CO2 MOBILITY CONTROL USING DIRECT\ud THICKENERS AND FOAMING AGENTS

There are two strategies for using CO2-soluble compounds to decrease the mobility of supercritical carbon dioxide. The first involves the “direct thickening” of CO2, which is accomplished by dissolving an associative thickener in the scCO2 that forms viscosity-enhancing macromolecules in solution. The second strategy is to inject a CO2 surfactant solution into the porous media (which contains both brine and oil) that will generate a low mobility system of CO2 droplets separated by surfactant-stabilized brine lamellae that bridge pore throats. \ud Direct thickening was accomplished with surfactants that formed cylindrical, rather than spherical, micelles in scCO2. The surfactants employed divalent cations (Ni, Co) rather than a monovalent cation (Na). Therefore, each surfactant had two tails (rather than one). Further, each tail was a double-tail or triple-tail that was tailored to be CO2-philic, consisting of either highly fluorinated alkanes or highly branched hydrocarbon groups. High pressure SANS was employed to establish whether the micelles were cylindrical or spherical. Further, the dimensions of the micelles were determined. Cloud point pressures of surfactant solutions (1-10wt% surfactant) were determined for the dry and wet (W=0–15, water/surfactant molar ratio) systems using a non-sampling technique, and viscosity was determined using a falling cylinder technique. The CO2 viscosity was doubled using several weight percent of a fluorinated surfactant in the presence of water.\ud Several commercially available, nonionic surfactants were identified that are capable of dissolving in carbon dioxide (CO2) in dilute concentration at typical minimum- miscibility-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 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 CO2 at 1,500psi and 25ºC, 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 transient mobility tests using Berea sandstone cores, and high-pressure computed-tomography (CT)-imaging tests using polystyrene cores. 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 Berea 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. \u