Decoupling instead of grid coarsening: how to achieve reservoir scale reactive transport simulations in highly heterogeneous settings. Example from CO2 storage

Abstract:

The characteristics of a typical CO₂ storage system allow simplification strategies for reactive transport simulations based on process decoupling. In such systems the feedback of the slow chemical reactions to hydrodynamics is low until the system reaches a substantial hydrodynamic equilibrium. Furthermore, the presence of CO₂ is the main driving force for chemical reactions, which are for most reactants kinetically controlled. Hence, the same reaction path is substantially replicated in all elements of the grid exposed to the injected CO₂, either in gaseous or in dissolved form. The analysis of
fully coupled 3D simulations of the Ketzin pilot site for CO₂ storage performed with the TOUGHREACT simulator confirms these hypotheses to a large extent, both in homogeneous and in heterogeneous settings. This allows the definition of a simplified one-way coupling combining independent non-reactive hydrodynamic and batch geochemical models. The exposure time to CO₂ of each grid element is estimated by the conservative simulations, then the outcome of one single geochemical model per lithofacies is applied to each grid element. A threshold value for the minimum concentration of dissolved CO₂ required to start chemical reactions permits to mitigate the discrepancy due to the lack of a mass balance between the independently run simulations. The comparison with fully coupled simulations validates the novel approach. The simplified coupling can tackle a wide class of problems, not only CO₂ storage; it allows calculating reactive chemistry on grids comprising millions of elements, overcoming a major limitation of reactive transport models, which are often bounded to 2D radial domains. This is particularly advantageous in highly heterogeneous settings with complex hydrodynamics. The new coupling is demonstrated at full scale for the Ketzin site with simulations up to 15000 years, a result which cannot yet be achieved by fully coupled simulations.