Phase Partitioning of Anionic and Nonionic Surfactant Mixtures

Abstract

Abstract The efficiency of oil recovery by waterflooding can be greatly improved by the addition of judiciously selected surfactants. Under certain conditions, when surfactant solutions are mixed with oil, microemulsions are formed that may be in equilibrium with an excess oil (Type I), an excess aqueous phase (Type II), or both (Type III). The partitioning of the surfactant between those coexisting equilibrium phases is important to consider in the design of microemulsion processes for oil recovery. This is particularly true because different phases generally move at differing velocities within the pore spaces of an oil reservoir and therefore fractionation will occur in successive stages along the flow path. This leads to chromatographic separation of the surfactant molecules. The problem is complicated because all surfactant systems are blends of molecules, and chromatographic separation will result in a change in the optimal salinity of the surfactant system. Thus, a system that is initially optimized will not remain optimized during the course of the flood. To examine the question of selective surfactant partitioning, we varied the composition of oil/water/surfactant equilibrated systems so that they would pass through the optimal formulation region. The partitioning of anionic surfactants into the oil in Type I and Type III phase systems was small. Furthermore, binary and ternary mixtures of these surfactants were found to copartition; that is, little if any fractionation was detectable. Their collective behavior was intermediate between those of the pure components. Nonionic surfactants, unlike anionics, partition substantially into the oil phase in Type I and Type III phase systems. This was found to be an intrinsic property of the surfactant structure and not unique to those nonionic surfactant systems that are polydisperse. Selective fractionation also was found for polydisperse nonionic surfactants.