We study the impact of pore space heterogeneity on mixing and reaction in porous media. We simulate the parallel injection of two streams of reactants at different pH in a three-dimensional microporous consolidated rock whose pore space was resolved by differential micro-CT imaging. As an exemplar of a heterogeneous medium, we consider the pore structure obtained from a Portland carbonate sample. We use direct numerical simulation to study the coupled impact of flow heterogeneity, characterized by a wide distribution of velocities, and chemical reversibility on multispecies reaction. The flow field is found from the Darcy-Brinkman equation while the advection-diffusion equation describes transport, which is coupled to a general multispecies geochemical solver for homogeneous reactions; precipitation and dissolution are ignored.

We observe a highly non-uniform spatial distribution of concentration and rates of formation and consumption. For advection-dominated transport, the heterogeneous flow field leads to significant transverse mixing in macropores at early times, followed by a slower mixing driven by diffusion between macro- and micropore regions. The effective rates of formation and consumption are species-dependent and distinct in macro- and microporosity: while some species reach an asymptotic rate in well-mixed regions, others still show a transient non-monotonic behaviour as a consequence of incomplete mixing. Our findings have important implications for the understanding of time- and space-dependent reaction rate behaviour: the coupled impact of pore space heterogeneity and reversible reactions need to be taken into account as key determinants to describe multispecies reactive transport.