Determination of effective grain geometry for electrical modeling of sedimentary rocks

Abstract

Accurate electrical modeling of sedimentary rocks is crucial for the interpretation of electrical survey data and must take into account the geometric information of the rock grains. We have developed an incremental model for the multiphase electrical conductivity of sedimentary rocks on the theoretical basis of differential effective medium models. The developed incremental model was first validated on clean and clay-rich sandstones and then applied to simulate the conductivity of five digitally created rock samples with different spectra of grain aspect ratios. The conductivity is calculated as a function of water conductivity at full water saturation, and an effective grain aspect ratio for a two-phase medium is determined to reproduce the electrical behavior of the digital rocks. It was found that the effective grain aspect ratio can either be determined analytically or estimated from the aspect ratio spectrum. A link between the obtained effective grain aspect ratio and the cementation coefficient widely used in petrophysics has been established. The cementation coefficient increases with the decrease of the grain aspect ratio (increasing the z-axis depolarization factor) and grain conductivity and the increase of sample porosity. It showed that the cementation coefficient can be interpreted as a measure of the connectivity of pores as well as the conductivity of the grains. Our results gave us new insights into the physical meanings of the grain aspect ratio and cementation coefficient, which could be used to facilitate the electrical modeling or interpretation of reservoir rocks.