Reduction of hydrocarbon reserves and optimum production is one of major concerns about future of petroleum industry. In the recent years, several enhanced oil recovery methods have been introduced, tested and modified. With increasing methods of improving oil recovery and production techniques, it will be necessary to perform further studies and research to check their feasibility and displacement efficiency. The aim of this research is to study the effect of slug size variation of injection on the displacement efficiency in different scenarios of water alternating CO2 injection method in a hypothetical reservoir by performing numerical simulations. The results indicate that varying slug size method can yield better performance than fixed water alternating gas ratio, both in terms of recovery factor as well as CO2 storage. This can open a new look for calculating optimum water alternating gas ratios. © Maxwell Scientific Organization, 2014.