Role of pore–scale heterogeneity on reactive flows in porous materials: validity of the continuum representation of reactive transport

Abstract

Understanding multi-component reactive transport in porous media is critical to a wide range of fields. It poses great challenges to theoretical, experimental, as well as numerical studies, because it usually involves multiple processes (advection, diffusion, reaction) and multiple scales (molecular, pore, laboratory, field). Current modeling approaches commonly employ a continuum description and rely on volume averages. Because in porous media, averages are taken over length scales much larger than typical grain sizes, spatial heterogeneity at smaller scales is unresolved. This loss of information includes pore-scale velocities and geometry, mineral texture, and other features. Therefore, understanding the impact of pore-scale spatial heterogeneity on reactive processes in porous materials is a key aspect in validating continuum models of reactive transport, and developing predictive capabilities of system behavior. However, due to the complexity of the multi-process, multi-scale problem, the majority of pore-scale modeling studies have focused on the basic transport properties of porous media including effective diffusion, conductivity, permeability, and elasticity. In this study, using a Lattice Boltzmann (LB) model for multi-component reactive transport in porous media, we investigate the role of spatial heterogeneity related to pore geometry on mass transport and reaction rates in porous materials and its impact on the validity of the continuum representation of reactive transport. Because of the ability of the LB method to accurately represent pore-scale phenomena, it provides the most comprehensive approach to investigate the influence of pore-scale heterogeneity on continuum formulations of reactive transport. We apply our model to various chemical systems for two-dimensional, artificially- constructed and natural multi-scale fractured and porous media. The reactive- transport processes are simulated at the pore scale, with systematic consideration of the pore-scale flow field, diffusion, homogeneous reactions among multiple aqueous species, heterogeneous reactions between the aqueous solution and minerals, as well as the resulting changes in solid and pore geometry. The results are averaged over vertical slabs which are considered as REVs, and are compared with one-dimensional continuum-scale simulations. From the LB simulations, it is possible to determine macro-scale properties of the medium such as tortuosity, permeability, and reactive surface area. In addition, the LB simulations also enable the most appropriate continuum formulation—single, dual, or multiple continua—to be determined. Our results confirm that a multiple continuum model is required to describe upscaled pore-scale results derived from multi-scale geometries.