(88d) CO2-Soluble, Non-Ionic, Water-Soluble Surfactants That Stabilize CO2-In-Brine Foams

Several commercially available, inexpensive, non-ionic surfactants were identified that are capable of dissolving in CO₂ in dilute concentration at typical MMP conditions and, upon mixing with brine in a high pressure windowed cell, stabilizing CO₂-in-brine foams.  These water-soluble surfactants include branched alkylphenol ethoxylates, branched alkyl ethoxylates, a fatty acid-based surfactant, and a predominantly linear ethoxylated alcohol.  The solubility of these surfactants in liquid CO₂ at 25^oC and 1300 psia (~9 MPa) was in the 0.02 – 0.10 wt% range.  When equal volumes of liquid CO₂ and synthetic brine (5wt% NaCl) were thoroughly mixed with an amount of surfactant approximately equal to that which could dissolve in the CO₂ at test conditions, an opaque, white foam usually formed and initially filled the entire high pressure view cell.  When mixing stopped, this foam gradually collapsed.  The most stable foams were attained with branched alkylphenol ethoxylates.  Relatively stable foams were also formed with some of the branched alkyl ethoxylates, a predominanatly linear ethoxylated alcohol, and Tween 80.  Foam stability tests were conducted with several promising surfactants using SACROC reservoir brine at field conditions of ~3200 (20.7 MPa) and 136 ^oF (58^oC). 

Several promising branched alkylphenol ethoxylates were studied further in high pressure small angle neutron scattering (HP SANS) tests, mobility tests in Berea sandstone cores, and CT imaging tests using polystyrene cores.  HP SANS analysis of foams residing in a small windowed cell using CO₂ and SACROC brine demonstrates that the nonylphenol ethoxylate Huntsman Surfonic N 150 (15 EO groups) generates emulsions with a greater quantity of droplets and a broader distribution of droplet sizes than the shorter chain analogues with 9 – 12 ethoxylates.  The in-situ formation of weak foams was then verified by measuring the pressure drop across a Berea sandstone core initially saturated with brine.    CT imaging of CO₂ invading a polystyrene core initially saturated with 5wt% KI brine indicated that despite the oil-wet nature of this medium, CO₂ fingers that formed in the absence of a surfactant were completely suppressed by foams formed due to the addition of 0.06wt% surfactant to the CO₂.