Birefringence analysis at Emilio Field for fracture characterization

Fractures in the earth tend to be aligned and vertical because of the stress regime. If these fractures are smaller than the seismic wavelength we do not sense the individual fractures but we get an average response. This averaging leads to a directionally dependent response. For example, a medium with a single set of vertical fractures leads to horizontal transverse isotropy (HTI). By measuring the suitable seismic attributes of this anisotropy we can infer information about these fracture systems. Fracture information is very important for in-fill drilling programmes. Typically, fractures are aligned perpendicular to the minimum horizontal stress (SHmin) direction and drain an ellipsoidal volume around the well. Thus, well spacing and distribution need to be considered to drain the reservoir volume. In addition, fracture information is important in EOR operations where injection is used. When fractures are present, the injector pattern must take them into account to prevent premature breakthrough and coning, thus ensuring a good sweep. In addition, fracture characterization is important for planning any horizontal wells to be drilled parallel to the minimum stress direction and perpendicular to the fracture strike, thus intersecting more fractures and draining a larger reservoir volume. 3D converted P-to-S waves (PS-waves) provide an excellent opportunity to exploit upgoing shear-wave (S-wave) birefringence (splitting) for delineating reservoir fractures. In azimuthally anisotropic media, fracture intensities and orientations are directly related to travel time differences between the fast and slow S-wave and the polarization direction of the fast S-wave, respectively. Potters et al. (1999) demonstrated the importance of S-wave vibrator data for fracture characterization over the Natih field in Oman. Ata & Michelena (1995) acquired three 2D lines centred on a well to aid directional drilling programmes, and observed that azimuthal anisotropy appeared to be caused by two fracture systems. A small 3D/3C survey collected in the Wind River basin in Wyoming, to calibrate a larger P-wave effort, had some measure of success in characterizing fracture-induced anisotropy (Gaiser 1999; Grimm et al. 1999). Although marine S-wave data routinely show the presence of birefringence in the North Sea (Gaiser 2000a; Probert et al. 2000) and in the Gulf of Mexico (Gaiser 2000b; Spitz et al. 2000), there have been no marine surveys specifically conducted for the purpose of fracture characterization. This paper, presenting results from the 3D/4C Emilio survey, acquired in the Adriatic Sea, demonstrates that these S-wave velocity properties, as well as the overburden S-wave splitting properties, can be determined from azimuth-supergather analyses, and average velocity ratios of the fast (PS1) and slow (PS2) converted waves. Birefringent velocity properties of the overburden must be determined and removed prior to estimating fracture properties at target horizons. Upon removing the splitting effects of the overburden, rotation (Alford 1986) and layer stripping analysis further allow for the determination of fracture properties at the reservoir level.