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.

Previous studies performed in Abu Dhabi oilfields, United Arab Emirates, revealed the direct link of seismic wave attenuation to petrophysical properties of rocks. However, all those studies were based on zero offset VSP data, which limits the attenuation estimation at one location only. This is due to the difficulty of estimating attenuation from 3D seismic data, especially in carbonate rocks. To overcome this difficulty, we developed a workflow based on the centroid frequency shift method and Gabor transform which is optimized by using VSP data. The workflow was applied on 3D Ocean Bottom Cable seismic data. Distinct attenuation anomalies were observed in highly heterogeneous and saturated zones, such as the reservoirs and aquifers. Scattering shows significant contribution in attenuation anomalies, which is unusual in sandstones. This is due to the complex texture and heterogeneous nature of carbonate rocks. Furthermore, attenuation mechanisms such as frictional relative movement between fluids and solid grains, are most likely other important causes of attenuation anomalies. The slight lateral variation of attenuation reflects the lateral homogeneous stratigraphy of the oilfield. The results demonstrate the potential of seismic wave attenuation for delineating heterogeneous zones with high fluid content, which can substantially help for enhancing oil recovery.

Seismic attenuation was estimated from compressional (P) and shear (S) waves carefully extracted from three-component vertical seismic profiling (VSP) data. A high sensitivity of attenuation to fluid content was noticed, which shows the advantage of its use as a seismic attribute for reservoir studies. Contrary to most observations in fully saturated sandstones, the magnitude of P to S wave attenuation in the studied carbonate reservoir zones is higher than one. This disagreement is most probably because the fluid flow models developed to describe the attenuation mechanisms in sandstones are not valid for carbonate rocks due their complex texture. Moreover, the magnitude of seismic attenuation in the reservoir zones is controlled by the saturation of pore fluids and clay content. Furthermore, the attenuation magnitudes obtained at sonic and VSP frequencies are of the same order, and compressional and shear attenuations show similar variation at both frequencies. This indicates frequency-independent attenuations in the studied oilfield.

Investigation of fractured carbonate reservoirs by applying shear-wave splitting concept