On the seismic response of a periodic sequence of three thin layers saturated by two-phase fluids

Abstract

The conversion of fast to slow diffusion P waves in fluid-saturated porous media induces attenuation and dispersion of waves at seismic frequencies. This effect, known as wave-induced fluid flow, occurs at mesoscopic scales, which are much larger than the average pore size and much smaller than the average fast P-wave wavelength. When analyzing this mechanism in hydrocarbon reservoirs with the pore space saturated by multiphase fluids, it is important to include capillary pressure effects and flow interaction between fluids, which cause additional attenuation and velocity dispersion of P waves. We have developed a procedure to determine the phase velocities and dissipation factors in a medium composed of a periodic sequence of three poroelastic thin layers saturated by two-phase fluids. The methodology consists of applying compressibility tests to representative samples of the material, which are defined as boundary-value problems solved using a finite-element procedure. First, we analyze the case of two-phase fluid saturation on each layer, and the results are compared with those of the single-phase (effective) fluid case. Then, several cases of patchy saturation are presented, indicating that residual and wetting fluid saturation play an important role in determining the P-wave velocities and dissipation factors.