Technology Research Of Improving The Oil Displacement Efficiency For CO₂ Flooding

CO2 flooding is one of the most effective way to enhanced oil recovery, both theory and practice indicate that the recovery ratio of CO2 miscible flooding is apparently higher than the recovery ratio of immiscible flooding. Base on the existing oil-displacement mechanism of CO2 flooding and aim at the actual situation of the reservoir at Shu101 Block of in Yushulin oilfield of Daqing, this paper had used the tubule experimentation to test the minimum miscible pressure(MMP) between the CO2 and crude oil with natural gas in this block, and the MMP which was obtained in experiment is 32.3MPa; and on the base of the tubule experimentation, had come up with a kind of multimode method for test the MMP between the CO2 and crude oil with natural gas, compared with the conventional tubule experimentation, the test result which was obtained by the new method is more accurate, and could accord with the situation in the actual reservoir; had ensured the main influence factors of the MMP between the CO2 and crude oil with natural gas in this block were the component of crude oil and the temperature in the reservoir; comparing the existing methods for reducing the MMP between the CO2 and crude oil with natural gas, had ensured the method for reducing the MMP between the CO2 and crude oil with natural gas in this block was adding reagent which was a kind of oil soluble surfactant which included tri-n-butyl citrate, Citric acid octyl ester, iso-butyl citrate, isopropyl citrates, isoamyl citrate and so on, and iso-butyl citrate and isoamyl citrate’s range of decreasing MMP were bigger, so ensured the adding reagents were so-butyl citrate and isoamyl citrate; had conducted laboratory experiments about test MMP and CO2 flooding before and after adding reagent, and the experimental results show that when the injection slug of iso-butyl citrate respectively is 0.001 PV,0.002 PV,0.003 PV and 0.004 PV, correspondingly the MMP between the CO2 and crude oil with natural gas is 28.2MPa, 27.3MPa, 25.1MPa and 24.8MPa, when the injection slug of isoamyl citrate respectively is 0.001 PV, 0.002 PV, 0.003 PV and 0.004 PV, correspondingly the MMP between the CO2 and crude oil with natural gas is 28.4MPa, 27.2MPa, 25.7MPa and 25.2MPa, so the optimal slug of adding reagent is 0.003 PV and the optimal adding reagent is iso-butyl citrate; ultimate overall recovery factor for different schemes respectively is 41.4%, 48.7%, 49.3%, 54.4%,and the recovery ratio in the stage of water flooding respectively is 35.9%, 36.6%, 35.0%, 37.2%, the recovery ratio in the stage of miscible flooding respectively is 5.5%, 12.1%, 14.3%, 17.2%, it can be seen from above that ultimate recovery factor after adding the slug of iso-butyl citrate has a substantial increase, when the slug of iso-butyl citrate respectively is 0.001 PV, 0.002 PV, 0.003 PV, the enhanced overall recovery ratio respectively is 7.3%, 7.9%, 13.0%, the above show that the displacement efficiency of CO2 miscible flooding has improved apparently after adding iso-butyl citrate.