Borehole geophysics

[BH-05] Estimation of P and S wave attenuation from VSP and sonic waveforms in a carbonate reservoir

*Fateh Bouchaala¹, Mohammed Y Ali¹, Jun Matsushima² (1. Khalifa University of Science and Technology (United Arab Emirates), 2. The University of Tokyo (Japan))

Despite the value added by incorporating shear (S) wave attenuation in reservoir characterization studies, so far, all attenuation carried out in Abu Dhabi oilfields have only been based on compressional (P) wave. This is due to the difficulty in extracting S waveforms from seismic data, especially from data acquired in complex media such as carbonate rocks. In this paper we present a workflow of extracting S downgoing waves from three-component walkaround VSP data which we properly processed, before applying the spectral ratio method to estimate attenuation. The results show a high sensitivity of attenuation to fluid content such as oil content. This confirms the importance of using attenuation as a seismic attribute for reservoir studies. Contrary to most observations in fully saturated sandstones, the magnitude of P to S wave attenuation was found higher than 1 in the reservoir zones, which are fully saturated with oil and water. This disagreement is most probably due to the fact that the fluid flow models developed to describe the attenuation mechanisms in sandstones are not valid for carbonate rocks due to their complex texture. Furthermore, attenuation magnitude in the reservoir zones is controlled by pore fluid saturation and clay content. This is due to the fact that fluid saturation has a direct impact on fluid-related mechanisms, while the clay controls the permeability and hence the amount of the frictional losses caused by the relative movement between pore fluid and solid grains. The shear and compressional attenuations show similar variation and frequency-independency.