ELASTIC PROPERTIES OF SATURATED POROUS ROCKS WITH ALIGNED FRACTURES

Abstract

Elastic properties of fluid-saturated porous media with aligned fractures can be studied using the model of fractures as linearslip interfaces in an isotropic porous background. Such a medium represents a particular case of a transversely isotropic (TI) porous medium, and as such can be analyzed with equations of anisotropic poroelasticity. This analysis allows the derivation of explicit analytical expressions for the low-frequency elastic constants and anisotropy parameters of the fractured porous medium saturated with a given fluid. The five elastic constants of the resultant TI medium are derived as a function of the properties of the dry (isotropic) background porous matrix, fracture properties (normal and shear excess compliances), and fluid bulk modulus. For the particular case of penny-shaped cracks, the expression for P-wave anisotropy parameter ε has the form similar to that of Thomsen (1995). However, contrary to the existing view, the compliance matrix of a fluid-saturated porous fractured medium is not equivalent to the compliance matrix of any equivalent solid medium with a single set of parallel fractures. This unexpected result is caused by the wave-induced flow of fluids between pores and fractures.