A new approach to thicken dense liquid CO(2) is described using the principles of self-assembly of custom-made CO(2) compatible fluorinated dichain surfactants. Solutions of surfactants in CO(2) have been investigated by high-pressure phase behavior, small-angle neutron scattering (HP-SANS) and falling cylinder viscosity experiments. The results show that it is possible to control surfactant aggregation to generate long, thin reversed micellar rods in dense CO(2), which at 10 wt % can lead to viscosity enhancements of up to 90% compared to pure CO(2). This represents the first example of CO(2) viscosity modifiers based on anisotropic reversed micelles.

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Several commercially available and a few experimental, nonionic surfactants were identified that are capable of dissolving in carbon dioxide (CO 2 ) in dilute concentration at typical minimum-miscibility-pressure (MMP) conditions and, upon mixing with brine in a high-pressure windowed cell, stabilizing CO 2 -in-brine foams. These slightly CO 2 -soluble, water-soluble surfactants include branched alkylphenol ethoxylates, branched alkyl ethoxylates, a fatty-acid-based surfactant, and a predominantly linear ethoxylated alcohol. Many of the surfactants were between 0.02 to 0.06 wt% soluble in CO 2 at 1,500 psia and 25 C, and most demonstrated some capacity to stabilize foam. The most-stable foams observed in a high-pressure windowed cell were attained with branched alkylphenol ethoxylates, several of which were studied in highpressure small-angle-neutron-scattering (HP SANS) tests, transient mobility tests using Berea sandstone cores, and high-pressure computed-tomography (CT)-imaging tests using polystyrene cores. HP SANS analysis of foams residing in a small windowed cell demonstrated that the nonylphenol ethoxylate SURFONIC V R N-150 [15 ethylene oxide (EO) groups] generated emulsions with a greater concentration of droplets and a broader distribution of droplet sizes than the shorter-chain analogs with 9-12 ethoxylates. The in-situ formation of weak foams was verified during transient mobility tests by measuring the pressure drop across a Berea sandstone core as a CO 2 /surfactant solution was injected into a Berea sandstone core initially saturated with brine; the pressure-drop values when surfactant was dissolved in the CO 2 were at least twice those attained when pure CO 2 was injected into the same brine-saturated core. The greatest mobility reduction was achieved when surfactant was added both to the brine initially in the core and to the injected CO 2 . CT imaging of CO 2 invading a polystyrene core initially saturated with 5 wt% KI brine indicated that despite the oilwet nature of this medium, a sharp foam front propagated through the core, and CO 2 fingers that formed in the absence of a surfactant were completely suppressed by foams formed because of the addition of nonylphenol ethoxylate surfactant to the CO 2 or the brine.

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Cobalt and nickel salts of the highly branched trichain anionic surfactant sodium 1,4-bis(neopentyloxy)-3-(neopentyloxycarbonyl)-1,4-dioxobutane-2-sulfonate (TC14) are shown to be soluble in dense CO(2) at concentrations up to 6 wt % at 500 bar pressure. This is a remarkably high solubility for such hydrocarbon transition metal surfactants in CO(2). High-pressure small-angle neutron scattering (HP-SANS) has been used to study the surfactant aggregates in a normal organic solvent, cyclohexane, dense CO(2), and also mixtures of these two pure solvents. The results show that transition metal TC14 derivatives are viable compounds for incorporating reactive and functional metal ions into CO(2).

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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.

Several commercially available, non-ionic surfactants have been identified that are capable of dissolving in CO 2 in dilute concentration at typical MMP conditions and, upon mixing with brine, stabilizing CO 2 -in-water or CO 2 -in-brine emulsions or foams. These surfactants include water-soluble branched alkylphenol ethoxylates, linear alkylphenol ethoxylates, and branched alkyl ethoxylates. At 25 o C, the solubility of these surfactants in liquid CO 2 at ~1300 psia (~9 MPa) is 0.02 -0.10 wt%. When equal volumes of liquid CO 2 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 CO 2 , and a clear CO 2 zone above the emulsion as the droplets of CO 2 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 CO 2 -in-brine emulsions containing 9-23vol% brine that were stable for more than five hours.