Abstract
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 CaCl2 and Na2CO3, 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.

Plain Language Summary
Understanding fluid movement inside rocks is crucial for enhancing CO2 storage and other underground applications. This study used neutron and X-ray imaging to explore how reactive fluids mix in basalt rocks and how this process forms calcium carbonate, aiding long-term carbon storage. We conducted experiments by pumping fluids through basalt samples at varying flow rates and employed advanced imaging to observe the mixing and subsequent calcite precipitation. Neutron imaging tracked fluid movement, while X-ray imaging revealed the pores filled with fluids and calcite. Our findings underscore the challenges and opportunities of using these imaging techniques to study pore-scale mixing and reactive transport, providing a framework for future research on optimizing mineral precipitation processes.

In many geological systems, the porosity of rock or soil may evolve during mineral precipitation, a process that controls fluid transport properties. Here, we investigate the use of 4D neutron imaging to image flow and transport in Bentheim sandstone core samples before and after in-situ calcium carbonate precipitation. First, we demonstrate the applicability of neutron imaging to quantify the solute dispersion along the interface between heavy water and a cadmium aqueous solution. Then, we monitor the flow of heavy water within two Bentheim sandstone core samples before and after a step of in-situ mineral precipitation. The precipitation of calcium carbonate is induced by reactive mixing of two solutions containing CaCl₂ and Na₂CO₃, either by injecting these two fluids one after each other (sequential experiment) or by injecting them in parallel (co-flow experiment). We use the contrast in neutron attenuation from time-resolved tomograms to derive three-dimensional fluid velocity field by using an inversion technique based on the advection-dispersion equation. Results show mineral precipitation induces a wider distribution of local flow velocities and leads to alterations in the main flow pathways. The flow distribution appears to be independent of the initial distribution in the sequential experiment, while in the co-flow experiment, we observed that higher initial local fluid velocities tended to increase slightly following precipitation. The outcome of this study contributes to progressing the knowledge in the domain of reactive solute and contaminant transport in the subsurface using the promising technique of neutron imaging.