4D Neutron Imaging of Solute Transport and Fluid Flow in Sandstone Before and After Mineral Precipitation

Abstract 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 CaCl 2 and Na 2 CO 3 , 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. Plain Language Summary Flow and mixing processes in porous media control many natural and industrial systems, such as microbial clogging, oil extraction, and effluent disposal. In many systems, the porosity may evolve during mineral precipitation, such as in rocks, and control fluid transport properties. Here, we use time‐lapsed three‐dimensional neutron imaging to explore fluid transport into Berea sandstone core samples during in‐situ carbonate precipitation. Neutron imaging can track fluid flow inside the rock, whereas X‐ray imaging illuminates the regions where mineral precipitation occurs. We control the precipitation of calcium carbonate in the rock through reactive mixing between solutions containing CaCl 2 and Na 2 CO 3 . By solving the adverse advection‐diffusion equation using the contrast in neutron attenuation from time‐lapse images, we derive the 3D velocity field of the injected fluids. Results show that under the effect of mineral precipitation, a wide range of local flow velocities develop in the sample, and we quantify the distribution of flow velocities in the sample. The finding of this experimental study is useful in progressing the knowledge in the domain of reactive solute and contaminant transport in the subsurface. Key Points 4D neutron imaging is used to image flow and transport in porous rock and investigate the effect of carbonate precipitation The velocity field of injected fluid is estimated by solving the advection‐dispersion equation using the contrast in neutron attenuation Carbonate precipitation widens the distribution of fluid velocities due to local pore‐clogging