A Nonequilibrium Description of Alkaline Waterflooding

Abstract

Summary Alkaline waterflooding is complicated because the surfactant species are generated in sits from acidic components in the crude oil rather than injected externally. We previously outlined an equilibrium displacement theory 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 laboratory-scale cores. Reversible or irreversible surfactant adsorption on reservoir rock is detrimental to alkaline water-flooding. 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. Introduction In the caustic oil recovery process, alkali contacts crude oil containing natural acids. Once the oil is bathed in alkali, the indigenous acids migrate to the oil/water interface and react with the alkali to produce organic salts, some of which are water-soluble and surface-active. The IFT may be reduced to a level that permits the mobilization of residual oil globules. Because many, if not most, acidic oils are viscous, significant mass-transfer resistances may occur in this extraction process. Diffusion resistances, however, are not the only, nor the most important, resistances in the alkaline recovery scheme. Recently, Rubin and Radke argued that the dynamic-tension minima for acidic oils in alkali are caused by significant sorption barriers at the oil/water interface. They present both theoretical and experimental evidence for this assertion. Unfortunately, these concepts are not directly applicable to the alkaline displacement process. This paper shows the application of their ideas to alkaline flooding of acidic crude oils. We formulate a quantitative description of linear, tertiary, alkaline waterflooding, including all relevant mass-transfer and interfacial-sorption resistances. The proposed chromatographic model permits the calculation of proposed chromatographic model permits the calculation of dynamic IFT's and, hence, of critical capillary numbers for displacement of tertiary oil globules as a function of time and distance on continual injection of an alkaline solution. Essentially, the theory is one of nonequilibrium oil displacement. Therefore, we scale the alkaline displacement process-that is, our calculations permit an estimate of the process-that is, our calculations permit an estimate of the time and distance scales necessary to achieve local equilibrium. If times and distances are long enough, local equilibrium is attained. Our model must reduce to its equilibrium subcase, which is outlined in Ref. 1. Our objective is to investigate the transient phenomena and, in particular, the transient tensions and accompanying transient capillary numbers that occur during alkaline displacement of oil. Before detailing the theory, we briefly outline the transport steps that are thought to occur during alkaline flooding.