Lab-based assessment of engineered CO­2 mineralization in mafic rock reservoirs

Understanding how in-situ mineralization of CO2 affects the porosity and permeability of the host rock is critical to assessing the viability of basalt reservoirs as carbon dioxide repositories. Precipitating carbonate minerals have the potential to fill primary porespace and decrease permeability, reducing injectivity and overall reservoir capacity. Laboratory experiments that induce carbon mineralization in basalt under controlled conditions can inform how fluid transport properties evolve in geological storage reservoirs. Here, we present time-resolved 3D datasets acquired using a novel x-ray transparent cell that allows carbon mineralization in basalt to be documented on the grain scale through time using x-ray microtomographic imaging (µCT). Our 4DµCT data aim to document the formation of carbonate mineral species, via ion exchange between dissolved inorganic carbon and the divalent cations of primary minerals in the basalt sample. We use the 4DµCT dataset to track sample deformation, changes in porosity, and to model the permeability evolution on the grain scale. Our 4DµCT data (Figure, below), and other post-reaction analyses, document the in-situ formation of carbonate mineral species. Our experiments utilise cylindrical cores of basalt with a diameter of 10 mm and a central 2 mm bore and react these with water-dissolved CO2. The second phase of the experiments is a switch of injection fluid to an aqueous solution of NaHCO3 equilibrated with CO2 During the ongoing experiment, the sample has been repeatedly sealed to maintain fluid pressure, disconnected from benchtop apparatus, and imaged using a µCT scanner.  Exceptionally long operando experiments such as ours can be of particular use in assessing reservoir potential of prospective carbon mineral storage sites by recreating subsurface conditions unique to each location. The apparatus can investigate the evolution of physical rock properties over time periods relevant to field operations (months/years).