Correlation of Emulsion Stability With Phase Behavior in Surfactant Systems for Tertiary Oil Recovery

Abstract

Abstract This paper describes a study of the emulsions which could occur during a pilot surfactant flood, such as that conducted by Phillips Petroleum Co. in the North Burbank Unit, Osage County, OK. The phase behavior of this surfactant system can be characterized by three types of microemulsions, with the transition from one type to another being a function of the salinity. The rate at which emulsions coalesce was seen to correlate directly with the type of microemulsion. Coalescence was slow for macroemulsions at low salinities, rapid at intermediate salinities (where the final state was a three-phase system), and varied from slow to rapid at salinities above the three-phase region. Knowledge of the correlation between phase behavior and emulsion stability can be useful in treating macroemulsions produced during a surfactant flood. Introduction With the increased emphasis currently being placed on the use of surfactants for tertiary oil recovery, a potential problem exists with emulsions which can be produced as a consequence of a surfactant flood. For example, if a channeling problem between an injection well and a production well should occur, it may be possible to produce relatively large amounts of surfactant at moderately high concentrations (0.2 to 2.0070). Under these conditions, emulsions of oil and brine could be stabilized by the presence of the surfactant and could pose a serious problem. Although these emulsions are thermodynamically unstable and ultimately should separate into bulk oil and water phases, the presence of surfactants can increase greatly the time required for such separations. Typical oilfield operations allow, at most, several hours for this separation of phases to occur, but some emulsions containing surfactants may require weeks or even months to separate. Thus, a definite need exists for being able to accelerate this coalescence process. Phillips Petroleum Co. is conducting a pilot surfactant flood in the North Burbank Unit (NBU) in Osage County.1,2 The work reported here was directed at developing a contingency plan for breaking emulsions which may be produced by this surfactant flood. The problem of studying emulsions produced by a surfactant flood has two aspects:the nature of the phases which result when thermodynamic equilibrium finally is attained andthe rate at which this equilibrium state is reached. This is not to imply that any emulsion can be described completely by characterization of these two properties but rather that these are the two properties most important to oilfield operations and, hence, form the basis for the work reported here. The next section discusses the equilibrium properties of surfactant systems and the one following discusses the coalescence of emulsions. The fourth section describes the use of chemical demulsifiers to accelerate coalescence. Equilibrium Phase Behavior The equilibrium phase behavior of systems of oil and water containing appreciable amounts of surfactant (i.e., 0.5%) is characterized by the presence of microemulsions.1,3-5 These microemulsion phases have a high degree of structure and may contain large amounts of both oil and water.