Smectite Crystallite Swelling Under High Pressure of Methane
Paweł P. Ziemiański, Arkadiusz Derkowski, Marek Szczerba, Stephen Guggenheim
Abstract
Clay minerals, along with organic matter, control methane (CH4) gas adsorption properties of sediments and sedimentary rocks. These natural, high-pressure environments of sediments and sedimentary rocks are particularly important in view of hydrocarbon exploration and global warming. The present study investigates swelling of smectites in dry methane at high pressure (1–60 bar), using in situ high-pressure X-ray diffraction and Grand Canonical Ensemble Monte Carlo simulations. The results show that CH4 molecules actively expand the interlayers of divalent cation-exchanged smectites at elevated pressure (e.g., from initial 12.1 Å at 1 bar of N2 to 13.4 Å at 60 bar of dry CH4 of Mg-montmorillonite and from 11.8 Å at 1 bar of N2 to 12.1 Å at 60 bar of dry CH4 of Ca-montmorillonite). The swelling is observed when the interlayer cations are hydrated at the appropriate level of H2O content adsorbed primarily around the interlayer cations. The H2O configuration leaves CH4-accessible interlayer space outside the coordination sphere of the cations. Higher amounts of H2O in the interlayer act as a steric hindrance for CH4 and effectively decrease CH4 intercalation, whereas insufficient H2O content produces interlayer contraction preventing CH4 from entering the interlayer. The swelling was not observed in monovalent cation-exchanged smectites. Adsorption isotherms measured using overdried and fully contracted smectite allowed the determination of the interlayer adsorption component for the higher hydration state of the same sample. The swelling of smectite at an elevated pressure of CH4 is reversible and is driven by energetic factors, whereas entropy change and mechanical work are minor components of the free energy of swelling. The observed phenomenon of smectite swelling explains the occurrence of hysteresis in high-pressure CH4 adsorption isotherms and provides a previously unknown component to methane interaction with sedimentary rocks.