Effect of gravity segregation on CO2 sequestration and oil production during CO2 flooding

In this study, we attempted to analyze CO2 sequestration and oil production in the environment of gravity segregation, and ultimately making a relationship between them during the CO2 flooding. In order to utilize the depleted oil field after finishing the CO2 flooding as CO2 storage, quantitative characteristics of CO2 storage efficiency to be possible in that oil field should be understood for preventing a leak through fractures generated by high-pressured CO2 of free gas state prior to the injection of CO2. As an experimental system, we designed a two dimensional CO2 flooding apparatus equipped with sandstone plate. The apparatus had dip angles of 0° (horizontal system) and 90° (vertical system) to simulate gravity segregation condition. Experimental results for the vertical system showed that oil production was heavily influenced by gravitational effects for immiscible condition. This situation is analogous to thick reservoirs in which gravity dominates over viscous forces. However, for near-miscible condition, the gravity override phenomenon was not significant, and these results suggest that reservoir thickness should not be a criterion when using CO2 flooding. CO2 storage efficiency for near-miscible system was found to be much better than that for immiscible system regardless of the gravitational effect. For designing the CO2 EOR scheme in terms of relationship between CO2 storage and oil production, particularly in the case of high permeable homogeneous sandstone, we found out that the trapped CO2 as free gas state and oil recovery during the CO2 flooding were monotonically decreased without critical value of inflection point corresponding to CO2 injection rate increase.