnes of carbon dioxide within the subsurface. The Otway Project is the first of its kind, where CO2 injection will be into a depleted gas reservoir and, therefore, the project will provide important experience for monitoring and verification under these conditions. The overall complexity of the field, its depth, small size and the presence of free and residual gas zones present a serious challenge for 4D seismic monitoring. Accordingly, we adopted a monitoring strategy which combines surface and borehole seismic measurements that optimises our ability to verify CO2 containment. Design of the monitoring program involved numerical modelling and, crucially, pilot field tests. Of particular interest was the application of borehole seismic methods as they can provide better resolution, higher signal to noise ratio and superior repeatability in comparison to surface seismic data. Data recorded from several test VSP surveys and 3D base line VSP survey are analysed for quality, consistency and repeatability.
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Integration of Structural Uncertainties into Reservoir Grid Construction
More LessStructural Uncertainties have a direct impact in exploration, production and drilling decisions. We suggest an approach to generate realisations of reservoir grid conditioned by structural uncertainties. In this paper we show how structural uncertainty can be estimated from geophysical data and used to constrain the shape of reservoir grid realisations. Firstly we present a practical and global approach for estimation of structural uncertainty (i.e. not limited to vertical displacements) which results in a 3D vectorial field attached to the structural model. Secondly we show how this vectorial field can be used in the process of constructing reservoir grids to produce realisations constrained by structural uncertainties. The presented technique enables the propagation of structural uncertainty in the computation of reservoir realisations, in such a way that a new dimension is added to probabilistic reserve calculation. Some examples have shown that including structural uncertainties in the generation of reservoir grid realisations is a major improvement for history matching.
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Objective Assessment of the Reliability of 4D Seismic for Value of Information Analysis
Authors P. Delfiner and C. HubansIn Value of Information (VOI) analyses seismic reliability is often obtained by expert assessments. These carry an inevitable subjectivity that is perceived as a significant weakness of VOI methodology. An alternative objective approach is presented for the case of 4D surveys. Reservoir property grids are converted to elastic parameters and the seismic response is simulated for different time intervals, producing several seismic cubes. Selected attributes representing essentially the seismic effect and the production effect are then classified in a two-way table from which the probabilities of observing a real and a false 4D anomaly (true and false positives) are computed. This method allows a comparative evaluation of the potential value of seismic acquisition surveys and contributes to the selection of the best option.
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Formalizing Geological Knowledge through Ontologies and Semantic Annotation
Authors L. Mastella, M. Perrin, Y. Aït Ameur, M. Abel and J. F. RainaudThe paper addresses the problem of integrating various applications related to geological interpretation within an integrated workflow. This supposes the exchange of different types of objects related to several knowledge fields (seismic, stratigraphy, structural geology), represented in various formats. The solution that we propose consists in linking specialized ontologies describing the different fields knowledge to a pivot ontology related to basic geological knowledge. The linkage between these ontologies and the earth modelling applications can be operated thanks to the semantic annotation technology. The paper presents some key-elements of this pivot ontology and gives an outline of the semantic annotation methodology, studied as a possible solution for modelling workflow integration.
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4D Chi Significance - Enhanced AVO Imaging of 4D Fluid and Pressure Changes
Authors T. A. Ricketts, M. Dyce and D. N. WhitcombeWe introduce a new technique to compensate for angle-dependent noise, in the analysis of 4D AVO data. AVO projections are a valuable tool for imaging production related pressure and fluid changes across a reservoir, but commonly have issues with high noise levels, and fully decoupling pressure and fluid effects from each other. Both issues can be addressed by a new approach that compensates for angle dependent noise. A trend of 4D noise as a function of angle is first extracted from the data and amplitudes are then normalised against this trend. Fluid and pressure projections clipped below the noise level are generated. The method improves the separation of pressure and fluid signals, enabling 4D attributes to be used with greater confidence.
