Experimental Analysis for the Relationship of Oil Recovery and CO2 Storage Efficiency During CO2 EOR

Abstract

CO2 EOR method has been a great attention due to the advantages of oil production enhancement as well as CO2 sequestration. When designing CO2 EOR, the relationship of oil production and CO2 storage efficiency should be understood for stably injecting CO2. When applying CO2 injection method, various factors affect the process performance including gravity segregation, miscibility of the system, and injection rate. In CO2 displacement process, CO2 front in contact with oil is gravitationally unstable due to the density difference between oil and CO2. In addition, the oil flow path may be altered because of dissolution or vaporization between oil and CO2. When CO2 is injected rapidly, it penetrates far into the oil system. If this phenomena generate viscous fingering and gravity segregation, early CO2 breakthrough will occur, resulting in low oil recovery and low CO2 storage efficiency. In this situation, proper injection rate is a critical factor from the beginning stage of CO2 EOR designing for stably injecting CO2. This study attempted for analyzing the relationship between oil recovery and CO2 storage efficiency under the environment of gravity segregation for different miscibility conditions. To this objective, two-dimensional experiments using the cylinder type holder designed in this work were performed in vertical and horizontal systems under the different environments of gravity segregation. As experimental results for the vertical system corresponding to miscibility condition, it showed that gravitational effect for immiscible condition heavily influenced oil production. This situation is analogous to gas flowing phenomenon in thick reservoir in which gravity dominates over viscous forces. However, in the case of near-miscible condition, the gravity override phenomenon was not significantly affected to oil recovery compared to the case of immiscible condition. According to these results, reservoir thickness was not a main factor to oil production in CO2 flooding. From the viewpoint of CO2 storage, CO2 storage efficiency for near-miscible process was found to be much better than that for immiscible process regardless of the gravitational effect. Based on the aforementioned results, in terms of the relationship of CO2 storage and oil production, it was resulted that the relationship between CO2 as free gas state and oil production was monotonically decreased as CO2 injection rate increased regardless of gravitational effect as well as miscibility condition. While, the CO2 as dissolved state was inversely increased with the decrement of oil production as CO2 injection rate increased.