Phase Behavior Effects On The Oil Displacement Mechanisms Of Broad Equivalent Weight Surfactant Systems

Abstract Multi-component micellar fluids containing broad equivalent weight surfactants, such as vacuum gas oil (VGO) and polybutene sulfonates, are being considered for commercial applications of micellar flooding. These surfactants are attractive for such applications because they are considerably less expensive than the more pure sulfonates. A phase behavior study of micellar fluids containing VGO sulfonate as a function of salinity, hardness, brine dilution, temperature, cosurfactant type, and surfactant-cosurfactant ratio was used to define the region within which stable micellar fluids can be formulated. The phase behavior study demonstrates that for micellar formulations within the stable region that contain 10 wt. % surfactant, the addition of an equal volume of oil (decane) normally produces Type I microemulsions (Winsor's notation). More dilute micellar fluids (1 wt. % surfactant) show Type III behavior. Effluent analyses from corefloods show that VGO sulfonate remains almost exclusively in the water phase (although a small amount of Type III microemulsion was observed in some of the effluent). This suggests that the recovery mechanism for the VGO sulfonates is donlinated by the Type I lobe of a Type III phase envirorunent. Phase behavior tests of polybutene sulfonates show that in the presence of calcium the sulfonate partitions into the oil phase forming microemulsions partitions into the oil phase forming microemulsions in a Type II lobe of a Type III phase environment. During displacement tests in the presence of calcium, polybutene sulfonates partition into the oil phase generating such microemulsions in situ. phase generating such microemulsions in situ. The use of a Type II to Type I phase transition in moving partitioned sulfonates was investigated. Type II microemulsions generated external to the core were injected to displace the in-place oil. These were followed by a fluid designed to produce a phase transition from Type II to Type I. The phase transition from Type II to Type I. The Type II microemulsion miscibly displaced the inplace oil. The phase transition microemulsion flood displaced oil more efficiently at higher salinity and hardness levels than has been obtained using conventional micellar fluid formulations of VGO and polybutene sulfonates. polybutene sulfonates Introduction Phase Behavior of Surfactant-Brine-Oil Systems Phase Behavior of Surfactant-Brine-Oil Systems The phase behavior of surfactants in hydrocarbon-brine systems has been the subject of interest in the literature for more than a quarter of a century. As seen in Figure 1, a surfactant brine-oil system can equilibrate as single, two-or multiple-phase, depending on the overall system composition. At higher surfactant concentration, all phase environments are single-phase. At lower surfactant concentrations in a Winsor Type I system two equilibrium phases are present. (To simplify this presentation the original phase behavior notation introduced by Winsor is used throughout.) The equilibrium tie lines in the two-phase region indicate one of the phases is essentially oil and the other phase is a microemulsion containing surfactant, brine, and oil. Because the compositions of the two equilibrium phases which result from all mixtures on a tie line are the same, the tie lines are also lines of constant interfacial tension. Type I microemulsions displace oil by a capillary number controlled process and a slug of Type I fluid is miscibly displaced by the polymer mobility (brine) with which it is soluble. Pope and Nelson predicted and observed the following characteristics of oil displacement by Type I microemulsions:Only one type of phase relationship can exist in the transition zone, essentially pure oil inequilibrium with brine-rich microemulsion.The volume fractions of oil-rich phase will decrease rapidly in the transition zone. P. 547