In Situ Study of CO₂ and H₂O Partitioning between Na–Montmorillonite and Variably Wet Supercritical Carbon Dioxide

Abstract

Shale formations play fundamental roles in large-scale geologic carbon sequestration (GCS) aimed primarily to mitigate climate change and in smaller-scale GCS targeted mainly for CO2-enhanced gas recovery operations. Reactive components of shales include expandable clays, such as montmorillonites and mixed-layer illite/smectite clays. In this study, in situ X-ray diffraction (XRD) and in situ infrared (IR) spectroscopy were used to investigate the swelling/shrinkage and H2O/CO2 sorption of Na(+)-exchanged montmorillonite, Na-SWy-2, as the clay is exposed to variably hydrated supercritical CO2 (scCO2) at 50 °C and 90 bar. Measured d001 values increased in stepwise fashion and sorbed H2O concentrations increased continuously with increasing percent H2O saturation in scCO2, closely following previously reported values measured in air at ambient pressure over a range of relative humidities. IR spectra show H2O and CO2 intercalation, and variations in peak shapes and positions suggest multiple sorbed types of H2O and CO2 with distinct chemical environments. Based on the absorbance of the asymmetric CO stretching band of the CO2 associated with the Na-SWy-2, the sorbed CO2 concentration increases dramatically at sorbed H2O concentrations from 0 to 4 mmol/g. Sorbed CO2 then sharply decreases as sorbed H2O increases from 4 to 10 mmol/g. With even higher sorbed H2O concentrations as saturation of H2O in scCO2 was approached, the concentration of sorbed CO2 decreased asymptotically. Two models, one involving space filling and the other a heterogeneous distribution of integral hydration states, are discussed as possible mechanisms for H2O and CO2 intercalations in montmorillonite. The swelling/shrinkage of montmorillonite could affect solid volume, porosity, and permeability of shales. Consequently, the results may aid predictions of shale caprock integrity in large-scale GCS as well as methane transmissivity in enhanced gas recovery operations.

On sorption and swelling of CO2 in clays

“…However, it is critical as it results in cation hydration at lower relative humidity, i.e., clay hydration, compared to the non-CO 2 case. This hypothesis is in agreement with Loring et al (2014) who showed an initial steep increase in CO 2 interlayer concentrations with increasing water contents in the range between 0 and 1 W, followed by a decrease towards higher hydration states.…”

“…These measurements were performed at 50°C and pressures up to 18 MPa. Loring et al (2014), using X-ray diffraction and IR spectroscopy at 50°C and 9 MPa on Na-SWy-2, studied relative CO 2 uptake as a function of clay water content, and demonstrated a steep increase in CO 2 interlayer contents at low water saturations. This steep increase seems to correspond to a step in hydration from 0 to 1 W. With an increase in water content, this CO 2 concentration decreases and almost disappears when the clay interlayer distance moves to 2 W. This clearly demonstrates that the interlayer CO 2 uptake capacity is strongly related to the water content.…”

FT-IR study of CO2 interaction with Na+ exchanged montmorillonite

“…This behavior indicates that the addition or removal of nonstructural or interlayer space H 2 O can be recognized based on the areas of peaks near 3450 cm − 1 and 3350 cm −1 . Recent work by Loring et al (2014) quantitatively correlated Na + -Mt clay mineral expansion with adsorbed H 2 O concentration in the presence of supercritical CO 2 .…”

“…Specifically, hydrated and relatively dehydrated Na + -STx-1 were exposed to anhydrous CO 2 and monitored with Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) to discern mechanistic information for CO 2 interaction with the Na + -STx-1 and examine any CO 2 -induced perturbations in vibrational modes associated with interlayer water or the silicate framework. The interaction of CO 2 with Na-STx-1 was investigated at pressures up to 5.9 MPa, a range that lies between CO 2 pressures of b120 Torr (b0.016 MPa) (Fripiat et al, 1974) and 9 MPa (Loring et al, 2012(Loring et al, , 2014 explored in previous FTIR studies on clay mineral-CO 2 interaction.…”

Mineralization of Carbon Dioxide: A Literature Review

“…The most recent research on CO 2 intercalation, however, provides an accurate molecular description of CO 2 for evaluating surface tension, contact angles, capillary flow, snap-off (or the disconnection of a continuous stream of the non-wetting CO 2 when it passes through pore constrictions, a form of residual trapping), and related phenomena associated with CO 2 trapping [84]. Intercalation could provide additional storage capacity for CO 2 in clay layers on reservoir rock coatings and in the caprock [51,58], and could also be a mechanism for controlling potential leakage [58,100,101]. Investigations into these aspects of scCO 2 behavior, at elevated pressures and temperatures characteristic of geologic carbon sequestration, have only recently begun [99,102].…”