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Polymer flooding is very promising chemical enhanced oil recovery technique because it has been widely field tested in many oil fields and commercially applied in several countries in onshore reservoirs. The understanding of polymer flooding mechanisms is still developing, even though the principal mechanism was sweep efficiency increase due to reduced mobility ratio of water and oil due to reduced mobility of water. The incorporation of polymer flooding mechanisms and practical challenges make some projects fail to attain economical or technical goal. For offshore reservoirs, the polymer flooding becomes more difficult because of limited space and harsh reservoirs. Although there were hundreds of polymer flooding field tests in onshore reservoirs, polymer flooding in offshore reservoirs remains limited. In this paper, the previous onshore polymer flooding lessons and findings were briefly reviewed to look into the mechanisms which can guide the design of polymer flooding in offshore reservoirs. Then, the lessons learned from previous offshore reservoirs were reviewed. Advices were given to improve the field test performance. it is conclude that low concentration polymer solution with moderately-low viscosity should be injected into offshore reservoirs to keep displacing pressure between injectors and producers. The injected polymers should have good transportation ability which avoids the formation blockage. The optimum injection timing remains to be further investigated because the evidences. The injection rate should be controlled to avoid well casing damage which has been observed in onshore reservoirs. Except for Bohai oilfield, the formation blockage was not reported in offshore reservoirs. However, the microfracture can form in injectors which improved the injectivity of polymers as long as the injected polymers have good transportation capacity.

CO2 flooding technology for EOR not only meet the needs of oilfield development, but also solve the problem of carbon emission, which has become a global research hotspot. CO2 flooding includes miscible and immiscible flooding. The advantage of immiscible CO2 flooding is the ability to achieve high recovery in different reservoirs or fluid conditions compared with miscible flooding. But there are no reports about quantitative analysis of immiscible CO2 flooding at the micro level due to the expense and complexity of the experiments. In this paper, the process of immiscible CO2 flooding was simulated based on the Navier-Stokes equation in porous media by COMSOL Multiphysics. An ideal homogeneous rock structure model was established to study the influence of interfacial tension, injection velocity, injection viscosity and gravity on immiscible CO2 flooding. The porosity of the model is 34.7% and the permeability is 36.9mD. The simulation of pressure is 10 MPa and the temperature is 80 ℃. It was found that with the injection of CO2, the contact interface of two phases gradually changes from near-piston flow to non-piston flow under immiscible condition. Decreasing the interfacial tension and increasing the injection velocity significantly change the flow paths of CO2 and increase the sweep area of CO2. The difference between CO2 and oil viscosity is one of the factors influencing the occurrence of fingering. Increasing the viscosity of CO2 injection effectively suppress viscous fingering and improve the sweep effect. Gravity is one of the factors affecting the effect of immiscible CO2 flooding. Phase field simulation was used to study immiscible CO2 flooding for the first time. It was found that increasing the viscosity of CO2 injection could significantly enhance recovery. In order to increase the viscosity of CO2, a thickener can be added to the supercritical CO2. This study provides micro-level theoretical support for the development of process parameters in oilfield, and further provides new ideas for CO2 EOR.