Effect of viscoelasticity on oil displacement in a microfluidic porous medium

Abstract

Oil displacement by an immiscible phase in porous media occurs in many important applications, including oil recovery, groundwater remediation and $\hbox {CO}_2$ sequestration. Usually the injected fluid has a much lower viscosity than the resident oil phase and the displacement flow in inherently unstable, with the formation of viscous fingers. Polymer can be added to the injected water phase to increase its viscosity, improve the stability of the displacement front and, consequently, the efficiency of the process. Recent experimental evidences have shown that the injection of viscoelastic polymer solution can improve not only the macroscopic efficiency of the flow, but also reduce the volume of oil that remains trapped by capillary forces in a region of a porous medium that has been swept by the water phase. Despite the recent progress, the pore-scale flow mechanisms by which viscoelastic forces mobilize trapped oil ganglia are still not clear and a matter of debate. This work studies the effect of viscoelastic forces on displacement flow in porous media by comparing the residual oil saturation and trapped ganglia size distribution of three different water phases: pure water, a more viscous Newtonian glycerol solution and a viscoelastic polymer solution with the same shear viscosity of the glycerol solution. The porous medium was a transparent micromodel that consisted of a regular network of constricted capillaries. The results show that at low flow rates (low capillary and Weissenberg number), the flow behavior of the glycerol and polymer solution is very similar. At high capillary/Weissenberg number, the extra pressure associated with the viscoelastic extensional dominated flow through the constricted capillaries mobilizes trapped ganglia, leading to a lower residual oil saturation and smaller oil ganglia after the injection of the polymer solution.