Phase Properties Of Oil-Recovery Systems Containing Petroleum Sulfonates

Abstract

Abstract The number and types of phases formed in surfactant-oil recovery systems have been investigated. Depending on the conditions, three types of phase separations are observed. These are (1) an phase separations are observed. These are (1) an aqueous-based microemulsion (labeled gamma-type) in equilibrium with an oil phase, (2) a microemulsion containing significant fractions of both water and oil (labeled beta-type) in equilibrium with an aqueous phase and an oil phase, and (3) an oil-external phase and an oil phase, and (3) an oil-external microemulsion (labeled alpha-type) in equilibrium wit an aqueous phase. The alpha-type microemulsion forms when the surfactant is relatively more soluble in oil than in water, the garma-type forms when the surfactant is more soluble in water, while the beta-type forms when the solubilities are comparable. For surfactant systems employing petroleum sulfonates and alcohol cosurfactants, the oil solubility of the surfactant relative to that in the water is increased when (1) the salt concentration increases, (2) the concentration of divalent ions relative to monovalent ions increases, (3) the relative solubility of the alcohol in oil vs that in water increases, (4) the sulfonate equivalent weight increases, (5) the average alkane chain length of the oil is decreased, or (6) the temperature decreases. Physical properties of the phases, such as Physical properties of the phases, such as electrical conductivity, density, viscosity, interfacial tensions, and compositions of the phases, indicate that the gamma-type microemulsion is water external, the alpha-type is oil external, while the beta-type is some sort of mixed external phase or bicontinuous structure. For the systems studied, the controlling interfacial tension has its minimum value in the three-phase region, and therefore, it should be expected that oil recovery will be maximized with a three-phase system. The magnitude of the minimum tension was seen to be lower when the three-phase region occurs over a narrow salinity range. Introduction Recently, surfactant systems (usually consisting of petroleum sulfonate, an alcohol, salt, and water) have been employed to recover oil from reservoirs that have been reduced to residual oil saturation by primary and secondary methods. It has been seen that physical properties important to oil recovery, such as interfacial tensions and emulsion stability, are strongly correlated with the phase behavior of these systems. Indeed, the actual phase behavior of these systems. Indeed, the actual tertiary recovery, obtained from laboratory simulations in cores, appears to be strongly correlated with the phase behavior. Therefore, it is imperative that we understand the phase behavior in order to identify fully the mechanisms by which surfactant systems recover oil. Recent developments in methods of representation of phase behavior in multicomponent systems should facilitate the understanding of surfactant systems and how they recover oil. In this work we will restrict attention to systems involving three pure components (water, salt, and an alcohol) and two pseudocomponents (oil and a petroleum sulfonate). The careful use of petroleum sulfonate). The careful use of pseudocom-ponents has been discussed in detail elsewhere and pseudocom-ponents has been discussed in detail elsewhere and need not concern us here. it has been found useful, under certain circumstances, to examine only a small portion of the quinary-phase diagram (a portion of the quinary-phase diagram (a four-dimensional figure) in screening surfactant mixtures as tertiary oil recovery agents. The analysis trill make extensive use of phase-volume diagrams since the shapes of these diagrams are indicative of nearness to critical points and hence to low interfacial tensions. The phase-volume diagrams employed herein were constructed by equilibrating two parts by weight of an aqueous surfactant system with one part by weight of oil and plotting the resulting phase volume vs some independent variable. Some examples of choices of this independent variable are salinity of brine in the surfactant system, salt type (monovalent ions, divalent ions, etc.), partition coefficient of the alcohol between oil and water, average equivalent weight of sulfonate, oil type (e.g., different choice of crude oil or pure hydrocarbon), and temperature. Fig. 1 illustrates three general types of phase behavior that often occur in these systems.