Mineral precipitation in geological formations occurs when reactive fluids of varying compositions mix, altering the porous microstructure of the rock. Basalt rocks are of particular interest for long-term CO2 storage due to their potential to rapidly mineralize CO2 into stable carbonate minerals. We investigated reactive fluid mixing and subsequent carbonate mineralization in porous basalt using time-lapse three-dimensional neutron and X-ray imaging. Two flow-through experiments with different injection rates were performed on basalt cores, where co-injected CaCl2 and Na2CO3 solutions were mixed within the porous network, leading to calcium carbonate precipitation. Time-lapse neutron imaging distinguished the two injected fluids and tracked their mixing. X-ray imaging was used to separate the solid matrix from the pore space to enable fluid analysis in the neutron images. A first experiment with a high flow rate induced a steady transverse mixing pattern, captured by a decay of the concentration variance through the sample, as measured by neutron imaging. A second experiment at a lower flow rate promoted more temporal fluctuations in the fluid distribution due to the multiphase flow of water and air in the rock. The analysis of neutron images showed a significant mixing of reactive fluids driven by these temporal fluctuations. Furthermore, a higher-resolution, synchrotron X-ray image of one of the sample rocks acquired after the experiment showed the formation of additional calcite resulting from long-term diffusive mixing. The results highlight the great potential and challenges of neutron and X-ray imaging in characterizing pore-scale mixing and precipitation in rocks.

Advances in micro-scale imaging techniques, such as X-ray microtomography, have provided new insights into a broad range of porous media processes. However, direct imaging of flow and transport processes remains challenging due to spatial and temporal resolution limitations. Here, we investigate the use of dynamic three-dimensional 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-lapse 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. These findings suggest that neutron imaging is a promising technique to investigate dynamics processes in porous media.

Induced earthquakes are still highly unpredictable, and often caused by variations in pore fluid pressure. Monitoring and understanding the mechanisms of fluid-induced fault slip is essential for seismic risk mitigation and seismicity forecasting. Fluid-induced slip experiments were performed on critically stressed faulted sandstone samples, and the evolution of the actively sent ultrasonic waves throughout the experiment was measured. Two different fault types were used: smooth saw-cut fault samples at a 35º angle, and a rough fault created by in-situ faulting of the samples. Variations in the seismic slip velocity and friction along the fault plane were identified by the coda of the ultrasonic waves. Additionally, ultrasonic amplitudes show precursory signals to laboratory fault reactivation. Our results show that small and local variations in stress before fault failure can be inferred using coda wave interferometry for time-lapse monitoring, as coda waves are more sensitive to small perturbations in a medium than direct waves. Hence, these signals can be used as precursors to laboratory fault slip and to give insight into reactivation mechanisms. Our results show that time-lapse monitoring of coda waves can be used to monitor local stress changes associated with fault reactivation in this laboratory setting of fluid-induced fault reactivation. This is a critical first step towards a method for continuous monitoring of natural fault zones, contributing to seismic risk mitigation of induced and natural earthquakes.