Wide-azimuth, long-offset seismic data allow estimating azimuthal anisotropy and direction resulting from the presence of natural fractures and anisotropic in-situ stress. This can be done by measuring how seismic velocities and amplitudes vary as a function of the offset and azimuth associated with the seismic reflection data. 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 with orthotropic symmetry (an orthotropic medium has three orthogonal planes of mirror symmetry), and quantitative interpretation workflows, are illustrated using high-resolution prestack 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 world wide-azimuth 3D data. Results indicate that amplitude versus azimuth (AVAz) analysis of wide-azimuth seismic measurements for orthotropic media, can be used to estimate anisotropy, principle directions, and to constrain the orientation and magnitude of the principal in-situ stresses.
In addition, the middle Bakken member can be fully resolved seismically through the application of a suitable, high fidelity, inversion algorithm to the azimuthal seismic data. This allows characterization of the middle Bakken member in 3D in terms of thickness, porosity and fracture density, as well as elastic and rock strength parameters. With the implementation of a suitable rock physics model, the inversion results can be used to estimate high-resolution fracture density and total porosity 3D volumes, and the orientation and magnitude of the principal components of the in-situ stress tensor.
Surface seismic measurements play a vital role in the characterization of unconventional reservoirs. Many operators worldwide are embarking on full-field evaluation projects that involve acquisition and processing or reprocessing of seismic data. Properly processed, imaged, and well calibrated surface seismic data can be used for the early delineation of 'sweet spots', for prioritization of drilling operations, and for providing valuable information on the elastic and rock mechanics properties of unconventional reservoirs, as well as anisotropy and in-situ principal stress components.