CO_2-cyclic solvent injection (CO_2-CSI) and gas/waterflooding in the thin post-CHOPS reservoirs

摘要

In this paper, a new combined CO2-cyclic solvent injection (CO2-CSI) and gasflooding (GF) process is proposed as an effective enhanced oil recovery (EOR) process in the post cold heavy oil production with sand (CHOPS) reservoirs. The synergy of the CO2-CSI and GF is explored and compared with that of the CO2-CSI and water-flooding (WF). A total of ten sandpacked laboratory tests were conducted to study and compare the production performances of the WF, CO2-CSI, combined CO2-CSI and GF/WF after the primary production. In the combined CO2-CSI and GF/WF processes, CO2/water is injected post, at the same time with, and prior to the CO2-CSI in the CSI + GF/WF, simultaneous (CSI + GF/WF), and GF/WF + CSI, respectively. Also the effect of CO2 injection rate is studied by injecting CO2 at three different flow rates in the CSI + GF. It is found that CO2-CSI + GF or CO2-CSI + WF performs better than the CO2-CSI or WF alone due to the extended foamy-oil flow. The combined CO2-CSI and WF outperforms the combined CO2-CSI and GF in terms of the heavy oil recovery factor (RF), heavy oil production rate, and cumulative gas-oil ratio (GOR). CO2 channeling is hindered by the subsequently injected water. In the combined CO2-CSI and GF, however, strong free-gas flow adversely affects the foamy-oil production in the later cycles of the CSI and in the subsequent GF. The best fluid injection timing in terms of the heavy oil RF is to inject CO2/water immediately after each cycle of the CSI production. A moderate CO2 injection rate gives the highest heavy oil RF of the CSI + GF. In conclusion, the combined CO2-CSI and GF/WF process is capable of recovering the remaining oil in the post-CHOPS reservoir.