Study of the Enhanced Oil Recovery Mechanism and Remaining Oil State of Heavy Oil after Viscosity-Reducer-Assisted CO2 Foam Flooding: 2D Microvisualization Experimental Case

Although traditional viscosity-reducer-assisted water flooding can improve heavy oil recovery, it does not have a sufficient advantage over expanding the sweep range and advancing the displacement front deeply. The CO2 foam can make up for the lack of a viscosity reducer very well in the flooding process and decrease the volume of green-gas CO2. However, the mechanism of combining the flooding of CO2 foam with a viscosity reducer is still unclear. To solve this problem, the flow characteristics and remaining oil state of heavy oil in the process of CO2 foam flooding assisted by a viscosity reducer are analyzed. In this study, an etched microglass and high-temperature and high-pressure displacement visualization device are used to simulate the flow behavior of heavy oil derived by CO2 foam and a viscosity reducer together. The state and location of the remaining oil in the etched glass are observed and compared at different flooding channel positions from macro and micro points of view. The experimental results show the following. (i) The oil recovery factor increases obviously in the early stage and tends to increase slowly in the later stage of CO2 foam flooding assisted by a viscosity reducer. (ii) The CO2 foam viscosity-reducer solution can block the water channel, adjust the streamline, and make the flooding front advance evenly. The sweep range expands to both sides, and an effective oil–water flow channel is also formed at the upsweep zone. (iii) Due to the synergistic viscosity reduction effect of CO2 and a foamy viscosity reducer, the oil–water interfacial tension decreases, the viscosity of crude oil decreases greatly, and the flow ability of crude oil in the pore is significantly improved. The 43.2% increase in oil recovery confirmed the flooding effect of a CO2 foam viscosity reducer.