Multispecies Reactive Transport in a Microporous Rock: Impact of Flow Heterogeneity and Reversibility of Reaction

Abstract

AbstractWe 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 nonuniform 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 macropore and micropore regions. The effective rates of formation and consumption are species‐dependent and distinct in macroporosity and microporosity: while some species reach an asymptotic rate in well‐mixed regions, others still show a transient nonmonotonic behavior as a consequence of incomplete mixing. Our findings have important implications for the understanding of time‐ and space‐dependent reaction rate behavior: the coupled impact of pore space heterogeneity and reversible reactions need to be taken into account as key determinants to describe multispecies reactive transport.