A Critical Review of Capillary Number and its Application in Enhanced OilRecovery

摘要

Capillary number(Ca),defined as dimensionless ratio of viscous force to capillary force,is one of themost important parameters in enhanced oil recovery(EOR).The ratio of viscous and capillary force isscale-dependent.At least 33 different Cas have been proposed,indicating inconsistencies between variousapplications and publications.The most concise definition containing velocity,interfacial tension andviscosity is most widely used in EOR.Many chemical EOR applications are thus based on the correlationbetween residual oil saturation(ROS)and Ca,which is also known as capillary desaturation curve(CDC).Various CDCs lead to a basic conclusion of using surfactant to reduce interfacial to ultra-low to get aminimum ROS and maximum displacement efficiency.However,after a deep analysis of Ca and recent newexperimental observations,the traditional definition of Ca was found to have many limitations and basedon misunderstandings.First,the basic object in EOR is a capillary-trapped oil ganglia thus Darcy's law isonly valid under certain conditions.Further,many recent tests reported results contradicting previous ones.It seems most Cas cannot account for mixed-wet CDC.The influence of wettability on two-phase flow isimportant but not reflected in the definition of the Ca.Then,it is certainly very peculiar that,when theviscous and capillary forces acting on a blob are equal,the current most widely used classic Ca is equal to2.2* 10-3.Ideally,the condition Ca ~ 1 marks the transition from capillary dominated to viscous-dominatedflow,but most Cas cannot fulfill this expectation.These problems are caused by scale dependent flowcharacterization.It has been proved that the traditional Ca is of microscopic nature.Based on the dynamiccharacterization of the change of capillary force and viscous force in macroscopic scale,a macroscopic Cacan well explain these complex results.The requirement of ultra-low IFT from microscopic Ca for surfactantflood is not supported by macroscopic Ca.The effect of increasing water viscosity to EOR is much higherthan reducing IFT.Realizing the microscopic nature of the traditional Ca and using CDCs based on the morereasonable macroscopic Ca helps to update screening criteria for chemical flooding.