Abstract
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
Proper Use of Capillary Number in Chemical Flooding