Abstract: Results of Newtonian Fluids flooding very homogeneous porous media show the displacement efficiencyof the system is determined by the capillary number of the driving fluid. The experience gained from chemicalflooding natural cores and reservoirs that are all micro-heterogeneous or macro-heterogeneous to some degree showsthat displacement efficiency is influenced, besides capillary number, by many other factors. Newtonian Fluids donot haveelasticproperties; when flooding, there is no change in the wettability of the system; there is no imbibition;emulsions are not formed; and there is no alteration of thegeometry of the pores. However, when chemicallyflooding, elasticity markedly effects the displacement efficiency; change in wettability could occur; emulsions welloften show up, and from core and field tests data, emulsions significantly increases the recovery, emulsions canincrease the volumetric sweep oftheporoussystems and markedly alter the phase permeability behavior and water cut. The wettability will change and imbibition will occur; pore geometry alteration by solution and precipitation will happen, which influences the permeability, imbibition and recovery. The above factors all markedly influence
the displacement efficiency of the system, especially when many factors act jointly. The concept of ultimate and economical displacement efficiency and its effect on the selection of actual field flooding systems is analyzed（Many papers analyze ultimate displacement efficiency. however, in oil field, usually economical displacement efficiency is more important）. Besides capillary number, the above factors should be considered when developing and selecting chemicals and flooding systems. The above insights can further deepen our knowledge of the mechanism of chemical flooding, promote further research in this area and form better criteria for designing, developing, screening and selecting EOR chemicals and flooding systems in oil field.

Key words: recovery ratio, oil displacement mechanism, displacement efficiency, chemical flooding, viscoelastic- fluid displacement