The paper addresses enhanced oil recovery in chalk and sandstone rocks by CO2 injection, with different wettability, porosity, and permeability as well as injection rate and flooding conditions. Results indicate that an increase in Bond number has a positive effect on oil recovery whereas for capillary number, there is a limit in which recovery is improving. This limit is estimated when the pressure drop by viscous force is approximately equal to the threshold balance between capillary and gravity forces. A dimensionless group is proposed that combines the effect of capillarity, injection rate, permeability, and CO2 diffusion on the oil recovery. Recovery from all experiments in this study and reported data in the literature shows a satisfactory relationship with the proposed group. 1. Introduction CO2 flooding is the most widely used method for medium and light oil recovery in sandstone and carbonate reservoirs during the last decades [1, 2]. In the past five decades, there have been extensive laboratory studies, numerical simulations, and field applications of CO2 EOR processes [1, 3–6]. The mechanisms affecting the displacement of oil by CO2 injection include oil swelling, IFT (interfacial tension), and viscosity reduction as well as increasing the injectivity index due to solubility of CO2 in water and subsequent reaction of carbonic acid with the minerals [7–9]. For immiscible CO2 flow in porous media, displacement efficiency could depend on the wetting properties of the fluids, the rate of CO2 injection and oil production, the difference of oil and CO2 density, the viscosity ratio of fluids, and the oil and CO2 relative permabilities [7]. Miscible CO2 EOR reduces viscosity and CO2 can vaporize and extract intermediate hydrocarbons from the reservoir crude oil and/or condense to the oil for developing miscibility under certain pressures and temperatures [10]. Displacement efficiency for miscible CO2 flooding is also influenced by wetting properties of the rock, injection and production rates, density difference between oil and gas, viscosity ratio of fluids, and oil/CO2 relative permabilities [10]. Investigations in the laboratories and in the field [11–18] suggest the importance of the gas-oil gravity segregation in CO2 EOR. Kulkarni and Rao [16] presented the effect of important dimensionless groups on the final oil recovery obtained from a number of miscible and immiscible CO2 gravity drainage experiments. They stated that the main design parameters on a laboratory scale to evaluate the feasibility of gas injection depend on reservoir
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