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Prediction of reactive transport in porous media remains challenging whenpore scale incomplete mixing is at play. Previous experimental studiesinvestigated chemical reactions in porous media by visualizing reaction productor reactants mostly in uniform flow. However, the local reaction rate, which isnecessary to infer mechanisms of reaction in pore space, could not be obtainedwithout considering transport of reaction products and reactants. Thus, theinterpretation remained elusive. We visualized the reaction rate field usingchemiluminescnece within index-matched 3D porous media under zero accelerationand constant acceleration flow fields to investigate how pore scale chaoticmixing and Darcy scale fluid acceleration rectify reactive transport. We foundthat the reaction rate kept increasing from upstream to downstream in constantacceleration field, whereas it increased only at the upstream zone in zeroacceleration field. The ratio of dispersion rate and size of the mixinginterface determined such an effect of acceleration. Moreover, the experimentalresults showed stronger dependency of reaction rate on velocity compared to thenumerical simulations that assume complete mixing in pore space. To explainthis, we suggested the mechanistic model that includes the pore scale foldingof lamellae due to chaotic mixing and the pore scale concentration gradientsagainst compression. Such a pore scale mechanism was consistent with theexperimentally observed change in reaction rate over the space. These resultsgive new insights on underlying mechanisms of reactive transport in porousmedia.

Impact of pore-scale chaotic mixing on Darcy-scale reaction rates