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Description
Carbon mineralization in porous basalt rocks offers promising long-term solutions for CO2 sequestration [1]. Current approaches involve the injection of a CO2-enriched acidic brine into the subsurface. The reactivity of the host rock is thereby leveraged as cations get progressively dissolved and subsequently precipitate as carbonate minerals further along the flow pathway, permanently trapping CO2 in solid form. Mineralization is thus triggered by the evolving brine composition resulting from rock dissolution and mixing with resident alkaline pore water [2]. While fluid mixing is well characterized in idealized systems such as homogeneous or two-dimensional porous media, natural rocks exhibit strong structural heterogeneities that fundamentally alter the flow. Under relevant injection conditions, subsurface transport is often advection-dominated. Pore-scale fluid mixing dynamics can thus generate sharp concentration gradients and may promote pore clogging and flow rerouting [3]. Yet, most reactive transport models rely on continuum approximations assuming well-mixed conditions, and therefore do not capture the feedback between localized reaction zones and evolving flow pathways at the pore scale. In addition, natural porous systems rarely satisfy ideal saturation conditions, and the presence of trapped or mobile secondary phases may further perturb flow and mixing dynamics. Direct observation of fluid mixing under realistic conditions is therefore key to better constrain these processes and improve reactive transport models [4].
In this work, we investigate mixing-driven mineralization in porous vesicular basalt samples using combined neutron and X-ray tomography. While neutron imaging enables time-resolved 3D tracking of the fluid distribution, X-ray tomography captures both the reference porous structure and its evolution due to mineral precipitation. Using co-injection of aqueous calcium and carbonate solutions, we perform flow-through experiments under fast reaction and advection-dominated conditions representative of natural subsurface systems. To account for both heterogeneity and saturation conditions, we consider two distinct regimes: (i) fluid mixing in an initially fully saturated sample, and (ii) flow perturbations induced by a mobile air phase. We show that the structure governs the evolution of the mixing front, which propagates along preferential pathways within the porous network. Additionally, the development of a dynamic secondary phase, whether solid or fluid, reroutes flow pathways through evolving pore-scale changes. This results in spatially heterogeneous mineralization patterns and a progressive reduction of porosity. Our observations provide experimental evidence of the feedback between mixing-induced reactions and flow redistribution in heterogeneous porous media, complementing ongoing pore-scale simulations.
| References | [1] Matter et al., Science (2016), [2] Menefee et al., Environ. Sci. Technol. (2018), [3] Heyman et al., Proc. Nat. Acad. Sci. (2020), [4] Shafabakhsh et al., Wat. Resour. Res. (2025) |
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| Country | Norway |
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