Effectiveness of modified CO2 injection at improving oil recovery and CO2 storage—Review and simulations

Abstract

Modified CO2 injection refers to the injection of CO2 premixed with one or more chemical additives that enhance miscibility between CO2 and oil. This paper has two main objectives. The first is to summarize published findings on how various additives, which we classify as polymers, alcohols, surfactants, or other chemicals, affect interfacial tension (IFT) between CO2 and oil, minimum miscibility pressure (MMP), and oil recovery. The second is to fill gaps in the literature by presenting field-scale numerical simulation of ethanol-treated CO2 injection.IFT has been reported to be lower using modified CO2 than using pure CO2. The various classes of additive have been found to bring about different reductions in MMP, with polymers attaining a reduction of 7.4–7.6 MPa, alcohols 9.4 MPa, and surfactants 1–6.1 MPa. However, these classes provided comparable increases in oil recovery. Studies attribute the increase in oil recovery to enhanced displacement efficiency during modified CO2 injection. However, they do not consider sweep efficiency as a possible factor, nor do they consider the effect of modified CO2 injection on CO2 storage. To fill this gap in the literature, we run numerical simulations for (a) pure CO2 injection and (b) ethanol-treated CO2 injection into a three-dimensional oil reservoir model. Ethanol-treated CO2 injection is found superior to pure CO2 injection in displacement efficiency, sweep efficiency, and CO2 storage. Improvement in CO2 storage and oil recovery is more pronounced at lower pressures, which suggests that modified CO2 injection is more suitable for low-pressure reservoirs.