Transient increase in reactive surface and the macroscopic Damköhler number in chalk dissolution

摘要

The quantification of surface area between mineral and reactive fluid isessential in environmental applications of reactive transport modelling. Thisquantity evolves with microstructures and is difficult to predict because themechanisms for the generation and destruction of reactive surface remainelusive. The challenge of accounting for the inherent heterogeneities ofnatural porous media in numerical simulation further complicates the problem.Here we first show a direct observation of reactive surface generation in chalkunder circumneutral to alkaline pH using in situ X-ray microtomography. Themomentary increase of reactive surface area cannot be explained by a change influid accessibility or by surface roughening stemming from mineralogicalheterogeneity. We then combine greyscale nanotomography data with numericalsimulations to show that similar temporal behaviour can be observed over a widerange of pH as porous media dissolve in imposed flow field. We attribute theobservation to the coupling between fluid flow and mineral dissolution andargue that the extent of surface generation is strongly correlated with theadvective penetration depth of reactants. To conclude, we demonstrate theapplicability of using a macroscopic Damk"ohler number as an indicator for thephenomenon and discuss its environmental significance beyond geologic carbonstorage.