Pore-Scale Evaluation of Dielectric Measurements in Formations With Complex Pore and Grain Structures

Abstract

Dielectric permittivity measurements are typically used to estimate water-filled porosity, since water’s dielectric permittivity is bigger than the permittivity of other constituents of the rock. The dielectric interpretation methods such as Complex Refractive Index Model (CRIM) (i.e., known as volumetric techniques) are extensively used to correlate dielectric permittivity of the fluid-bearing rocks to petrophysical properties such as porosity and water-filled porosity. However, volumetric techniques usually oversimplify the rock structure. These techniques do not take into account the impact of complex pore structure and spatial distribution of solid and fluid components on dielectric properties of the rock. The lack of reliable rock physics models to interpret dielectric permittivity measurements can lead to huge uncertainty in estimates of water-filled porosity.

This paper introduces a pore-scale numerical simulation method to quantify the impact of pore and grain structures and heterogeneity on dielectric permittivity measurements, and a new method to improve assessment of water-filled porosity in formations with complex pore/grain structure using dielectric measurements. In order to quantify the structure of pore and grain networks, we introduce the directional tortuosity factor and investigate the correlation between the directional tortuosity and the corresponding dielectric permittivity.

We applied the introduced techniques on three-dimensional (3D) computed tomography (CT) scan images of sandstone and carbonate core samples and synthetic organic-rich source rocks. We showed that our modified CRIM method is more reliable in sandstone and carbonate formations for the assessment of fluid saturation, compared to the conventional CRIM method. In the case of synthetic organic-rich source rocks, we observed that (a) the presence of kerogen affects the dielectric permittivity measurements, but it does not significantly affect estimates of water-filled porosity, and (b) the presence of pyrite and its spatial distribution significantly affect the dielectric permittivity of organic-rich source rocks, and failure to consider the influence of pyrite and its spatial distribution on dielectric permittivity will cause large uncertainty in estimates of water-filled porosity.