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2nd EAGE St Petersburg International Conference and Exhibition on Geosciences
- Conference date: 16 Oct 2006 - 19 Oct 2006
- Location: Saint Petersburg, Russia
- ISBN: 978-90-73781-64-1
- Published: 16 October 2006
41 - 60 of 209 results
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Multicomponent OBC (4C) Time Imaging over Pamberi, Offshore Trinidad
Authors T.D. Johns, C. Vito, R. Clark and R. SarmientoResults from the P-P and P-Sv prestack time migration of a four-component (4C) OBC seismic swath test acquired in 2004 over the Pamberi area in Block LRL offshore Trinidad, are presented. A description of the processing applied to the multicomponent seismic data, through curved-ray anisotropic Kirchhoff prestack time migration for both the compressional P-wave and mode-converted PSv-wave, is provided. Mode-converted shear-wave data acquired from 4C surveys allow for imaging where conventional seismic data are perturbed due to the presence of shallow gas or fluid in the pore spaces of the rock. Furthermore, mode-converted shear waves propagate with a different raypath than that of the compressional wave, thereby providing an alternative illumination of the subsurface target. As both the P-wave and S-wave record independent measurements of the same subsurface, more reliable rock properties can be uniquely determined, allowing for improved reservoir characterization and lithology prediction. A conventional towed-streamer 3D was acquired over the Pamberi area in 2003, but failed to resolve adequately the target reflectors comprising the reservoir under the main growth fault. Therefore, the purpose of the 4C survey was to evaluate the potential of long-offset multicomponent technology for resolving stratigraphic interpretation in areas of complex challenging geology.
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Shear-Wave Seismic and VSP with Virtual Sources
Authors A.V. Bakulin and R.W. CalvertShear-wave seismology holds great promise but always remains small niche activity due to a variety of operational and subsurface limitations. In this study we demonstrate that the Virtual Source method can overcome many of these limitations and revive shear-wave seismology. With an array of sources at the surface this method allows us to obtain a Virtual Shear Source at the location of each downhole geophone in a well. Firstly, in certain cases it allows us to generate pure shear-wave energy without P-wave contamination using conventional P-wave sources even in a marine environment. Secondly, we can construct SS images of the subsurface even through a complex near-surface for which the velocity model is unknown. Thirdly, we can control the polarization. All this is at a price of placing geophones in the subsurface and making downhole recordings. This price tag is expected to decrease with greater use of permanent downhole monitoring, cheap wells and instrumented oilfields.
We will show examples of SS images obtained through a complex near-surface and show shear-wave checkshots generated with the help of P-wave sources even in a marine environment. This
development may lead to a renaissance of shear-wave seismology especially for permanent seismic monitoring.
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Full-Wave Imaging Projects Using Multicomponent Digital Sensors in Russia
Authors T.E. Galikeev, A.P. Zhukov, I.P. Korotkov and A. BurlakovTwo 3-D multicomponent datasets were acquired in Western Siberia and analyzed. The same type of digital 3-C accelerometers used for data recording were used with different types of sources (dynamite and VibroSeis). Data analysis suggests that the source and near surface conditions can impact the quality of recorded converted wave data, while the vertical P-wave component is of a high quality and equally interpretable when compared to geophone data acquired in the same area. Absorbing properties of the overburden in Western Siberia limit frequencies of converted waves up to 20-30 Hz depending on intensity of near-surface frost penetration. Therefore, an adequate effort should be spent on generating low frequencies in the source’ spectrum. Production sweeps rarely start below 10 Hz due to equipment restrictions and applied sweep tapers further reduce full energy effort spent on low frequencies. Absence of low frequencies in the source manifests itself by a very narrow frequency spectrum of converted waves in the data acquired with VibroSeis. Although, during processing and utilizing data rotation (data were recorded with 3-C digital sensors) the quality of C-wave stacks were improved.
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Advances in Multicomponent Exploration
Authors S. Grion, S. Roche, J. Gibson, B. Mattocks, S. Ronen, H. Watt and K. PoplavskiiMulticomponent (MC) seismic data are gaining importance as an exploration tool for difficult targets. Vector acquisition of seismic data in three orthogonal directions (3C) allows for the recording of both compressional (P) and shear (S) waves. These respond differently to fluid and solid combinations in the subsurface and the joint use of both wave modes provides many potential benefits. Applications relate to structural imaging (seeing through gas clouds, better fault definition, improved near-surface resolution), fracture characterization (density and orientation), fluid discrimination and estimation of lithology (sand/shale, dolomite/anhydrite). Furthermore, vector recordings allow for improved noise attenuation with respect to conventional P-wave records.
