The Mechanism and Simulation Research of "Foamy Oil" During CO 2 Flooding

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 asphatic 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 withthat in dissolved gas flooding. During CO2 floodingthe‘’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, whichsimilar 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 wassteady gas/oil mobility helping in maintaining high pressure in the reservoirs. Then, the compositional model considering bubble nucleation, growth and coalescenceand 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 floodingwas 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