Effects of variations in fluid properties and fracture geometry on dispersion, anisotropy and reflection in media with planar fractures

Our objective was to discover the effect of variations in fluid properties and fracture geometry on the velocity of seismic wave propagation in fluid-saturated media with parallel planar fractures. We used numerical models calculated by analytical solutions to examine the behaviour of P-wave phase velocity dispersion in the normal direction to layering, in non-porous and porous media with planar fractures. We also examined the anisotropy of low frequency phase and group velocities of fast and slow P-waves and angular dependent reflection coefficients in media with planar fractures, under conditions of saturation by fluids with varying bulk moduli, densities, and fracture apertures. We defined several parameters: rHL, rhl, and Ic characterising dispersion, ϵ′ characterising anisotropy, rFS characterising the difference between fast and slow modes, R0 and G′ characterising reflection. Our results show that the behaviour of dispersion shows wider stopbands in the case of gas saturation. Concavity indicator of dispersion Ic for gas saturation was greater than that for liquid saturation, and is usually greater than 1. Anisotropy is more sensitive to bulk modulus contrast than to density contrast between the solid and fluid, and rFS is more sensitive to density contrast than to bulk modulus contrast. The case of gas saturation usually had a greater negative R0 and a greater value of G′ compared with those of brine, heavy and light oil saturations. Our results are helpful in distinguishing fluid types saturating geophysical fractures and estimating the aperture and spacing of planar fractures. In seismic exploration, bulk modulus and fluid density can provide useful information in distinguishing among brine, oil, and gas; fracture geometry is important to estimate the permeability of reservoirs. This article is protected by copyright. All rights reserved