The results from novel in situ high-pressure nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, and X-ray diffraction (XRD) investigation of the interaction of the smectite hectorite with variably wet supercritical methane (scCH4) at 90 bar and 323 K (hydrostatic conditions equivalent to ∼1 km depth) show that CH4 occurs in the clay interlayers, in pores external to the individual clay particles, and as bulk fluid. The occupancy of each environment depends on the relative humidity (RH) of the CH4-rich fluid and the hydration energy and size of the charge-balancing cation. As RH increases, the fraction of interlayer and interparticle CH4 decreases, although with Cs+, addition of a small amount of H2O initially increases CH4 uptake. Maximum interlayer CH4 adsorption occurs when the mean basal spacing just permits methane intercalation (∼11.5 Å) and never below this basal spacing. It is also higher with divalent cations than with monovalent cations. The data show that CH4 adsorption occurs predominantly via a weak dispersion interaction with the clay and that its intercalation occurs via a passive space-filling hydrophobic mechanism. The results suggest that, under reservoir conditions, smectite interlayers may provide a reservoir for CH4 under low-water conditions.

This paper presents the results of experiments performed in situ at temperature and pressure relevant to reservoir conditions (T = 323 K and P fluid = 90 bar) to evaluate whether clay minerals can react with supercritical CO 2 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 nm 2 ) 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 (Q 2 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 13 C NMR can effectively characterize the amorphous and crystalline products of such reactions. For all three laponites, IR and NMR data show that HCO 3 − ions form at water content as small as ∼5 H 2 O molecules/exchangeable cation. When the exchangeable cation is Ca 2+ , 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 scCO 2 , 13 C NMR shows the presence of a greater number of more mobile HCO 3 − ions and a poorly crystalline or amorphous hydrous magnesium carbonate/bicarbonate phase that forms from Mg 2+ released by clay dissolution. The 100% RH sample with exchangeable Ca 2+ also forms calcite, vaterite, and aragonite precipitates. Comparison of these and previously published results suggest that a high edge site Q 2 fraction is crucial to the dissolution−reprecipitation process occurring on a short time scale. In the Pb-exchanged hectorite exposed to scCO 2 , once a critical humidity threshold of ∼78% is reached, cerussite (PbCO 3 ) forms rapidly concurrent with replacement of interlayer Pb 2+ by H 3 O + formed by reaction of CO 2 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.

Several strategies for mitigating the build-up of atmospheric carbon dioxide (CO 2 ) bring wet supercritical CO 2 (scCO 2 ) in contact with phyllosilicates such as illites and smectites. While some work has examined the role of the charge-balancing cation and smectite framework features on CO 2 /smectite interactions, to our knowledge no one has examined how the polarizability of the charge-balancing cation influences these behaviors. In this paper, the scCO 2 adsorption properties of Pb 2+ , Rb + , and NH 4 + saturated smectite clays at variable relative humidity are studied by integrating in situ high-pressure X-ray diffraction (XRD), infrared spectroscopic titrations, and magic angle spinning nuclear magnetic resonance (MAS NMR) methods. The results are combined with previously published data for Na + , Cs + , and Ca 2+ saturated versions of the same smectites to isolate the roles of the charge-balancing cations and perform two independent tests of the role of charge-balancing cation polarizability in determining the interlayer fluid properties and smectite expansion. Independent correlations developed for (i) San Bernardino hectorite (SHCa-1) and (ii) Wyoming montmorillonite (SWy-2) both show that cation polarizability is important in predicting the interlayer composition (mol% CO 2 in the interlayer fluid and CO 2 /cation ratio in interlayer) and the expansion behavior for smectites in contact with wet and dry scCO 2 . In particular, this study shows that the charge-balancing cation polarizability is the most important cation-associated parameter in determining the expansion of the trioctahedral smectite, hectorite, when in contact with dry scCO 2 . While both independent tests show that cation polarizability is an important factor in smectite-scCO 2 systems, the correlations for hectorite are different from those determined for montmorillonite. The root of these differences is likely associated with the roles of the smectite framework on adsorption, warranting follow-up studies with a larger number of unique smectite frameworks. Overall, the results show that the polarizability of the charge-balancing cation should be considered when preparing smectite clays (or industrial processes involving smectites) to capture CO 2 and in predicting the behavior of caprocks over time.