Noticeable progress in understanding the high efficiency of "foamy oil" in solution gas drive in heavy oil reservoirs has been made in recent years. However, "foamy oil" during CO2 flooding was still a special and novel phenomenon when CO2 flood asphaltic oil. The basic mechanism and corresponding numerical equation of seepage flow of ‘foamy oil’, in connection with CO2 flooding, have not been reported. This paper presented the mechanism analysis and simulation study that addressed this issue.

A new viewpoint of foamy oil during CO2 flooding was proposed with contrasting the similarity of "foamy oil" during CO2 flooding front with that in dissolved gas flooding. During CO2 flooding the "foamy oil" could form when CO2 contacted with oil because precipitated asphaltene could facilitate bubble nucleation, decrease the critical super-saturation and help in maintaining the dispersed gas flow by suppressing bubble coalescence, which similar to ‘foamy oil’ in depletion drive. Two main mechanisms were proposed. The first was enhancing oil recovery obviously by decreasing viscosity of crude oil, reducing interfacial tension, and swelling oil. The second mechanism was steady gas/oil mobility helping in maintaining high pressure in the reservoirs.

Then, the compositional model considering bubble nucleation, growth and coalescence and CO2 flowing characteristic was established, combining the advantages of "equilibrium model" with that of "dynamic model". With the establishment of the model, the flowing characteristic of "foamy oil" during CO2 flooding was regarded as the function of time and flow condition. The relative permeability curve and critical saturation of bubble were modified to determine the influence of "foamy oil" on oil recovery.

Taken one oil field in north China as example, the compositional model calculation results indicated that bubble was mainly located at the front of carbon dioxide flooding. High efficient flooding potential was achieved with "foamy oil" existed in carbon dioxide displacing front which were in accordance with the anomalously good performance in oil production. This method of calculating foamy oil properties would provide the basics for developing numerical simulation models of foamy oil flow during CO2 flooding.

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