Investigation of fluid-flow and mineral carbonation reaction processes in vesicular basalt by computed x-ray tomography

Mineral carbonation of subsurface basalt by CO2-rich fluids is a proven CO2 sequestration method. Aqueous and supercritical CO2 fluids permeate through variably porous lava layers away from the injection well, and somewhere along this flow pathway, mineral carbonation reactions initiate. Mineral carbonation is a two-stage process of dissolution of silicate phases by carbonic acid followed by precipitation of carbonate minerals from solution. Where along the flow pathway and when, relative to the start of injection, mineral carbonation begins is largely unconstrained. Sub-millimetre-scale x-ray tomography reveals the vesicle (i.e. porosity) structure in 3D. Scans reveal a bimodal vesicle size distribution in macroscopically vesicle-rich samples. Small (<1 mm-diameter) vesicles are isolated pores formed either by volatile exsolution (i) into a liquid (a sub-spherical bubble) or (ii) during groundmass crystallization (interstitial ‘ditytaxitic’ pores). A few anomalously large voids of connected porosity are formed when individual bubbles nucleate - these represent the only connected porosity and efficient permeability in unfractured basalt. We investigate further if mineral carbonation reactions can reach and act on isolated pores. Millimetre-sized samples were scanned after being immersed in aqueous CO2 for between 2 and 4 months. Some but not all samples show evidence of cation-dissolution along fractures and external surfaces. New results will be presented.