The Optimal Salinity Concept For Oil Displacement By Oil-external Microemulsions And Graded Salinity Slugs
Authors: Chou S.I. , Shah D.O. 
Published: March 2010

Abstract
Our laboratory studies showed that the concept of the optimal salinity, as derived from phase behaviour or interfacial tensions of microemulsions, is applicable to oil displacement by soluble oils in porous media. Maximal oil recovery was obtained when the salinity of connate water and polymer solution was at the optimal salinity of the surfactant formulation. In the present study, it is shown that the salinity of polymer solution is far more important than the salinity of connate water. When the salinity of polymer solution was at the optimal salinity of the surfactant formulation, high oil recovery efficiency was obtained over a wide range of connate water salinities. Evidence showed that the phase behaviour of the surfactant slug in porous media is largely determined by the salinity of the polymer solution.
For better mobility control and minimum surfactant loss, a two-slug design of the surfactant formulation was employed. In this design, the first surfactant slug has an optimal salinity close to the connate water salinity and the second surfactant slug has a much lower optimal salinity. The polymer solution salinity is made equal to the optimal salinity of the second surfactant slug. With this design, high oil recovery in Berea cores can be obtained even in the presence of high-salinity (6% NaCI + 1% CaCl2) connate water.
The optimal salinity concept is further extended to include the effects of mobility control and surfactant dispersion and entrapment in porous media. The proposed salinity shock design of mobility polymer solutions employs two slugs of polymer solution in which the first polymer slug is at the optimal salinity of the preceding surfactant formulation and the second polymer slug is at a much lower salinity. With this unique design, high oil recovery and high surfactant recovery can be obtained for soluble oil flooding in sand packs, and the polymer consumption can be greatly reduced.

INTRODUCTION
Surfactant-polymer flooding processes for enhanced oil recovery ate influenced by a number of parameters. These include reservoir capillary properties(1%4), rock wettability(4%6), reservoir brine and mineralogy(7%14), and rock-fluid interactions (15%19). Connate water salinity and reservoir minerals can affect surfactant flooding through their effects on phase- behaviour, interfacial tensions with slug and residual oil, and mobility control, as well as on surfactant loss due to adsorption, precipitation and phase entrapment processes. By incorporating small amounts of clays or minerals in sand packs, it was shown (7) that the oil recovery efficiency of soluble oil flooding processes greatly diminishes due to the presence of the clays and minerals. Even when clays or divalent cations are absent or in trace amounts, NaCI alone can effectively deteriorate the performance of oleic(8) and aqueous surfactant flooding processes (11%14). Such detrimental effects can often be minimized by means of preflush and optimal salinity design (11,20) of the surfactant slug for a given surfactant and cosurfactant system. The design basis is that for a given surfactant formulation and oil; there exists a salinity at which minimum interfacial tensions occur when surfactant, cosurfactant, oil and brine are equilibrated.