Evaluation Of The Optimal Salinity Concept For Design Of A High Water Content Micellar Fluid
Authors: Rathmell J.J. , Smith F.W. , Salter S.J. , Fink T.R. 
Published: December 2007

Abstract
A high water-content micellar fluid has been designed and evaluated in the laboratory for possible field application in the Bell Creek Muddy reservoir. The concept of optimal salinity was applied through measurement of phase volumes and interfacial tension in order to identify potentially effective surfactant-cosolvent mixtures for subsequent displacement testing. In this process design, goals were to optimize the oil recovery-chemical cost ratio for preflush, micellar, and polymer fluids all having the same salinity that was near the current reservoir salinity. The micellar fluid contained only that oil normally associated with a petroleum sulfonate of 60-percent activity.
The compositional variables of the injection fluids were optimized through core displacement testing in Berea cores. Oil recoveries, fluid mobilities, and effluent fluid compositions were monitored as performance indicators. The effects on oil recovery of crude oil composition, core length, and rock type also were evaluated.
High recoveries were achieved in 4-ft Berea cores when a low-calcium brine was in place and with effective preflushing when a high-calcium brine was in place. Oil recovery was found to be sensitive to core length, crude oil composition, and rock type. Although not consistent with the design philosophy, reduced polymer costs can be achieved by lowering the polymer brine salinity below that of the other fluids. polymer brine salinity below that of the other fluids

Introduction
A high water-content micellar-flooding process has been designed for the Bell Creek field, Unit A, Muddy sandstone reservoir in Powder River and Carter Counties, Mont. The micellar fluid contained no oil other than that contained in the petroleum sulfonate surfactant. The work was part of a project partially funded by the Dept. of Energy (Contract partially funded by the Dept. of Energy (Contract EY-77-C-02-4207; Div. of Oil, Gas and Shale Technology).
Limited time did not permit evaluation of more than one approach to designing a micellar process; only one option out of a number of possibilities was examined. Employing the concept of optimal salinity as a screening and development tool, a micellar fluid effective in laboratory core tests was achieved. In order to reduce the deleterious effects of reservoir mixing, a major feature of the design was to maintain a constant salinity throughout the entire sequence of injected fluids.
A number of factors important to the success of a micellar flood have been discussed previously These include rock stratification and heterogeneity, reservoir brine composition, reservoir mineralogy, rock wettability, and rock capillary properties. Although these matters were properties. Although these matters were studied in considerable detail, in this paper we have described only laboratory performance of the recovery method resulting from the screening and design criteria employed.
PROCESS DESIGN CONCEPTS PROCESS DESIGN CONCEPTS The basis for the concept of optimal salinity in micellar flooding has been presented by Healy et al. They have shown that the phase behavior of oil-brine-surfactant systems reflects the magnitude of the interfacial tensions among the phases. Furthermore, other papers have demonstrated the importance, as well as the potential effectiveness, of immiscible displacement in micellar flooding. If sufficiently low interfacial tensions prevail, high recoveries of oil can be achieved even in the absence of miscibility.
Fig. 1 illustrates the significance of optimal salinity with respect to both phase behavior and interfacial tension. A series of equilibrated samples of oil, surfactant, and brine of various salinities may exhibit a range of phase behavior. If certain conditions, such as surfactant concentration, surfactant type, cosolvent type, temperature, etc., are appropriate, some of the samples at intermediate salinities will form three phases.