Novel Approach to Enhancing Tight Oil Recovery Using N_2-Assisted CO_2 Flooding and Its Potential for CO_2 Storage

摘要

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%.