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Uncertainty Study of Time-lapse Seismic Data in a Carbonate Field
Authors S. Vega and K. BerteussenTo fully evaluate the results and achievement of a time-lapse seismic, we need to determine the uncertainties of the seismic signature changes. In this paper, we study the uncertainty of a time-lapse seismic in a carbonate reservoir in the Middle East. We combine statistics and analytical methods to study the uncertainties in compressional velocity, acoustic impedance and seismic amplitude. We find, in general, that the uncertainty increases with thickness. The uncertainty at the facies is considered intermediate as it honors the geology but it carries variations in fluid content along. In addition, Gassmann predictions seem to under-estimate fluid changes. We propose an approximated equation to evaluate uncertainties in seismic amplitude. Finally, we calculate that the signal-to-noise ratio should be higher than 6 (15.56 dB), for detecting physical time-lapse seismic changes in this data.
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4-D Petrophysical Seismic Inversion on the Troll West Field
Inversions of seismic surveys often give low-resolution results that are difficult to compare with well data and geological models. We are therefore developing a multi-vintage seismic inversion based on a Petro-Elastic Model, called PetroSI-4D. This involves input from the geological model and control on the petrophysical correlation between porosity and PEM-based velocities. All vintages and angle stacks are combined to jointly invert for layer thickness, rock properties and saturations. Perturbations of selected properties are introduced using a simulated annealing algorithm to optimise the match between the synthetic and the real angle stacks. The base inversion result is delivered in porosity cubes instead of ordinary acoustic impedances. The 4D inversion is planned to deliver saturation cubes instead of cubes of acoustic impedance changes. These parameters should be easier to compare and use in update of geological and reservoir models to validate the results. The Troll West Field shows that the Petrophysical seismic inversion with rock physics PEM connected to the inversion, gives good results and comparison with the porosity input model is shown. The purpose of 4D petrophysical inversion is to make use of all available seismic data simultaneously, multi-vintage and pre-stack, to constrain the evolution of the saturation field.
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Comparison of Probabilistic and Forward Modeling Workflow Approaches for Integrating 4D Seismic into Reservoir Models (Best of AAPG)
Authors S. A. Castro, J. Caers, H. Meisingset, T. Høye, P. Gomel and E. ZachariassenThe ultimate goal of reservoir modeling is to obtain a model of the reservoir that is able to predict future flow performance. Achieving this challenging goal requires the model to honor all available static (well log, geological information and 3D seismic) and dynamic (4D seismic and production) data. This paper presents a general methodology and workflow for reservoir modeling that integrates data from multiple sources, using a fully probabilistic approach. The main contribution of this paper is the inclusion of 4D seismic data, which has previously not been accounted for within this workflow. 4D seismic data not only helps monitoring pressure and saturation changes in the reservoir due to production, but also identifying the facies where those changes are occurring. Two general approaches can be followed to include 4D seismic data into the reservoir modeling workflow. The first approach consists on modeling the information content of 4D seismic through a spatial probability of facies occurrence; and the second approach consists of matching 4D seismic along with historical production data. This paper presents the methodology followed to incorporate 4D seismic data using the two proposed approaches, while aiming for applying them on a reservoir located in the North Sea.
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R We There Yet?
More LessI compare stress and strain sensitivity of vertical seismic velocity using two rock physics models. Third order elasticity relates tensor strain changes to changes in anisotropic velocity. The R-factor rock physics model (linking vertical velocity change to change in vertical strain, Hatchell and Bourne, 2005) can be derived from third order elasticity by neglecting horizontal strain changes and subsequent Taylor approximation. In the overburden, the influence of horizontal stress and strain changes on vertical velocity increases the predictions for the R-factor. Inside the reservoir the influence of horizontal stress changes decreases estimates for the R-factor. This may explain the observation that the R-factor in field observation of time-lapse seismic data is consistently larger in the overburden than in the reservoir.
