The Role of Co-Solvents and Co-Surfactants in Making Chemical Floods Robust

Abstract

The ability to select low-cost, high-performance surfactants for a wide range of crude oils and reservoir conditions has improved dramatically. There are thousands of possible combinations of chemicals and components (including differing salinities, hardness, and oil concentrations) that could be tested on each crude oil. Each chemical combination requires many observations at reservoir temperature for proper evaluation. Therefore it would take too long, be too expensive, and in many cases not even be feasible to test all combinations. In practice we use our scientific understanding of surfactant/cosurfactant/co-solvent characteristics to match a chemical formulation to the properties of a crude oil and formation brine salinity at reservoir temperature. Typically the surfactant is mixed with other chemicals such as polymer in brine and injected at optimum salinity (the ASP slug). It is important that the ASP slug be a clear, stable solution since precipitation or phase separation will interfere with surfactant and polymer transport deep into the reservoir. Co-solvents and/or co-surfactants are often needed to make the primary surfactant sufficiently soluble. Core floods are presented that show the oil recovery is significantly increased by simply adding a small concentration of co-surfactant to make the ASP slug clear. In such cases, the use co-solvent/ co-surfactant is economically beneficial in terms of higher oil recovery because of the improved surfactant performance, lower surfactant retention and improved effectiveness of the polymer for mobility control.

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