Residual oil after water flooding appears in three types: oil drops, films and clusters. Work performed using Newtonian fluids shows that to mobilize the residual oil remaining after water flooding, the driving forces, on condition of oil-water IFT (interfacial tension) being constant, need to be increased by one thousand to ten thousand times over that of water-flooding before the capillary forces retaining the residual oil can be overcome[1] . Viscous forces cannot be increased by such a large magnitude in the field.

Polyacrylamide (PAM) solutions have little effect on the oil-water IFT. However, results in the laboratory and field show that after flooding by polymer fluids, all types of microscopic scale residual oil in porous media are lowered[2] . The amount that is lowered is related to the elasticity of the fluids. Other conditions being the same, the higher the elasticity, the lower the residual oil saturation.

The results of lab tests (including microscopic visualization cores) show that viscoelastic fluid flooding can lower the residual oil saturation in cores of different wettabilities. The main forces to mobilize the residual oil by viscoelastic fluids are not entirely the same as that of Newtonian fluids. It is not only a force perpendicular to the oil-water interface overcoming the restraining capillary force, but also a viscous dragging force parallel to the oil-water interface that mobilizes the residual oil.

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