EFFECTS OF PORE-SCALE HETEROGENEITIES ON CONTINUUM-SCALE RATES OF REACTIONS ASSOCIATED WITH ANORTHITE DISSOLUTION

Geochemical reaction rates are usually measured in well-mixed laboratory systems using single minerals, while natural subsurface systems consist of porous media with a wide range of physical and chemical properties.  The heterogeneities in properties of natural systems occur over a range of length scales, including the pore scale. Pore-scale heterogeneities can affect continuum-scale reaction rates in porous media, although such effects are usually ignored in geochemical modeling.

In this work, we use pore-scale network models to study the effects of pore-scale heterogeneities on continuum-scale reaction rates, and identify the conditions under which these effects are significant. The model system is a CO₂-brine-rock system with 10% of the solid phase being reactive (anorthite, kaolinite and gibbsite), and the aqueous phase being brine saturated with high-pressure CO₂. We compare the simulated continuum-scale reaction rates from the network model with those from a continuum model, which represents the porous medium as homogeneous.

Simulation results show that concentration heterogeneities at the pore scale cause up-scaled reaction rates to be significantly different from those that would be predicted from simple continuum modeling. The values of [Ca²⁺] can range over more than two orders of magnitude, and pH can vary over an order of magnitude.

Differences in rates between the network model and the continuum model depend on the types of reactions, the hydrodynamic conditions, and the spatial distributions of reactive minerals. For anorthite dissolution, the continuum model predicts the same reaction direction as the network model, but overestimates its rates under all hydrodynamic conditions; for kaolinite and gibbsite, the continuum model predicts the same reaction direction at slow and fast flow conditions, but opposite reaction directions in medium flow conditions, as compared to predictions from the network model.  For all three reactions, larger reactive cluster sizes in the porous medium lead to larger differences between the continuum-scale rates from the two models. These results provide guidelines on conditions under which pore-scale heterogeneities should be incorporated into continuum-scale reaction rates.