Experimental Study on Enhanced Shale Oil Recovery and Remaining Oil Distribution by CO2 Flooding with Nuclear Magnetic Resonance Technology

The recovery factor of shale oil is extremely low. CO2 flooding is considered a promising way to improve the recovery factor of shale oil. The pressure gradually increases during the actual injection of CO2. CO2 and oil can go from immiscible to near-miscible and finally to miscible during the whole displacement process. Therefore, a continuous multipressure point displacement experiment (progressive flooding) with nuclear magnetic resonance technology is conducted. The experimental pressure is increased continuously from 0.7 to 11 MPa, which realizes the immiscible flooding change to near-miscible flooding and finally to miscible flooding, simulating the actual continuous displacement process of a reservoir. The results show that from immiscible flooding to near-miscible flooding and finally to miscible flooding, the cumulative oil recovery factor exhibits a step-like growth trend under continuous multipressure point displacement, and the increase in the amplitude of the recovery rate at different displacement states decreases in turn. In addition, the cumulative recovery factor of differently scaled pores shows different bench-type growth trends. When immiscible flooding changes into near-miscible flooding, the oil in the macropores is completely displaced, and the oil recovery of the mesopores increases more than that of the micropores. When converting from near-miscible flooding to miscible flooding, the increase in the amplitude of oil recovery from the micropores is higher than that from the mesopores. Under the conditions of transitioning from immiscible flooding to miscible flooding, CO2 first forms a miscible state with macroporous oil, second with mesopores, and finally with micropores. The research results provide theoretical guidance and reference for the field practice of CO2 flooding.