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On the Feasibility of Using Marine CSEM for Reservoir Monitoring
Authors L. O. Loeseth and L. AmundsenMarine CSEM exploration for hydrocarbons exploits that thin resistive layers in the subsurface transport EM energy with higher velocity and less attenuation than the conductive surroundings. The successful application of marine CSEM for exploration suggests that the method can be used for reservoir monitoring. Conventional survey design for exploration uses low-frequency signals from an electric dipole source towed close to the seabed and an array of receivers situated on the seabed for measuring the electric and magnetic fields. Low-frequency signals lead to low resolution and the attenuation of field response with offset implies high sensitivity to source and receiver positioning. Thus, when evaluating if monitoring of a reservoir is feasible, it is important to investigate the dependency of the EM response to parameters such as source frequency, overburden conductivity, and reservoir thickness and conductivity. We have considered stratified media and used analytical expressions for the EM fields when studying how the reservoir response depends on these parameters. Moreover, we have derived simple expressions for estimating the reservoir conductivity or thickness from two or more offset measurements when one knows the overburden conductivity as well as expressions for estimating conductivity or thickness changes from time-lapse data.
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Feasibility of EM Monitoring in Land Reservoirs
Authors C. Schamper, S. Spitz and A. TabbaghMarine controlled-source electromagnetic method (CSEM) is increasingly used today in oil exploration in conjunction with seismic data. It helps to decide whether or not a structure displays the resistive anomaly characteristic to the presence of hydrocarbons. In this respect, marine CSEM is intended to provide a non seismic attribute that can be considered as a Direct Hydrocarbon Indicator. However, since resistivity is closely related to the nature of the fluids in the reservoir, one can also expect that changes in the fluid flow can induce resistivity changes that can be measured performing time-lapse measurements. In this paper we simulate a lateral variation of the brine-hydrocarbon contact and give an estimation of the 3D corresponding time lapse signal in a land environment.
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Radar Satellite Observations for Reservoir Monitoring - Case Histories
Authors F. Novali, A. Ferretti, F. Rocca and S. CespaReconciling reservoir production and surface deformation data is not an easy task to perform but remote sensed data can significantly improve our ability to measure surface displacement fields with sub-centimeter accuracy. Satellite data powerfully complements ground-based techniques, such as tiltmeters and GPS stations and permits measurements in areas where access is difficult or expensive. In addition, PSInSAR™ data allow the analysis of wide areas very quickly compared campaign-style traditional time-consuming surveys. The cost is lower than traditional leveling and GPS surveys and can be applied as a monitoring tool, having typically a monthly update. The large archive of data, acquired by the ESA ERS satellites since 1992 allows one to analyze past phenomena in different areas. The new SAR sensors recently launched (e.g. TerraSAR-X, ALOS PALSAR, Radarsat-2, Cosmo-SkyMed) will provide continuity of radar data and will further improve the quality of InSAR surveys.
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Fluid Substitution in Rocks Saturated with Heavy Oil
Authors D. Makarynska, B. Gurevich, R. Ciz and K. OsypovHeavy oils are defined as having high densities and extremely high viscosities. Due to their viscoelastic behaviour the traditional rock physics based on Gassmann theory becomes inapplicable. In this paper, we use effective-medium approach known as coherent potential approximation (CPA) as an alternative fluid substitution scheme for rocks saturated with viscoelastic fluids. Such rocks are modelled as solids with elliptical fluid inclusions when fluid concentration is small and as suspensions of solid particles in fluid when solid concentration is small. This approach is consistent with concepts of percolation and critical porosity, and allows one to model sandstones and unconsolidated sands. We test the approach against known solutions. First, we apply CPA to fluid-solid mixtures and compare obtained estimates with Gassmann results. Second, we compare CPA predictions for solid-solid mixtures with numerical simulations. Good match between the results confirms the applicability of the CPA scheme. We extend the scheme to predict effective frequency- and temperature-dependent properties of heavy oil rocks. CPA scheme reproduces frequency-dependent attenuation and dispersion which are qualitatively consistent with laboratory measurements and numerical simulations. This confirms that the proposed scheme provides realistic estimates of the properties of rocks saturated with heavy oil.
