PS-Wave Birefringence Analysis at the Emilio Field for Fracture Characterization

By

J. Gaiser¹, E. Loinger², L. Vetri², R. Walters³

(1) WesternGeco, N/A, Egypt (2) ENI-Agip, N/A, (3) WesternGeco, Gatwick, United Kingdom

 3-D converted P to S-waves (PS-waves) provide an excellent opportunity to exploit up-going shear-wave (S-wave) birefringence for delineating reservoir fractures. In azimuthally anisotropic media, fracture intensities and orientations are directly related to travel-time differences between fast and slow S-wave, and the polarization direction of the fast S-wave, respectively. The Emilio 3-D survey, acquired over a fractured carbonate reservoir in the Adriatic Sea, demonstrates that these S-wave velocity properties can be determined from azimuth-supergather analyses, average velocity ratios between the fast (PS1) and slow (PS2) converted waves, and 2Cx2C Alford rotation and layer stripping. Exploiting the high uniform fold of coverage at all offset and azimuth ranges, separate volumes for the different wavefield propagation directions were produced (eight PS-wave restricted azimuth datasets). The PS-wave azimuth volumes were combined through Alford rotation for the final fracture analysis. Birefringent velocity properties of the overburden must be determined and removed prior to estimating fracture properties at target horizon. Upon removing the splitting effects below the sea-bottom (almost isotropic), further Alford rotation and layer-stripping analyses allow for the determination of birefringent properties at intermediate levels and the target level in the carbonate Scaglia formation. The results from this analysis at target level near the well locations were in good agreement with the borehole information.