We have conducted a miscible CO 2 flood using carbonate cores and 31° API west Texas oil under four injection modes: simultaneous water and CO 2 (SWACO 2 ), water alternating CO 2 (WACO 2 ), continuous gas injection (CGI), and waterflood (WF). Then, a pseudo-miscible black oil simulator was used to match the experimental results. The matched simulation model was extended to conduct a sensitivity study on permeability variation in the core, WACO 2 ratio and slug size, and SWACO 2 ratio. Three permeability values were chosen to represent a flow barrier, average matrix permeability, and high permeability streak. These values were arranged to represent a layered reservoir and then aligned in series to represent a sequenced reservoir. The four injection modes were conducted in all six permutations of these three values in both the layered and sequenced arrangements. The oil recovery from the coreflood experiments was measured to be 90, 86, 75, and 54 % of OIIP for SWACO 2 , WACO 2 , CGI, and WF, respectively. The simulation results may be summarized as follows: for all injection modes during the layered permeability arrangements, the best oil recovery was obtained when the flow barrier is in the middle of the core. Conversely, lower recoveries were obtained when the barrier was not in the middle and the high permeability was located at the top with significant decrease in oil recovery (up to 40%). In sequenced reservoirs, each injection mode showed different preference to the permeability arrangements. For example, CGI showed higher recovery (5% increase) when the low permeability is near the outlet which allowed for more controlled displacement but with high differential pressure across the core. The WACO 2 ratio study in the homogenous (base) case showed that a 1:2 ratio had the highest oil recovery but the optimum ratio was 1:1 based on the amount of injected CO 2 . In layered reservoirs, each permeability arrangement had higher recovery at different WACO 2 ratios depending on the location of the flow barrier. However, in most cases 1:1 ratio was the optimum except for two cases, where the high permeability was at top, the 3:1 (increasing water ratio) was the optimum WACO 2 ratio.