Effects of Mixing at Pore Intersections on Large-Scale Dissolution Patterns and Solute Transport

The flow-induced dissolution of porous rocks governs many important subsurface processes and applications. Solute mixing, which determines pore-scale concentration fields, is a key process that affects dissolution. Despite its importance, the effects of pore-scale mixing on large-scale dissolution patterns have not been investigated. Here, we use a pore network model to elucidate the mixing effects on macroscopic dissolution patterns and solute transport. We consider two mixing rules at pore intersections that represent two end members in terms of the mixing intensity. We observe that the mixing effect on dissolution is the strongest at moderate Damkohler number, when the reactive and advective time scales are comparable. This is the regime where wormholes spontaneously appear. Incomplete mixing is shown to enhance flow focusing at the tips of the dissolution channels, which results in thinner wormholes and shorter breakthrough times. These effects on passive solute transport are evident independent of initial network heterogeneity. When a reactive fluid infiltrates the rock, the dissolution channels (wormholes) can spontaneously form, in which the flow and transport of reactant focus. The formation and growth of such channels is a complex phenomenon in which the processes taking place at the micro-scale are strongly coupled with the macro-scale patterns. One of these processes is the mixing of reactant-saturated water at pore intersections. In this paper, we study how the intensity of the mixing process impacts the shapes and growth velocities of the dissolution channels. We find that when the mixing at pore intersections is relatively weak, the flow focuses more strongly in front of the wormhole tip, which reduces the width of the wormhole and leads to its faster propagation and early breakthrough. These effects are also evident from tracer breakthrough curves. Our findings contribute to the understanding of dissolution-induced patterns, with implications to subsurface flow-related processes such as karst formation and contaminant migration. Mixing at pore intersections can have a major impact on macroscopic dissolution patterns and solute transportMixing effect is the strongest when reactive and advective time scales in the system are comparableMixing effect on dissolution increases as network heterogeneity decreases