In the past, the attention of CO2 flooding is merely paid to the CO2 Enhanced Oil Recovery (CO2-EOR) of low permeability reservoirs. However, facing the worldwide warming impact caused by excessive CO2 emissions, improving the gas sequestration efficiency in a reservoir within the process of CO2 displacement is becoming the research hotspot. In this work, we investigated the oil production characteristics and gas sequestration law in the pores of reservoirs by conducting several CO2-induced experiments. The nuclear magnetic resonance (NMR) test is used to monitor the seepage law of oil and gas in the pores of core samples during CO2 flooding. The underlying mechanisms of improving oil recovery and CO2 storage efficiency in different water-cut reservoirs are also investigated through Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and wettability angle analyses. The experiment results show that the CO2 flooding and gas storage efficiency are mainly related to the factors of water-cut fw of reservoirs, CO2 flooding pressure, and the geophysical properties changes induced by the CO2-brine-oil-rock reaction. Compared with the amount of oil extraction in the oil-saturated rock samples by CO2 injection, much more formation oil can be produced, and laying more amount of CO2 to be stored under the reservoir with fw=50% by immiscible CO2 flooding with the mechanism of gas breakthrough prevention. The oil yield and CO2 storage efficiency in reservoirs can also be effectively enhanced by rising the CO2 flooding pressure and prolonging the contact time of CO2 with the formation brine and rocks. Compared with the reaction results of CO2-brine-oil-rock that occurred in the reservoir with only bound water, the reaction that occurred in the reservoirs with fw=50% is more intense, leading to more minerals dissolution, thus increasing the larger pore volume (PV) of rocks and making the rock surface more hydrophilic. Otherwise, the hydrophilic wettability modification of rocks can improve the CO2 displacement and gas storage effect within the reservoir due to the capillary resistance reduction in the process of CO2 flooding oil. The experimental evidence additionally indicates that CO2 flooding applied within the reservoir after a stage of water flooding is more suitable to boost the oil displacement and gas sequestration effect.
Keywords CO2 flooding and gas sequestration, oil recovery, gas sequestration efficiency, water-cut