This paper provides an overview of multicomponent technology and focuses on the techniques and advances that are improving viability and includes theory and practical examples of how multicomponent technology is improving the ability of the industry to find and produce more oil and gas.
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Minimization of the 4C Alford Data Matrix for Non-Orthogonal PS-Wave Reflection Modes
By J.E. GaiserJoint inversion of P-waves (compressional waves) and PS-waves (converted P to S-waves) is becoming more important for pore pressure prediction, time-lapse effects and characterizing the stress and fracture state of reservoirs. However accurate results will depend on how well the principal S-wave (shear-wave) directions can be determined in anisotropic media. PS-wave reflection coefficients in azimuthally anisotropic media produce a set of non-orthogonal amplitude modes that can complicate the estimation of principal S-wave directions.
The purpose of this study is to show that a conventional Alford rotation provides the correct principal direction for non-orthogonal PS-wave modes in azimuthally anisotropic media when reflection coefficients vary azimuthally. Fast and slow S-wave components of 4C (four component) Alford data are examined in terms of azimuthally anisotropic PS-waves reflection coefficients. Results for orthorhombic media illustrate that the Alford data matrix is diagonally dominant when properly aligned with the principal directions. A synthetic data set consisting of a 5-layer model representing vertical fractures of different orientations demonstrates that conventional Alford rotation and layer stripping will give the correct principal directions when the off-diagonal traces are minimized.
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Numerical Modelling of Seismic Character of Depositional Sequences
Authors B. Gurevich, A. Gerhardt, G. Lambert, C.M. Griffiths and C. DytAnalytical and numerical models of sediment transport and deposition are being increasingly used to investigate controls on sediment variability in depositional sequences. In this paper we consider the stratigraphic response to eustatic sea-level (amplitude and period) and local controls, such as sediment supply and subsidence, and their impact on the resulting reflectivity along a notional well situated in an outer neritic location. The development of key stratal surfaces shows a systematic relationship with respect to the various stages of an aggradation curve.
The results of seismic modelling indicate that seismic events related to sediments deposited under the influence of systematic changes in sea level show a distinctive pattern that approximates the derivative of the original seismic wavelet, while their amplitudes indicate the degree of evolution along the stages of the aggradation curve. If appropriate conditions are met, this distinctive character can be used to distinguish such seismic events from the ones associated with unconformities or layers not directly associated with these controls.
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3D Finite-Difference Modeling of Borehole Wave Propagation
Authors V.A. Tcheverda, V.I. Kostin, D.V. Pissarenko and G.V. ReshetovaFinite-difference (FD) method for 3D simulation of sonic waves propagating in a borehole and surrounding 3D heterogeneous elastic medium is presented. It is based on explicit second-order staggered grid FD scheme that solves the first-order elastic wave equations in cylindrical coordinates. Special modification of Perfectly Matched Layer (PML) for cylindrical coordinate system is developed and implemented. Essential 3D nature of the waves processes for realistic models claims necessity to use parallel computations. Parallelization is performed on the base of domain decomposition approach and implemented under Message Passing Interface (MPI). Result of numerical experiments is presented.
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Wave Propagation in Rocks and Granular Materials Modelled as 1D Heteromodular Elastic Medium
More LessWe consider 1D non-stationary essentially nonlinear dynamic problem for heteromodular elastic medium. Heteromodular medium reacts differently to the tension and compression and therefore presents strongly nonlinear behaviour at infinitisemal deformations. It is subjected to the harmonic force which was "switched on" at zero moment of time. We consider the case when the difference in elastic moduli for tension and compression is small, and construct asymptotical solutions for large times. The analytical results are verified numerically. We construct both near and far-field asymptotics, analyse spectral properties of the problem.
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Tube-Wave Interaction with a Fluid-Filled Circular Fracture of a Finite Radius
Authors S.R. Ziatdinov, A.V. Bakulin and B.M. KashtanTube waves in boreholes are used for characterizing formation properties and hydraulic properties of fluid-filled fractures and permeable zones intersecting wellbores. At low frequencies there is a wellknown approximate formalism describing reflection/transmision of tube waves on layer boundaries and infinite fluid-filled fractures. However for fractures or washouts of finite size one can only use numerical methods such as finite difference that are time-consuming and do not provide physical insights.
