Mixing of complex fluids in confined porous media

Convective mixing in porous media plays a central role in a wide range of geophysical and environmental processes, including geological CO2 storage, groundwater contamination, and reactive transport in subsurface formations. We investigate how fluid properties and boundary conditions control solutal convection and mixing in confined porous media. The system consists of two miscible fluid layers initially separated by a horizontal interface, where density variations are induced by solute concentration. Mixing can locally increase fluid density, triggering buoyancy-driven instabilities that enhance mass transport. The relative importance of convective and diffusive mechanisms is quantified by the Rayleigh-Darcy number. Using high-resolution numerical simulations, we explore mixing dynamics at high Rayleigh-Darcy numbers (O(10,000)) for fluids with different density-concentration relationships, including linear, parabolic, and piecewise non-monotonic laws. These scenarios are representative of realistic fluids encountered in subsurface applications, such as CO2-brine mixtures or chemically reactive solutes. We analyse how (i) the density contrast between the mixed fluid and the initial layers, and (ii) the concentration at which density is maximized relative to the initial conditions, influence the onset and efficiency of convective mixing. Across all cases considered, we find that the mixing process is controlled by the mean scalar dissipation rate, allowing us to develop simple physical models that capture the observed behaviour. We further assess the impact of boundary conditions on the mixing rate and identify configurations that promote efficient mixing. Finally, we compare two- and three-dimensional systems and discuss the implications of our results for predicting and optimizing solute transport in geophysical porous-media flows. Funded by the European Union (ERC, MORPHOS, 101163625). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them.