Micro-continuum models using the COMSOL-CrunchFlow framework for multi-scale simulations of reactive transport processes

Water-rock interactions are important processes that govern the evolution of many natural and engineered geosystems in the subsurface. Mineral precipitation and dissolution can alter the morphology of pores and fractures, which, in turn, influence flow and transport properties in porous media. Due to complex multiscale features of real geosystems, it is increasingly evident that, a pore-scale perspective is needed for mechanistic understanding and for improving predictive capabilities, while the continuum scale approach continues to provide important insights into bulk properties and larger scale processes. Reactive transport models that solve a set of conservation equations, are a valuable tool to investigate the coupled processes encountered in natural geosystems. We employ a micro-continuum model based on the Darcy-Brinkman-Stokes formulation for simulations of multiscale phenomena. Instead of a full Navier-Stokes approach in the entire domain, this approach solves for the Stokes flow in pore space and Darcy flow in porous media. This model is implemented by coupling a multiphysics code (COMSOL Multiphysics) with a reactive transport code (CrunchFlow). This modeling framework is validated by comparing the simulation results of a benchmark problem that describes the dissolution of a single calcite crystal, with those from other established numerical codes. Furthermore, the approach is then used to investigate fracture alteration and altered layer development, with a focus on the impacts of mineral composition on altered layer properties and the evolution of fracture hydraulic properties.