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Microscale Modeling of Heavy Oil Rocks
Authors K. Stuermer, R. Ciz, S. A. Shapiro, E. H. Saenger and B. GurevichDue to decline in conventional oil and gas reservoirs, heavy oil has recently become an important resource. In consequence of this demand, it is necessary to have a rock physical model considering the viscoelastic behavior of heavy oils. Such effective properties are modeled on microscale using numerical simulations. Heavy oil rock sample is represented by the realistic digital 3D model. To obtain the effective velocities of such a digital sample, we use the viscoelastic rotated staggered grid FD algorithm. Numerical experiments are performed for transmission and reflection. Synthetic data obtained from numerical simulations are compared with recently developed model of generalized Gassmann equations for porous media with a viscoleastic solid material filling the pore space. The results show a very good agreement and confirm the applicability of the new rock physics model proposed for the modeling of heavy oil rocks.
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Generalization of Gassmann Equations for Porous Rocks Saturated with a Solid Material - Theory and Applications
Authors R. Ciz, K. Stürmer, C. H. Arns, M. A. Knackstedt and S. A. ShapiroGassmann equations predict effective elastic properties of an isotropic homogeneous bulk rock frame filled with a fluid. This theory has been generalized for an anisotropic porous frame by Brown and Korringa equations. Recently, Ciz and Shapiro (2008) developed a generalization of Gassmann equations for porous media saturated with a solid material. In this paper we present an overview of the new theory and compare to numerical simulations of effective elastic properties of porous rocks filled with an additional solid component. Moreover, we model the effective elastic prroperties of clay-sand mixtures and rocks saturated with heavy oils. The latter application we validate by comparing to experimental data. A very good agreement between numerical and experimental data has been obtained. Thus, this new generalized Gassmann's model is directly applicable as useful rock physics model, e.g.: in proper seismic data interpretation.
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Penetrating Neutron Radiation Enhances Physical Properties of Rock (Best of SAGA)
Authors F. C. De Beer, M. F. Middleton and V. ZadorozhnayaKnowing the physical properties of rock, enhances the knowledge of engineers to predict more accurately e.g. the flow of water / gas or oil, the production rate and/or capacity of reservoirs and the composition of resources beneficial and important to mankind. This paper focuses on radiation based analytical techniques such as neutron and X-ray radiography and tomography to determine and validate existing data about some important physical properties of rock. Through this study, it is seen that neutrons have the ability to penetrate more easily geological laboratory samples and provide important information to geologist and scientists on a non-destructive basis.
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Experimental Compaction of Kaolinite Aggregates - Effects of Grain Size on Mudrock Properties
Authors N. Mondol, K. Bjørlykke and J. JahrenThis study investigates the effects of grain size on petrophysical and acoustic properties of experimentally compacted synthetic mudstones. Four brine-saturated kaolinite aggregates sorted by the grain size were compacted in the laboratory under vertical effective stress up to 50 MPa. Results show that the kaolinite aggregates compacted differently and have atypical acoustic and petrophysical properties as a function of grain size. The maximum compaction was observed in a mixture which contained mixed grain sizes (composite aggregates) whereas the minimum compaction was found in the mixture containing the smallest grain size (<2 microns). The composite aggregate compacted to 27% porosity at 20 MPa effective stress while the aggregates containing the <2 micron grains retained porosity of about 34% at same effective stress. This could be explained by the distribution of effective stress over a larger number of grain contacts in the fine-grained mixture compared to coarser-grained mixtures. The finest grained sample has the lowest density, permeability and velocity compared to the other mixtures. To our knowledge this is the first time grain size related changes in physical properties during compaction has been demonstrated in kaolinite aggregates. These results will have practical application in rock physics, seismic and well log interpretation.
