A Three-Parameter Representation of Surfactant/Oil/Brine Interaction

When optimal salinity and solubilization parameter are augmented by oil molar volume, the resulting three-parameter representation provides a more precise description of microemulsion phase behavior than has previously been available. It then becomes possible to introduce the idea of equivalent oils (Eqo's) as a replacement for the equivalent alkane carbon number (EACN), which is shown to lack some of the properties needed to implement efficient preliminary screening of microemulsions for EOR. Broadly speaking, oils are ''equivalent'' when they have the same molar volumes, optimal salinities, and solubilization parameters. If, in addition to equivalence, oils are required to have equal viscosities and similar phase behavior as a function of surfactant concentration, then it may be possible to replace microemulsion floods of live crude at high pressure with floods of appropriately diluted dead crude at low pressure. This paper places EACN in perspective by means of the three-parameter representation, explores parallel effects of temperature and alcohol cosolvents, and reveals essential nonlinearities in optimal salinity as a function of oil composition (and hence molar volume) for mixtures of various oils. Much of this is subsequently used to develop methods for preparation of Eqo's and the more complex but evidently essential equivalent systems (EqS's)more » needed to model live crudes.« less