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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
1 - 20 of 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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