Assessing the carbon sequestration potential of basalt using X-ray micro-CT and rock mechanics

Mineral carbonation in basaltic rock provides a permanent storage solution for the mitigation of anthropogenic CO2 emissions in the atmosphere. 3D X-ray micro-CT (XCT) image analysis is applied to a core sample from the main basaltic reservoir of the CarbFix site in Iceland, which obtained a connected porosity of 2.05–8.76%, a reactive surface area of 0.10–0.33 mm−1 and a larger vertical permeability (2.07 × 10−10 m2) compared to horizontal permeability (5.10 × 10−11 m2). The calculations suggest a CO2 storage capacity of 0.33 Gigatonnes at the CarbFix pilot site. The XCT results were compared to those obtained from a hydromechanical test applied to the same sample, during which permeability, electrical resistivity and volumetric deformation were measured under realistic reservoir pressure conditions. It was found that permeability is highly stress sensitive, dropping by two orders of magnitude for a −0.02% volumetric deformation change, equivalent to a mean pore diameter reduction of 5 μm. This pore contraction was insufficient to explain such a permeability reduction according to the XCT analysis, unless combined with the effects of clay swelling and secondary mineral pore clogging. The findings provide important benchmark data for the future upscaling and optimisation of CO2 storage in basalt formations.