Impact of Mixing-Controlled Reactions on Subsurface Fate and Transport: Pore-Scale Simulation and Experiments

Several studies have demonstrated the important role played by mixing-controlled reactions in porous media. For example, transverse mixing of nutrients along the fringes of a contaminant plume is often the limiting step that controls overall degradation rate during natural or engineered in situ bioremediation. Similar mixing processes can promote precipitation/dissolution reactions during geological sequestration of carbon dioxide. Field and laboratory investigations have demonstrated that the length scale of transverse mixing zones can be very small, often on the order of centimeters or less. To study dispersion, mixing and reaction at this scale, we use pore-scale numerical simulation models and micro-fluidics laboratory experiments. I will present an overview of our methods and findings, including comparisons between direct numerical simulations and laboratory experiments. Recent results showing coupling of precipitation/dissolution reactions with porosity reduction will be emphasized. In order to match qualitatively the experimental results, the simulation model must include the impact of pH upon carbonate speciation and CaCO3 reaction rate constant, the effect of changing reactive surface area, and the impact of pore blockage from CaCO3 precipitation on diffusion and flow. Our work has improved our understanding of coupled flow and reactions processes; however, there remain significant challenges in extending the results to larger field scales.