The Salinity-Requirement Diagram - A Useful Tool in Chemical Flooding Research and Development

Optimal salinity, the level of brine salinity at which a chemical flooding surfactant displaces oil most efficiently, is related to midpoint salinity and to the range of salinity over which the phase environment is Type III. These three conditions are functions of surfactant concentration-usually decreasing as surfactant concentration decreases, particularly if the brine contains multivalent cations. A salinity-requirement diagram, constructed from phase equilibrium data, expresses quantitatively the dependency of midpoint salinity and the Type III range on surfactant concentration. Because surfactant concentration decreases as a flood with a small-pore-volume chemical slug proceeds, a salinity-requirement diagram can provide insight into the performance of chemical floods. Examples are presented that support the proposal that chemical flooding is most efficient when conducted in a salinity gradient. A phenomenon in which a "wedge" of oil is left on the bottom of the core by a chemical flood is related to the salinity-requirement diagram for the system, and the effect of ion exchange on such diagrams is discussed. Introduction In the nest paper of this series, we concluded that chemical flooding under normal reservoir flow rates can be treated as an equilibrium process. Results of laboratory chemical floods, some of which were reported in that paper, indicate that the phases that form when oil, surfactant, and brine are mixed and allowed to equilibrate in sample tubes also form in the pores of reservoir rock during a chemical flood. Consequently, a reservoir under chemical flood can be visualized as a series of connected mixing cells, with phase equilibrium attained in each cell. Mathematical simulators of the chemical flooding process based on this physical model have been published, and a general theory of multicomponent, multiphase displacement in porous media has been described and illustrated. We defined in that first paper three types of phase environment: Types II(-), II(+), and III. We concluded from our experiments that one objective in designing chemical flooding, systems should be to keep as much of the surfactant as possible in the Type III phase environment, near midpoint salinity, for as long as possible during, the course of the flood. In the second paper of the series, we emphasized that the salinity requirement of a chemical flooding system usually changes during a chemical flood as adsorption and dispersion cause the surfactant concentration to decrease. The salinity requirement is the salinity required for the surfactant/brine/oil system to be at midpoint salinity (i.e., that point in the Type III phase environment where the concentration of oil equals the concentration of brine in the microemulsion, middle phase). Healy and Reed and others have observed that optimal salinity for oil displacement is at or near midpoint salinity. We use midpoint salinity rather than optimal salinity in our work because midpoint salinity is defined precisely. We assume that the sum of the micro emulsion/excess oil and micro emulsion/excess brine interfacial tensions is minimal and oil displacement efficiency is maximal near midpoint salinity. Our experimental results are consistent with this assumption. For most anionic surfactants. midpoint salinity decreases as surfactant concentration decreases, particularly in the presence of multivalent cations. The dependency on surfactant concentration of midpoint salinity and of the range of salinity over which the system is Type III is depicted on a salinity requirement diagram. Such diagrams are similar to those presented by Glover et al. The utility of salinity-requirement diagrams was illustrated in Ref. 9 by considering the results of four laboratory chemical floods that differed only in the salinity of their polymer drives. SPEJ P. 259^