A micellar flood was run in a Berea core using an isomerically pure nonionic surfactant. Phase behavior studies, interfacial tension measurements, and adsorption tests were conducted prior to flooding. High solubilization parameters, low interfacial tensions, and acceptable adsorption values were obtained for this surfactant. Nonetheless, the micellar flood failed to recover the anticipated amount of oil. Compositional analyses of flood effluent indicate that the micellar slug fell apart resulting in an increase in interfacial tensions and significant surfactant loss due to partitioning into the oleic phase.
The purpose of this study is to address the issue of whether nonionic surfactants can be used effectively to recover oil, Prior literature alludes to both unacceptably high adsorption, as well as failure to achieve ultralow interfacial tensions, as downfalls of these materials. Many reservoirs contain harsh brines which are not amenable to sulfonates. Thus, the motivation for consideration of nonionics is their insensitivity to such harsh brines, by which we mean they remain soluble and have less tendency to form precipitate phases than sulfonates. phases than sulfonates. Many of the early studies of nonionic surfactants as oil recovery agents were conducted before Reed and coworkers delineated the important relationship between surfactant-oil-brine phase behavior and oil recovery efficiency. It is thus often difficult to correlate these older studies using today's concepts. For example, many early adsorption studies were performed without regard for the conditions under which the surfactants would be effective oil recovery agents (so called "optimum" conditions).
Commercial nonionic surfactants, often polyethoxylates or propoxylates, are diverse mixtures polyethoxylates or propoxylates, are diverse mixtures of species. Lipophiles range from narrow to fairly broad distributions, while hydrophiles are generally Poisson distributions of ethylene oxide numbers (EON). The effects of these mixtures have been shown to be important in understanding surfactant-oil-brine phase behavior. Partitioning of surfactant between the microemulsion and excessoleic phases leads to fractionation of the different EON species, where the lower EON species prefer the oleic phase. Such fractionation can be very important to a micellar flooding process. To fully consider this effect, however, requires coupling the component partitioning as described by the phase behavior with the dynamics (flow) of the phase behavior with the dynamics (flow) of the micellar process. Consideration of this phenomenon is outside the scope of this work. We have chosen instead to work with single-specie nonionic surfactants, eliminating one potential mechanism for process breakdown. process breakdown.
In the selection of a surfactant system for micellar flooding, phase behavior, physical properties, adsorption (retention), and oil recovery properties, adsorption (retention), and oil recovery effectiveness are often considered. The methodology chosen here is to consider each of these factors for a single chemical system, rather than to perform a detailed investigation on any one of these perform a detailed investigation on any one of these areas. We have chosen to work with Berea core because of availability and reproducibility. The oil chosen was n-octane because it generates phase behavior similar to crude oil, yet allows much simpler chemical analysis. All brines studied are solutions of sodium chloride in deionized water.
In choosing a surfactant for this study several factors were important. First, we wanted a surfactant which would generate high enough solubilization parameters to ensure acceptably low interfacial tensions. The second factor dealt with cloud point temperature, (the temperature where phase separation begins in dilute aqueous phase separation begins in dilute aqueous surfactant solutions). Above the cloud point, surfactant-brine solutions separate into two phases, a surfactant-rich phase and a brine-rich phase.