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

Surface area is an important input parameter for reactive transport modelling and it changes with time when rocks dissolve. Here we show direct observations of increased surface area due to chalk (predominantly CaCO₃) dissolution, using 3D in situ X-ray microtomography. This transient increase cannot be explained by changes in fluid accessibility or by surface roughening that arises from preferential leaching. We use model simulations to explain how this increase in surface area correlates with the size of the Damköhler space, defined macroscopically using a phenomenological dissolution rate law and the cumulative surface of the microstructure. We attribute this transient increase to the coupling between fluid flow and mineral dissolution and argue that the extent is determined by the advective penetration distance of the reactants. We conclude that using a macroscopic dimensionless number as a qualitative indicator for microstructure evolution has limited applicability.