Key variables that govern oil displacement in a micellar flood are capillary number (velocity x viscosity/interfacial tension) and chemical loss. At high capillary numbers, oil displacement is very efficient if various phases propagate at the same velocity. Chemical loss, however, is not always low when oil displacement efficiency is high. Compositions developed in situ often alter the ability of the micellar fluid to displace oil. Oil recovery can be predicted from static equilibrium fluid properties, providing the in situ compositions are known.The displacement of the wetting phase requires a capillary number of 10 times higher than that required to displace the nonwetting phase. This implies less efficient oil displacement in oil-wet systems. The correlation of oil recovery vs capillary number also varies with rock structure and wettability. Hence, for field application, immiscible oil displacement with micellar fluids should be determined in reservoir rocks. The decrease in final oil saturation with increase in capillary number indicates that relative permeability changes with capillary number. A numerically study showed that both the end-points and the shape of the relative permeability curves affect oil recovery at high permeability curves affect oil recovery at high capillary number in a slug process. The shape of the relative-permeability curves also affects the design of micellar slug viscosity. Thus, for field application, it is important to know the shape of relative-permeability curves at anticipated capillary numbers. Introduction In a micellar flood, the injected fluid banks interact with one another and with the reservoir brine, crude oil, and reservoir rock. This places stringent requirements on the design of the micellar flood. Initially, the micellar fluid may be miscible with crude oil and reservoir brine. However, because of dilution and surfactant adsorption, the flood can degenerate to an immiscible displacement. If low interfacial tension (IFT), or more specifically, high capillary number (velocity x viscosity/IFT) is maintained between all the phases, the displacement efficiency is good.There are many phenomena that can decrease oil recovery efficiency. The most important are chemical (surfactant or sulfonate) losses from adsorption by the rock, precipitation by high-salinity and high-hardness brines, interaction with polymer, partitioning into an immobile phase, and trapping of partitioning into an immobile phase, and trapping of the surfactant-rich phase. Recovery efficiency also can be poor when unfavorable in situ compositions develop. This occurs when the micellar fluid is diluted, develops undesirable salinity and hardness environment, experiences selective adsorption of surfactant, or undergoes selective partitioning of components into phases moving at different velocities.A micellar phase (or microemulsion) can exist in equilibrium with excess oil, water, or both. Winsor designated such phase behavior as Type I, II, and III, respectively. More recently, Healy et al. identified this behavior as lower phase (where the micellar phase is in equilibrium with excess oil), upper phase (where the micellar phase is in equilibrium with excess water), and middle phase (where the micellar phase is in equilibrium with excess oil and water). The importance of phase behavior has been the subject of considerable discussion in the literature.Since the function of the micellar fluid is to displace crude oil, not water, it would be desirable if the micellar fluid remained miscible with oil and immiscible with water during the immiscible displacement portion of a flood. This is achieved with upper-phase micellar systems. Since only a small bank of micellar fluid is injected, it must be displaced effectively by the succeeding polymer water bank. However, the upper-phase micellar fluid is not miscible with the polymer water; therefore, some of the micellar phase may be trapped as an immobile saturation (much as residual oil is trapped). SPEJ p. 116

Laboratory core tests show that small polymer-driven micellar slugs displace tertiary oil, polymer-driven micellar slugs displace tertiary oil, efficiently. Surfactant adsorption studies reveal nonclassical behavior. Polymer requirements are decreased by permeability-reducing micellar/clay interaction and by reduced losses behind a micellar slug. The required volume of polymer slug increases when the pore volume that is inaccessible to the polymer increases. Long-core tests with multiple polymer increases. Long-core tests with multiple pressure taps reveal the existence of a high-mobility pressure taps reveal the existence of a high-mobility oil-water bank and a low-mobility oil-micellar mixing zone. Introduction Micellar fluids that use petroleum sulfonate surfactants have been tested by several organizations as potential candidates for secondary and tertiary oil recovery operations. Typically, a micellar flood consists of a brine preflush to condition the formation, a bank of micellar fluid (5 to 40 percent PV) that displaces the oil, a mobility buffer (polymer) bank to drive the micellar slug, and a chase-water bank. Micellar flooding is attractive as an improved oil recovery process because it is not severely affected by gravity segregation and is not limited by ultimate surfactant availability. DEVELOPMENT OF MICELLAR FLUIDS The micellar fluids that have been developed by our laboratory for miscible waterflooding are microemulsions of high water content (85 to 95 percent by weight). These fluids generally are percent by weight). These fluids generally are prepared with 4 to 10 weight percent oil-soluble prepared with 4 to 10 weight percent oil-soluble hydrocarbon sulfonate (with an equivalent weight, EW, from 350 to 475) and an oil- or water-soluble alcohol cosurfactant. The cosurfactant performs several functions. In many cases it aids the water solubility of the sulfonate. Gale and Sandvik reported that systems that do not use cosurfactants require sulfonates with a broad EW range. The low-EW sulfonates provide water solubility for the high-EW material. Since the systems discussed here use a cosurfactant to perform this function, the EW range of the sulfonates we used is much narrower. An additional benefit of the cosurfactant is that sulfonate adsorption by the rock surface is reduced. SCREENING PROCEDURE Before core testing, crude oils that are potential candidates for micellar flooding are screened qualitatively against a number of micellar compositions of varying surfactant/cosurfactant ratio, monovalent ion concentration, and water content. Divalent ion tolerance and temperature are also examined. Preliminary qualitative screening tests are used to visually examine the degree of miscibility between the crude oil and the micellar solution. If a micellar fluid shows potential for oil displacement and is economically attractive, a core testing program is initiated. Typically, tertiary floods are conducted at reservoir conditions in fresh 2-in.-diameter, 4-ft-long Berea sandstone cores mounted in Hassler holders. A small volume of micellar fluid (from 2.5 to 10 percent PV) is injected at a linear advance rate of percent PV) is injected at a linear advance rate of about 2 ft/D and is followed by a bank of low-salinity, polymer-thickened water. If the tertiary recovery is encouraging, the micellar fluid is evaluated in detail. This evaluation involves extensive adsorption studies to optimize the fluid composition and long-core tests to examine the propagation and interaction of the fluid banks. propagation and interaction of the fluid banks. MICELLAR FLUID DEVELOPMENT FOR A PARTICULAR RESERVOIR The Second Wall Creek reservoir of the Salt Creek field north of Casper, Wyo., has been selected as one of the potential candidates for micellar flooding. The reservoir has a temperature of 110 degrees F, a crude oil viscosity of 4.0 cp, and an average permeability of 50 md. Analyses of the Second Wall Creek formation water and the Madison Field water (to be used as the micellar and polymer bank makeup water) are shown in Table 1. Both waters produce stable one-phase micellar fluids since the produce stable one-phase micellar fluids since the total dissolved solids and divalent ion contents are low. SPEJ P. 633

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