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Differential Acoustic Resonance Spectroscopy (DARS) Measurements
Authors B. B. S. A. Vogelaar, D. M. J. Smeulders and J. M. HarrisThe objective of current research is to discuss the experimental relation between permeability and compressibility of eight natural and artificial porous samples estimated by two distinct laboratory set-ups. The first method to calculate compressibility is the conventional ultrasonic measurement (~ 1 MHz) of the dry rock velocities. The second is the so-called Differential Acoustic Resonance Spectroscopy (DARS) set-up (~ 1 kHz), which relies on the shift in resonance frequency between an empty and a sample-loaded fluid-filled cylindrical resonator. We conclude that for the five reference solids and three low-permeable samples the compressibility estimated by both methods are comparable. However, for one medium- and four high-permeable samples the compressibility estimated by both methods differs. Apparently, the compressibility measured by the DARS-method is related to the permeability of the sample.
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Generalized Anisotropy Parameters for Attenuative Media of Arbitrary Anisotropy Type
More LessSome authors have recently published different parametrizations of viscoelastic anisotropic media of particular symmetry type (e.g., transversely isotropic or orthorhombic).The aim of this paper is to extend such work to media of arbitrary anisotropy type, that is to say to triclinic media. In contrast to previous work, our parametrization uses complex anisotropy parameters and constitutes a "natural" extension to viscoelastic media of previous work dealing with non-attenuating elastic media of arbitrary anisotropy type. The new parametrization allows to compute the complete directional dependence of approximate velocities and quality factors Q of the 3 bulk waves. These approximate formulas are checked on qP-wave experimental data in a moderately anisotropic sandstone sample. Compared to the exact solutions, the errors observed on the approximate velocity are small and hardly exceed 1%. The errors on the approximate quality factor Q is reasonable, typically smaller than 6%, considering the huge directional dependence of Q (with variations of more than 160%).
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Modelling Effective Elastic Properties of Fractured and Partially-saturated Rocks
Authors F. Wenzlau and T. M. MüllerWave-induced flow of pore fluids is considered as one of the prominent loss mechanisms in hydrocarbon reservoirs. It occurs during wave propagation around heterogeneities on sub-wavelength scale, if there is a local contrast in rock stiffness. Using quasistatic numerical experiments, the complex effective moduli of a partially saturated and a cracked synthetic rock sample are determined accurately. From the elastic moduli, velocity dispersion and attenuation are derived. Hence, it is demonstrated that poroelastic finite-difference modelling is a valuable tool for the investigation of frequency-dependent effective elastic rock properties and for the verification of new rock physics theories.
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Log Quality Assessment and Data Correction for AVO
Authors E. Anderson, J. Yan, R. Lubbe, K. Waters and N. PillarAVO modelling requires the relationships of porosity, shale, water saturation, mineral and fluid properties plus P and S-wave sonic and density logs, which in turn provide useful information for rock physics study. Despite modern logging developments, the bad quality of log data caused by borehole environment or mud-filtrate invasion is still problematic and can affect the AVO analysis so that synthetic gathers cannot be obtained accurately. The bad log data must be recognized and corrected and could be divided into three classes: (a) relatively easy routine log curve editing, (b) environmental and (c) mud-filtrate invasion. The detailed methods with a case study are given for the application of synthetic AVO modelling. These include using Biot-Gassmann fluid substitution and an improved time-average method to outline the petrophysical process in an invaded hydrocarbon zone, using the Zoeppritz approximation, the input P-wave, S-wave sonic and density logs are firstly converted from depth to two-way travel time and the reflectivity at each given angle is then calculated. The result shows a satisfactory agreement between a synthetic seismic gather (generated using the corrected logs) and a gather substituted by Gassmann fluid substitution (80% oil saturated).
