Critical Water Coverage during Forsterite Carbonation in Thin Water Films: Activating Dissolution and Mass Transport

Abstract

In geologic carbon sequestration, CO2 is injected into geologic reservoirs as a supercritical fluid (scCO2). The carbonation of divalent silicates exposed to humidified scCO2 occurs in angstroms to nanometers thick adsorbed H2O films. A threshold H2O film thickness is required for carbonate precipitation, but a mechanistic understanding is lacking. In this study, we investigated carbonation of forsterite (Mg2SiO4) in humidified scCO2 (50 °C and 90 bar), which serves as a model system for understanding subsurface divalent silicate carbonation reactivity. Attenuated total reflection infrared spectroscopy pinpointed that magnesium carbonate precipitation begins at 1.5 monolayers of adsorbed H2O. At about this same H2O coverage, transmission infrared spectroscopy showed that forsterite dissolution begins and electrical impedance spectroscopy demonstrated that diffusive transport accelerates. Molecular dynamics simulations indicated that the onset of diffusion is due to an abrupt decrease in the free-energy barriers for lateral mobility of outer-spherically adsorbed Mg2+. The dissolution and mass transport controls on divalent silicate reactivity in wet scCO2 could be advantageous for maximizing permeability near the wellbore and minimize leakage through the caprock.

Thin Water Films Enable Low-Temperature Magnesite Growth Under Conditions Relevant to Geologic Carbon Sequestration

“…Forsterite carbonation experiments were carried out at 25, 40, and 50 °C (25C_Time_Dep, 40C_Time_Dep, and 50C_Time_Dep, respectively; see Table ). Water was added until the H 2 O coverage was above a measured threshold of 1.5 ML needed for carbonate precipitation . While (bi)­carbonate surface complexes predominate below 1.5 ML, carbonates precipitate above this threshold coverage because the H 2 O film is thick enough to enable the transport of ions (e.g., Mg 2+ and HCO 3 – ) to nucleation sites for carbonate growth. , For 25C_Time_Dep, 47 H 2 O additions over a period of 4.9 days were required to titrate to 64% H 2 O saturation, which corresponds to an H 2 O coverage of 1.82 ML; for 40C_Time_Dep, 13 H 2 O additions over a period of 1.3 days were required to titrate to 58% H 2 O saturation, which corresponds to an H 2 O coverage of 1.73 ML; and for 50C_Time_Dep, 9 H 2 O additions over a period of 22 h were required to titrate to 65% H 2 O saturation, which corresponds to an H 2 O coverage of 1.78 ML (see also Figure S2d–f).…”

“…Examples of IR spectra collected as a function of time during experiment 40C_Time_Dep are shown in Figure a; example spectra from experiments 25C_Time_Dep and 50C_Time_Dep are in Figures S2–S4. The background spectrum was forsterite exposed to 90 bar CO 2 at 0% H 2 O saturation, which effectively removes most of the spectral contributions from (bi)­carbonate surface complexes. , The choice of this background, however, resulted in negative-going features related to forsterite dissolution and positive-going features due to amorphous silica [SiO 2 (am)] precipitation in the SiO stretching region between 820 and 1160 cm –1 . Spectra of unreacted forsterite, as well as fumed SiO 2 as a model for SiO 2 (am), are shown in Figure S6a.…”

“…A comparison of E a Mgs to the activation energy reported by Saldi et al (80.2 kJ mol –1 ) for the growth of magnesite particles in bulk water would be useful to help elucidate the reaction mechanism for magnesite growth in thin H 2 O films. Processes that could be underlying the net reactions dictating E a Mgs in thin H 2 O films could include mass transport . Yet, in order to compare with Saldi et al, we must correct for SI Mgs within the H 2 O films at each temperature .…”

“…Reservoir modeling that includes reactive transport requires good characterization of the initial site geochemistry as well as a good understanding of the reaction networks and kinetics that will be important during CO 2 injection and plume migration. While most laboratory studies have focused on reactivity in bulk water, significant carbonation also occurs in scCO 2 within Å-to-nm thin H 2 O films adsorbed on silicate surfaces. − Nanoconfined H 2 O has unique properties that lead to reactivity that can differ significantly from those in bulk liquid. − The activity of H 2 O in nanoscale H 2 O films is low compared to bulk water because the chemical potential of H 2 O in the overlying scCO 2 is low and because interactions between adsorbed H 2 O and the mineral surface disrupt the hydrogen bonding structure and lower the dielectric constant. ,,− Thin H 2 O films also have extraordinarily high mineral-surface-to-water-volume ratios; thus, minor amounts of mineral dissolution can rapidly yield high degrees of supersaturation with respect to secondary phases …”

Carbonation Reaction Mechanisms of Portlandite Predicted from Enhanced Ab Initio Molecular Dynamics Simulations

“…Moreover, the adsorbed H2O molecules are strongly surface- We relate the variation of the free-energy barriers in the formation of the bicarbonate complex to the interfacial structure of water [98,99] which is, in turn, dictated by the surface chemistry and the degree of nanoconfinement [87,88]. A threshold H 2 O film thickness is required for carbonate precipitation, and it has been recently shown [100] that carbonate precipitation becomes predominant at about 1.5 monolayers of water, below which the reaction products are limited to (bi)carbonate surface complexes. The arrangement of the water molecules in the vicinity of the three surfaces is distinct (see the density profiles and H 2 O orientation plots in Figure 6), and this explains the variation in the carbonation reaction mechanisms.…”