An experimental study on mineral sequestration of CO2 in basics and ultra basics rocks

The first part of the thesis is dedicated to dissolution data of siderite FeCO3 and magnetite Fe3O4 which have been monitored in situ on the FAME beamline of the european synchrotron radiation facility in Grenoble. Iron in solution close to siderite single crystals is shown to be divalent hydrated. The small size of the experimentally investigated volume of solution (200 *400 micrometer and 3 mm height) allowed to work with single crystals in well defined geometries. No specific interaction was observed between iron (II) and dissolved inorganic carbon, suggesting that modelling siderite evolution under high CO2 pressures by using CO2-less very acidic (pH 1-2) solutions is adequate. Using initial reaction rates, we get an activation energy for siderite dissolution of 62 kJ.mol-1, consistent with existing literature data. Such a value is suggestive of a mineral/solution interface mechanism.. Data from this study and from literature are consistent over a temperature range 25 C - 125 C and a pH range pH 1-7 with an empirical law: pk = pH + Ea/(ln(10)*RT(K)) - log(S/V) - 10,5 where Ea = 62 kJ.mol-1 and S/V is the ratio between solid surface S and fluid volume V. A value of activation energy of 73.5 kJ.mol-1 is obtained in the case of magnetite, also consistent with mineral/solution processes. The second and major part of the thesis work is the realization of analogical experiments for simulating carbonation of basic and ultra basic minerals. Experiments were carried out on consolidated rock cores at 90 C and 280 bar of CO2 (low temperature experiments) and on powders contained in metallic capsules at 400-500 C and 1000-1700 bars of CO2 (high temperature experiments). The rate of mineral storage of CO2 was defined as the molar ratio of solid carbonate formed over total CO2 injected. It is of about 1% in three months in low temperature experiments whereas it reaches several tens of percents per hour in high temperature experiments. In all cases, carbonation is shown to proceed according to dissolution/precipitation mechanisms. A quasi stoichiometric coupling is evidenced between carbonation and (proto) serpentinization in high-temperature experiments. Newly formed carbonates are mostly magnesite MgCO3 with Fe and Ca in solid solution. In low temperature samples, the silicates are covered with a thin silica layer and with carbonate spherules consisting of ankerite CaFe(CO3)2 - dolomite CaMg(CO3)2 - siderite cores surrounded by magnesite overgrowth. In CO2-saturated water, peridotites are more reactive than serpentinite and basalts, in accordance with thermodynamic modelling whereas serpentinites are the most reactive in the supercritical CO2 phase, showing stronger reactivities in this latter phase than in CO2-saturated water. In high temperature experiments, the rate of mineral storage is larger at 400 C than at 500 C, in agreement with thermodynamic modelling of the system. High water fugacities and high fluid salinities are shown to have a positive effect on mineral storage rates. Isotopic mass balance of carbon have evidenced that about 15% of the mineral storage consist of a reduced carbon phase, also identified by transmission electron microscopy as an ill-organized graphite phase. (author)