Rock physics modelling of elastic properties of rocks saturated with heavy oils

Abstract

Success with heavy oil production depends as much on knowing the geology of reservoir as it does on understanding the fluid properties of the reservoir. Heavy oils are defined as having high densities and extremely high viscosities. Due to their viscoelastic behaviour the traditional rock physics based on Gassmann theory becomes inapplicable in principle. In this paper, we use effective-medium approach known as coherent potential approximation or CPA as an alternative fluid substitution scheme for rocks saturated with viscoelastic fluids. Such rocks are modelled as solids with elliptical fluid inclusions when fluid concentration is small, and as suspensions of solid particles in the fluid when the solid concentration is small. This approach is consistent with the concepts of percolation and critical porosity, and allows one to model both sandstones and unconsolidated sand. To test the approach against the known solutions, first, we apply CPA to a rock with fluid pore fill and compare the obtained estimates with Gassmann results. Second, we compare CPA predictions for solid-solid mixtures with numerical simulations. Good match between the results confirms the usefulness of the proposed fluid substitution scheme. We extend the CPA scheme to predict the effective frequency- and temperature-dependent properties of heavy oil rock. We also use viscoelastic extension of Hashin-Shtrikman (H-S) bounds to obtain alternative estimates. The proposed fluid substitution scheme provides realistic estimates of the properties of a rock saturated with heavy oil which lie between H-S bounds.