Results

SUMMARY

One of the fields in the Norwegian Sea has been imaged several times over the past decades, both with conventional narrow azimuth seismic surveys as well as with ocean bottom seismic. The extensively faulted structure of the field and the possible presence of a salt diapir cause imaging problems in some areas. Therefore, a simulation study has been initiated to judge whether or not a marine full azimuth acquisition geometry improves the image of the subsurface.

For this simulation study, simplified velocity and density models of the field were created, containing the main features characterizing it, as well as the problem areas. The simulation was done with 3D finite difference (FD) modeling. Data sets with and without free surface multiples were generated, and imaging from a full azimuth acquisition geometry was compared with imaging from a conventional narrow azimuth geometry.

FD modeling shows that the full azimuth design generally leads to a better suppression of noise in the data, mainly due to increased fold. Depth slices show that fault edges are imaged sharper in a full azimuth geometry. Also, the image of and below the salt/limestone structure is improved. However, attenuation of multiple energy due to increased cross line fold is less than expected, except for the first seabed multiple. The low maximum frequency used in FDmodeling may have limited the increase in image quality with the full azimuth modeling.

INTRODUCTION

The field of interest in this study is located in the Norwegian Sea, west of the coast of Mid-Norway. The geological strata below the Base Cretaceous Unconformity (BCU) are intensely faulted and the reservoir of the field is segmented into many compartments. The field was put on stream in the mid-nineties, but a few parts are still not fully understood because of imaging problems. In particular, a dome shaped feature in what is called the M-segment, causes a very disturbed image. Earlier, it was assumed that the dome is caused by a salt diapir sourced by Triassic salt. A recent study shows that there are some high impedance limestone banks of seep deposits located around the dome. In addition, in some specific areas of the field, faults and dip directions are unclear, giving rise to dipping conflicts below the BCU. In spite of several vintages of seismic data, it has not been possible to obtain a much clearer seismic image of these problem areas, although multiples caused by the strong impedance variations around the dome seem to be attenuated better in ocean bottom seismic (OBS) data.

For a good seismic image of a subsurface point, a proper illumination is needed. That means, one needs an adequate distribution of offsets and azimuths, or in other words, a look from all directions. Conventional survey geometries tend to look in one direction only, i.e., sources and receivers are placed along a line with a uniform distribution of offset values. In a marine full azimuth (FAZ) geometry, the objective is not only to vary the offset, but also the azimuth.

SUMMARY

One of the most important mechanisms of P-wave attenuation at seismic frequencies is known as "mesoscopic loss", and is caused by the presence of heterogeneities larger than the pore size but smaller than the predominant wavelengths (mesoscopic-scale heterogeneities). These effects are caused by equilibration of wave-induced fluid pressure gradients via a slow-wave diffusion process. To perform numerical simulations in these type of media using Biot''s equations of motion at the macroscale, it is necessary to employ extremely fine meshes to properly represent these mesoscopic heterogeneities and their attenuation effects on the fast waves, which makes this procedure computationally very expensive or even not feasible. An alternative approach is to employ the numerical upscaling procedure recently presented by the authors, to determine equivalent undrained complex frequency dependent plane wave and shear moduli defining at the macroscale a viscoelastic medium behaving in similar fashion as the original Biot medium. These equivalent moduli are determined performing numerical compressibility and shear tests on a representative sample of heterogeneous bulk material, allowing to reduce in several orders of magnitude the degrees of freedom needed to characterize the material. In this paper we present a finite element procedure, formulated in the space-frequency domain, to perform numerical simulations of wave propagation in highly heterogeneous fluid-filled porous sandstones employing a viscoelastic model with complex moduli determined using the mentioned upscaling procedure. The methodology is first validated by comparison with previous numerical experiments using Biot''s equations of motion. Finally, the procedure is applied to simulate and analyze the effect of underground carbon dioxide (CO2) accumulations on the amplitude and attenuation of seismic waves.

