CO₂-soluble/ nonionic, water-soluble surfactants that stabilize CO₂-in-brine foams

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- misci-bility-pressure (MMP) conditions and, upon mixing with brine in a high-pressure windowed cell, stabilizing CO₂-in-brine foams. These slightly CO₂- soluble, water-soluble surfactants include branched alkylphcnol cthoxylatcs, branched alkyl cthoxylates, 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 CO₂ at 1,5(X) 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 alkylphcnol cthoxylates, several of which were studied in high-pressure small-anglc-ncutron-scattering (HPSANS)tests, transient mobility tests using Bcrea 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® 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 cthoxylates. 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 CO₂/surfactant solution was injected into a Bcrea sandstone core initially saturated with brine; the pressure-drop values when surfactant was dissolved in the CO₂ were at least twice those attained when pure CO₂ 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 CO₂. CT imaging of CO ₂ invading a polystyrene core initially saturated with 5 wt% KI brine indicated that despite the oil-wet nature of this medium, a sharp foam front propagated through the core, and CO₂ fingers that formed in the absence of a surfactant were completely suppassed by foams formed because of the addition of nonylphenol ethoxylate surfactant to the CO₂or the brine.