Influence of Smectite Structure and Hydration on Supercritical Methane Binding and Dynamics in Smectite Pores

While significant effort has gone into studying H2O-mineral interactions over the past few decades, less attention has been paid to the interactions of hydrophobic fluids with mineral surfaces, particularly at temperatures and pressures relevant to tight petroleum and natural gas reservoirs that are of increasing importance to our energy future. Here, we show that CH4 resides within the interlayer nanopores of smectite clays, in pores between the clay particles (interparticle pores), and in bulk fluid using in situ high-pressure C-13 Bloch decay magic angle spinning nuclear magnetic resonance (MAS NMR), exchange NMR spectroscopy (EXSY), and infrared (IR) spectroscopy at T = 323 K and P-fluid = 90 bar CH4 pressure. All three CH4 environments interact with the clay surfaces and, under vacuum-dried conditions, are in dynamic exchange, with the specific rates depending on the two sites involved in the exchange process. The (13)CH(4 )chemical shifts for these three environments are well resolved, with the C-13 shift becoming more positive with decreasing physical space accessible by the CH4. This effect is most pronounced for interlayer-sorbed CH4, where the layer charge, size of the charge-balancing cation, and relative humidity of the fluid all influence the interlayer pore size, (CH4)-C-13 chemical shift, and amount of interlayer CH4. Layer charge has the strongest influence. 2D EXSY NMR spectra of dry Cs-Laponite show that CH4 is exchanging among all three environments, with interlayer CH4 exchanging with interparticle and bulk CH4 at a broad range of frequencies between 10(0) and 10(4) Hz. In contrast, exchange between interparticle and bulk CH4 occurs at frequencies < 10 Hz. With increasing H2O in the clay, the progressively more negative C-13 chemical shifts of the interparticle CH4 and reduction in the peak area for interlayer CH4 suggest that CH4 is displaced from the interlayers and smaller interparticle pores by H2O, causing a greater fraction of the CH4 to spend more time in larger pores and bulk-like environments, consistent with earlier literature. However, the data show that the influence of H2O on the interlayer CH4 chemical shift is complicated, with increasing H2O leading to larger interlayer basal spacings and decreased lateral pore space that may lead to either larger or smaller pores depending on the relative affinity of the charge-balancing cation for the clay surfaces vs hydrating H2O. Clay T-O-T layers with larger lateral dimensions (basal surface area) exhibit greater structural and/or dynamical heterogeneity than those with smaller particle sizes, suggesting that dynamic basal spacing fluctuations with characteristic wavelengths between 10 nm and 1 mu m may be important to fundamental understanding of molecular behavior in clay interlayer galleries.