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In-situ CO2 storage through carbonate mineralization in offshore mafic rocks offers a promising pathway for long-term anthropogenic carbon sequestration. Although the chemical viability of basalt carbonation is well-established, there has not yet been a single experiment that fully integrates CO2-rich seawater transport, mineral dissolution, and secondary carbonate precipitation at temperatures below 120°C without adding alkaline base. To characterize the kinetics of reaction at such conditions, we used a dual experimental approach: static batch reactors and dynamic flow-through slim-tubes.
Initial batch experiments were conducted at 70 and 120°C using Mg-Fe-rich crystalline basalt powder (80 – 150 µm), and synthetic normal/desulfated seawater under a PCO2 of 50 bar. Results showed the formation of Fe-Mg-carbonates after four months, where the temperature is the main driver of the mineralization kinetics. Desulfated seawater experiments display similar results as the normal seawater ones.
A dynamic multi-stage slim-tube apparatus was developed to incorporate potential transport limitations into the assessment of chemical processes. The system consists of six titanium tubes (10 cm length 3.85 mm ID) connected in series to form the equivalent of a 60 cm porous medium. The tubes were packed with the same crystalline basalt powder as used in the batch experiments, resulting in an average porosity of ~ 28% and an initial permeability of ~50 mD. Synthetic desulfated seawater was injected at a controlled flow rate of 1.5 μm /min at 100 bar total pressure (PCO2 = 50 bar).
A 41 days cumulative flow experiment at 120°C revealed significant spatial heterogeneity in mineralogical alterations. SEM-EDS analysis showed intense dissolution of olivine crystals, often resulting in “skeletonized” mineral morphologies, while pyroxenes and plagioclase remained relatively stable. Secondary Fe-Mg(-Ca) carbonates (10 - 28 μm diameter) were identified within the primary porosity. Unlike the batch experiments, these carbonates exhibited distinct chemical zonation, with cores slightly enriched in calcium compared to the rims. Precipitation occurred mainly in the primary pore space, not in secondary porosity from olivine dissolution, indicating that pore-scale transport and local saturation control nucleation sites.
These dynamic experiment results demonstrate that even under flow conditions, basaltic carbonation is viable at 120°C, though the presence of calcium in the precipitates suggests a more complex ion exchange in porous networks that predicted by static models . On-going experiments at 45°C will further elucidate the temperature dependence of these transport-limited reactions.
This work provides critical data for optimizing numerical models at the laboratory scale, serving as a prerequisite for future pilot-scale offshore CO2 sequestration.
| Country | France |
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