Convective-Reactive CO2 Dissolution in Aquifers With Mass Transfer With Immobile Water

Abstract

The objective of this paper is to study the impact of immobile water, its fraction, and its mass transfer with the flowing region on efficiency of CO2 dissolution in aquifers with an immobile water zone. A continuum scale code is developed with underlying assumptions of spatially homogeneous and temporally invariable partitioning fraction of the porous media, first-order mass transfer between the mobile and immobile zones, and simplified reaction of CO2 aqueous solution with calcium carbonate rock. Using ranges of values for Damköhler number (Da), fraction of the total pore volume, and mass transfer coefficient rate (α), 96 simulations are conducted. It is shown that due to a lower intensity of reaction in the mobile region, intermediate values of α serve as a threshold below which the mass transfer coefficient is not affecting the overall CO2 storage and above which overall CO2 storage increases as a function of mass transfer coefficient. Additionally, we found that (i) when α is high and geochemistry is intensive (high Da), the overall CO2 storage decreases with increase in fraction of mobile water. This is because CO2 storage through consumption of rock in immobile water with higher geochemistry is reduced. (ii) When α is high but Da is low, the system is effectively a single porosity medium with no chemistry-influenced discrimination between mobile and immobile zones, and therefore, overall CO2 storage increases with fraction of mobile water. (iii) When α is low, the magnitude of Da does not influence the overall CO2 storage.