High-temperature CO2 mineralisation in basaltic rocks: Inconsistencies between laboratory experiments and numerical modelling

The Nesjavellir high temperature geothermal reservoir, located in southwest Iceland, was one of the test sites for the 'Geothermal Emission Control' (GECO) project. The project involves the reinjection of exhaust gases from geothermal power plants into the subsurface for permanent storage. Numerical modelling and field data from the nearby CarbFix2 storage site at Hellisheiði indicate that even at high temperatures (< 280°C), large quantities of CO₂ can be mineralised. To complement these data, we investigated the potential for CO₂ sequestration in the Nesjavellir reservoir by conducting a 260 °C batch reaction experiment using a basaltic drill core sample and effluent water from the Nesjavellir injection well. We also simulated the experiment using the PHREEQC geochemical modelling program and observed significant inconsistencies between the modelled and experimental results. The experiment produced a secondary mineral assemblage dominated by zeolites, chlorites and anhydrite, with no carbonates observed. In contrast, the model predicted the formation of calcite, which did not occur during the experiment. This discrepancy is due to the model's inability to handle solid solutions and non-ideal phases adequately. During the experiment, Ca was primarily incorporated into anhydrite and a Na-Ca-zeolite, which resembles a solid solution of wairakite and analcime. However, the model did not consider this phase, which resulted in Ca being incorporated into calcite instead of zeolite.

The experimental results are consistent with previous studies that show limited carbon mineralization at higher temperatures (> 180 °C) due to the competition between carbonates and silicates for the uptake of divalent cations. In addition, a comparison with the numerical model shows that simulations of high-temperature CO₂ sequestration can be misleading as they may not be able to reproduce the complexity of non-ideal silicate mineral formation, resulting in an overestimation of carbonate formation.