Thickening Carbon Dioxide With the Fluoroacrylate-Styrene Copolymer

Summary The fluoroacrylate-styrene copolymer is the first associative thickener that has been identified for carbon dioxide. Fluoroacrylate is highly carbon dioxide-philic, enhancing the solubility of the copolymer in carbon dioxide. Styrene is relatively carbon dioxidephobic, but promotes viscosity-enhancing, intermolecular associations. The copolymer used in this study had a composition of 29 mol% styrene - 71 mol% fluoroacrylate, a number-average molecular weight of 540,000, and a polydispersity index of 1.63. The copolymer was sufficiently soluble in CO2 at typical reservoir flooding conditions to induce a significant increase in viscosity. Falling cylinder viscometry measurements at 298 K demonstrated that the viscosity enhancement of CO2 increased with increasing copolymer concentration, decreasing shear rate, and decreasing temperature. Mobility measurements of the fluoroacrylate-styrene copolymer-CO2 solutions flowing through 80 to 200 md Berea sandstone cores at superficial velocities of 0.00035 to 0.028 cm/s (1-80 ft/D) at 298 K also indicated that the copolymer was an effective thickener. At a superficial velocity of 0.00035 m/s (1ft/ D), the addition of 1.5 wt% copolymer increased the CO2 viscosity by a factor of 19 relative to neat CO2. Smaller increases in viscosity occurred at lower copolymer concentrations and higher velocities. These results demonstrate that it is possible to design a compound that can enhance the viscosity of dense carbon dioxide. Fluoropolymers are characterized by environmental persistence, high cost, and unavailability in bulk quantities, however. Therefore nonfluorous, inexpensive thickeners are currently being designed.