Characterization of Produced and Residual Oils in the CO2 Flooding Process

In immiscible CO2 flooding processes, the injected CO2 extracts light and intermediate components from reservoir hydrocarbons, changing the properties of both the produced and residual oils. CO2 injection also increases asphaltene precipitation, which further causes adverse effects on the oil effective permeability, operational facilities, and the oil recovery. In this study, six CO2 flooding followed by blowdown processes are carried out in the laboratory under four CO2 injection pressures and two temperatures, and the properties of produced and residual oils are further characterized. It is found that uneven residual oil distribution occurred due to the coupled effects of viscous fingering and oil properties changes during CO2 flooding process. Characterization of the produced oil shows that both the produced oil density and viscosity decrease when CO2 injection pressure increases. The asphaltenes content of the oil produced after CO2 breakthrough is found to be relatively lower than that of the oil samples collected before CO2 breakthrough. In addition, the produced oil becomes lighter as the CO2 injected volume increases. The saturates content in the residual oil decrease considerably in comparison with original crude oil in the CO2 miscible flooding process. Furthermore, this study discovers that asphaltene component in produced oils is more stable than that in residual oils based on asphaltene-to-resin ratio (ATR) analysis. Finally, incremental oil recoveries of 2.78%11.72% can be experimentally achieved through blowdown processes that are carried out after a 30 min soaking period. The laboratory study not only shows that blowdown processes can be successfully applied to improve the performance of CO2 flooding processes, but also provides a deep understanding of properties changes of produced and residual oils in CO2-enhanced oil recovery processes.