Competitive Adsorption of H₂O and CO₂ in 2-Dimensional Nanoconfinement: GCMD Simulations of Cs- and Ca-Hectorites

Abstract

The intercalation of H2O, CO2, and other fluid species in expandable clay minerals (smectites) may play a significant role in controlling the behavior of these species in geological carbon sequestration and enhanced petroleum production and has been the subject of intensive study in recent years. This paper reports the results of a computational study of the effects of the properties of the charge balancing, exchangeable cations on H2O and CO2 intercalation in the smectite mineral, hectorite, in equilibrium with an H2O-saturated supercritical CO2 fluid under reservoir conditions using Grand Canonical Molecular Dynamics (GCMD) methods.The results show that the intercalation behavior is greatly different for the cations with relatively low hydration energies and high affinities for CO2 (here Cs + ) than for cations with higher hydration energies (here Ca 2+ ). With Cs + , CO2 intercalation occurs in a 1-layer structure and does not require H2O intercalation, whereas with Ca 2+ the presence of a sub-monolayer of H2O is required for CO2 intercalation. The computational results provide detailed structural, dynamical and energetic insight into the differences in intercalation behavior and are in excellent agreement with in situ experimental XRD, IR, quartz crystal microbalance, and NMR results for smectite materials obtained under reservoir conditions.

Tuning the Hydrophobicity of Layer-Structure Silicates To Promote Adsorption of Nonaqueous Fluids: Effects of F– for OH– Substitution on CO2 Partitioning into Smectite Interlayers

“…These changes are probably driven by a combination of two factors: (i) disruption of the H-bonding network among H2O molecules due to the presence of CO2 and (ii) positive non-bonded interactions among CO2 molecules resulting in their clustering. [39][40][41][42]49 The intercalation behavior of smectite interlayers depends upon many factors, including total structural charge, charge location, the properties of charge compensating cations, orientation of structural OHgroups, the composition and properties of the external fluid phase, and as this study shows, the F -/(F -+OH -) ratio of the octahedral sheet. The increasing CO2/(CO2+H2O) ratio with F -/(F -+OH -) ratio suggests that Ffor OHsubstitution should result in increased capacity to sorb non-polar species such as CH4 and other small hydrocarbons.…”

“…Cs + ). [39][40][41] The calculations presented here show that at a given basal spacing (interlayer thickness) increasing Ffor OHsubstitution causes a progressive decrease in the amount of intercalated H2O and an increase in the CO2/(CO2+H2O) ratio. The structural sites occupied by Fand OHin the tetrahedral-octahedral-tetrahedral (T-O-T) structure of 2:1 layer silicates are part of the central, octahedral sheet and are therefore not part of the basal surface oxygen (Ob) layer with which the interlayer species interact most directly ( Figure 1).…”

“…Details of the simulation methods and analysis are discussed in greater detail in our previous papers and in the Supplementary Material of this paper. [38][39][40][41]45,46 For the remainder of this discussion, we define the 1L spacing for each clay composition as the one with the maximum CO2 content (near 12.2 Å), and the 2L spacing as the stable state determined from the immersion energies, as discussed below.…”

Understanding methane/carbon dioxide partitioning in clay nano- and meso-pores with constant reservoir composition molecular dynamics modeling

“…This structuring is consistent with the well-known ability of CO 2 to enter smectite clay interlayers. 6,[20][21][22][23][24][25][26][27][28][31][32][33][34]37,38,40 In contrast, the lack of structuring in the CH 4 peak nearest the surface and the equal distances between the three layers suggest that these molecules are interacting much more weakly with the surfaces and behave like hard spheres with the layers packed on one top of each other. This conclusion is consistent with the incorporation of CH 4 into the interlayer galleries of expandable clays occurring by a passive, space filling mechanism with basal spacings similar to 7.5 Å.…”

“…Further details about the analysis methods are described elsewhere. 10,11,14,15,[31][32][33] We performed the molecular dynamics simulations in the canonical NVT ensemble at 323 K using the LAMMPS simulation package. 63 A Nosé-Hoover thermostat was used to control the temperature.…”