A 3D Darcy-scale reactive transport modeling of experimental wormhole formation in limestone under geological CO2 storage conditions

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. The ability to understand and evaluate such alterations benefits an improved understanding of 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. We show that the pore space heterogeneity controls chemical reactions from the very beginning of acid injections while the acid type becomes progressively important as the reaction front further penetrates the rock. The complete dissociation of HCl as a strong acid leads to a compact dissolution pattern, 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 a 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. 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.