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EAGE Workshop on Seismic Attenuation
- Conference date: 28 Oct 2013 - 30 Oct 2013
- Location: Singapore, Singapore
- ISBN: 978-90-73834-64-4
- Published: 28 October 2013
37 results
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Imprints of Earth Transmission on Seismic Reflections
Authors E.F. Herkenhoff, J.D. Cocker, K.T. Nihei, J.P. Stefani and J.W. RectorBroadband seismic acquisition and advances in amplitude preserving processing have driven the industry toward pre-stack reflection amplitudes that are proportional to the earth’s subsurface reflectivity; resulting in improved drilling success rates and increased production. The amplitude and phase of a reflection is a function of spreading loss, inelastic attenuation, interbed multiples and transmission loss interacting with the spatial distribution of elastic (Vp, Vs, anisotropy and density) and inelastic (Qp, Qs) earth properties. Effective Qp (absorption plus scattering) for near-surface layers is on the order of 5-10 (Mangriotis, et al., 2013) and typically increases with depth to 50-130 for water saturated sediments. Vertical and wide-angle scattering can reduce amplitudes by factors of 2 to 3. Taken together these effects can alter absolute amplitudes by factors of 50 and relative event amplitudes vs. offset by factors of 0.3 to 3.
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Poroelasticity Theory and Wave Attenuation in Porous Rocks
By T.M. MuellerPoroelasticity theory provides a general framework for seismic wave propagation in fluid-saturated porous media. This framework entails four wave modes. The fast P- and S-waves describe the in-phase compressional and shear motions. The slow P- and S-waves describe the corresponding out-of-phase motions. In heterogeneous poroelastic media wave mode conversions can occur. The fast to fast wave conversion scattering is important for the quantification of scattering attenuation. The fast to slow wave conversion describes dissipation processes. Analysis of the slow to slow wave conversion process allows us to understand the relation between permeability and seismic attenuation.
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Rigorous Bounds for Seismic Attenuation and Dispersion in Poroelastic Rocks
By B. GurevichThe Hashin-Shtrikman (HS) bounds define the range of bulk and shear moduli of an elastic composite, given the moduli of the constituents and their volume fractions. Recently, the HS bounds have been extended to the quasi-static moduli of composite viscoelastic media. Since viscoelastic moduli are complex, the viscoelastic bounds form a closed curve on the complex plane. When the medium is poroelastic (a composite of an elastic solid and a viscous fluid), the viscoelastic bounds for a bulk modulus are represented in the complex plane by a semi-circle and a segment of the real axis, connecting the formal HS bounds. Furthermore, these bounds are independent of frequency. The complex bulk modulus describing attenuation and dispersion due to squirt flow in a porous medium of a particular geometry spans the entire bounding region. This shows that the bounds for the bulk modulus are attainable (realizable). These bounds account for the viscous shear relaxation and squirt-flow dispersion, but not for Biot’s global flow dispersion. This is to be expected, since the bounds are quasi-static whereas the global flow dispersion is largely controlled by inertial forces.
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Seismic Attenuation Prediction by Dynamic-equivalent-medium Approach
Authors Y. Kobayashi and G. MavkoWe developed a new approach to model seismic velocity and attenuation dispersion caused by mesoscopic wave-induced fluid flow due to sub-log-scale heterogeneity (patchy saturation). The proposed procedure does not require a-priori information on spatial scale of heterogeneity, but utilizes existing velocity measurements, like the Gassmann equation (Gassmann, 1951) in standard fluid substitution problems. One of the practical benefits of the Gassmann equation is that it does not require detailed information about the solid grain geometry. Starting with the bulk modulus of a rock saturated with one fluid, which can be measured at field in-situ conditions, the Gassmann equation allows us to predict the bulk modulus of a rock saturated with another fluid. Proposed procedure also starts from one measured velocity and predicts velocity and attenuation (inverse quality factor) at another frequency without knowing the spatial heterogeneity scale explicitly in advance. Furthermore, this approach can allow us to model both velocity and attenuation from only a velocity measurement at one frequency. Application to laboratory velocity and attenuation measurements confirms the validity of the method. This new approach will become a first step in forward attenuation modeling for quantitative interpretation of seismic attenuation attributes.