INTRODUCTION

The analysis of attenuation at seismic frequencies due to wave-induced fluid flow caused by mesoscopic-scale heterogeneities has been the object of many studies, such as White J.E. et al. (1975); Pride S. et al. (2002); Muller T. M. and Gurevich B. (2005); Mason Y. and Pride S. (2006); Carcione J.M. and Picotti S. (2006); Rubino, J. G. et al. (2007), among others. These heterogeneities in the solid frame and fluid properties, typically on the order of centimeters, are much smaller than the wavelengths of the fast P and S waves travelling in Biot''s media. Consequently, the huge number of degrees of freedom (DOF) needed to represent these heterogeneities and their attenuation effects at the macroscale in any finite element or finite difference based numerical procedure employing Biot''s equations renders such approach not feasible. In Santos, J. E. et al. (2007) the authors presented a novel numerical upscaling approach to tackle this problem. The idea is as follows: take a representative sample ?R of bulk material and perform (local) numerical oscillatory compressibility and shear tests to determine the equivalent undrained complex frequency dependent plane wave Mc(w) and shear modulus N(w) associated with WR in the range of frequencies at which the material is going to be tested by acoustic methods.

These local compressibility and shear oscillatory tests are defined as boundary value problems formulated in the space-frequency domain assuming that the sample obeys Biot''s equations of motion.

Summary

The noise attenuation aspects of single-sensor acquisition are discussed in this paper. The paper shows examples from 3D seismic data acquired in West Kuwait in 2006. It is often difficult to make direct comparisons between singlesensor and existing conventional data because of the differences in acquisition geometry and processing sequence. In an attempt to get a more direct comparison and a better understanding of the uplift in quality of the recently acquired single-sensor data, this data set was used to simulate conventional acquisition geometries. Equivalent data processing sequences were used to make such comparisons.

Summary

In the recent years, many wide-azimuth(WA) surveys have been conducted, the main purpose of them is wide-azimuth subsurface complex structural imaging using P waves. By using wide-azimuth acquisition, more information from complex subsurface areas was acquired. In Liaohe Basin, we face a challenge that the subsurface complex structural areas can not be properly illuminated by using conventional narrow azimuth survey, so it failed to image these areas. By properly using target illumination oriented acquisition design, advanced wide-azimuth pre-processing, wideazimuth tomographic velocity and anisotropic parameter inversion and amplitude-preserved anisotropic pre-stack depth imaging, etc., satisfactory subsurface imaging result has been obtained. In addition, more information, such as anisotropic parameters, azimuthal anisotropic velocity, fracture density and direction, etc, can also be obtained, and all of them can assist reservoir characterization.

SUMMARY

Knowledge of interval attenuation can be highly beneficial in reservoir characterization and lithology discrimina- tion. Here, we combine the spectral-ratio method with velocity-independent layer-stripping to develop a tech- nique for estimation of the interval phase attenuation coefficient from reflection seismic data. The algorithm is designed for arbitrarily anisotropic target layers, but the overburden is assumed to be laterally homogeneous with a horizontal symmetry plane. Although no velocity in- formation about the overburden is needed, interpretation of the anisotropic attenuation coefficient in the target layer requires estimation of the velocity function.

The interval phase attenuation in a reservoir can be used to predict the presence and distribution of hydrocarbons. For example, our method can help to distinguish between steam and heat fronts in heavy-oil reservoirs. Azimuthal variation of the interval attenuation in fractured formations can provide sensitive attributes for fracture charac- terization and reconstruction of the stress field.

INTRODUCTION

Attenuation analysis can provide valuable information about lithology, presence of fluids, and physical proper- ties of subsurface rocks. In particular, attenuation may serve as an indicator of permeability, mobility of fluids, and fluid saturation (e.g., Batzle et al., 2006; Behura et al., 2007).

A number of laboratory measurements (Hosten et al., 1987; Tao and King, 1990; Prasad and Nur, 2003; Behura et al., 2006) and field case studies (Ganley and Kanasewich, 1980; Liu et al., 2007; Maultzsch et al., 2007) indicate that attenuation can be strongly anisotropic (directionally-dependent) because of the pref- erential alignment of fractures, interbedding of thin at- tenuative layers, and nonhydrostatic stress. The field study of Maultzsch et al. (2007) finds the symmetry of attenuation anisotropy to be different from that of veloc- ity anisotropy. Therefore, measurements of attenuation anisotropy may provide additional information about the fluid properties of fractured reservoirs (Liu et al., 2007).

