Electrical Efficiency: A Pore Geometric Model For The Electrical Properties Of Rocks Available to Purchase

An understanding of the way pore geometry affects the conductivity of rocks is important for proper interpretation of log and laboratory resistivity data when evaluating sedimentary rocks for hydrocarbon potential. The basis for virtually all formation evaluation from logs is the empirical Archie equation. This equation does not have a theoretical basis; rather, it is an ad hoc relationship derived from a graphical analysis of data from clean sandstones, No entirely satisfactory physical model based on pore geometry has been found which leads naturally to Archie's equation. We have formulated a new resistivity interpretation model based on the efficiency with which rocks conduct electric current and including an explicit parameter to account for pore geometry. The basis of the model is that electric current densities are not uniform throughout the conducting phase in the pore system of a rock. Numerical model studies show high current density in pore throats and low current density in nearly stagnant volumes in isolated parts of the pore system. The electrical efficiency is quantified by comparing the conductivity of water in a rock to the conductivity of a straight tube containing the same volume of water as the rock. The efficiency is inherently independent of the bulk volume of water in the rock, and is determined only by the non-uniform current distribution due to pore geometry and the distribution of hydrocarbons in the pore system. Moreover, the electrical efficiency can be calculated a priori given the geometrical distribution of the conducting phase. Variations in the distribution of water in a pore system due to diagenetic, depositional, or hydrocarbon emplacement processes result in varying electrical efficiency. Empirical relationships between electrical efficiency and water content can be used to calculate hydrocarbon saturations from log data independently of Archie's equation. The parameters in Archie's equation are explained using electrical efficiency theory.