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The Myth of Low Frequency Shadows
By A.E. BarnesLow frequency shadows have long been hailed as direct hydrocarbon indicators on the order of bright spots and flat spots (Balch, 1971; Sheriff, 1975; Taner et al, 1979). Time has not diminished their appeal. Indeed, recent years have seen renewed interest, in connection with methods of spectral decomposition (Castagna et al., 2003; Welsh et al., 2008; Nebrija et al., 2009). In spite of this, few examples of shadows have ever been published, and few of those are convincing as hydrocarbon indicators, discounting those caused by shallow gas. This contrasts with bright spots, flat spots, and AVO, for which published examples are numerous and credible.
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Seismic P-wave Attenuation in Fractured Rocks
Authors J.G. Rubino, T.M. Müller, L. Guarracino and K. HolligerIn this work, we have used numerical oscillatory compressibility simulations based on the quasi-static poroelastic equations to study the role played by fracture connectivity on the characteristics of seismic attenuation due to wave-induced fluid flow (WIFF). We verified that, in absence of fracture connectivity, mesoscale fractures oriented perpendicularly to the direction of seismic wave propagation generate important levels of attenuation, which are produced by WIFF between fractures and the embedding porous matrix. In addition, as soon as they are intersected by other fractures, the seismic signatures change rather dramatically. In particular, a decrease in the attenuation peak related to the unconnected scenario together with the appearance of an additional attenuation peak can be observed. The spatial distributions of the local energy dissipation allowed us to confirm that the additional manifestation of WIFF arising in presence of fracture connectivity is produced by fluid flow within fractures. We also corroborated that in presence of connectivity seismic attenuation is sensitive to key hydraulic parameters, namely permeabilities, lengths, aperture and intersection angle of the fractures, as well as to the connectivity degree of the fracture network. Correspondingly, a better understanding of this topic may allow to extract these key properties from seismic data.
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Attenuation Anisotropy in Fractured Media
More LessIn order to observe the effects of fractures on seismic response, a finite-difference wave equation method is developed to model wave propagation in anisotropic media, in which the elastic constants represent fractures effectively. As these elastic constants are frequency dependent, wave simulation is implemented in the frequency domain. While diffractions due to scattering of fractures can clearly identify the position of a fracture gallery, based on strong, fanlike energy mass at the high frequency spectrum, this paper focuses on the investigation of fractures with different viscosity. For viscosity, frequency dependency analyses show that low-frequency bands (<120 Hz) generally show much more variations in seismic responses than high-frequency bands. When frequency increases, the significance of viscosity is decreasing gradually. The investigation reveals that, for a plane wave with constant frequency, fracture porosity has a linear relationship with the attenuation anisotropy, and the matrix of fracture infill materials plays an important role in attenuation anisotropy.
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Stress-associated Scattering Attenuation from Ultrasonic Measurements
More LessAcoustic attenuation has been proved to be an indicator of stress changes in solid structures. Acoustic coda, as a superposition of incoherent scattered waves, reflects small-scale random heterogeneities in solids. Acoustic coda attenuation contains information on stress changes as a result of changes in the physical state of small-scale heterogeneous structures. We measure ultrasonic properties of a cylindrical rock sample with intra-grain pores and fractures under different effective stresses to study the effect of pore-pressure induced stress changes on coda attenuation as a combination of intrinsic attenuation and scattering attenuation. We investigate the stress-associated coda attenuation quality factors Qpc and Qsc as a function of frequencies and characterize its scale dependence on stress variations in rocks by comparing with the intrinsic attenuation quality factors Qp and Qs, calculated from ultrasonic measurements. Comparisons of the P- and S-coda attenuations versus frequencies under different effective stresses demonstrate that the scattering of the S-coda is much stronger because of its shorter wavelength. The intrinsic and coda attenuations versus stress variations present quite different non-linear features, where Qp, Qs, Qpc and Qsc increase with increasing effective stress, but Qpc and Qsc show a greater sensitivity to pore pressure than Qp and Qs.
