Interaction of Hydrocarbons with Clays under Reservoir Conditions: In Situ Infrared and Nuclear Magnetic Resonance Spectroscopy and X‑ray Diffraction for Expandable Clays with Variably Wet Supercritical Methane

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.