An Experimental Study of Miscible Displacement Regimes in A Microscopic Heterogeneous Porous Media.

Abstract:

When a less viscous fluid displaces a more viscous fluid and they are first contact miscible, the flow is dominated by instabilities arising from viscous fingering and total dispersion between two fluids. However, the relationship between viscous fingering and dispersion has not been well examined, especially at pore-scale. Here we design a microfluidic-visualization system to perform the displacement of two miscible fluids in an inherently heterogeneous microscopic porous media. Experiments of degassed water displacing glycerol are conducted at five flow rates (0.05~5μL/min), four viscosity ratios (50~1000). In the pore-scale miscible displacement, we discover two main mechanisms of flow instability including viscous fingering and velocity-induced dispersion (VID). At viscous-dominated flow, a series of nonlinear finger interactions including spreading, shielding, coalescence and even tip-splitting are observed. But when flow is governed by VID, instability of viscous fingers becomes less important. It is further found that the effect of the log-viscosity ratio R on the total flow instability depends on the flow rate because the main flow regime changes from viscous fingering to VID as flow rate increases. Finally the results of quantitative analyses of mixing length, normalized concentration gradient(NCG) and breakthrough time confirm the validation of our observations.