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Seismic Attenuation from VSP and Well Log Data: Approaches, Problems and Relative Contribution of Scattering
Authors R. Pevzner, T. Muller, R. Galvin, A. Alasbali, M. Urosevic and B. GurevichAll methods for quantitative interpretation of seismic data are based on the analysis of amplitudes of seismic (reflected) waves. Seismic attenuation along the ray path of a wave significantly affects this amplitude information. As such, understanding of this phenomenon has a huge impact for the industry. For the last sixty years vertical seismic profiling (VSP) was an obvious method of choice for exploring this phenomenon in-situ. A large number of different approaches for attenuation estimation were introduced. We have tested a large number of these methods and developed a reasonably robust workflow for attenuation estimation based on the modified centroid frequency shift method. Seismic attenuation measured from seismic data (so-called apparent attenuation) comprises two different components, namely, transfer of the energy into heat (absorption) and scattering. We employ seismic modelling using finely-layered model of the medium obtained from the log data as a part of the workflow to estimate relative contribution of scattering. In order to investigate causes and mechanisms of seismic attenuation we select ~70 wells from several areas in Australia (primarily from NW Shelf) with publically-available high-quality well log, VSP data and geological information. In this presentation we show some preliminary results from this study.
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Estimation of Scattering Attenuation from Zero-offset VSP Data: CO2CRC Otway Project Case Study
Authors T.M. Müller, R.J. Galvin, R. Pevzner and B. GurevichSeismic attenuation consists of anelastic absorption and scattering loss. Due to the dominance of stratification, the scattering attenuation in the sedimentary crust is dominated by 1-D scattering. In this study we applied an integrated workflow for estimation of attenuation from ZVSP and log data to a comprehensive dataset acquired at Otway basin. Both 1D reflectivity modeling and application of generalized O’Doherty-Anstey theory to the Otway log data shows that the 1-D scattering component of attenuation gives Q of over 200. At the same time, average Q estimated from field VSP data value is close to 60. Hence we conclude that scattering plays a relatively minor role in the study area. Further research is required to understand whether this conclusion holds in other areas. In particular, scattering attenuation might be larger in environments with larger variability of elastic properties between layers, such as in areas with laminated coal layers.
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Stable Q Analysis on Vertical Seismic Profiling Data
By Y. WangVertical seismic profiling (VSP) provides a direct observation of seismic waveforms propagating to different depths within the Earth subsurface. Q analysis based on individual waveforms at various depths however suffers from a problem of instability commonly due to fluctuations inherent in the frequency spectrum of each waveform. To improve the stability, it is suggested that, instead of doing Q analysis on individual waveforms, conducting analysis on an integrated observation which considers both frequency and time. The time- (or depth-) frequency domain spectrum is transformed to a 1-D attenuation measurement with respect to a single variable, the product of frequency and time. While this 1-D measurement has higher signal-to-noise ratio than the 2-D spectrum in time-frequency domain, it can also be used to generate a further stabilized compensation function. Then two stable Q analysis methods are implemented by data fitting in least-squares sense to either the attenuation measurement or the data-driven gain function. These two methods are theoretically consistent and practically robust, for conducting Q analysis on field VSP data.
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Effects of the Near Field on Source-independent Q Estimation
Authors R. Shigapov, B. Kashtan, A. Droujinine and W.A. MulderWe consider the problem of Q estimation from microseismic and from perforation shot data. Assuming that the source wavelet is not well known, we focused on the spectral ratio method and on source-independent viscoelastic full waveform inversion. We derived 3-D near-field approximations of monopole and dipole Green's tensors in a homogeneous viscoelastic medium. We show that the spectral ratio method is not applicable in the near-field region, but a two-step source-independent viscoelastic full waveform inversion strategy applied to synthetic data can first recover the purely elastic velocities and then provide an attenuation estimate.
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Continuous Mapping of Velocity Dispersion Using Full-waveform Multi-channel Sonic Logging Data
Authors F. Sun, B. Milkereit and A. CampbellContinuous mapping of velocity dispersion can provide important information on rock physical properties, and thus is desirable. Here we present a method to continuously measure velocity dispersion using full-waveform multi-channel sonic logging data. This method involves techniques of band-pass filtering, beam forming, and cross-correlation of matrices. Using this method, velocity dispersion profiles of P and Stonely waves are obtained using the field sonic data from 5L-38 Mallik gas hydrate research well. The results match very well with the local geological settings. This further proves the robustness of this method, and demonstrates that continuous mapping of P wave velocity dispersion is potentially a promising tool for reservoir characterization.
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The Robust Frequency-depended Attenuation Extraction from Multichannel Sonic
By V.I. RyzhkovDespite obvious benefit of using the attenuation as additional petrophysical parameter it still not widely used. The problem is the difficulty of measurement from fullwave sonic. We propose a modification of the spectral relations method basing on reciprocity principle. Synthetic tests demonstrate a significant improvement of accuracy and reliability of estimates. Real data processing showed that P-wave attenuation is very small in acoustic frequency range and is not a diagnostic parameter. S-wave attenuation waves increases sharply in interval of gas- and oil-saturated reservoir and can be used for joint logs interpretation.
