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, CO₂ has low miscibility pressure causing the swelling of crude oil and reducing its viscosity therefore improving the macroscopic sweep process. However, 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 (GOR). 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 facing this technique. In this study, a novel, soluble, and cost-effective thickener is proposed to directly increase the CO₂ viscosity. In this study, a PVT high pressure and high temperature (HPHT) apparatus was used to evaluate the compatibility and the solubility of the thickener in dense CO₂. Also, a custom designed apparatus was used to measure the viscosity of dense CO₂ in the presence of the thickener at different conditions. The assessment was conducted at different experimental pressures, temperatures, and thickener concentrations. The effect of pressure on the solubility of the thickener in CO₂ and on the measured viscosity of CO₂ was evaluated at 1500, 2000, 2500, and 3000 psi. Also, the influence of temperature was evaluated at 25 and 50°C. Moreover, the concentrations used to study the effect of thickener concentration on the measured viscosity of CO₂ ranged between 0.10-2 %. The results from laboratory experiments clearly demonstrated that the addition of the thickener at certain conditions can significantly impact the dense CO₂ viscosity. The results revealed that there must be a minimum pressure at which the thickener dissolves in the dense CO₂. The solubility of the thickener can occur when the CO₂ is either in the liquid or supercritical phase. The results also pointed out that the CO₂ viscosity increased as the pressure increased. The increase of CO₂ pressure can significantly impact the solubility of the thickener in the dense CO₂ and consequently the CO₂ viscosity. The increase of the thickener concentration also had a significant impact on the measured CO₂ viscosity. The results showed that the CO₂ viscosity increased with the thickener concentration. The CO₂ viscosity increased 100 to 1200 -fold as a result of adding the thickener depending on the experimental conditions.