Localized versus distributed dissolution in carbonate rocks: The key role of microstructure

Carbonate rocks are widespread and highly reactive, making their study a key topic for various environmental and anthropogenic issues. 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. To investigate the specific impact of microstructure on dissolution pattern, 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 distinct dissolution patterns are evidenced by the integration of CT-scanner and thin section observations, implying significant microstructural control. For Euville samples, dissolution is distributed across the entire width of the sample, whereas for Lavoux samples, dissolution is localized in wormhole-type channels. These contrasting responses are explained by differences in pore connectivity, reactivity and accessible specific surface areas. Conventional expressions of Péclet and Damköhler dimensionless numbers fail to differentiate the two studied limestones in a Pe - Da diagram despite their different behavior. Specific expressions are then defined to account for the specific accessibility to the intragranular microporosity and the associated specific surface area. The proposed approach allows a clear positioning of both limestones in areas of Pe - Da diagram consistent with the observed dissolution patterns.