Scale effect on the time dependence of mineral dissolution rates in physically heterogeneous porous media

Abstract

Hydrologic heterogeneity complicates the time dependent behavior of mineral dissolution rates and introduces large uncertainty in effective surface area estimation. In order to resolve the coupled evolution of mineral dissolution rates and mineral surface area over time in various fluid flow conditions, this study presents multiple reactive transport simulations of anorthite dissolution over 3000 years with 2D latticed random permeability fields on small (1 m × 1 m) and large (400 m × 400 m) domains. Physical heterogeneity is controlled by varying integral scales (λmin and λmax) and variance in permeability distribution (σlnk2) conditions. Chemistry is simplified to a single dissolving primary mineral (anorthite) distributed homogeneously throughout the domain and a single secondary mineral (kaolinite) that is allowed to dissolve or precipitate. In a homogeneous domain, non-normalized effective mineral dissolution rate decreases over time as depletion of the dissolving mineral reduces reaction product concentration. When the reaction rate is normalized either by local or domain-averaged mineral surface area, the rate increases over time approaching the value of the predefined intrinsic rate constant as the effect of saturation state becomes negligible. In a small heterogeneous domain, the effect of physical heterogeneity is largely negligible regardless of the changes in control parameters. In large domains, however, large λ and σlnk2 contribute to develop low-reactive zones that preserve the reactive mineral from dissolution by limiting solute transport to slow diffusion. This preserved mineral fraction provides reactants in late time period resulting in gradual decrease of non-normalized effective mineral dissolution rate. If the rate is normalized by domain-averaged specific surface area, these remaining surface areas over-normalize effective dissolution rates resulting in ∼1.5 orders of magnitude reduction from the homogeneous case.