Review of Capillary Number in Chemical Enhanced Oil Recovery

Abstract After decades of development, great progress has been made in capillary number theory and it has important but often incorrect application in EOR. Investigation into progress on capillary number theory and some misuse of capillary number theory helps to make better use of it. Latest progress concerning with capillary number theory and its application in chemical EOR is reviewed by studying the experiments data and checking its model hypothesis. Classic Capillary Desaturation Curves (CDC) are summarized and new CDC is introduced. Typical classical CDC showed larger capillary number lead to lower residual oil saturation and when capillary number increased to a certain critical value(first critical value), the residual oil saturation could drop to a minimum value even zero. CDC shapes were different in water-wet and oil-wet media. Recovery can be improved by increasing flooding rate, though invalid and impractical, displacement phase viscosity or/and reducing oil/water interfacial tension, which are actually adopted by chemical flooding. Guided by this theory and also first critical capillary number value requirement, it lead to the pursuit of low interfacial tension to largest extent and the requirement of ultra-low interfacial tension (10-3mN/m) in surfactant screening. However, experiments data showed that residual oil saturation was not always decreasing as capillary number increased. After capillary number increased to a certain value (second critical value), the residual oil saturation may increase or decrease as capillary number increase. What was more, the final residual oil saturation was quite more than zero and this CDC was regarded as the new CDC. Experiments in heavy oil laboratory tests showed that smaller injection rate lead to higher recovery which seemed contrary to capillary theory.