relative costs have been reduced dramatically and the information provided by
walkaway vertical seismic profile (VSP) surveys has improved. In addition to ...
Walkaway Borehole Seismic Surveys Clearer definition of zones of interest Applications ■■
Seismic scale measurement of amplitude variation with offset (AVO) under ideal conditions
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Reduced rig time Improved surface seismic processing using fully calibrated 1D earth model Improved earth model from more quantitative offset-dependent amplitudes Assessment of shale caprock anisotropy impact on amplitude versus angle (AVA) responses to improve prestack time surface migration Clearer definition of zones of interest with zero-phase and multiple-free seismic image
Features ■■
Computation of effective anisotropy
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Geometrical spreading
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Direct attenuation measurement
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Offset-to-angle transform measurement from the direct compressional arrival attributes Computation of the effective overburden anisotropy Computation of intrinsic anisotropy, local to the receivers
Since the introduction of long array tools, such as the VSI* versatile seismic imager, the relative costs have been reduced dramatically and the information provided by walkaway vertical seismic profile (VSP) surveys has improved. In addition to providing a high-resolution seismic image around the wellbore, walkaway VSP surveys also offer direct measurement of attenuation and anisotropy, which assist in surface seismic processing and interpretation.
Direct measurement of overburden effects
Walkaway VSP data provides measurements of the overburden propagation effects that affect quantitative surface seismic AVO analysis. Effective anisotropy, geometrical spreading, the anelastic attenuation of seismic waves (Q), and the offset-to-angle transform can all be measured from the direct compressional arrival attributes (Fig. 1). These measurements are combined with surface seismic processing sequences to compute more quantitative offsetdependent amplitudes. If several array settings are acquired over a depth interval in the well, multiple depth targets can be calibrated and depth-dependent anisotropy and Q can be deterimined, further improving the calibrated one-dimensional (1D) earth model used in surface seismic processing (Fig. 2).
Direct measurement of anisotropy
There are several approaches to polar anisotropy—also called vertical transverse isotropy (VTI)—calibration Figure 1. Determination of effective Q values from walkaway VSP data. using walkaway VSP data. Polar anisotropy local to the receiver array can be estimated using two methods. Techniques that fit direct-arrival times or differentiated direct-arrival times, called phase slowness methods, work best offshore —or when statics are not relevant—and in areas in which the velocity profile exhibits no velocity inversions. These methods assume a laterally invariant medium with horizontal slowness constant everywhere along the ray that Figure 2. Effective anisotropy measurements from walkaway VSP connects source and receiver. direct P travel times and their impact on NMO correction.
Walkaway Borehole Seismic Surveys Horizontal slowness is calculated by differentiation across offset (feasible only when static corrections are not significant) and is best measured by recording turning rays, which is possible only in the absence of velocity inversions. When these conditions cannot be met, the second method, which uses apparent slowness across the receiver array and wavefield polarizations, can be used.
AVO measurement for surface seismic calibration
AVO calibration integrates dipole sonic and density logs with zero-offset VSP, specially designed walkaway VSP, and, if available, surface seismic gathers. With this data, a calibrated elastic or anelastic anisotropic VTI model (including Q) will be built that will produce the most reliable modeled AVO response. In addition, it will provide seismic AVO measurements over the reservoir sequence and a quantification of the overburden propagation effects. Fluid substitution is performed to produce all-water or all-gas models, which are then used to produce common midpoint synthetics to compare anisotropy/isotropy in the shales and water/gas in the sands. The models are also used to produce walkaway synthetics to compare against the processed walkaway data. Tuning/layer thickness effects are also evaluated. No matter how good a model is, it is still a model. There is no replacement for an actual seismic measurement. The walkaway data provides a seismic scale measurement of AVO under ideal conditions. AVO walkaways differ from imaging walkaways, because the array tool is clamped just above the target reservoir sequence. Therefore, the downgoing wavelet is virtually the same as the one causing the reflections. After careful processing using proprietary software, the walkaway VSP seismic image is zero-phase and multiple-free. After normal moveout (NMO) correction using the calibrated VTI model, the real walkaway AVO measurement is compared to the model (Fig. 3).
Figure 3. AVO calibration. 0.5 0.4 0.3 Vertical slowness, s/km 0.2 0.1 0 –0.4
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–0.1 0 0.1 0.2 Horizontal slowness, s/km
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Figure 4. Phase slowness method: Slowness data (black dots) derived from walkaway VSP data. The best-fitting anisotropic slowness curve is indicated with a red line. Also shown are an isotropic slowness curve (green line) and an elliptical slowness curve (blue line).
Figs. 4 and 5 illustrate the phase slowness method and the slowness polarization method.
Downgoing SV
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Downgoing P
Apparent phase slowness, s/km
Isotropic model Anisotropic model (TI) Vp 3.50 km/s Vs 1.53 km/s ε 0.58 δ –0.05 Symmetry axis tilt 6° Well deviation 34°
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Upgoing P
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Upgoing SV –0.50
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Polarization angle, degrees
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Figure 5. Slowness polarization method: A slowness polarization plot for a 240-shot walkaway VSP in Algeria, with slownesses measured across a five-receiver subarray. (Graphic reproduced with permission from the In Amenas consortium [BP, Statoil, and Sonatrach]; from Leaney, 2008.)
www.slb.com/boreholeseismic *Mark of Schlumberger Other company, product, and service names are the properties of their respective owners. Copyright © 2011 Schlumberger. All rights reserved. 10-DC-0105
