Coupling Neutron and X‐Ray Imaging of Fluid Mixing and Precipitation in Rocks: Challenges and Opportunities

Solute mixing in rocks plays a central role in a wide range of reactive processes. However, how the complex 3D pore structure of rocks governs mixing rates remains largely unknown. Moreover, some mixing‐driven reactions—such as dissolution and precipitation—can modify the pore space, with poorly understood consequences for mixing itself. Recent advances in X‐ray imaging techniques have significantly enhanced our ability to visualize the pore‐scale rock architecture of rocks and a wide range of fluid processes. However, capturing solute mixing and its impact on chemical reactions—such as mineralization—remains a major challenge. Here, we investigated the potential of coupling time‐lapse 3D neutron and X‐ray imaging to characterize reactive fluid mixing and subsequent calcium carbonate mineralization in porous basalt. Two flow‐through experiments were performed with co‐injected CaCl 2 and Na 2 CO 3 , leading to precipitation. Neutron imaging tracked fluid mixing, while X‐ray imaging distinguished the solid matrix from pore space for fluid analysis. The first experiment showed steady transverse mixing, while a second experiment revealed temporal fluctuations due to trapped air, causing multiphase flow. Neutron images indicated significant fluid mixing driven by these fluctuations. A synchrotron X‐ray image post‐experiment indicated additional mineral precipitates from long‐term diffusive mixing. Despite the promising results, several challenges remain, including resolution limits, temporal synchronization between modalities, and accurate fluid phase segmentation. Overall, our findings highlight both the potential and limitations of integrated neutron and X‐ray imaging for studying pore‐scale reactive transport and mineralization processes.