Cosurfactants in Micellar Systems Used for Tertiary Oil Recovery

Abstract

Abstract The influence of alcohols used as cosurfactants on several micellar systems was investigated. These alcohols modify phase behavior and control the amount of brine or hydrocarbon that a microemulsion can "solubilize." Also, viscosity can be adjusted using the right cosurfactant for mobility control. Relationships between cosurfactant concentration and electrolyte concentration, hydrocarbon type, or temperature are presented. Introduction Micellar slugs used as oil-displacing agents in extra-oil recovery projects are usually composed of surfactant(s), hydrocarbon, water, electrolyte, and one or more alcohols that serve as cosurfactants. Physicochemical properties of these systems have Physicochemical properties of these systems have been presented by several authors; phase behavior has received special attention. Such emphasis is justified because of displacement-mechanism considerations and micellar-slug deterioration by reservoir rock and fluids. Other work has been concerned with defining the role of particular sulfonates in oil displacement and loss of surfactant to reservoir rock. Healy et al. presented detailed analyses of a single surfactant/cosurfactant combination in micellar systems and laboratory oil recovery experiments. Papers have presented qualitative information on the role of cosurfactants. Gogarty and Tosch found that the amount of 2-propanol (IPA) required to produce phase-stable systems depended on produce phase-stable systems depended on hydrocarbon type. They and others reported that IPA makes a micellar slug more compatible with reservoir brines, whereas oil-soluble alcohols enhance compatibility with crude oils. Cosurfactants have various effects on viscosity; IPA usually causes a reduction. Healy and Reed noted a decrease in viscosity upon addition of tertiary amyl alcohol (TAA). For the system examined, TAA increases the size of the water-external region in the ternary phase diagram. Others have reported that cosurfactants increase water solubility of sulfonates and reduce adsorption on reservoir rock. Although cosurfactants have been shown to perform a variety of functions, no systematic quantitative study of their effect on physicochemical properties of micellar systems has appeared in the petroleum literature. This paper will show how cosurfactants affect phase behavior, control hydrocarbon and brine solubility, interact with electrolyte concentration, and change viscosity and electrical conductivity. Systems examined include a low-water-concentration, oil-external microemulsion; a water-external microemulsion; and "intermediate" systems for which the continuous phase is not clearly defined. Each type of micellar system has been tested in field applications of the Maraflood oil recovery process. process. MATERIALS AND METHODS Compositions of the six micellar systems studied are shown in Table 1, which also indicates the source and equivalent weight of the petroleum sulfonate surfactants used. Table 2 lists information about the alcohols used as cosurfactants. Viscosities were measured at 72 deg. F and at 3 or 6 rpm with a Brookfield Model LVT viscometer equipped with a UL adaptor. Shear rates at this speed are about 5 to 10 sec-1. All phase-behavior experiments were performed at 72 deg. F, with the exception of the relationships shown in Fig. 11. Systems were observed for a minimum of 24 hours. PHASE BEHAVIOR WITH ALCOHOL PHASE BEHAVIOR WITH ALCOHOL Alcohols as cosurfactants modify the phase behavior of a brine-hydrocarbon-surfactant micellar system. For example, Composition A in Table 1, composed of an oleophilic surfactant (470-equivalent-weight petroleum sulfonate), 28-weight-percent water, petroleum sulfonate), 28-weight-percent water, and hydrocarbon (light, straight run gasoline), separates into an aqueous phase in equilibrium with a microemulsion phase. SPEJ P. 161