Pore-scale controls on calcite dissolution rates from flow-through laboratory and numerical experiments.

A combination of experimental, imaging, and modeling techniques were applied to investigate the pore-scale transport and surface reaction controls on calcite dissolution under elevated pCO2 conditions. The laboratory experiment consisted of the injection of a solution at 4 bar pCO2 into a capillary tube packed with crushed calcite. A high resolution pore-scale numerical model was used to simulate the experiment based on a computational domain consisting of reactive calcite, pore space, and the capillary wall constructed from volumetric X-ray microtomography images. Simulated pore-scale effluent concentrations were higher than those measured by a factor of 1.8, with the largest component of the discrepancy related to uncertainties in the reaction rate model and its parameters. However, part of the discrepancy was apparently due to mass transport limitations to reactive surfaces, which were most pronounced near the inlet where larger diffusive boundary layers formed around grains and in slow-flowing pore spaces that exchanged mass by diffusion with fast flow paths. Although minor, the difference between pore- and continuum-scale results due to transport controls was discernible with the highly accurate methods employed and is expected to be more significant where heterogeneity is greater, as in natural subsurface materials.