Abstract
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.
Highlights
Seismic wave attenuation illustrates the energy losses during wave propagation and is often quantified through the inverse of the quality factor, Q
The aim of this study is to estimate attenuation from downgoing P and S waves that are properly extracted from three-component (3C) vertical seismic profiling (VSP) data acquired over an offshore Abu
Based on the simulated travel time of downgoing S waves, calculated by using a 1D sonic log (Figure 6), we suggest that the selected downgoing event is generated from a P conversion close to the top of the Upper Cretaceous Fiqa Formation, which is dominated by argillaceous limestone and calcareous shales [Ali and Farid, 2016]
Summary
Seismic wave attenuation illustrates the energy losses during wave propagation and is often quantified through the inverse of the quality factor, Q. The energy losses are mainly the result of scattering and fluid-related mechanisms. Scattering describes the losses due to multiple small reflections caused by the heterogeneities of the subsurface [Wu and Aki, 1988], while fluid-related mechanisms such as waveinduce fluid flow (WIFF) describes the energy losses due to the frictional movement between solid grains and pore fluids [e.g., Murphy et al, 1986, Müller et al, 2010]. Several subsequent studies investigated attenuation in partially saturated media [e.g., Bouchaala and Guennou, 2012, Müller et al, 2010], and most of them confirmed the strong magnitude of attenuation as observed by Mavko and Nur [1979]. The direct link between fluid viscosity and attenuation allowed Shabelansky et al [2015] to use the latter as a proxy to monitor the viscosity changes during the heating of heavy oil reservoirs, and to manage the production
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