Existing attenuation estimates from reflection data (e.g., Vasconcelos and Jenner, 2005) are obtained for the whole section above the reflecting interface. Dasgupta and Clark (1998) introduce a technique for estimating interval at- tenuation from reflection data based on the spectral ra- tio method. This algorithm, however, is restricted to zero-offset attenuation and requires the knowledge of the source signature. Moreover, Dasgupta and Clark (1998) apply the NMO stretch prior to attenuation analysis, which may distort the estimated attenuation values.

Here, we present a method for computing the interval attenuation coefficients using an extension of the layer- stripping technique originally introduced by Dewangan

METHODOLOGY

Although our technique of estimating interval attenu- ation can be applied in 3D, this work is restricted to 2D models. Let us consider a pure-mode reflection in a medium that consists of an anisotropic, heterogeneous target layer under a laterally homogeneous overburden with a horizontal symmetry plane (Figure 1). To make wave propagation two-dimensional, the vertical incidence plane has to be a plane of symmetry in all layers. This restriction can be relaxed in the 3D version of the method operating with wide-azimuth data. The exact interval traveltime-offset function in the tar- get layer can be constructed by combining the target event with reflections from the bottom of the overburden.

Summary

When imaging geologic structures below an overburden with laterally varying attenuation (Q) properties, it is important to account for the variable amplitude dimming, frequency loss, and phase distortion. Migration algorithms based on the viscoaoustic wave-equation have been developed to handle a 3-D Q model, but they require unmigrated input data and require significant run time. Fast 1- D Q-filtering techniques can be applied to compensate for Q effects and can be applied to migrated traces, but they don''t account for 3-D physical geometries. Here we present a pseudo migration approach that is applied to migrated seismic data. This approach utilizes the ray paths through the model and restores amplitude and phase according to the integration of the Q-effect along these ray paths. The computational efficiency of this algorithm is similar to that of 1-D filtering, yet it accounts for the 3-D geometries. The validity of our technology is illustrated by a 2.5-D synthetic data example.

Summary

We analyze the relationship between seismic attenuation and rock properties in well 11-25-13-17W3 from the Ross Lake heavy oilfield, Saskatchewan. Well log analysis indicates that the main lithologies in this well are shale and shaly sandstone. Interval Q values for the P wave and shear wave were estimated by applying the spectral ratio method on near-offset VSP data (which used both vertical and horizontal vibrators). The Q values are most reliable from 400m to 1050m for the P wave and from 225m to 1050m for the shear wave. The Q values correlate interestingly with petrophysical variables. Qp values increase with Pand S-velocities and decrease with Vp/Vs and porosity. Shaly sandstone shows more attenuation than pure shale and sandstone. A crossplot between Qp and clay-bound water indicates more attenuation in shaly sandstone possibly caused by the interaction between mobile water and clay-bound water. Q values for S-waves also display a similar relationship. Velocity dispersion was observed between VSP-derived velocities and sonic velocities. On average, sonic log velocities are 3.4% higher than VSP velocities for the P wave, and 4.8% higher for shear waves.

Summary

A new method uses eigenimages to construct a coherent noise model in a localized time-space window and performs the noise attenuation by adaptively subtracting the noise model from the input data. Advantages to this method include minimum spatial-amplitude smearing, effective attenuation on various types of coherent noise such as ground roll, air waves and near-surface scattered energy as well as handling both the aliased and non-aliased noise quite well. This new nonlinear filter significantly outperforms conventional techniques. We demonstrate the performance of this local-nonlinear filter with real data examples.

Summary

In the winter of 2006/7 the BP Wamsutter team conducted an extensive seismic field trial including a world-first deployment of cable-less full-wavefield single-sensors in a 3D onshore survey, in addition to a 3DVSP and cross-well seismic. Despite weather and mechanical challenges, the acquisition operation successfully delivered a unique high density, full azimuth, full-wavefield dataset for analysis. Special care & ''designer'' processing was applied to this unconventional data to optimize resolution and retain robust amplitude and azimuthal information. This has successfully yielded a full stack image comparable in quality to that of conventional geophone arrays, despite the lower-than-anticipated trace recovery, boding well for future surveys where significantly greater recovery can be expected. Additional Prestack AVO (amplitude with offset) and AZAVO (azimuth with offset) attributes are shedding new light on a reservoir previously only imaged with full stack data; the converted-wave component is also of unusually high quality and is ultimately expected to contribute to describing the reservoir to new levels of detail. Learnings from the field trial are already impacting the onshore seismic strategy both within the Wamsutter field and across BP’s tight gas business.