Here, we extend existing formalism to analyze reflection/transmission of tube waves on a circular fluid-filled fracture/washout of an arbitrary finite size. We break the problem into three tasks. First, conversion of tube waves into guided waves is modeled using method of Kostek et al. (1998). Second, we derive new analytical solution for reflection of diverging guided slow wave from a fracture tip using 2D approximation. Finally, we derive conversion coefficient describing transformation of imploding guided into tube waves in a borehole. Combining three solutions, we obtain simple analytical representation of a total wavefield in the borehole as a superposition of upgoing and downgoing tube waves generated at the fracture intersection and borne by incoming guided waves and their multiples. New solution is in good agreement with finite difference computations.
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The Analysis of Microseisms Spectrum at Prospecting of Oil Reservoir on Republic Tatarstan
Authors E.V. Birialtsev, I.N. Plotnikova, I.R. Khabibulin and N.Y. ShabalinAre represented the results of the analysis of a microseisms spectrum, which is received at prospecting of small oil reservoirs on territory of Republic Tatarstan. For this analysis it is used the method based on revealing of peak abnormal high amplitude in a range of 1-10 Hz. Are discussed two alternatives hypothesis about nature of this peak over oil reservoirs. Based on experimental data it is shown, that the hypothesis of resonant amplification of microseisms in the resonator between daylight surface and oil reservoir is more adequate. It is shown, that it is can observe oil reservoirs, the border a sedimentary cover - the crystal and of other geological structures, such as active fault, as a peak on microseism spectrum.
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Microseismic Monitoring of Hydraulic Fracturing - Physical Fundamentals of Interpretation
Authors S.A. Shapiro, C. Dinske and E. RothertSeveral basic dynamic processes related to propagation of hydraulic fracturing modify the effective stress in rocks and, therefore, they are relevant for triggering of microseismicity. For instance, these are the creation of the new fracture volume, fracturing fluid loss and its infiltration into reservoir rocks as well as diffusion of the injection pressure into the pore space of surrounding rocks and inside the fracture. Here, using real data we show these processes can be seen from features of the spatio-temporal distributions of the induced microseismicity. Especially, the initial stage of the fracture volume opening as well as the back front of the induced seismicity starting to propagate after termination of the fluid injection can be well identified and used for reservoir engineering.
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Seismic Location of Emission Centers - A New Technology for Monitoring the Production of Hydrocarbons
Proposed is a technology of Seismic Location of Emission Centers (SLEC) – a new modification of passive seismic monitoring of processes taking place in reservoirs in course of production. SLEC technology features the use of standard seismic equipment for the surface acquisition of seismic wavefield and application of “Slionkin Focusing Transform” (SFT) which provides high spatial selection (focusing), noise immunity, and prompt interactive processing of results of seismic observations. Presented are results of industrial testing of the SLEC technology for monitoring hydrofracturing operations, fluid injection performed to maintain the formation pressure and natural fluid dynamics in reservoir during production.
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Synthesis of 30 Years of Seismicity Induced by the Exploitation of the Lacq Gas Field
Authors G. Sénéchal, T. Bardainne and N. Dubos-SalléeDuring the last 30 years, more than 1700 earthquakes have been recorded near the Lacq gas field. This large database has been recorded by a local network of 16 1-component seismometer from 1974 to 1997. Considering this seismicity as the brittle response of stress variations due to the depletion, we determine a synthetic model of deformation for the reservoir. This model is constrained by the spacial distribution of seismological events and by the geometry of the main faults obtained from 3D seismic data. This geometry appears strongly coherent with the analog model of deformation proposed by Odonne et al. (1999). Moreover, we observe a temporal evolution of the spacial distribution of the earthquakes: they are mainly located in the center of the structure during the first years and, later, we clearly observe a spacial migration of the events, showing a ring pattern distribution. Based on this various results, we propose a model of deformation of the Lacq gas field. New data, recently recorded with the new local network deployed in the vicinity of Lacq, are coherent with the proposed model.
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Earthquakes and Hydrocarbons
By M. PuckettIs there any scientific connection between earthquakes and hydrocarbons? Do geophysicists in the energy industry become involved in the study of seismic events? Geophysicists in the petroleum industry do monitor seismic events as they relate to hydraulic fracturing and reservoir monitoring. Borehole seismology in the energy sector involves detection of seismic events on a microseismic, i.e., Richter magnitudes less than 0.0, scale. Hydraulic fracture monitoring using microseismic techniques operational setup issues and two case studies are presented in this discussion.
