Surfactant Flooding With Microemulsions Formed In-Situ - Effect Of Oil Characteristics

Abstract

Abstract Aqueous surfactant flood systems can be designed to generate multiphase microemulsions in-situ as a result of their mixing with the resident oil in the reservoir. The effectiveness of such a process is dependent on the characteristics of process is dependent on the characteristics of the microemulsion generated and its associated phases. These characteristics are determined by phases. These characteristics are determined by the overall composition of the surfactant flood system. This paper discusses results of laboratory studies with these so-called in-situ partitioning processes and the extent to which these processes processes and the extent to which these processes are effected by the characteristics of the oil displaced. These results demonstrate the importance of adjusting the composition of the aqueous surfactant system to the characteristics of the crude oil to be displaced. A method has been developed for quantifying crude oils for purposes of surfactant flood design. This method is based on an effective alkane carbon number (EACN) determined by comparing the phase behavior of crude oils with that of pure alkane hydrocarbons. In general, optimal salinities increased with increasing EACN which is consistent with published results. Displacement tests showed that the salinity corresponding to maximum displacement of a given oil (either a pure hydrocarbon or crude oil) shifted coincident with the optimal salinity, although some deviation from this trend was observed for displacement of high EACN oils. The significance of this deviation is discussed. Results are presented which demonstrate the importance of designing surfactant flood systems based on "live" or simulated "live" crude oil. Loss of volatile components from crude oil can significantly alter the phase behavior of the surfactant flood system. Introduction Considerable research effort has been directed towards developing surfactant flooding (also known as micellar flooding, micellar polymer flooding, microemulsion flooding) into a viable enhanced oil recovery technique. Over the years this effort has led to a rather wide range of surfactant flood formulations in particular with respect to the concentration of surfactant used and dispersing medium used in the surfactant slug formulation. Surfactant slug compositions which have been studied range from essentially total aqueous systems to those prepared in a predominantly hydrocarbon medium. In many of the studies, compositions were designed such that they were initially (locally) miscible with oil or brine or both. However, practical surfactant flooding prevents maintenance practical surfactant flooding prevents maintenance of a miscible condition throughout the life of a process and hence, must be largely affected by process and hence, must be largely affected by immiscible displacement. Recognition of these facts has led to rather extensive studies of the multiphase behavior of surfactant flood systems, in particular in relation to their interfacial tension properties. At sufficiently low interfacial tensions, immiscible displacement from porous media approaches the efficiency of a miscible process. Healy and Reed have demonstrated that immiscible microemulsion systems can be effective in displacement of waterflood residual oil. Indeed under certain conditions, oil recovery as a function of salinity was maximized using a microemulsion formulation which was immiscible with both oil and brine phases. This led to the so-called optimal salinity concept for surfactant flood design. One approach to immiscible microemulsion processes is to use an aqueous surfactant formulation processes is to use an aqueous surfactant formulation which is capable of generating an immiscible microemulsion system in-situ. This type of process design was used in the surfactant flood pilot test in the North Burbank Unit.