Basic plutonic rocks: an unconventional solution for CO2 storage through mineral carbonation?

Abstract

CO2 capture and geological storage (CCS) is recognized as an essential technolo-gy to achieve carbon neutrality and the Paris Agreement targets. The success of in-situ mineral carbonation of CO2 in basalts accomplished in the Carbfix project, opened up the prospect for considering other igneous rocks as viable targets for CO2 storage. The InCarbon project embraced the challenge to test the potential for mineral carbonation in basic plutonic rocks, with a chemical composition similar to basalts, but with much more challenging textural and petrophysical conditions Samples from basic intrusions in south Portugal were tested for mineral carbona-tion potential under laboratory conditions, by promoting reaction with an aqueous solution with varying degrees of saturation in supercritical CO2. To ensure realis-tic conditions, and unlike previous laboratory studies, the liquid phase was a very saline brine or sea water, and the experimental design mimicked the reservoir conditions (80 bar and 40°C). Four experiments were conducted with a duration of up to 120 days each. A multi-analytical methodology was applied to monitor the chemical variations of the brine and the textural, mineralogical and chemical variations of the rock specimens. The experiments were followed by geochemical modeling with Crunch Flow@. The results show that brine supersaturated with CO2 promotes the increase of roughness on specimens’ surface due to the dissolution of silicates and results in an increase in silica, alumina and some other major elements (e.g. calcium, mag-nesium and iron) in solution. For longer experimental times (120 days), a de-crease in silica and aluminum concentrations is associated with crystallization of zeolite and clays. The rate of dissolution of Ca, Fe, Mg decreases and coincides with crystallization of trace magnesite and dolomite carbonates. The modelling results globally supports the obtained experimental data. Moreo-ver, the experimental conditions, not catalyzed by any additive, and in particular by using sea water, point to a viable solution for CO2 storage. Although these rock types present low porosity, essentially provided by the fracture network, the favorable mineralogy and the large volume associated to mafic intrusions may be promising for the use of this CCS technique for small-scale sources of CO2. Keywords: carbon capture and storage, carbonation, mafic plutonic, supercritical CO2, geochemical experiments.