Experimental study on oil recovery mechanism of CO2 associated enhancing oil recovery methods in low permeability reservoirs

Abstract

The low permeability reservoirs is difficult to be economically developed by conventional water flooding. In this work, five enhancing oil recovery (EOR) methods including water flooding, modified water flooding, CO2 flooding, water alternating CO2 flooding (WAC) and surfactant alternating CO2 flooding (SAC) are systematically evaluated by core-injection experiments. In addition, the microscopic oil-displacement mechanism for these EOR methods are revealed by nuclear magnetic resonance (NMR) technique. The experiment indicates that the SAC performs the best among the five flooding methods, which not only effectively improves the oil recovery (recovery factor up to 50%), but also effectively decreases the water cut. By using this method, gas phase can recovery more oil from smaller pores due to the reduction of interfacial tension and easy accessibility of these pores, while surfactant reduces oil-water interfacial tension and generates foam, significantly improving the oil displacement efficiency in larger pores. Therefore, SAC combines the advantage of gas and water which can recovery the oil both within smaller pores and larger pores in a higher proportion. The results of this work provides us fundamental understandings of the mechanisms in enhancing oil recovery of the CO2 associated EOR methods.