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Test of Rock Physics Models for Prediction of Seismic Velocities in Shallow Unconsolidated Sands
Authors C. F. Andersen, T. A. Johansen and M. PrasadIn this paper various rock physics models are tested for the prediction of seismic parameters in shallow unconsolidated sands. For evaluation of the models we use well log data containing P- and S-velocities, porosity, water-saturation and mineralogy. Our main approach implies the use of various contact theories, as Hertz-Mindlin and Walton with rough or smooth grain contact points, to model the elastic properties of the high porosity end member. Then bounding models, such as the Hashin-Shtrikman lower bound or the Reuss bound, are used to estimate the properties for porosities within the high porosity and mineral end member. Additionally, we check the empirical models with parameters from laboratory measurements made by Zimmer et al (2007). Using either Hertz-Mindlin or Walton with rough bounds to define the high porosity end member gives an over prediction of the velocities: P-velocity by a factor of ~1.4 and S-velocity by a factor of ~1.7. Combining Walton with smooth grain contacts with Hashin-Shtrikman lower bound gave reasonable results, except for the low porosity interval. Walton with smooth grain contact points, combined with Reuss bound describes the data very well. The empirical models also gave a very good fit to data.
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In Situ Estimation of Third-order Elastic Parameters - Theory and Example from the Gulf of Mexico
By R. BachrachA rock physics strategy for estimating in situ third-order elasticity (TOE) parameters is presented in this paper. The methodology uses estimates of vertical effective stress as well P- and S-wave velocity data derived from well logs and combine these data with a rock physics model to generate all three needed TOE parameters that will characterize the non-linear behavior of the sediments under general stress conditions. The theory is applied on data from deepwater Gulf of Mexico data.
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From Dynamic Moduli to Static Moduli in Arbitrary Anisotropic Rocks with Preisach Viscoelastic Models
More LessIt is a classical result that the elastic moduli of rocks measured with a "static" mechanical press can be substantially smaller than the elastic moduli measured with ultrasonic devices. This result is often incorrectly attributed only to the dispersion effect due to the jump in the frequency band of observation, which is abusively summarized in the terms "static moduli" and "dynamic moduli". Without neglecting the frequency dependence of the mechanical properties, here we show the importance of another fundamental parameter, namely the strain level, to explain this mismatch, which is not commonly appreciated. We propose a theoretical model for describing all these phenomena in arbitrarily anisotropic media. The model is based on the synthesis between viscoelasticy for frequency dependence, and nonlinearity and hysteresis for strain level dependence.
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Application of Shear Wave Anisotropy for Natural Fracture Detection in a Cased Well (Best of AAPG)
Authors S. T. Grayson, A. Donald, T. Bratton, R. Prioul, M. Carlsen and L. SwagerFracture detection in an open borehole has become a routine process through the use of resistivity imaging. However, once the well has been cased, this technique is no longer viable and fracture detection becomes difficult. Certain acoustic measurements can still be utilized, even with the presence of cement and casing. Shear waves transmission is known to become anisotropic when fractures alter the intrinsic nature of the rock, affecting the amplitude and velocity of the waves that cross them. In addition, dispersion analysis, comparing shear slowness to frequency response, shows a unique signature when this intrinsic anisotropy exists. New acquisition hardware, software, and interpretation techniques have been developed to observe this occurrence in cased wells. On a recent cased well, in which openhole data were limited, these techniques were used to identify fractured zones. Waveform, dispersion, and spatial analysis were performed on the oriented shear sonic data and then combined with knowledge of the geologic model to select perforation intervals to maximize production from the fractured zones.
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Multipole Dispersion Tracking for Accurate Slow Shear Determination in LWD Sonic Data
By J. MarketSlow shear determination from LWD sonic data is considerably complicated by the substantial presence of the drill collar in the well bore. Within the industry, monopole, dipole, and quadrupole modes are used to derive shear in slow formations, with a variety of dispersion correction techniques. A multipole slow shear inversion method is described here, which makes use of multiple borehole modes simultaneously both to characterise the tool/collar influence on the dispersion characteristics of the modes and to reduce the uncertainty in the inversion parameters.
