paimans

• Shafabakhsh, Paiman; Cordonnier, Benoit; Borgne, Tanguy Le; Mathiesen, Joachim; Linga, Gaute & Pluymakers, Anne [Show all 9 contributors for this article] (2025). Coupling Neutron and X‐Ray Imaging of Fluid Mixing and Precipitation in Rocks: Challenges and Opportunities. Water Resources Research. ISSN 0043-1397. 61(12). doi: 10.1029/2025wr041911. Full text in Research Archive Show summary

  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.

• Shafabakhsh, Paiman; Cordonnier, Benoit; Pluymakers, Anne Marie Henriette; Borgne, Tanguy Le; Mathiesen, Joachim & Linga, Gaute [Show all 9 contributors for this article] (2024). 4D Neutron Imaging of Solute Transport and Fluid Flow in Sandstone Before and After Mineral Precipitation. Water Resources Research. ISSN 0043-1397. 60(3). doi: 10.1029/2023WR036293. Full text in Research Archive Show summary

  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.

• Shafabakhsh, Paiman; Borgne, Tanguy Le; Renard, Francois & Linga, Gaute (2024). Resolving pore-scale concentration gradients for transverse mixing and reaction in porous media. Advances in Water Resources. ISSN 0309-1708. 192. doi: 10.1016/j.advwatres.2024.104791. Full text in Research Archive Show summary

  Mixing-limited reactions are central to a wide range of processes in natural and engineered porous media. Recent advances have shown that concentration gradients sustained by flow at the pore-scale influence macroscopic reaction rates over a large range of reactive transport regimes. Yet, resolving concentration gradients driven by fluid mixing at the pore-scale is challenging with current simulation methods. Here, we introduce a computational methodology to resolve concentration gradients at the pore scale in mixing-limited reactions. We consider a steady-state reactive transport problem characterized by reactive fluids flowing in parallel in a porous material. Given a mesh representation of the pore space and a steady velocity field, we solve the steady advection-diffusion equation for conservative scalar transport using a stabilized finite-element method combined with mesh refinement adapted to local scalar gradients. Based on this solution and assuming instantaneous reaction kinetics in the fluid, we infer the distribution of species involved in an irreversible bi-molecular reaction. We validate the method by comparing our results for uniform flow with analytical solutions and then apply it to simulate mixing-limited reactions in a three-dimensional random bead pack and Berea sandstone sample. Chaotic flow within the pore space leads to sustained concentration gradients, which are captured by our numerical framework. The results underscore the ability of the methodology to simulate transverse mixing and mixing-limited reactions in complex porous media and to provide bottom-up numerical data to improve the prediction of effective reaction rates at larger scales.

• Shafabakhsh, Paiman; Ataie-Ashtiani, Behzad; Simmons, Craig T; Younes, Anis & Fahs, Marwan (2021). Convective-reactive transport of dissolved CO2 in fractured-geological formations. International Journal of Greenhouse Gas Control. ISSN 1750-5836. 109. doi: 10.1016/j.ijggc.2021.103365. Full text in Research Archive
• Shafabakhsh, Paiman; Fahs, Marwan; Ataie-Ashtiani, Behzad & Simmons, Craig T (2019). Unstable density-driven flow in fractured porous media: the fractured elder problem. Fluids. 4(3). doi: 10.3390/fluids4030168. Full text in Research Archive

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