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An Experimental Study of Attenuation in Sandstones at Seismic Frequencies
Authors V. Mikhaltsevitch, M. Lebedev and B. GurevichWe present the results of our measurements conducted on two Donnybrook (A and B)and one Harvey (C) sandstone samples with high (590 mD) and low (7.8 mD and 9.6 mD) permeability. There were no significant attenuation and dispersion observed in the high-permeability sample A. Two distinct inter-related effects have been indicated in the water/brine saturated low-permeability samples B and C. Prominent peaks of extensional attenuation were found at frequency of 0.8 Hz in sample B and at frequencies ~7 Hz (at effective pressure of 23 MPa) and ~20 Hz (at effective pressure of 9 MPa) in sample C. The dispersion of the bulk moduli of both samples in the frequency range from 0.1 to 100 Hz was also detected. The dispersion of the bulk and shear moduli of the samples in the dry state was within the accuracy of our measurements. Our results demonstrate that the low frequency limit of acoustic dispersion for low-permeability rocks can correspond to seismic or even teleseismic frequency band.
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Velocity Dispersion in Sandstones with Oil and Brine: Experimental Measurements and Theoretical Predictions
More LessWe measured ultrasonic P- and S-wave velocities of sandstone on 36 fine-grained and 14 medium-grained samples collected from Changyuan area of Daqing oilfield, northeast China at dry, full brine- and oil- saturated conditions. Relations between acoustic velocity and saturation were measured on 6 samples saturated with oil-brine mixture under hydrostatic pressure at 25MPa and pore pressure varying between 0.1 to 10MPa. The Biot-Gassmann theory was used in the analysis of velocity dispersion. Its predictions of P-wave in full-brine-saturated rocks were consistent with the measurements for most samples, whereas those of S-waves were less than measurements. Comparisons also show that Biot-Gassmann theory could be applicable for describing the changes of Vp with brine(oil) saturation in the oil-brine mixture-saturated sandstones.
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Measurement of P-wave Attenuation of Heavy Oil for Viscoelastic Modeling
Authors A. Kato, S. Onozuka and F. KonoUltrasonic P-wave velocity and attenuation measurements for Canadian heavy oil are conducted in wide temperature range. In the attenuation measurement we use a sweep-type wave and the continuous wavelet transform to obtain stable and broad amplitude spectrum, resulting in that attenuation data with high consistency among three experiments are successfully obtained. The estimated attenuation shows the maximum at about 20˚C, which nearly coincide in the highest rate of P-wave velocity increase with decreasing temperature. The measurement data clearly show existence of additionally induced bulk modulus by the bulk viscosity. The estimated P-wave attenuation and velocity data are expected to help us better understanding for the modeling.
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An Exploratory Study of the Seismic Properties of Thermally Cracked, Fluid-saturated Aggregates of Sintered Glass Beads
Authors E. David, Y. Li and I. JacksonA synthetic rock analogue with simple microstructure was used to advance our understanding of the influence of cracks and pore fluids on seismic properties. The glass beads with ~ 300 μm diameter were sintered near the glass transition with average 1~2% porosity and subsequently quenched from high temperature into water at room temperature to introduce cracks with uniformly low aspect ratio α ~ 0.0007. Jackson-Paterson attenuation apparatus was used for both torsional and flexural mode forced oscillations at seismic frequencies to extract shear and Young’s modulus respectively, with or without the presence of pore fluids (e.g. argon, water) of varying viscosities. In-situ permeability was extracted by using pore-fluid re-equilibration method. Shear modulus is found lower with longer oscillation periods for the cracked and argon pore fluid saturated material possibly indicating the pore-fluid relaxation mechanism at sufficiently longer periods, with minimal strain-energy dissipation 1/Q < 0.003. The averaged elastic moduli for different oscillation periods and permeability are discovered to be extremely sensitive to variation of effective pressure. The crack closure effects can be observed easily at the effective pressure level at ~ Eα, consistent with the theoretical prediction.
