Effects of fluid saturations on undrained poroelastic constants in layered media

Abstract

SUMMARY Understanding the fluid dependence of the poroelastic stiffness constants of a layered porous package is of great importance in subsurface exploration and development. While the effects of the pore-fluid distribution caused by coarse-scale heterogeneities within an isotropic medium have been studied for several decades, the role of these heterogeneities on the poroelastic constants of a finely layered package is still largely unexplored. In this study, we apply the poroelastic upscaling methods to estimate the fluid-dependent poroelastic stiffness constants of a layered package at the coarse scale. The numerical results show that the refined Gassmann's fluid substitution formulae presented in this paper is applicable if a single fluid phase is uniformly saturated within a layered package. The stiffness constants (${c_{11}}$ and ${c_{33}}$) of the layered package with patchy saturations are always higher than or equal to those obtained for the medium with homogeneous saturations, the stiffness constants predicted by the refined fluid substitution formulae for the package simultaneously saturated with different fluids fall between them. Experimental results confirm the relationship between the undrained vertical stiffness constant and the effective pore-fluid bulk modulus for the patch saturated package, indicating that a reasonable result can be achieved by properly choosing an effective poroelastic model that accounts both for the fluid hydraulic communication and the anisotropy of the medium. The results improve the understanding of the coarse-scale fluid dependence of the poroelastic stiffness constants of a layered package, and therefore, it can be used to interpret the seismically inverted elastic parameter for the petrophysical properties in heterogeneous reservoirs.