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Real-time Completion Monitoring with Acoustic Waves
Authors A. V. Bakulin, A. Sidorov, B. M. Kashtan and M. JaaskelainenDeepwater production is challenged by well underperformance issues that are hard to diagnose early on and expensive to deal with later. Problems are amplified by reliance on few complex wells with sand control media. New downhole data is required for better understanding and prevention of completion and formation damage. We introduce Real-Time Completion Monitoring, new non-intrusive surveillance method for identifying impairment in sand-screened completions that utilizes acoustic signals sent via the fluid column. These signals are carried by tube waves that move borehole fluid back and forth radially across the completion layers. Such tube waves are capable of "instant" testing of presence or absence of fluid communication across the completion and are sensitive to changes occurring in sand screens, gravel sand, perforations, and reservoir. The part of completion that has different impairment from its neighbors will carry tube waves with modified signatures (velocity, attenuation) and also would produce reflection from the boundary where impairment changes. The method would rely on permanent acoustic sensors performing acoustic soundings at the start of production and then repeating these measurements during the life of well. Thus, it could be thought of as "miniaturized" 4D seismic and "permanent log" in individual wellbore.
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Experimentally Achieving Borehole Radar Antenna Directivity in the Time Domain in the Presence of Strong Mutual Coupling (Best of SAGA)
Authors D. Vogt and T. NyareliIt is difficult to achieve significant directivity in the radial direction of a borehole radar antenna, because the spacing of antenna elements is typically constrained by the borehole diameter to be considerably less than a wavelength. Previously published borehole radar antennas have achieved directivity by post processing data received in the frequency domain, or by constructing an aperture antenna, where borehole dimensions allowed this. In this paper, a time-domain technique is investigated for determining the radial direction of reflectors detected in borehole radar images. The antenna itself is an array of four elements and the delay in arrival of the signal between elements is used to determine its direction. We show here that a relativity slow sampling rate is adequate to resolve the small time intervals between signals received on different antenna elements. Mutual coupling between the antenna elements does affect the relative timing, but does not prevent the extraction of usable directional data. Experimental data from a test tank confirms that estimates of reflector direction can be made within about +15°/-5° of the true direction for antenna elements 20mm apart in water, excited with a pulse that has a centre frequency of 250 MHz.
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Comparison of Uncertainty Estimates from Deterministic and Geostatistical Inversion
Authors M. S. Sams and D. SaussusEstimating uncertainty is an important aspect of seismic reservoir characterisation. One reason, amongst others, given for carrying out geostatistical inversion is to quantify uncertainty. Recently a number of authors have shown that uncertainties can be approximately captured from deterministic inversion. Given that deterministic inversion is much quicker than geostatistical inversion, it is worth understanding the differences that can be expected in terms of uncertainty estimates from the two processes. In this paper, we compare the inversion of multi-angle stack data using a simultaneous constrained sparse-spike algorithm for deterministic inversion with the inversion obtained using a Markov Chain Monte Carlo algorithm for geostatistical inversion. The deterministic inversion yields a single model of elastic properties that can be subsequently manipulated to provide estimates of lithology probability and net pay. The geostatistical inversion produces multiple models of elastic properties, reservoir properties (net pay) and lithology so that uncertainty estimation is straightforward without ad hoc procedures. The inversion results are compared in terms of lithology probability estimates and net pay maps.
