Novel Approach to Enhancing Tight Oil Recovery Using N2-Assisted CO2Flooding and Its Potential for CO2Storage

Four types of gas injection experiments─CO2flooding, N2flooding, alternating CO2and N2flooding, and CO2pre-slug N2flooding─were conducted in a 28.42 cm long core plug to study the production performances of a CO2and N2flooding model in enhancing the tight oil flooding efficiency. Experimental results indicate that the following. (1) For CO2and N2flooding, the highest tight oil flooding efficiency is achieved at a gas injection rate of 0.10 mL/min, with a CO2flooding efficiency of 68.75%, a gas breakthrough time of 0.33 PV, a CO2utilization of 11.35 Mscf/stb, an N2flooding efficiency of 38.46%, and a gas breakthrough time of 0.42 PV. (2) For alternating CO2and N2flooding, the highest tight oil flooding efficiency is 56%, and the CO2utilization is 7.97 Mscf/stb. (3) The highest tight oil flooding efficiency is 53.08% for CO2pre-slug N2flooding. A comparison of the experimental results indicates that a larger CO2slug leads to a higher oil flooding efficiency. The CO2plug size has less influence on the recovery rate before breakthrough. The larger the CO2plug after breakthrough, the higher the recovery rate. The optimized N2injection time is the CO2breakthrough time (0.33 PV). In the optimized case, flooding efficiency and contribution indicate the optimal CO2slug size at 0.50 PV. When pure CO2is 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 CO2is 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 N2is 0.17 PV after CO2breakthrough, and the oil flooding efficiency is 63.27%.