Carbonate rocks are characterized by an important reactivity to fluid circulation, leading to dissolution phenomena associated to significant changes in their hydromechanical (petrophysical and mechanical) properties. These processes play a key role in the genesis of karstic environments, which constitute major reserves of drinking water, and must be integrated to ensure large-scale deployment of geological storage of CO₂ in carbonate saline aquifers.

Dissolution of carbonate rocks is controlled by different factors, related to the fluid (chemical composition, pH, flow velocity) or the rock (mineral content, porosity, permeability, microstructure), and results in different dissolution patterns. Even though microstructural controls on dissolution processes have been previously evidenced, the specific contribution of the double porosity (i.e., the intergranular macroporosity and intragranular microporosity, commonly found in carbonate rocks) remains unclear, and is so far poorly considered in experimental and numerical studies.

To investigate the specific impact of the double-porosity interconnectivity on the dissolution of carbonate rocks, we perform controlled dissolution experiments under identical experimental conditions on two nearly pure calcite limestones (Euville and Lavoux) characterized by different grain and pore structures. Two flow rates are applied to investigate the effect of various hydrodynamic conditions. At high flow rates, two distinct dissolution patterns are evidenced: for Euville samples, dissolution is distributed across the entire width of the sample, whereas for Lavoux samples, dissolution is localized in wormhole-type channels. At lower flow rates, similar wormhole-type dissolution patterns are observed for both Euville and Lavoux samples. We explain these contrasting responses by differences in pore interconnectivity, resulting in different accessible specific surface areas and reactivity.

Finally, numerical transport modeling approach using Lattice-Boltzmann Method is proposed to support our hypotheses.

Théo Briolet, LGENS.

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