Chemical Trapping of CO₂ by Clay Minerals at Reservoir Conditions: Two Mechanisms Observed by in Situ High-Pressure and -Temperature Experiments

This paper presents the results of experiments performed in situ at temperature and pressure relevant to reservoir conditions (T = 323 K and Pfluid = 90 bar) to evaluate whether clay minerals can react with supercritical CO2 to produce carbonate phases by ion exchange–precipitation reactions and dissolution–reprecipitation reactions. The results show that both can occur on a time scale of hours under the conditions of our studies. The dissolution–reprecipitation mechanism was examined using Ca-, Cs-, and tetramethylammonium (TMA+) laponite, a synthetic smectite analogous to hectorite that has small particles (basal dimensions of ∼10 × 10 nm2) and a high fraction of edge sites where only two of the usual three bridging oxygen atoms are shared with other tetrahedra in the silicate sheet (Q2 sites), making it an excellent case for examining the role of T–O–T edges. The ion exchange–precipitation mechanism was observed for a Pb-exchanged natural low-Fe smectite (hectorite). Novel X-ray diffraction and NMR and infrared (IR) spectroscopic tools provide in situ observation of these reactions in real time supported by a suite of ex situ analyses. The results demonstrate for the first time that 13C NMR can effectively characterize the amorphous and crystalline products of such reactions. For all three laponites, IR and NMR data show that HCO3– ions form at water content as small as ∼5 H2O molecules/exchangeable cation. When the exchangeable cation is Ca2+, the IR data show the formation of carbonate anions at low water content as well, with the NMR spectra showing formation of amorphous calcium carbonate in vacuum-dried samples. For laponites equilibrated at 100% RH at atmospheric conditions and then exposed to scCO2, 13C NMR shows the presence of a greater number of more mobile HCO3– ions and a poorly crystalline or amorphous hydrous magnesium carbonate/bicarbonate phase that forms from Mg2+ released by clay dissolution. The 100% RH sample with exchangeable Ca2+ also forms calcite, vaterite, and aragonite precipitates. Comparison of these and previously published results suggest that a high edge site Q2 fraction is crucial to the dissolution–reprecipitation process occurring on a short time scale. In the Pb-exchanged hectorite exposed to scCO2, once a critical humidity threshold of ∼78% is reached, cerussite (PbCO3) forms rapidly concurrent with replacement of interlayer Pb2+ by H3O+ formed by reaction of CO2 with water on the clay surface. This type of reaction is not observed on a similar time scale with Ca- or Na-exchanged natural hectorite and other smectites, and the low solubility of cerussite appears to be the thermodynamic driving force for this process