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.