Improvement of CO2 EOR performance in water-wet reservoirs by adding active carbonated water

• A method of applying active carbonated water as a pre-flush is described.
• Water blocking effect has been eliminated.
• Better contact between CO2 and residual oil via oil redistribution.
• A great potential of the proposed method for field application has been found.

A significant amount of oil is trapped by the continuous water phase in water-wet reservoirs during waterflooding and CO2 water-alternating-gas (WAG) flooding processes. In tertiary CO2 injection, such a phenomenon may cause part of the residual oil to be inaccessible to the injected CO2 and remain trapped by water. As a result, the displacement efficiency of the CO2 WAG process is reduced, as is the oil recovery. This paper discusses the experimental investigation of a method of pre-flushing the reservoir using active carbonated water (ACW) before a CO2 flood. A total of 10 coreflood tests were conducted to investigate the effects of different injection strategies, slug sizes, and core lengths on the tertiary oil recovery. A comparison of the four laboratory tests with different injection strategies demonstrates that applying the active carbonated water as a pre-flush before a CO2 flood is an effective strategy for enhancing the tertiary oil recovery. Oil recovery was increased by 35.5%, compared to 16.6% from injecting CO2 alone. Results of the other seven tests using different slug sizes of active carbonate water indicate that the optimal slug size of 0.6 pore volume (PV) achieved the maximum improvement in oil recovery of CO2 injection. In addition, a test in a longer core shows the extension of core length does not influence the oil recovery, which indicates the adsorption of surfactant is not significant in this study.