Frequency dependent anisotropy of porous rocks with aligned fractures

Abstract

One of the main issues in the characterization of any reservoir is the ability to predict the effect of fluid properties on seismic characteristics. This effect is studied by modelling fractures as very thin and highly porous layers in a porous background. Elastic moduli of a porous rock permeated by a system of such fractures distributed periodically are obtained using the result of Norris for elastic properties of layered poroelastic media. When both pores and fractures are dry, such material is equivalent to a transversely isotropic elastic porous material with linear-slip interfaces. When saturated with a liquid this material exhibits significant attenuation and velocity dispersion due to wave induced fluid flow between pores and fractures. At low frequencies the material properties are equal to those obtained by anisotropic Gassmann theory applied to a porous material with linear-slip interfaces. At high frequencies the results are equivalent to those for fractures in a solid (non-porous) background. The characteristic frequency of the attenuation and dispersion depends on the background permeability, fluid viscosity, as well as fracture density and spacing.