Inversion of azimuthal velocity and amplitude variations for seismic anisotropy
Khaled Al Dulaijan
Natural fractures can play a key role in production of hydrocarbon in the form of increased porosity and permeability for efficient fluid flow especially for unconventional reservoirs of low matrix permeability. Thus, knowledge related to fracture orientation and intensity is vital for the development of unconventional hydrocarbon reservoirs, such as tight sand oil and shale gas reservoirs. The most productive horizontal wells are those crossing the most vertical fractures. A pattern of vertical fractures causes the seismic wavefield to exhibit azimuthal anisotropy. The best way known to detect fractures, at large scales, is by recognizing the effect of them on seismic data in attempt of inversing it. The Altamont-Bluebell play is within the Uinta Basin in northeast Utah, and is considered an unconventional play in the sense that natural fractures act as fluid storage and conduits in mostly the tight sandstones and partially in the tight carbonates. Consequently, analyzing the azimuthal variation in the observed amplitudes and velocities of the 3D seismic data acquired over the Altamont-Bluebell field is of great value in ascertaining important and relevant reservoir conditions in terms of porosity and permeability. In the Altamont-Bluebell field, azimuthal anisotropy was analysed using two types of data (3D surface seismic and VSP) and using three different methods (inversion of azimuthal amplitude, inversion of azimuthal travel times, and S-wave splitting). To use the VSP data, several types of VSPs were processed from field files to final products (P and S wavefield images and velocities). All results of different methods and data types were correlated to each other where similarities were pointed out and differences were explained.
Numerical seismic modeling provides a valuable tool for geophysicists to test and validate their methodologies. Fractures make numerical modeling more complicated and introduce complexities that might even require geophysicists to validate their numerical models before using them to assess their methods. Scaled-down physical modeling of seismic surveys provides a unique opportunity to test, validate, and develop methods for characterizing fractured reservoirs, because it can produce experimental data from known physical properties and geometries that can be comparable with both numerical and field seismic data. Therefore, physical modeling is utilized to determine stiffness coefficients associated with the anisotropic material and validate techniques used for anisotropy, such as S-wave splitting.