A coupling alternative to reactive transport simulations for long-term prediction of chemical reactions in heterogeneous CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; storage systems

M De Lucia,T Kempka,M Kühn

doi:10.5194/gmd-8-279-2015

Abstract

Abstract. Fully coupled, multi-phase reactive transport simulations of CO2 storage systems can be approximated by a simplified one-way coupling of hydrodynamics and reactive chemistry. The main characteristics of such systems, and hypotheses underlying the proposed alternative coupling, are (i) that the presence of CO2 is the only driving force for chemical reactions and (ii) that its migration in the reservoir is only marginally affected by immobilisation due to chemical reactions. In the simplified coupling, the exposure time to CO2 of each element of the hydrodynamic grid is estimated by non-reactive simulations and the reaction path of one single batch geochemical model is applied to each grid element during its exposure time. In heterogeneous settings, analytical scaling relationships provide the dependency of velocity and amount of reactions to porosity and gas saturation. The analysis of TOUGHREACT fully coupled reactive transport simulations of CO2 injection in saline aquifer, inspired to the Ketzin pilot site (Germany), both in homogeneous and heterogeneous settings, confirms that the reaction paths predicted by fully coupled simulations in every element of the grid show a high degree of self-similarity. A threshold value for the minimum concentration of dissolved CO2 considered chemically active is shown to mitigate the effects of the discrepancy between dissolved CO2 migration in non-reactive and fully coupled simulations. In real life, the optimal threshold value is unknown and has to be estimated, e.g. by means of 1-D or 2-D simulations, resulting in an uncertainty ultimately due to the process de-coupling. However, such uncertainty is more than acceptable given that the alternative coupling enables using grids of the order of millions of elements, profiting from much better description of heterogeneous reservoirs at a fraction of the calculation time of fully coupled models.

Highlights

Long-term, reservoir-scale, multi-phase reactive transport simulations in heterogeneous settings are computationally extremely challenging, often forcing to set up oversimplified models if compared to purely hydrodynamic simulations
There is no example of fully coupled reactive transport simulations considering complex chemistry on spatial discretisations with a resolution comparable with the one usually adopted for pure hydrodynamic simulations
We can anticipate here that none of the reactive simulations for the case study of this paper showed possible salt precipitation, but this could be due to the coarse discretisation used; in the specific case of the Ketzin site, there was no sign of injectivity loss throughout operations, and repeated Pulsed Neutron Gamma (PNG) logging campaigns interpreted very moderate, spatially concentrated and probably transient salt precipitation around the borehole (Baumann et al, 2014), confirming its negligibility

Summary

Introduction

Long-term, reservoir-scale, multi-phase reactive transport simulations in heterogeneous settings are computationally extremely challenging, often forcing to set up oversimplified models if compared to purely hydrodynamic simulations. Most studies of reactive transport only consider very simple geometries and homogeneous media, disregarding spatial heterogeneities at reservoir scale, which in turn are routinely considered by the usually much more detailed geologic models and pure hydrodynamic reservoir simulations. These oversimplifications concern the chemistry as well, leading one to consider only a subset of the potentially reactive minerals. There is no example of fully coupled reactive transport simulations considering complex chemistry on spatial discretisations with a resolution comparable with the one usually adopted for pure hydrodynamic simulations

Results

Discussion

Conclusion