Thickening Carbon Dioxide with the Fluoroacrylate-Styrene Copolymer

Abstract The fluoroacrylate-styrene copolymer, polyFAST, is the first associative thickener that has been identified for carbon dioxide. Fluoroacrylate is highly carbon dioxide-philic, enhancing the solubility of polyFAST in carbon dioxide. Styrene is relatively carbon dioxide-phobic, but promotes viscosity-enhancing, intermolecular associations. The polyFAST 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. Carbon dioxide density values of 0.65-0.85 g/cc were required to dissolve 0.25-2.0wt% polyFAST in the 298 – 373 K temperature range. Falling cylinder viscometry measurements at 298 K demonstrated that the viscosity enhancement of CO2 associated with polyFAST increased with increasing polyFAST concentration, decreasing shear rate and decreasing temperature. The non-Newtonian falling cylinder viscometry results at 298 K were fit to a power law model with copolymer concentration-dependent coefficients. Mobility measurements of polyFAST-CO2 solutions flowing through 80-200 md Berea sandstone cores at superficial velocities of 0.00035 – 0.028 m/s (1 - 80 ft/day) at 298 K also indicated that polyFAST was an effective thickener. At a superficial velocity of 0.00035m/s (1ft/day), 1.5wt% polyFAST increased the CO2 viscosity by a factor of 19 relative to neat CO2. Smaller increases in viscosity occurred at lower polyFAST concentrations and higher velocities.