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3D Q Tomographic Inversion Using Adaptive Centroid Frequency Shift for Compensating Absorption and Dispersion
More LessIn this paper, we propose a new Q tomographic inversion approach using centroid frequency shift information. An adaptive correction is applied to the observed centroid frequency to account for any deviation from the explicit relationship through tabulating the absorption effect for different accumulated dissipation time. These adaptively corrected centroid frequency shift data will then be fed to reconstruct the attenuation distribution tomographically. We will demonstrate how our approach can accurately estimate Q model and can be included in the Q compensation process to fully account for attenuation and dispersion.
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FWI-guided Q Tomography and Q-PSDM for Imaging in the Presence of Complex Gas Clouds: Case Study from Offshore Malaysia
More LessThe presence of gas clouds has long been recognized as a significant problem in the seismic data processing. To compensate the phase, frequency and amplitude loss due to the gas absorption in the QPSDM migration, both the geometry and the Quality Factor values within these anomalous attenuation regions need to be accurately estimated. In recent years, Ray-based Q tomography has been successfully applied to field data to estimate the anomalous Q model. However, when the distribution of the gas charged sand bodies gets more complex, Q tomography itself often fails to provide necessary resolution to generate a geological plausible absorption model. Joint inversion for velocity and Q with visco-acoustic full waveform inversion has attracted a lot of interest from the industry; however, it remains a very challenging topic as the joint inversion for both Vp and Q is an ill-posed problem, as they are coupled. In this paper, we propose a new approach which uses the highly detailed velocity information from the 3D frequency domain FWI to guide the Q tomography inversion. We demonstrate with a production example offshore Malaysia that our method can effectively reconstruct the Q model and improve the seismic imaging in the presence of complex gas clouds.
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Wave-equation Migration Q Analysis (WEMQA)
Authors Y. Shen, B. Biondi, R. Clapp and D. NicholsQ model building, which is conventionally done in the data space using ray-based tomography, is a notoriously challenging problem due to issues like spectral interference, low signal-to-noise ratio, diffractions, and complex subsurface structure. To produce a reliable Q model, we present a new approach with two major features. First, this method is performed in the image-space, which uses downward-continuation imaging with Q to stack out noise, focus and simplify events, and provide a direct link between the model perturbation and the image perturbation. We develop two methods to generate the image perturbation for the following scenarios: the model with sparse reflectors and the model with dense reflectors. Second, this method uses wave-equation Q tomography to handle the complex wave propagation. Two synthetic tests on two different 2-D models with a Q anomaly shows the capability of this method on the model with sparse events. Tests with a modified SEAM model also demonstrate the feasibility of this method for the model with dense events.
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Q-compensation of a Complex Synthetic
Authors M. Cavalca, R.P. Fletcher and M. RiedelWe apply and compare three model-based Q-compensation approaches. The first two approaches rely upon ray-based attenuated traveltime (t*) modelling operators. One is applied prior to migration, the other one is embedded into a Kirchhoff depth migration algorithm (KDM). The third approach is based on wavepath-consistent t* modelling and is embedded into reverse time migration (RTM). Benefits and drawbacks of each of these methods are discussed and illustrated on a visco-acoustic version of the complex BP 2004 model. While ray-based Q-compensation performs well, especially when embedded within KDM, wavepath-consistent Q-compensation within RTM compensates more complex events and seems worthwhile in highly complex media. However, all approaches are prone to noise amplification issues which must be accounted for.
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Compensating Attenuation Due to Gas Cloud through QPSDM: Case Study from Offshore Brunei
Authors K.H. Teng, J. Zhou, X. Wu, Y. Zhou, T. Brothers, S. Bergler and C. GibsonMarine seismic broadband solutions, as e.g. variable-depth streamer acquisitions, have shown great potential in providing high resolution seismic imaging and better low frequency penetration compared to conventional data (Lin et al., 2011). The increased low frequency content as well as signal to noise ratio play an important role in velocity model building, especially in the presence of gas. It benefits the PreSDM with absorption compensation technology (QPSDM) as low frequency energy may penetrate better through gas clouds (Xie et al., 2009). In this paper, we will show the methodology to compensate for Q absorption effects for a case study offshore Brunei. The step change in image quality shows that it is a viable solution to improve imaging through gas clouds by combining broadband acquisition and advanced depth imaging technology (Q tomography and QPSDM).
