Preparation and thickening mechanism of copolymer fluorinated thickeners in supercritical CO2

Supercritical carbon dioxide (SC-CO2) dry fracturing is considered to be an ideal choice for low permeability, tight and other unconventional reservoir development, because SC-CO2 has the characteristics of inhibiting water sensitivity damage and improving flowback efficiency. However, the low viscosity of SC-CO2 limits its application. At present, the most effective way is to thicken CO2 with thickeners. In this work, seven heptadecafluorodecyl acrylate/styrene (HFDA/STY) copolymers were synthesized, their solubility and thickening properties in SC-CO2 were evaluated. With the increase of phenyl content in the copolymer, the dissolution pressure and thickening ability of the copolymer in SC-CO2 increased. The solubility pressure of P(HFDA0.70-co-STY0.30) reached 22.2 MPa at 5 wt%, and the viscosity of SC-CO2 was increased by 297 times. The increase of temperature leads to the increase of molecular thermal motion, which leads to the increase of solubility pressure but the opposite of thickening ability. Molecular dynamics (MD) simulation method was used to reveal the thickening mechanism of SC-CO2. The MD simulation results show that fluorocarbon headgroups in copolymers can significantly reduce its dissolution pressure and phenyl group plays the key role in improving the viscosity of CO2, which is consistent with laboratory experiment results.