Fluid effect on shear wave splitting in a porous fractured reservoir

Abstract

The presence of fractures in a reservoir is a main cause of azimuthal anisotropy of its elastic properties. A shear wave propagating in an azimuthally anisotropic medium splits into two components with different polarizations if the source polarization is not aligned with the principal symmetry axis. If the direction of shear wave propagation is not parallel to the plane of fracturing, shear-wave splitting will depend upon the normal fracture compliance, which in turn depends upon the properties of the filling fluid. If the system of pores and fractures in a fluid-saturated rock is interconnected, then fluid flow between pores and fractures must be taken into account. How shear-wave splitting varies with fluid properties depends upon the assumptions that are made regarding the pressure relationship existing between pores and fractures. In this paper we use the anisotropic Gassmann equations, and existing formulations for the excess compliance experienced due to fracturing, to estimate the splitting of vertically propagating shear waves as a function of the fluid modulus. This is done for a porous medium with a single set of dipping fractures and with two conjugate fracture sets dipping with opposite dips to the vertical. The estimation is achieved using two alternative approaches. The first approach assumes that the fractures and pore space are in full pressure equilibrium with respect to fluid flow. That is, the frequency of the elastic disturbance is low enough to allow enough time for fluid flow between the fractures and the pore space. In the second approach each of the fracture sets are in full pressure equilibrium with the surrounding pore space, but not with the other fracture set. That is, the frequency is low enough to allow fluid flow between a fracture set and the surrounding pore space, but high enough so that there is not enough time during the period of the elastic disturbance for fluid flow between fracture sets to occur. It is found that the second approach yields a much stronger dependency of shear-wave splitting on the fluid modulus than the first one.