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

Abstract

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

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