Seismic Fracture Characterization of a Gas Sand Reservoir - A Comparison Study of Two AVAz Methods

In this work we present a comparison of two AVAz inversion methodologies for seismic characterization of fractured reservoirs, one based on Ruger’s approximation and a second one based on the singular value decomposition of AVAz data by Varela and coauthors. The comparison is performed in a pilot case study using a patch covering 33 square kilometers from a 3D AVAz seismic dataset. The studied reservoir is a gas sand reservoir. The studied area is centered at a well location where image logs revealed the existence of almost vertical open fractures with a high fracture density. The fracture density distribution obtained through the model-based SVD inversion allowed characterising an area of high fracture density in the vicinity of the well located in the centre of the studied survey. These results are consistent with existing FMI log data. The outcome of this study also indicates that the high degree of anisotropy induced by the fractures cannot be described by Ruger’s approximation. The analysis at two markers above the fractured reservoir showed that the fracture density distribution is significantly different than at the top of the fractured target. Therefore, footprint acquisition effects do not seem to influence the fracture density results.