A Pilot Multi-azimuth VSP Study for Fracture Characterization Using Seismic Anisotropy

We present the analysis of a multi-azimuth VSP that was designed as a pilot study in one of our tight gas fields to evaluate the quality of seismic anisotropy measurements in this area, with the objective of using them for fracture characterization. We investigate various different anisotropy attributes, which are shear-wave splitting, azimuthal variation of P-wave traveltimes, and azimuthal AVO. The assessment of these attributes provides important guidance for the analysis of anisotropy from surface seismic data in the area. We find clear evidence of azimuthal anisotropy in the VSP data with the most robust measure being shear-wave splitting, but also P-wave traveltime anisotropy can be observed. Other borehole data from the same well suggest that no open fractures intersect the well itself. We resolve this apparent discrepancy between VSP and borehole measurements by seismic modelling with discrete fracture zones that do not intersect the well, but are within the VSP investigation radius.