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Abstract

Wide-azimuth, long-offset seismic data allow azimuthal anisotropy and direction resulting from the presence of natural fractures and anisotropic in situ stress to be estimated. This can be done by measuring how seismic velocities and amplitudes vary as a function of the acquisition azimuth. Rock intrinsic properties like anisotropy, azimuth of fast and slow directions, fracture density, and total porosity, as well as the in-situ principal stress components, can be inferred and used to help with well design, placement, and completion strategies.

Recent advances in seismic azimuthal analysis of media having orthotropic symmetry (an orthotropic medium has three orthogonal planes of mirror symmetry), and quantitative interpretation work flows are illustrated using high resolution pre-stack seismic inversion in an unconventional play in the Williston basin in North America. The algorithm, valid for orthotropic symmetry, was first tested on synthetic data, and then applied to real wide-azimuth 3D data. Results indicate that AVAz (Amplitude Versus Azimuth) analysis of wide-azimuth seismic data for orthotropic media can be used to estimate anisotropy, azimuth, and to constrain the orientation and magnitude of the principal in situ stresses.

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