Atefeh Vafaie, Josep M Soler, Jordi Cama, Iman R. Kivi, Victor Vilarrasa

Geologic CO2 storage is projected to play a key role in mitigating the climate change crisis. Changes in pore structure and hydraulic properties are likely to occur in carbonate rocks when they interact with CO2 as an acid-producing agent, potentially affecting CO2 flow and storage behavior in the subsurface. Here, we combine laboratory experiments and numerical simulations of CO2-saturated water and HCl solution injections into limestone specimens to develop an improved understanding of reactive flow in these rocks. We employ a digital rock approach based on X-ray micro-computed tomography (µCT) to construct heterogeneous rock permeability maps, fed as inputs into 3D Darcy-scale reactive transport models of the experiments. The simulations satisfactorily reproduce measured changes in effluent chemistry, porosity and permeability as well as the observed dissolution features in reacted rock samples. The complete dissociation of HCl as a strong acid results in compact dissolution, numerically captured using the classical Kozeny-Carman porosity-permeability relationship. In contrast, the partial dissociation of aqueous CO2 as a weak acid and the related pH-buffering effect drive strong feedback between fluid flow and dissolution, leading to wormhole formation. This dissolution pattern can be only reproduced by a large exponent (15 to 27.6) in the porosity-permeability relationship. We show that dimensionless Péclet and Damköhler numbers alone cannot predict the observed dissolution patterns in the rock. The obtained results highlight the primary control of small-scale heterogeneities and acid type on coupled flow and chemical reactions in permeable limestones and the need for a rigorous upscaling approach for field-scale studies.