Frequency-dependent attenuation of compressional wave and seismic effects in porous reservoirs saturated with multi-phase fluids

Varying fluid types and saturations in porous media cause attenuation of compressional seismic waves and thus contribute to frequency-dependent reflections. We analyze the frequency-dependent attenuation and the corresponding seismic reflection behavior by taking into account the effects of the multi-phase fluid on the wave-induced fluid flow in porous reservoir. Fluid viscosity, the property that largely controls pore-fluid mobility, is proportional to the relaxation time, which in turn influences the frequency-dependent velocities. Here, we describe the variation of effective viscosity varies with saturation for a sandstone reservoir saturated for different multi-phase fluid mixtures using the Refutas equation. Then we explore the frequency-dependent velocity and inverse quality factors under different fluid saturation cases by employing equivalent-medium theory of Chapman's model. Next, we present a frequency-dependent phase-shift wavefield extrapolation method to simulate frequency-dependent seismic wavefield on 4-layer models. Comparative analyses indicate that the frequency-dependent attenuation and the seismic behaviors are functions of fluid viscosity associated with hydrocarbon saturations. Hydrocarbon saturations in fluid mixture strongly affect attenuation and characteristic frequencies. The frequency-dependent attenuation within the seismic frequency range can be increased due to the presence of hydrocarbons in multi-phase fluids. Synthetic seismic records indicate that the frequency-dependent attenuation of compressional wave and seismic reflection signatures are significantly dependent on hydrocarbon saturation, in terms of amplitude, waveform and traveltime of the reflection located at the base of the saturated reservoir and the underlying shale layer.