Abstract Taking PI1-2 oil-bearing formations of northeast block in Lamadian oilfield as an example, laboratory experiments of high-concentration polymer flooding in artificial five-spot flat-panel core model are conducted, the effect of polymer molecular weight, concentration, slug size on oil displacement effect are evaluated and the influence on injection pressure of those parameters are analyzed. At the same time, injection parameters, such as polymer molecular weight, concentration and slug size etc, are optimized combined with actual injection ability of high-concentration polymer flooding from the pilot site. The results show: high-concentration polymer flooding after polymer flooding can further improve recovery, in addition, episode recovery degree can be improved with the increase of concentration and high-concentration slug; the theory of similarity on injection-production differential pressure between laboratory flat model experiment and actual reservoir is put forward, and parameters information of injection pressure in real reservoir are obtained through upscaling the flat model; the optimized polymer injection parameters for the pilot site in Lamadian oilfield are molecular weight of 25 million, concentration of 3,500mg/L, slug size of 0.5 PV, and the recovery can be improved 8.47%OOIP through using those optimal parameters. Introduction Polymer flooding, as the leading post-waterflood production technology in major oil reservoirs of Daqing oil field(1-4), has been put into the large-scale field application since 1996. At present, sixteen polymer-flooded blocks with geological reserves of 2.75×108 t OOIP are in the follow-up water flooding stage. The technology to further improve the recovery after polymer flooding is the true sense of the secondary development of oilfield technology (5-6), it is one of the key technology to realize the secondary development of Daqing oil field. In 2002 Daqing oil field company Technology Symposium, Academician Wang Demin proposed that the increased oil recovery by high-concentration polymer flooding is higher than that of conventional polymer flooding, and adopting high-concentration polymer in different stages of conventional polymer flooding can also yield good effects. Xia Huifen et al.(7-9) conducted the laboratory study on the microscopic displacement mechanisms of polymer solution, and concluded that increased recovery in microscopic displacement mechanism of polymer solution is due to the viscoelastic effect of polymer solution. Lu Xiangguo et al.(10) carried out the research on the optimization of polymer molecular weight, and proposed that if the formation and surface conditions allow, we should choose the higher molecular weight as the oil displacement agent; Wu Wenxiang et al. (11) did the research on the effect of high-concentration polymer injection timing and slug combinations on oil displacement effect, the results showed that the earlier the conventional polymer flooding converts to high-concentration polymer, the higher the oil recovery will achieve. Conventional polymer flooding in PI1-2 oil-bearing formations in Lamadian oilfield was switched to inject high-concentration polymer as a test in the course of the middle stage of conventional polymer flooding, the results of this field test show that 6% OOIP incremental oil recovery over the conventional polymer flooding was obtained. Now we plan to carry out a field test that injects high-concentration polymer after conventional polymer flooding in 106 meters well spacing in PI1-2 oil-bearing formations of northeast block in Lamadian oilfield.

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