Azimuthal anisotropy analysis applied to naturally fractured unconventional reservoirs: A Barnett Shale example

Abstract

Studying the seismic responses of velocity and amplitude on wide-/full-azimuth seismic data is now common for unconventional reservoir characterization. Velocity variation with azimuth (VVAz) and amplitude variation with azimuth (AVAz) are two of the most popular tools to map not only the relative intensity and orientation of natural fractures but also the strength and orientation of the maximum horizontal stress SH. We prestack time migrated a wide-azimuth Barnett Shale survey in North Texas into eight azimuths and reduced noise on the gathers using prestack structure-oriented filtering. We then computed the envelope, spectral peak frequency, and prestack P-wave impedance attributes for each azimuthally limited seismic volume. We compensated the VVAz effects by flattening each sector along the Barnett Shale key horizons, thereby registering the gathers for subsequent AVAz analysis. The results indicate the intensity, orientation, and confidence of azimuthal anisotropy effects on seismic velocity and amplitude, which can be referred to smaller scale vertical cracks or natural fractures. Our analysis reveals four zones of high anisotropy intensity that can be tied to either the regional structures or paleo stress field. Analysis of production data indicate from the anisotropy interpretation results that vertical, sealed fractures are the dominant cause of anisotropy and those specific fractures inhibit production. This observation and results indicate that horizon-based azimuthal anisotropy analysis avoids the VVAz effect and can be applied to fractures and regional stress field prediction.