Summary The use of entrainers (cosolvents) to improve CO2 mobility control for EOR was investigated. The cosolvent serves as a miscible additive that modifies the phase behavior of supercritical solvents and enhances the solubility of crude oil components in the CO2-rich phase. The presence of the cosolvent increases the viscosity and density of the gas phase. The improvement in supercritical extraction power and enhancement of bulk fluid properties result in improved mobility ratios. Introduction Considerable interest has been directed toward the use of CO2 for EOR. CO2 supercritical extraction of hydrocarbons is one of the major mechanisms in CO2 miscible or immiscible displacement processes. CO2 can achieve dynamic miscibility by extracting hydrocarbons from crude oil. The extraction of a broad range of hydrocarbons from crude oil in reservoirs induces dynamic miscibility within reasonable distances from reservoir injection. The efficiency of the process has been hindered, however, by poor sweep efficiency resulting from the unfavorable mobility of gaseous CO2 and gravity segregation. The basic contributing factors to this problem are the low density and viscosity of CO2 at reservoir conditions. Several methods have been proposed to provide a solution to the mobility-control problem. The concept of adding a small amount of a miscible component to pure supercritical solvents to increase the solvent power of gases was first proposed by Peter et al. The increase in solvent power when cosolvent is added has also been noted in recent publications. Much of the attention in this area has focused on applications in coal extraction and within the food industry. Our work presents am extension to EOR. The definition of entrainer (cosolvent), based on this approach, is a chemical additive that enhances the solubility of crude oil components in the CO2-rich phase, resulting in a "thickened" gas phase. We present the results of an initial evaluation of the effect of adding cosolvent on the viscosity and density of the supercritical CO2 phase. Increasing the density and viscosity of the gas phase by adding cosolvent would by itself provide improved mobility control. The ability to modify the phase behavior results in an improvement in the extraction of higher-molecular-weight hydrocarbons present in oil. This increased extraction power provides further adjustment of the mobility ratio. The selection of cosolvents depends on the reservoir operating conditions. Additives with the following properties are most suitable for use as cosolvents in this application:appreciable solubility in the gas phase,ability to enhance the solubility of crude oil components in the gas phase,high viscosity, andlow solubility in water. Some candidate cosolvents including higher-molecular-weight alcohols and hydrocarbons (straight-chain and branched) and ethoxylated compounds were examined in this work. Experiments were performed to test the increase in CO2 density and viscosity caused by the addition of representative cosolvents. Comparison studies on improving solubilization of higher-molecular-weight hydrocarbons into the gas phase also were performed under conditions that promoted density and viscosity enhancement for selected cosolvents. This study's results indicate that the addition of entrainers (cosolvents) results in a preferential increase in the extraction of the heavier components of a synthetic oil (three-component system).

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