Multi-Tracer µCT Characterisation of Basaltic Microporosity, Transport Visualisation, and Carbonation-Related Changes

Geological storage of CO₂ in basaltic formations enables permanent sequestration via in situ mineral carbonation, where CO₂‑rich fluids dissolve silicate minerals (e.g., basaltic glass, olivine), releasing divalent cations that react with dissolved inorganic carbon and precipitate as carbonate minerals such as calcite and magnesite. Basalts display highly heterogeneous pore networks and alteration textures, where fluid accessibility, reactive surface area, and mineralogy govern the location and rate of coupled dissolution–precipitation processes. The role of microstructure and micro-porosity in controlling mineralisation efficiency and rock property evolution remains poorly quantified. A better core-scale understanding of these features is required to optimise CO₂ injection strategies and interpretation of geophysical monitoring at pilot sites.

This work is an experimental investigation into CO₂-mineralisation in Icelandic basalts using X‑ray micro‑computed tomography (μCT) enhanced by contrast‑agents, and laboratory petrophysical measurements. Four Icelandic borehole cores with varying mineralogy, pore structure, and degrees of geothermal alteration were analysed. Sub‑samples from one core were exposed to CO₂‑rich brine for two months at 50 °C and 20–30 bar in a batch reactor, while in situ pH evolution was monitored to track bulk reaction progress. Before and after exposure, effective porosity, permeability, and P‑ and S‑wave velocities were acquired, enabling correlation between reaction progress, flow properties and acoustic response.

A novel multi‑tracer μCT workflow was developed to resolve fluid pathways and quantify sub‑resolution microporosity. Cylindrical core plugs (diameter and height ~6 mm) were scanned at a voxel size of ~2.45 μm.  Using a time-lapse sequence of 3D volumes, we visualised pore network transport using three high-attenuation contrast agents: aqueous CsCl, NaI (1 mol/L), and gaseous Xenon. Difference imaging via the voxel-wise subtraction of baseline scans from contrast-filled states revealed advective and diffusive tracer invasion within vesicles, fractures, and the fine-grained matrix, demonstrating the respective accessibility of each tracer.

Microporosity was quantified using two complementary approaches. First, the discrepancy between μCT‑derived porosity and laboratory‑measured effective porosity was interpreted as accessible pore volume below the imaging resolution. Second, we utilise a partial-volume model where the relative increase in the attenuation of a single voxel, relative to the known attenuation of the pure tracer indicates the portion of the voxel filled, thereby providing an estimation of the sub-resolution microporosity. As a µCT-resolvable analogue for mineralisation-relevant cations (Ca²⁺, Mg²⁺), the CsCl tracer revealed preferential Cs⁺ uptake within zeolite channels in vesicular basalts. This spatial enrichment corroborates that zeolite-mediated cation exchange may facilitate carbonate precipitation at rates exceeding stoichiometric silicate dissolution (Alqahtani et al., 2025).

Post‑reaction μCT volumes show localised mineral precipitation, remobilisation of palagonite, and spatial correlation of new mineralisation with Fe–Ti oxides, suggesting they may serve as an additional iron source for carbonate growth. Measurable changes to mineralogy, porosity and acoustic velocities demonstrate how CO₂‑induced mineralisation modifies storage‑relevant properties at the core scale. Quantification of basaltic microporosity and improved understanding of transport behaviour can clarify mineralisation controls and help optimise future CO₂ injection strategies.

Alqahtani, A., Addassi, M., Hoteit, H., Oelkers, E., 2025. Rapid CO2 mineralization by zeolite via cation exchange. Sci. Rep. 15, 958. https://doi.org/10.1038/s41598-024-82520-6