A Three-Parameter Representation of Surfactant/Oil/Brine Interaction
Abstract When optimal salinity, C, and solubilization parameter Vo/Vs are augmented by Oil molar volume, V mo, the resulting three-parameter representation provides a more precise description of microemulsion phase behavior precise description of microemulsion phase behavior than has previously been available. It then becomes possible to introduce the idea of equivalent oils (Ego's) possible to introduce the idea of equivalent oils (Ego'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 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, 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 Ego's and the more complex but evidently essential equivalent systems (EqS's) needed to model live crudes. Introduction An essential step in design of a microemulsion flood is to test the proposed system and optimize it by using reservoir conditions. fluids, and rock. However, especially when pressure and temperature are high and there is gas in solution, this can be very complex and time consuming, so that it is preferable to minimize this aspect of the total design procedure. Under reservoir conditions, surfactant system phase behavior is also difficult to accomplish and assess in a satisfactory way. In fact, an opaque crude sometimes causes discrimination of the various kinds of phases and emulsions to be problematical. Therefore, it has long been a goal to replace live crude with a pure oil or mixture of pure oils. If this could be accomplished, then phase behavior and the bulk of screening floods could be done at reservoir temperature, but under low pressure, considerably easing the design process. It should be stressed, however, that laboratory tests conducted under the most realistic conditions still are required in final phases of design work. During the attempt to formulate a live-crude replacement algorithm, it became evident that the existing description of surfactant/oil/brine phase behavior was not unique. For example, a single surfactant at fixed temperature can exhibit different interfacial tensions (IFT's) for certain nonhomologous pure oils and yet all tensions can correspond to the same optimal salinity. Or a collection of oils can be found that all furnish the same middle-phase solubilization parameters but have different optimal salinities. Hence, a parameter is needed that characterizes the oil in addition to optimal salinity and solubilization parameters. In this paper, oil molar volume is proposed as one such additional parameter, and the extent to which this improves the characterization of phase behavior is discussed. The resulting three-parameter correlation then is used to replace dead or live crude with pure oil and/or pure-oil/crude-oil mixtures that are equivalent in a pure-oil/crude-oil mixtures that are equivalent in a certain sense related to phase behavior and flooding performance. performance. SPEJ p. 669