(352f) Olivine Dissolution in CO2 Saturated Water Under Reservoir Conditions

Geological carbon storage (GCS) is one of the most promising and reliable techniques to mitigate the atmospheric CO₂ concentration. Among the four trapping mechanisms of GCS, CO₂ mineralization provides the most permanent way for CO₂ fixation by converting CO₂ into carbonate rocks. Olivine is one of the most promising minerals for CO₂ mineralization because it is abundant in Earth's upper mantle. The Olivine dissolution kinetics under reservoir conditions are important to predict the long-term fate of the injected CO₂. In this work, experimental studies on Olivine dissolution in CO₂ saturated water at pressures from 76 to 155 bar, and at temperatures from 373.15 to 473.15 K, were conducted.

Experiments were performed with a batch reactor system, and liquid samples were withdrawn from a reactor with an auto-sampling system. The schematic of the experimental system is shown in Fig. 1. The two batch reactors were equipped with a heater jacket, a thermocouple, a pressure sensor, and a stirrer shaft assembly with impellers. The liquid from reactor 1 was transferred to reactor 2 via a transfer tubing between them. The CO₂ was pressurized by a syringe pump. The auto-sampling system was used to collect the filtered liquid samples from reactor 2. Two six-port sampling valves were used: one for collecting liquid samples, the other for collecting in-line solvent. N₂ was used to purge the in-line solvent together with the liquid sample to one of the centrifuge tubes containing some pre-added additional solvent, of the fraction collector.

Ultra-pure water and gem-quality Olivine particles were added to reactors 1 and 2, respectively, before injecting CO₂ and heating up the batch reactor system. Once CO₂ and water reached equilibrium in reactor 1, the CO₂ loaded water was transferred from reactor 1 to reactor 2 to provoke the Olivine-CO₂-water reaction under stirring. Liquid samples that were withdrawn from reactor 2 periodically were analyzed by ICP-MS, so the concentrations of Mg, Si and Fe were obtained. The recovered Olivine particles were analyzed by XPS. The initial dissolution rate of Olivine was determined by the following equation:

r = (dc/dt)(V/A). (1)

Here, r denotes the dissolution rate, V denotes the volume of the solution, A denotes the mineral exposure area to the solution, c denotes ion concentration, and t denotes time.

The XPS analysis results of the particles showed that during the Olivine dissolution, secondary phase was formed on the particle surface along with the Olivine dissolution. Silica was mainly formed at low temperature, while Fe-rich secondary phase was mainly formed at higher temperatures. The formation of the Fe-rich secondary phase can be first easily identified by the change of the colour of the particle, and can also be revealed by the decreasing trend of the concentration of Fe during the reaction. The Olivine initial dissolution rate declined first and then inclined with the increase of temperature, when the pressure was fixed. This non-monotonic trend showed that the dissolution rate did not only depend on temperature, but the Fe-rich secondary phase on the particle surface has a stronger inhibition of Olivine dissolution. Fig. 2 shows some results of initial dissolution rates against temperature. Here the dissolution rates of Mg and Si were first calculated, after which the stoichiometric Olivine dissolution rate was converted based on the chemical formular, finally an average of the rates calculated by Mg and Si was taken. Fig. 2 also shows that the influence of pressure on olivine dissolution was slight until the temperature was increased to 473.15 K.

The results from this study extend the database of Olivine dissolution rates up to a temperature of 473.15 K. This study helps understand the long-term fate of the injected CO₂ and the geochemical reaction under reservoir conditions, and helps optimize the design of a GCS project.