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Continued Development of Full Waveform Redatuming for Gas Cloud Imaging
Authors A.R. Ghazali, M.H. Mad Zahir, N.B. Darman, Y.B.M. Yusoff and N.A. RadziIn Malaysian Basins, offshore Malaysia, there are many imaging problems related to the so-called ’gas clouds’ or ’gas chimneys’. Conventional imaging processes do not offer satisfactory solutions, because of the complex propagation effects that occur. Due to the complex wave propagation through the anomaly and the transmission imprint on the reflections from below these complexities, the image below the anomaly is usually not properly recovered. We aim at constructing full waveform transmission operators (including the codas) from the gas cloud reflection response via an effective medium representation. True amplitude imaging is achieved via multi-dimensional deconvolution for these full waveform transmission operators. In this paper, we present a full waveform redatuming approach on a field in Malaysia, using a non-linear inversion to invert the gas cloud reflection response for its effective medium parameters to calculate the redatuming transmission operator. This imaging approach resulted in improvement in reflector continuity and amplitude restoration below the gas cloud overburden.
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Visco-acoustic Least-squares Reverse Time Migration of Cross-well Data
Authors K. Lu, W. Dai and G. SchusterThe viscoacoustic wave equation is applied into least squares reverse time migration(LSRTM) algorithm, to compensate the energy attenuation and modify the phase shift caused by the anelastic property of the media. Test is conducted on cross-well seismic data of the Friendswood site in Texas. Compared to standard LSRTM, higher image quality is produced by Visco-acoustic LSRTM with Q compensation, especially in the low Q value area. It suggests that by using the viscoacoustic wave equation, the blurring and distortion in the migration image due to the attenuation effect can be well corrected.
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Estimation of Gas Saturation by Frequency-dependent Amplitude-versus-offset Analysis
Authors X.Y. Wu, M. Chapman and E. AngererWe present a new method for the determination of gas saturation from frequency-dependent amplitude-versus-offset analysis. The method works by combining measurements of attenuation with a “low-frequency” acoustic impedance to resolve gas saturation variations. The concept is validated on synthetic data before being applied to field data from a producing gas reservoir. We present both the pre-drill and post-drill analyses, and conclude that the method shows great potential for improving our ability to estimate rock and fluid properties from seismic data.
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Evaluating Feasibility of Estimating Attenuation Changes from 4D Seismic
Authors V.I. Ryzhkov, M. Rapoport and M. DenisovThe main problem in measuring of the frequency-dependent attenuation is non-flat and variable spectrum of reflections caused by interference of seismic waves in thin-layered medium. 4D seismic observations provide an opportunity to reduce (almost eliminate) this effect using an assumption that the layered medium does not change (changes slightly) over the time interval between repeated seismic surveys (several months). We analysed 4D seismic data obtained on offshore field with the purpose to study the possibility to use seismic attenuation as a parameter which is sensitive to changes of the amount of the hydrocarbon fluid in pores. The effects of different factors such as changes of seabed depth, repeatability of source position, etc., have been studied. The presented method of 4D attenuation estimation allows us to study block structure of oilfield and separate operating and idle blocks. 4D analysis showed a decrease of frequency-dependent attenuation during oil production. The anomalous zones for all 4D parameters (amplitudes, time, attenuation decrement) are aligned.
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Effect of Attenuation on Reflections from Interfaces Showing Phase Changes
Authors M. Chapman, X.Y. Wu and E. AngererPhase changes are a common phenomenon encountered in amplitude-versus-offset analyses for interfaces with a weak contrast in elastic properties. The effect is usually modelled under the assumption of no attenuation. In this paper, we study the effect of introducing attenuation into the analysis. We find that such reflections are very sensitive to the presence of attenuation, and that the amplitude-versus offset (AVO), frequency-dependent amplitude-versus-offset and phase-versus-offset behaviours are all significantly altered by attenuation. We show a field data example of the phenomenon, and indicate how the effect was used to constrain an AVO inversion.
