Azimuthal investigation of compressional seismic-wave attenuation in a fractured reservoir

Three-dimensional vertical seismic profiling data acquired from an oilfield located in Abu Dhabi, United Arab Emirates, were used to obtain a high-resolution multioffset azimuthal estimate of compressional seismic wave attenuation. On the basis of the assumption that the fracture strike corresponds to the azimuthal direction $φ0$ at which the attenuation is minimized, fracture orientations were obtained in three reservoir units. Two approaches were used to determine $φ0$⁠: first from the variation of the absolute attenuation $Q−1$ with the azimuth and second from the variation of the relative attenuation $ΔQ−1$ with the azimuth. The rose diagrams of $φ0$ estimated from the $Q−1$ variation indicated better agreement with those showing the strikes of open and cemented fractures obtained from core interpretation than with either of those showing the two types of fractures separately. However, the rose diagrams of $φ0$ estimated from the variation of $ΔQ−1$ were more similar to those showing the strikes of open fractures obtained from core and Fullbore Formation Microimager data. This observation can be explained by the fact that in the first approach, all types of fractures contribute to the scattering and fluid-related mechanisms of $Q−1$⁠. However, in the second approach, $φ0$ is obtained from a least-squares fitting of the variation of $ΔQ−1$ with the azimuth, which is based on the squirt flow mechanism that is caused by the movement of fluid between grain pores and fractures. Therefore, a comparison of the orientations obtained using these two approaches can be an efficient way to separately determine the orientations of open and cemented fractures.