Cosurfactant-Enhanced Alkaline Flooding

Abstract

Abstract Many crude oil candidates for enhanced oil recovery by alkaline flooding produce their lowest interfacial tension at very low concentrations of alkali. Alkaline consumption by the rock makes propagation through the oil reservoir of such propagation through the oil reservoir of such dilute alkaline solutions prohibitively slow. The dilemma of having to choose between highest displacement efficiency (lowest interfacial tension) and satisfactory displacement rate can be resolved by adding cosurfactants to the alkali. Low concentrations of properly chosen cosurfactants raise the concentration of electrolyte required for minimum interfacial tension to alkali concentrations high enough for satisfactory propagation of the alkaline bank. That is, just as in chemical flooding, a cosurfactant can be used to raise the "salinity requirement" of an alkaline flood. Activity Maps, similar to the Salinity Requirement Diagrams of chemical flooding, are useful in formulating and understanding the results of cosurfactant-enhanced alkaline floods. Alkaline flooding systems, formulated by the methods discussed in this paper, recover as much oil in laboratory core floods as well-formulated chemical flooding systems. Introduction Johnson defined four mechanisms of enhanced oil recovery by alkaline flooding:"Emulsification and Entrainment" in which the crude oil is emulsified in-situ and entrained by the flowing aqueous alkali,"Wettability Reversal (Oil-Wet to Water-Wet)" in which oil production increases due to favorable changes in permeabilities accompanying the change in wettability,"Wettability Reversal (Water-Wet to Oil-Wet)" in which low residual oil saturation is attained through low interfacial tension and viscous water-in-oil emulsions working together to produce high viscous/ capillary number, and"Emulsification and Entrapment" in which sweep efficiency is improved by the action of emulsified oil droplets locking the smaller pore throats. Castor, et al proposed a fifth mechanism, "Emulsification and proposed a fifth mechanism, "Emulsification and Coalescence," in which unstable water-in-oil emulsions form spontaneously in the alkaline solution, then break to create local regions of high oil saturation, hence, increased permeability to oil. Because alkaline flooding research predated surfactant flooding research in Shell and basic concepts of salinity control and low IFT viscous/ capillary mechanisms were developed in those early alkaline studies, we have generally considered both in-situ generated surfactant flooding and preformed surfactant flooding to be special cases preformed surfactant flooding to be special cases of the same process. That is, we view alkaline flooding as a type of chemical flooding in which the surfactant is formed in-situ as the alkali converts petroleum acids in the crude oil to soaps. Our objective is to improve the cost- effectiveness of alkaline flooding by applying principles developed through recent research on principles developed through recent research on chemical flooding. As in chemical flooding, high oil-displacement efficiency depends upon attaining and maintaining conditions of "optimum salinity." When optimum salinity cannot be achieved by simple adjustments in salinity, we use cosurfactants, just as in chemical flooding. In keeping with the terminology of chemical flooding, we call the petroleum soaps formed during and alkaline flood the "primary surfactant" and any added preformed surfactant the "cosurfactant." Hence, supplementing an alkaline flood with preformed surfactants, added to the alkaline slug preformed surfactants, added to the alkaline slug before injection, becomes "cosurfactant-enhanced alkaline flooding." P. 413