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Characterization of Gas Clouds Using Non-linear Full Waveform Inversion
Authors M.F. Abd Rahim, E.U. Ulugergerli, T. Konuk and A.R. GhazaliDue to complex wave propagation, anelastic losses, multiple scattering and strong contrast, it becomes a belief that gas cloud area especially the overburden is not invertable. In this paper we demonstrate that the so-called overburden gas cloud is invertable using non-linear full waveform inversion; Case study shown are based on synthetic and field data example from Malaysian basin. In our inversion strategy, we aim to invert the gas cloud overburden reflectivity response to achieve an effective medium representation which will be implemented in the full waveform redatuming method (see companion paper, Ghazali 2013). The depth interval to be inverted is shallow and the purpose of the inversion was not to have a full quantitative interpretation of the shallow overburden, but only to describe the overburden in terms of properties in a way sufficient to derive full waveform redatuming operators that remove the overburden imprint on the deeper targets (Ghazali et al., 2009; Ghazali, 2011). The actual derivation of the redatuming operators is not within the scope of this paper
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In Situ Acoustic Wave Attenuation Measurements of Ocean Water
Authors R.A. Clark, S. Osborne and J. CooperP-wave attenuation in water in the industry-seismic bandwidth is usually regarded as so low that it’s ‘ignored’, and, if quantified, given semi-arbitrary frequency-independent Q values (e.g. 103, 104) when standard acoustics models give 108-1011: hence, we measured in-situ effective attenuation from decay of direct P wave amplitudes along conventional towed streamers, and from comparison of primary and multiple seabed reflections. Neither experiment, with any dataset we used, delivered expected values. The direct wave gave Qeff increases from ~80 to 180 over 30-70Hz, and from ~30 to 90 over 20-90Hz. The primary-multiple comparison gave Q-1eff=-0.0015±0.0042 i.e. Q=-663, nominally consistent with the standard model of intrinsic attenuation but nevertheless indicating a large but negative Q. Apparent attenuation dominates for both horizontal (direct-wave) and vertical (reflection) ray-paths, raising questions about Q values to use in quantitative modelling and analysis, and implying a more insightful appreciation is needed of acoustic wave propagation in ocean water. Of various reasons that could produce apparent attenuation, to date we have investigated quantitatively dispersive sea-floor reflectivity. We find that an intrinsic Q of ~15-30 in the sea-floor sediment is a credible explanation, in whole or part, for the apparent attenuation that we observe on the vertical raypaths.
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Reflectivity Dispersion for Gas Detection
More LessSeismic wave energy attenuation and velocity dispersion happen when wave propagating viscoelastic rocks. A well known effect of attenuation is the change in frequency content and amplitude of a pulse propagating through an attenuating medium. Another is the presence of frequency dependent reflection coefficients. In viscoelastic media, frequency dependent reflection coefficients bring seismic energy dispersion of wave propagation, especially in seismic exploration where the attenuation is mainly due to the fluid content of the considered media. For the wave propagation in gas reservoirs, even no contrast in acoustic impedance, there is a reflection coefficient depending on the reflectivity dispersion. But most reflection coefficient equations cannot explain the above phenomena. Based on linear viscoelastic theory of velocity dispersion and the linearization of Zoeppritz equations, a new approximate reflection coefficient Rf is presented, which is pore fluid related and adds the effects of frequencies and quality factors. With Rf, the reflected energy change induced by reflectivity dispersion can be calculated and used to directly detect fluid. In our work, this method has been successfully applied to characterize the distribution of tight gas sands in a Triassic clastic sedimentary basin of southwest China. And the results can offer reliable foundations for well designing.
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Ultrasonic Attenuation in Shale
By A.F. SigginsInterpretation of the elastic wave response in shales is a challenging task due to the difficulty of understanding the roles of various elements in the shale microstructure, including clay platelets, rigid grains, pore structure and saturating fluids. In sandstones, the velocity-effective stress response is pronounced and follows a log-linear relationship. The influence of pore fluids on velocity is well described by Gassmann (1951). In contrast, the velocity-effective pressure behaviour of shales does not exhibit the same sensitivity at ultrasonic frequencies. The role of pore fluids and microcracks on the seismo-acoustic response of sandstones is reasonably well understood whereas similar mechanisms are difficult to identify in shales. Probably, some form of local flow in response to propagating elastic waves occurs but the mechanism is unknown at this stage. One experimental approach is to use an extended spectral ratio method (Toksoz et al, 1975, Siggins and Dewhurst, 2011). This technique was used to process the P-wave waveform data obtained with ultrasonic measurements on a Norwegian sea shale, at a range of effective stresses with pore pressure held constant. A generalised standard linear solid model (GSLS), was then fitted to the observed spectral responses.
