A Compositional Global Implicit Approach for Modeling Coupled Multicomponent Reactive Transport

Reactive transport modeling has become widely used to help improve understanding of hydrogeochemical processes from the pore scale to the watershed scale. In recent years, the scope of reactive transport applications has increased toward a higher level of complexity and process coupling. For example, the production and consumption of water as well as porosity evolution associated with the dissolution and precipitation of hydrated minerals can impact system evolution. Waste rock weathering, carbon sequestration, or the degradation of engineered barriers in radioactive waste repositories all constitute applications in which geochemistry and hydrodynamics can strongly influence each other. For these purposes, the traditional formulation of reactive transport simulators, which decouples groundwater flow and reactive transport processes, is limited. We present a global implicit compositional approach, which integrates the flow processes directly into the reactive transport and geochemical framework. This approach solves the flow field implicitly with the reactive transport equations, simultaneously accounting for water consumption and production due to geochemical reactions. Applications show that the model allows tackling complex reactive transport problems while accounting for intra-aqueous reactions, redox reactions, and reactions involving mass transfer with the gas and solid phases. The presented simulations also demonstrate that the compositional and traditional approaches yield similar results for complex geochemical systems with relatively low reactivity.