Assessing natural basalt reactivity for carbon storage in mafic rocks

Understanding the geochemical reactivity of basaltic rocks with carbon dioxide (CO2)-rich fluids is integral for advancing subsurface technologies involving gas-fluid-rock interactions, including carbon mineralization, enhanced geothermal systems, and critical mineral extraction. One application is geologic carbon sequestration (GCS). During GCS, CO2 is injected into the deep subsurface, where it reacts with the geologic formation to precipitate carbonate minerals. While mineralization has been predicted to take thousands of years at some proposed GCS sites, rapid mineralization has recently been reported in mafic and ultramafic rocks, such as basalt. In the current study, we delve into the extent of CO2 geochemical interactions with different natural basaltic rocks over 30 days of reaction. Secondary mineral precipitation was investigated on basalt surfaces using Raman spectroscopy and scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX). Changes to the pore structure were investigated using micro X-ray computed tomography (µXCT). We found that carbon enrichment of the mineral surface was associated with higher iron-content, rather than magnesium or calcium, and that this enrichment correlated with decreases in the connected porosity. These findings have implications for site selection based on both formation reactivity towards CO2 and the impact of reaction on formation injectivity.