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Determination of Seismic Wave Attenuation Profile in Carbonate Rocks
Authors F. Bouchaala, M.Y. Ali and A. FaridThe Abu Dhabi underground medium is composed by carbonate rocks, in such medium the seismic wave is highly attenuated and scattered during its propagation. In this abstract we determine robustly the total quality factor profile from Vertical Seismic Data(VSP) and also the scattering quality factor profile from the sonic log. The comparison between the two profiles shows that the scattering is bigger than the total attenuation, this is explained by the difference between the frequency range of the VSP(6-120Hz) and that of the sonic data(in order of mega Hz). The big values of the scattering indicate the large degree of heterogeneity in the medium.
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Enhanced 4D Imaging in West Africa Using High-resolution Tomography and Q-imaging
Authors M. Cavalca, J. Penwarden, A. King, S. Knapp, X.G. Meng and S. MondzielA high-resolution tomography and imaging approach is used to enhance the 3D and 4D responses of a complex production field, West Africa. The approach relies upon building a high-resolution anisotropic vertical transverse isotropic (VTI) earth model including anisotropic velocity and 1/Q components and applying amplitude and phase Q-compensation within Kirchhoff prestack depth migration. Velocity and absorption effects associated with overburden heterogeneities are nicely mitigated, leading to enhanced amplitudes and resolution in the prestack gathers and 3D/4D images.
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Seismic Attenuation in Partially Saturated Porous Rocks
Authors E. Caspari, Q. Qi, J.G. Rubino, S.C. Lopes, M. Lebedev, T.M. Mueller and B. GurevichWe provide an overview on recent developments on the acoustics of partially saturated porous rocks. The focus is on the mesoscopic flow induced by seismic waves and leading to wave attenuation and dispersion. At the laboratory scale recent core plug imbibition experiments with simultaneous acquisition of X-ray CT and ultrasonic waveforms allow us to retrieve the saturation dependence of velocities and attenation. Fluid patches and their evolution at the milimetre scale are observed. To model these relations in a consistent manner we invoke the concept of fluid patch membrane stiffness. The latter accounts for the net effect of capillary forces at the macroscale. We further extract the velocity saturation relation from time-lapse sonic logs aquired during CO2 injection into a sandstone formation. It is shown that this velocity-saturation relation can be also modelled using the mesoscopic wave-induced flow effect. Simulation results give further support that mescopic fluid patches on the centimetre scale have a first-order effect on seismic amplitudes provided that the fluid bulk modulus contrast across the patch boundaries is suffiently large. This is typically the case in the presence of a gas phase. We conclude that fluid patches on the milimetre-to-centimetre scale have important implications for attenuation estimates.
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Simultaneous Determination of Q and AVO from Surface Seismic Data
Authors J.A. Beckwith and R. ClarkKnowledge of the interval quality factor value, Q, is of great interest in seismic exploration. Interval Q values can be used as a lithology and pore fluid discriminator as well as in processing of seismic data to restore high frequencies to deep reflectors. Q values are generally obtained from VSP data which has a spatial coverage limited to borehole locations therefore a method to derive Q from surface seismic data is desirable due to the much larger spatial coverage of the derived Q values. Pre-Stack Q Inversion is a method whereby not only is interval Q calculated from pre-stack data but as a by product attenuation-free AVO is produced. Here we introduce updates and extensions to the PSQI algorithm including an s-tau -p transform designed to overcome aliasing problems in tau-p domain data as well as inversion for powerlaw frequency dependent Q. The inversion intercepts are also analyzed with a view to deriving estimates of the p-wave, s-wave and density of the region surrounding and including the interval of interest. It is found that it is possible to constrain the highest interval velocity surrounding the region of interest through a Monte Carlo simulation.
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Detection of Low-frequency Shadow Zones using Quantum Swarm Evolutionary Matching Pursuit Decomposition (QSE-MPD)
Authors A. Semnani, M. Nabi Bidhendi and B. Nadjar AraabiLow frequency anomalies associated with the presence of hydrocarbons, plays the role of frequency dependent attenuation that can be considered as a direct hydrocarbon indicator. Matching pursuit decomposition (MPD) is a suitable tool for spectral decomposition of seismic data to be able to detect and visualize low frequency zones. However, this method has high computational cost. In this work a combination of two artificial intelligence method which are quantum-inspired evolutionary algorithm and particle swarm optimization, has been applied to accelerate the performance of MPD. The proposed method called quantum-swarm evolutionary-matching pursuit decomposition (QSE-MPD) has been utilized to detect low-frequency shadow zones of an offshore gas reservoir of Iran.
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