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A fractured reservoir in the light of seismic wave attenuation attribute.

Abstract

In this paper, we investigate a fractured reservoir zone in an oilfield located in the Emirate of Abu Dhabi, United Arab Emirates, by using seismic wave attenuation attribute.

The dense geometry of a 3D VSP walkaway spiral survey carried out in the oilfield, permits to perform a high resolution (15o spacing) azimuthal estimation of the attenuation at different offsets. By assuming the azimuth of the minimum attenuation as the strike direction of fractures, we obtain this direction in the three reservoir units, R1, R2 and R3 at several offsets. The comparison between the estimated directions and those based on core data interpretation is quite satisfactory. This is a good indication about the potential of seismic wave attenuation attribute for fracture characterization.

Our results show that the effect of fractures on seismic wave attenuation is frequency dependent. By modifying the frequency range, the dominant orientation of fractures changes. This is related to scattering and intrinsic attenuation mechanisms that are strongly dependent on frequency. More investigation on this dependence can be useful to provide information about fracture sizes and their fluid content. This a great advantage of attenuation attribute, compared to the classical seismic attributes, such as curvature and ant tracking, which are widely used to extract fracture orientations from seismic data.