Carbon dioxide (CO2) injection is an attractive EOR technique that allows not only the improvement of oil recovery but also the protection of the environment owing to the reduction in the amount of CO2, which is recognized as one of greenhouse gases 1),2). The key for successful CO2 injection is to achieve miscibility that maximizes local displacement of oil by CO2 3),4). Another key is to improve sweep efficiency because gas injection has a relatively low sweep efficiency owing to gravity segregation induced by the low density of injectant 5). To improve the sweep efficiency, some injection strategies have been proposed, and water-alternating CO2 injection is frequently applied to oil fields 6),7). During this process, not only CO2 is dissolved with oil to achieve miscibility but it is also dissolved with water, leading to the formation of carbon _ dioxide-saturated water, that is, carbonated water. Carbonated water injection is also recognized as an EOR technique. Various laboratory scale studies have been conducted since 1940 s 8) 12). Core flooding experiments using a sandpack were performed by Menteclaire Research and the results demonstrated that the residual oil decreased by 15 % PV if carbonated water was injected 8). Core flooding experiments using freshly sampled cores were also performed, and the results showed that after carbonated water injection, the residual oil saturation reduced by 2-26 % PV, as compared to the values for water flooding. Johnson et al. performed carbonated water injection in tertiary mode using sandpack 9). The increase in oil recovery was between 15 % and 25 %. Martin performed experiments using carbonated water, and the results showed an increment of 12 % oil recovery 10). He also investigated the effects of oil recovery on the carbonation level of carbonated water and API oil gravity. Holm conducted core flooding experiments using carbonated water and CO2 slug, and the results demonstrated an additional oil recovery of 20 % in the case of carbonated water injection as compared to that in water flooding 11). Dong et al. conducted sandpack flood experiments 19