Novel Approach to Enhancing Tight Oil Recovery Using N2-Assisted CO2 Flooding and Its Potential for CO2 Storage

Four types of gas injection experiments─CO2 flooding, N2 flooding, alternating CO2 and N2 flooding, and CO2 pre-slug N2 flooding─were conducted in a 28.42 cm long core plug to study the production performances of a CO2 and N2 flooding model in enhancing the tight oil flooding efficiency. Experimental results indicate that the following. (1) For CO2 and N2 flooding, the highest tight oil flooding efficiency is achieved at a gas injection rate of 0.10 mL/min, with a CO2 flooding efficiency of 68.75%, a gas breakthrough time of 0.33 PV, a CO2 utilization of 11.35 Mscf/stb, an N2 flooding efficiency of 38.46%, and a gas breakthrough time of 0.42 PV. (2) For alternating CO2 and N2 flooding, the highest tight oil flooding efficiency is 56%, and the CO2 utilization is 7.97 Mscf/stb. (3) The highest tight oil flooding efficiency is 53.08% for CO2 pre-slug N2 flooding. A comparison of the experimental results indicates that a larger CO2 slug leads to a higher oil flooding efficiency. The CO2 plug size has less influence on the recovery rate before breakthrough. The larger the CO2 plug after breakthrough, the higher the recovery rate. The optimized N2 injection time is the CO2 breakthrough time (0.33 PV). In the optimized case, flooding efficiency and contribution indicate the optimal CO2 slug size at 0.50 PV. When pure CO2 is injected at 0.50 PV and the gas is switched to N2, the recovery is 5.82%, and the contribution is 9.20%. When pure CO2 is injected at 0.50 PV, followed by the continuous injection of pure CO2, the recovery rate is 8.59% and the contribution rate is 12.49%. The difference between the recovery rates of the two methods is small, so the best injection timing for N2 is 0.17 PV after CO2 breakthrough, and the oil flooding efficiency is 63.27%.