How does imaging help unveil chaotic mixing in porous rocks?

Geochemical reactions in porous rocks are typically scaled up using effective reaction parameters derived under well-mixed conditions. Such well-mixed conditions are often absent in natural settings. While conventional transport theories based fundamentally on diffusion and dispersion processes can not fully capture the state of mixing, several lines of evidence point to the dominance of chaotic solute mixing. Yet, proving the existence of chaotic mixing in porous rocks remains unresolved mostly due to the limitations in directly observing pore-scale processes. In this work, we present direct evidence of chaotic microscale trajectories in porous rock samples by performing fast high-resolution X-ray tomography at the European Synchrotron Radiation Facility (ESRF). We utilize a custom-designed core holder and highly permeable sandstone and sand pack samples to achieve notably high Peclet numbers during the co-injection of two miscible, highly viscous mixtures of glycerin and brine. These high Peclet numbers are crucial for visualizing chaotic trajectories within the rock pores, as they allow the deformation of fluid fronts to dominate before molecular diffusion blurs the patterns. The existence of such trajectories could significantly enhance microscale concentration gradients, potentially leading to chemical reaction rates that differ from conventional reactive transport model predictions. This difference underscores the need to update kinematic models to incorporate the coupling between chaotic mixing and chemical reactions in porous media for a better understanding and quantification of transport and storage processes in the subsurface.