Prediction of lateral variability in fracture intensity using multicomponent shear-wave surface seismic as a precursor to horizontal drilling in the Austin Chalk

Abstract

A method of multicomponent multisource shear-wave interpretation, utilizing the output of the Alford rotation for separating the fast and slow particle motion stacks in azimuthally anisotropic media, yielded a qualitative prediction of lateral fracture intensity variation prior to horizontal drilling. The method exploits a fundamental difference between the two particle motion stacks when vertical, aligned fractures are the source of the shear-wave splitting. The fast shear wave (S1) section is insensitive to the lateral variation in fracture intensity since its particle motion is parallel to the fractures; hence, reflectivity on the S1 CDP-stacked section should be likewise insensitive to fractures. The slow shear wave (S2) section, however, possesses particle motion perpendicular to the fracture planes, so variation in lateral fracture intensity will affect velocity and reflectivity. Examination of the reflectivity on the two sections for a seismic line reveals laterally consistent reflector strength on the S1 section for the Austin Chalk, the horizon of interest, but laterally inconsistent reflector strength for the same horizon on the S2 section in the study area. Reflector dimming on the S2 section was interpreted as indicating fracture intensity maxima. Horizontal drilling confirmed both fracture azimuth and intensity predictions, and resulted in significantly higher initial hydrocarbon production than for surrounding wells.