Carbonated water injection (CWI) is the process of injecting CO2-saturated water into hydrocarbon reservoirs as a displacing fluid. As CO2 is dissolved in and transported by the flood water, CO2 is more evenly distributed within the reservoir, improving sweep efficiency. This is beneficial to watered-out oil reservoirs, where high water saturation can adversely affect the performance of conventional CO2 injection. In this study, the effects of increasing CO2 concentration in water using silica nanoparticles, and of pressure on the CWI process were investigated through a number of high-pressure coreflooding experiments. The experiments were performed in a highly water-wet core, using normal decane as the oil phase. The results showed an increase in ultimate oil recovery as the level of CO2 concentration in water increased. It was also observed that in the application of nanoparticles, an optimized concentration of nanoparticles must be used to obtain the maximum oil recovery factor. CWI showed a higher recovery factor both in the secondary and tertiary modes at higher pressures, owing to the increased solubility of CO2 in water at high pressures. The results of this study suggest that secondary CWI performs better than tertiary recovery, due to the high probability of contact between the oil and the CO2 gas and a growing concentration of CO2 in the water.