Experimental Study of Organic Ligand Transport in Supercritical CO2 Fluids and Impacts to Silicate Reactivity

Abstract

The interactions between water-saturated supercritical carbon dioxide, organics, and minerals are relatively unknown despite being important to carbon sequestration and enhanced hydrocarbon recovery activities. The goals of this study include verification of organic transport through scCO 2 and exploration of whether organic ligands can impact carbonate formation. An in situ near-infrared spectroscopic technique was used to probe supercritical CO 2 (scCO 2 )-organic mixtures at 35 °C and 100 bar. Observation of C-H bands in the spectra collected from scCO 2 equilibrated with a Ca-acetate solution (1.7 m) provided direct evidence of organic partitioning into the scCO 2 phase. A series of high pressure X-ray diffraction experiments at 50 °C and 90 bar were performed to investigate how citrate (0.01-0.5 m) affected the coupled dissolution of forsterite and precipitation of magnesium carbonates in scCO 2 . In control experiments where no citrate was present, nesquehonite (MgCO 3 •3H 2 O) was initially produced as a metastable intermediate that was then converted to magnesite (MgCO 3 ). However, experiments with citrate promoted magnesite, rather than nesquehonite, precipitation, and at the highest concentrations reduced the extent of the carbonation reaction. This paper discusses these unique findings on organic and scCO 2 interactions that are not currently being considered in the fate and transport of CO 2 in the subsurface.

Quantitative Review of Olivine Carbonation Kinetics: Reactivity Trends, Mechanistic Insights, and Research Frontiers

“…Two of the low-temperature (<150 °C) studies , shown in Figure A examined carbonation when olivine was exposed to H 2 O-saturated scCO 2 (wet scCO 2 ), a non-aqueous reactive phase . Dissolution and carbonate precipitation in wet scCO 2 are facilitated by the formation of angstrom- to nanometer-scale water films on hydrophilic mineral surfaces. ,,, Work in this area, including studies of olivine carbonation, ,,,,,,,,− has revealed unique reaction mechanisms and pathways for mineral carbonation that cannot be attained in aqueous media. For instance, the properties of Si-rich surface precipitates on carbonating silicates in wet scCO 2 are distinct from those that develop in aqueous experiments, and their development is remarkably sensitive to pressure–temperature conditions of the CO 2 . ,− Also, critical water film thicknesses are required for continuous coupled dissolution and precipitation. ,, Lastly, nucleation and growth of magnesite at low temperatures (≲65 °C) is promoted in wet scCO 2 , ,, likely due to the reduced level of hydration of Mg 2+ in nanoscale interfacial water films. , …”

“…At higher temperatures, including those relevant to CO 2 -enhanced geothermal systems, substantially more H 2 O can dissolve into the CO 2 -rich fluid, although the exact consequences for reactivity are unclear. For instance, at 250 °C and 500 bar along a prototypical geothermal-pressure gradient, the equilibrium concentration of water increases to ∼35 mol % (Figure B); such a dramatic increase would likely lead to enhanced solvation of both inorganic , and organic dissolved species. The increased water activity and clustering , at elevated temperatures will also likely induce ion formation, which has not yet been detected in wet scCO 2 .…”

“…The increased water activity and clustering , at elevated temperatures will also likely induce ion formation, which has not yet been detected in wet scCO 2 . Organic ligands and metal–organic complexes, ubiquitous in diverse geologic settings, can be stable at >200 °C and may contribute to reactions in the bulk CO 2 fluid in addition to those in adsorbed nanoscale water films. , These non-aqueous reactions will lead to carbon cycling among organics, carbonate minerals, and CO 2 fluids. It is also unclear how kinetics, thermodynamics, and reaction mechanisms of serpentinization and other metamorphic processes would be influenced by CO 2 -dominant fluids.…”

“…Red plotted points denote the P – T conditions of compiled studies that evaluated interactions of minerals, rocks, cement, or steel with water-bearing scCO 2 . Blue points denote wet scCO 2 experiments conducted with olivine. ,,,,,,− Dashed drop-down lines are a visual reference for interpreting the location of the points in three-dimensional space, and they illustrate the full range of solubility of water in CO 2 for the specified P – T conditions. (B) Phase relationships of CO 2 and H 2 O determined experimentally by Takenouchi and Kennedy .…”

Surface-Catalyzed Oxygen Exchange during Mineral Carbonation in Nanoscale Water Films

“…Here, we investigate CO 2 –H 2 O O-exchange in adsorbed H 2 O films on synthetic forsterite (Mg 2 SiO 4 ). In contrast to silica, the Mg 2+ content gives forsterite a particularly high potential to form various magnesium carbonate minerals in CO 2 –H 2 O fluids, such as those relevant to geologic carbon storage. − The goals of this study are twofold: first, we establish the dominant process by which O-exchange occurs in thin H 2 O films on forsterite and determine whether H 2 O structure is the key controlling parameter, as is the case for silica. Second, we determine if there is a relationship between O-exchange rates and the reactivity of forsterite toward carbonation in variably humidified supercritical CO 2 (scCO 2 ), conditions that have direct relevance to CO 2 fluids in deep geologic reservoirs. ,− Specifically, previous studies have shown that a threshold H 2 O film thickness is necessary for rapid and continuous transformation of forsterite to magnesium carbonates in wet scCO 2 , although the exact origins of this reactivity threshold have remained unclear.…”