Azimuthal Anisotropy - VVAZ and AVAZ
All unconventional gas plays rely on the presence of natural fractures to enhance or create permeability in the reservoir. Fractures cause significant, measureable changes in 3D seismic data. These changes appear as variations in seismic amplitudes (AVAZ) and velocities (VVAZ) with shot-receiver azimuth and incident angle (or offset), and are known as seismic azimuthal anisotropy. These are best observed on pre-stack data displayed as Common-Offset-Common-Azimuth (COCA) cubes.
Wide-azimuth acquisition is required to provide sufficient azimuthal information to measure these effects, and multicomponent data can provide additional information from analysis of the converted waves.
Velocity Variation with Angle and aZimuth (VVAZ)
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VVAZ in a COCA cube. The seismic is arranged in offset groups
(separated by yellow dashed lines) ordered by shot-receiver azimuth. F] |
VVAZ appears as variations in traveltime with shot-receiver azimuth in a COCA cube. These variations increase in intensity with increasing shot-receiver offsets and are caused by azimuthal variations in the velocity layer above, which are assumed to be due to the presence of fractures in the overlying strata.
CGG have developed new processing algorithms to analyze and remove these effects, providing measurements of the magnitude and direction of the anisotropy. After removing the effect of VVAZ, seismic data is much better suited for the extraction of amplitude attributes.
Amplitude Variation with Angle and aZimuth (AVAZ)
AVAZ appears as variation in amplitudes with shot-receiver azimuth. This AVAZ effect is due to local changes in the direction and intensity of the azimuthal anisotropy of both the rocks’ rigidity and its seismic velocity, usually caused by changes in the direction and intensity of fracturing and/or stress. CGG can measure the AVAZ effects and use this to create an estimate of the crack density and strike, using FracVista and FractAL techniques.
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Detail map of AVAZ intensity and strike indicating the relative fracture density and strike at this location in the reservoir. Note the well path marked in white, which crosses this level at the cross-hairs indicating moderately intense fracturing with a strike of NNE at the well location. |
The AVAZ measurements come from seismic amplitude data and have similar resolution, while VVAZ measurements have much lower resolution. Reservoir caprocks are generally not fractured and so do not exhibit this azimuthal anisotropy in rigidity. Different rocks behave differently under the same stress loads, e.g. sandstone tends to fracture, while shales tend to flex under similar stress loads. So AVAZ can be an indicator of both the presence of fractured reservoirs and of unbreached caprock.
CGG have achieved a technical success rate for identifying fractures upwards of 80% in unconventional gas plays using seismic azimuthal anisotropy measurements.
This can significantly impact drilling success in areas where success rates are low. These measurements also show the fracture strike and so, by identifying where the gas is coming from, they can be used to avoid drilling into depleted pools. New methods of calculating the stress state can now identify the best areas for fracking to ensure maximum productivity from every well.
