Characterization of multiphase electrokinetic coupling using a bundle of capillary tubes model

Abstract

A simple bundle of capillary tubes model has been used to investigate electrokinetic coupling during the flow of water and an immiscible second phase such as air or oil. It is shown that the total electrokinetic coupling is the sum of the individual phase contributions. The electrokinetic coupling depends on the capillary size distribution and wettability. When water is the wetting phase and the second phase is nonpolar and does not contain an excess of charge, the relative coupling decreases with decreasing water saturation. Similar behavior has been observed in geologic porous media. However, when water is not the wetting phase, the coupling increases with decreasing water saturation before falling sharply to zero at the irreducible water saturation. This behavior has not been predicted before. If water is the only phase that contains an excess of charge, then the electrokinetic coupling can be described in terms of the water relative permeability and relative electrical conductivity. The model predictions suggest that multiphase electrokinetic coupling in geologic porous media depends on rock type and wettability and can be described in terms of commonly measured petrophysical properties. However, the predictions should be applied with caution because the pore space topology and pore occupancy of the model is very simple. Moreover, if the second phase is polar and contains an excess of charge, preliminary work suggests that the multiphase coupling can be significantly enhanced.