IN SITU INFRARED SPECTROSCOPIC STUDY OF NANOCRYSTALLINE FAYALITE CARBONATION IN WET SUPERCRITICAL CO2

Mineral trapping, the most permanent CO₂ storage option, is a process where metal silicate minerals are reacted with CO₂ to form solid metal carbonates. Reservoirs that are best suited for mineral trapping are those in volcanic flood basalts. Olivine is a major basalt component with high reactive potential for mineral trapping because it dissolves to release divalent metal cations that readily precipitate as carbonates. Olivine has complete solid solution between magnesium-rich forsterite, Mg₂SiO₄, and iron-rich fayalite, Fe₂SiO₄. Carbon dioxide injected into basalt reservoirs will initially reside as a supercritical fluid (scCO₂) containing varying amounts of dissolved water. While many previous studies have focused on forsterite carbonation in wet scCO₂, few have examined the fayalite reactivity in this low-water high-pressure fluid.

In this current study, the reaction of nanocrystalline fayalite with variably wet scCO₂ was investigated at 50 degrees C and 90 bar using a combination of in situ probes, including IR spectroscopic titrations and high pressure XRD, and ex situ analyses, including TGA-MS and SEM. While no reaction occurred in anhydrous CO₂, reaction products varied depending on the adsorbed H₂O concentration. At lowest adsorbed H₂O concentrations, water was consumed by reacting with CO₂ to ultimately form adsorbed bicarbonate and a protonated fayalite surface. At moderate concentrations, either surface complexed carbonate or (sub)nanocrystalline siderite formed, but the reaction was not continuous. Above a threshold adsorbed H₂O concentration, siderite precipitated continuously. These results are discussed in comparison with those from similar studies of forsterite, with the differences attributed to siderite versus magnesite solubility.