A Nonequilibrium Description of Alkaline Waterflooding

Summary Alkaline waterflooding is complicated because the surfactant species are generated in situ from acidic components in the crude oil rather than injected externally. We previously outlined an equilibrium displacement theory1 that captured the essential features of the process. Assumption of local equilibrium, however, does not allow for a careful examination of the complex transport and kinetic phenomena that occur. Our non-equilibrium theory models the pertinent mass-transfer and kinetic resistances affecting interfacial tensions (IFT's) during displacement and oil-recovery efficiency. The model shows that when natural acid in a trapped oil blob contacts alkali, it diffuses to the interface, adsorbs, reacts with alkali, desorbs, and convects into the bulk aqueous phase. We quantify these transport steps, calculate concentration profiles in the oil and flowing aqueous phases during a linear displacement, and determine how transient tension behaves as a function of time and distance. Mass-transfer resistances are insignificant, but sorption resistances at the oil/water interface affect the transient evolution of IFT during alkaline displacement. Interfacial sorption barriers are modeled with first-order kinetics. Desorption resistances may be quite large, so low IFT's may not be established in typical laboratoryscale cores. Reversible or irreversible surfactant adsorption on reservoir rock is detrimental to alkaline waterflooding. Rock adsorption reduces the amount of surfactant available to the oil/water interface and may raise the IFT to levels that are ineffective in mobilizing residual oil globules. Most importantly, we provide a conceptual framework to design a successful alkaline waterflood.