CO2-Soluble Surfactants for Improved Mobility Control

Abstract Several commercially available, non-ionic surfactants have been identified that are capable of dissolving in CO2 in dilute concentration at typical MMP conditions and, upon mixing with brine, stabilizing CO2-in-water or CO2-in-brine emulsions or foams. These surfactants include water-soluble branched alkylphenol ethoxylates, linear alkylphenol ethoxylates, and branched alkyl ethoxylates. At 25 °C, the solubility of these surfactants in liquid CO2 at ~1300 psia (~9 MPa) is 0.02 – 0.10 wt%. When equal volumes of liquid CO2 and brine (5wt% NaCl) are mixed with these surfactants, an opaque, white emulsion forms that initiallyfills the entire high pressure view cell. This emulsion collapses, yielding a clear aqueous zone below the emulsion as the brine drains from the continuous films of surfactant-stabilized brine that separate the cells of dense CO2, and a clear CO2 zone above the emulsion as the droplets of CO2 coalesce. The stability of these emulsions, as measured by the rate of their collapse, is strongly influenced by the architecture of the surfactant. The most stable emulsions were achieved with water-soluble, branched alkylphenol ethoxylates, such as nonylphenol ethoxylates with 9-15 ethylene oxide repeat units, and linear alkylphenol ethoxylates, which yielded CO2-in-brine emulsions containing 9-23vol% brine that were stable for more than five hours.