Ion Exchange With Clays in the Presence of Surfactant

Abstract

Abstract The transport of sodium and calcium through porous media in the presence of clays and surfactant has been calculated. The exchange of the cations with both the clays and the surfactant micelles is assumed to result entirely from electrostatic association. The results show that a system in which the preflood, slug, and drive have the same sodium and calcium concentration can have a significant increase in the calcium concentration in the surfactant bank and a significant decrease in calcium concentration in the drive because of ion exchange. A process with a salinity gradient design can have a decrease in calcium concentration in the surfactant bank compared with the injected slug because of ion exchange. Introduction The exchange of sodium and calcium with clays is important to surfactant flooding because calcium has a much greater effect than sodium on the phase behavior and interfacial tension. This exchange phenomenon is well understood in the absence of surfactant. Experiments with surfactants have shown that the surfactants have a significant effect, but the interaction of the cations with them is not well understood. The previous works assumed that the flowing surfactant either was dissociated completely or formed a calcium complex of some form, independent of the sodium concentration. This work models the exchange of sodium and calcium with the micelles as if the association results entirely from electrostatic association. The calculations are compared with observations of corefloods. Assumptions A previous paper described the association of sodium and calcium with surfactant micelles. It was shown that the association can be described by electrostatic models such as the Donnan equilibrium model. It was shown that the "selectivity coefficient" with this model is not a constant but is only a weak function of the electrolyte composition at sufficiently high electrical potentials if the Stem layer thickness is large compared with the Debye-Huckel characteristic thickness of the electrical double layer. In this work it is assumed that the selectivity coefficient is directly proportional to the surfactant concentration but otherwise independent of the electrolyte composition. Further assumptions areco-ion exclusion is negligible,surfactant monomer concentration is negligible,dispersion is negligible,surfactant adsorption can be described by an adsorption isotherm that is dependent only on the surfactant concentration and the adsorbed surfactant associates sodium and calcium in the same way as the micelles,all anions other than the surfactant are treated as chloride ions,volume fraction of the flowing phase occupied by surfactant is small - i.e., the total concentration of unassociated ions per unit of pore volume is equal to the concentration of that ion in the electrolyte solution - andthere is local equilibrium. Only single-phase flow is considered here. The presence of an oil phase will change the velocity of the waves and is described in Ref. 1. The Ion Exchange Equilibrium Relationships The Donnan equilibrium models is the stalling point for the models for association of sodium and calcium with both the clays and the micelles. The concentration of the associated species is denoted as C and C to represent the concentration of species i associated with the clays and micelles, respectively, expressed as equivalents per unit of pore volume. SPEJ P. 181^