Unraveling dissolution regime transitions in carbonates during CO2-rich water injection

Carbonate dissolution by CO₂-rich brine (carbonic acid) can strongly modify pore structure and flow pathways in subsurface systems relevant to geological CO₂ storage. However, predicting the resulting dissolution regimes remains challenging, as widely used transport–reaction scaling approaches based on Péclet and Damköhler numbers often fail to reproduce experimentally observed dissolution patterns. Here, we present a new set of core-scale dissolution experiments designed to directly observe the coupled evolution of pore structure, flow, and reaction-front migration during CO₂-rich water injection. Experiments were performed on cylindrical limestone cores with a diameter of 12 mm and a length of 36 mm from two formations exhibiting contrasting pore-scale heterogeneity: (1) Ketton limestone, representing a relatively homogeneous system, and (2) Estaillades limestone, representing a heterogeneous system. Carbonated water with an initial pH of 3 was injected into three samples of each limestone at ambient temperature and a pore pressure of 50 bar under constant flow rates of 0.1, 1, and 10 ml/min. Dissolution processes were monitored using time-lapse X-ray microcomputed tomography at approximately 6 µm spatial resolution. Scans were acquired under initial dry conditions, fully water-saturated conditions, and after successive intervals of 100 injected pore volumes of CO₂-rich water, enabling four-dimensional visualization of dissolution pattern development. Across both lithologies, systematic transitions in dissolution behaviour are observed with increasing flow rate: compact or inlet-localized dissolution at low flow rate, dominant wormhole formation at intermediate flow rate, and increasingly distributed, multi-branch, or ramified wormholing (nearly uniform) at the highest flow rate. While pore-scale heterogeneity influences the geometry and symmetry of the resulting dissolution structures, the overall regime transitions remain consistent across both carbonate systems. We observe that dissolution patterns cannot be solely explained by classical Pe-Da scaling based on initial flow and kinetic conditions. Instead, the results demonstrate that the spatial persistence of fluid reactivity governs both the extent and morphology of dissolution across flow rates and lithologies with contrasting heterogeneity. These experiments show that accounting for the evolution of fluid reactivity and reaction-front migration is essential for more accurate prediction of carbonate dissolution during CO₂ injection.