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New Data and New Tools That Can Enhance Exploration Success
More LessExplorationists often need to evaluate prospects in new areas on very short notice. A company’s ability to respond to new venture opportunities on short notice relies on rapid access to relevant information. Mining this information is time consuming and frequently requires specialized personnel and software that may not be available. A global geochemistry data portal addresses these issues by providing access to more than one source of information such as: Non-exclusive spec studies for petroleum basins around the world where the geochemical and geologic information has already been incorporated into a cohesive report. Links to scientific journals and conference proceedings. This information is typically housed in many disparate sources that make their efficient assimilation nearly impossible. It is now possible to search over 125 technical sites/publications with a single search engine. Consequently, both legacy data and recent articles can be obtained in an efficient manner. Access to a global geochemistry database with web-based tools that allow users to view and interrogate large volumes of data. This data can then be used to evaluate new areas of interest in which the explorationists may not be familiar or may be used to augment a company’s existing data.
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Impact of the Physical-chemical Interactions on the Sealing Capability of a Cap Rock for CO2 Containment in Aquifers
More LessThis study investigates the impact of dissolution and precipitation of the minerals that exist in a cap rock on CO2 containment in saline aquifers. A 2D model consisting of a storage reservoir and a cap rock disturbed with a fault is created using TOUGHREACT, a code for modeling reactive transport in porous media. Typical fluid and mineralogical compositions and petrophysical parameters are introduced. The storage mechanisms are evaluated and quantified based on the simulation results. The possible leakage amounts are determined based on variations of relevant parameters. A maximum of 0,1% of the injection rate is calculated. The effects of the dissolution and/or precipitation of the minerals on the storage capacity of the reservoir and on the sealing capability of the cap rock are estimated. The numerical results indicate significant porosity-permeability reductions in the fault in response to precipitation, which is expected to strengthen the cap rock integrity.
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Possibilities of Carbon Sequestration and Storage in Depleted Gas Fields in Poland
By J. StopaCoal may be important source of energy for Europe on the condition of introducing clean and safe technologies including carbon storage and sequestration (CSS). On the other hand, there are many depleted or partially depleted oil or gas reservoirs, which are producing bellow or near economical limit and may be easy converted into underground waste storages. This paper presents the experience in underground storage of acid gas in Poland. According to actual ecological tendency, since 1993 the acid gas from sour gas sweetening facilities is injected into one oil and one gas reservoir. This process eliminates sulfur compounds and carbon dioxide emission into atmosphere and gives necessary experience for large scale carbon sequestration. On the basis of the above mentioned experiences, the convertibility of the oil and gas reservoirs into underground storage of CO2 emitted by typical power plant is investigated. The full scale compositional simulation showed that initially CO2 injection could be carried out simultaneously with production but later, the production is to be terminated due to the changes in produced gas composition. Finally the life time of the project is estimated.
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Carbon Dioxide Geological Storage Potential of Deep Saline Aquifers of the Central Bohemian Upper Palaeozoic Basin
Authors Z. Tasáryová, R. Lojka and V. HladíkCO2 storage potential assessments were performed in the Czech Republic and suitable structures were identified. Distribution of big stationary CO2 emission sources predestines the Central Bohemian Upper Palaeozoic Basin as the most prospective structure for geological storage of CO2. Considering the criteria - porous reservoir rock sealed by impermeable rock presence at depths below 800 m – the study focused on sandstones of Nýřany Mb. and Týnec Fm.; and on mudstones of overlying Mšec Mb. and Líně Fm. - in areas of Žatec (ŽB) and of Mnichovohradiště sub-basin (MnB). Based on the dataset provided by the Czech Geological Survey: 1) Aquifer body was defined with the volume of 24 km3 and the CO2 storage capacity of 434 Mt CO2, consisting of 150-250 m thick fluvial complex of the Nýřany Mb. and Týnec Fm.; and isolated by 200 m thick mudstones of the Líně Fm in the ŽB. 2) Aquifer body was defined with the volume of 56 km3 and the CO2 storage capacity of 705.6 Mt CO2, consisting of 120-250 m thick fluvial complex of the Nýřany Mb. and Týnec Fm.; and isolated by 150 m thick complex of mudstones of alternating Hředle Mb. and Mšec Mb. in the MnB.
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