Experimental insights into gravity-driven flows and mixing in layered porous media

The heterogeneity of permeability in a porous medium considerably alters the behaviour of density-driven flows from what is observed in a medium of homogeneous permeability, and significantly enhances the mixing between the dense and light fluids during the flow. In this work, we present results from laboratory experiments performed in heterogeneous media consisting of horizontal layers of different permeabilities, investigating their effects on gravity current flows. We find that the mixing in our heterogeneous experimental set-ups can be ${O}(2)$ greater than that in a homogeneous medium of similar depth-averaged properties. The enhanced mixing in this setting is primarily because of transverse gravity-driven fingers and produced blunt front, which is the direct result of the layered structure. This enhanced rate of mixing dictates the gravity current height and length, making the current lose its long and thin shape much faster than a comparable current in a homogeneous medium. We discuss the experimental observations in detail and present relevant physical interpretations. Based on the experimental measurements and dimensionless modelling, we also derive semi-empirical formulas for predicting the gravity current length, height and mixing in a heterogeneous medium. Results from this work can be used in predicting the scale of mixing between different density fluids during contaminant transport in the subsurface environment.