Direct Thickening of Supercritical Carbon Dioxide Using CO2-Soluble Polymer

Two major applications of injecting dense carbon dioxide (CO2) into the petroleum reservoirs are enhanced oil recovery and sequester CO₂ underground. For enhanced oil recovery applications, the low viscosity of injected CO2 compared with the reservoir fluids causes the fingering of CO2, which may lead to bypassing huge amount of oil, early breakthrough of CO2, and increasing the gas to oil ratio. The use of direct thickeners, such as polymers, is one of the techniques used to increase the CO2 viscosity. Nevertheless, the solubility of polymers in CO2 and the high cost of soluble polymers are the main challenges. In this study, a novel, soluble, and cost effective thickener is proposed to directly increase the CO2 viscosity.

A PVT swelling apparatus is used to assess the compatibility and solubility of the thickener in dense CO2. Also, a custom designed apparatus is used to measure the viscosity of the mixture of thickener and dense CO2 at reservoir conditions. The assessment is conducted at different pressures, temperatures and thickener concentrations.

The results reveal that there must be a minimum pressure at which the thickener dissolves in the dense CO2. The minimum solubility pressure is found to be around 1500 psi at 25 ^oC, and it increases with temperature. The results also demonstrate that the viscosity increases as the pressure increases. The increase of thickener concentration has a significant impact on CO2 viscosity. The measured CO2 viscosity at 0.30 wt% thickener concentration, 3000 psi, and 50^oC is 3.88 cP. The addition of 0.60 wt% at the same experimental conditions results in a viscosity of 36.04 cP. This enhancement ranges between 65 and 1200 fold over pure sc-CO2 viscosity.

The injection of viscous CO2 has the potential to mitigate the gravity override and solve the CO2 mobility challenges.