Full Wave Synthetic Acoustic Logs In Saturated Porous Media Part Ill: Pore Shape And Pore Geometry Effects

Abstract

Homogenization theory states that the viscous and mass coupling coefficients of a fully saturated porous medium depend upon the pore shape and the pore geometry. Using an approximate formula, the three body wave velocities and attenuations are analyzed for different configurations. The results show a strong influence of the mass coupling coefficient on these properties. The greater it is, the less biphase the porous medium is. Hence, in the high frequency range, the body wave velocities and attenuations decrease. Using the discrete wavenumber method, synthetic microseismograms are computed in an axisymmetric water-filled borehole. In the presence of a given porous formation with a permeable interface, whatever the mass coupling coefficient, Stoneley wave attenuation and dispersion characteristics are only slightly affected. The reliability of the indirect permeability determination is then ensured. On the other hand, the pseudo-Rayleigh modes are less attenuated with increasing mass coupling coefficient. In the absence of any borehole fluid attenuation, they may become dominant with a high source center frequency. This emphasizes the requirement for low band pass frequency records to be able to easily determine the rheological parameters of a formation. Complex geometries are shown which would allow one to take into account an anisotropic permeability distribution.