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Theory of the Virtual Source Method
Authors A.V. Bakulin and V. KorneevThe Virtual Source method (VSM) has recently been proposed as a new practical approach to reduce distortions of seismic images caused by shallow heterogeneous overburden. VSM is demanding at the acquisition stage because it requires placing downhole geophones below the most complex part of the heterogeneous overburden. Where such acquisition is possible, however, it pays off later at the processing stage because it does not require knowledge of the velocity model above the downhole receivers. This paper demonstrates that VSM can be viewed as an application of the Kirchhoff-Helmholtz integral (KHI) with an experimentally measured Green’s function. Direct measurement of the Green’s function ensures the effectiveness of the method in highly heterogeneous subsurface conditions.
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Virtual Source Method for Imaging and Monitoring Below Complex Near-Surface
Authors A.V. Bakulin and R.W. CalvertWe propose the Virtual Source method (VSM) that is designed to provide imaging and monitoring in case of very complex overburdens.
VSM acknowledges upfront that our data inversion techniques are unable to unravel the details of the complex overburdens to the extent necessary to remove the distortions caused by them. Instead VSM suggests placing downhole geophones below that most complex overburden while still exciting signals with a surface sources. Receivers may sit in horizontal or slanted wells, which may be producers/injectors or dedicated sidetracks and first applications include drilling such wells below complicated near-surface, basalt or salt layers. By performing data-driven redatuming with measured Green\'s functions, these data can be recast into complete downhole dataset with buried Virtual Sources located at each downhole geophone. This step can be effectively thought of as a time reversal and it's remarkable feature is that velocity model between sources and receivers is not required to perform it. Therefore not only we can image through the overburden of any complexity, but also we can effectively monitor even if those overburdens change between the surveys. We illustrate VSM by applying to synthetic and monitoring 4D field case.
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The Correction of the Surface Heterogeneity Irregularity at Processing 2D/3D Seismic Data (Gazprom,Western Siberia)
Authors A.D. Avramenko and E.K. VyalkovaThe processing of seismic data of complex shallow structures over the territory of Yamal-Nenets autonomous district requires specific software and techniques to correct the shallow structure heterogeneity. The application of the proven processing techniques depends on the cost, Seismic-geological conditions and on the improved procedure. The article describes the theoretical view points on the heterogeneity correction and gives the examples of processed data results with allowance to the shallow structure heterogeneity over certain areas of Western Siberia, Gazprom.
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Velocity Determination in 3D Seismic
Authors A.A. Shevchenko and B.R. ZavalishinVelocity analysis still remains a weak link of 3D seismic inheriting the profiling (2D) technologies. The development of modern 3D seismic resulted in significant growth of number of channels in acquisition systems and use of migration of raw seismograms, that making it possible to enhance the resolution provided that a velocity model of medium is specified adequately. However it is difficult to count upon a noticeable increase in detail level of a survey where velocities are determined and, hence, smoothed, over several-kilometer-long arrays.
The proposed approach to determining seismic velocities is based upon two principles: Controlled Directional Reception (CDR) – i.e. slant stacking along a plane and the theory of diffraction by aperture. Amplitude analysis on sets of stacked traces computed with increasing aperture makes it possible to measure the first Fresnel zone radius of any regular wave that is proportional to its effective velocity. The ray parameters and effective velocities of all regular waves measurable at any point of a single 3D seismogram make it possible to construct more detailed depth-velocity model. Robustness of the first Fresnel zone radius measurement is discussed here.
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Seismic Imaging with and without Velocity
Authors T.J. Moser and E. LandaA new type of seismic imaging, based on Feynman path integrals for waveform modeling, is capable of producing accurate subsurface images without precise knowledge of a reference velocity model.
Instead of the usual optimization of the semblance objective function, a weighted summation over all representative curves avoids the need of velocity analysis, with its common difficulties of subjective and time consuming manual picking.
The path integral imaging can be applied to stacking to zero-offset, time- and depth migration.
In all these cases, a properly defined weighting function plays a vital role, to emphasize contributions from traveltime curves close to the optimal one and suppress contributions from unrealistic curves.
The path integral method is an authentic model-independent technique, in the sense that there is strictly no parameter optimization or estimation involved.
Synthetic and real data examples shows that it has the potential of becoming a fully automatic imaging technique.
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True Amplitude Thin Slab Multi-One-Way Inversion - Application to Marine-Type Seismic Data
Authors D.A. Kiyashchenko, R.E. Plessix, B.M. Kashtan and V.N. TroyanTrue-amplitude wave-equation migration can be obtained by using a modified imaging principle and a multi-one-way scheme. In order to avoid the artifacts of the multi-one-way scheme due to the turning rays, a thin-slab approach is investigated. A wave-equation inversion formula is proposed to recover the reflectivity. Under the high-frequency approximation, it is similar to the pseudo-inverse formula. This formula is applied on a synthetic and real example.
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