Production from many shale- and tight-gas reservoirs recovers ~10% of the available methane. Improving these yields requires development of a detailed, molecular scale understanding of where methane occurs in the pore network and what conditions promote its release from the different environments. Using natural and synthetic hectorite clays (a trioctahedral smectite similar to montmorillonite), we show that 13C MAS NMR of supercritical CH4 at reservoir conditions (90 bar, 323 K; ~1 km depth) is an effective probe of the environments available to CH4 and the exchange dynamics between them. In laponite (100% OH- synthetic hectorite), natural San Bernardino hectorite (Clay Mineral Society SHCa-1, 55% F- for OH- substitution) and a synthetic 100% fluorohectorite, we find that CH4 trapped in the interlayer nanopores on a ~300 ms timescale appears at chemical shifts between -4 ppm and -7 ppm, with the shift becoming more negative with increasing hydrophobicity of the clay, increasing layer charge, and decreasing free volume in the interlayer. CH4 also occupies the meso-scale pore network between clay particles and we observe a unique spectral signature indicative of a range of exchange rates between nano- and meso-pores. In general, we observe increased quantities of nanopore-confined CH4 when the number of interlayer cations is small, the hydrophobicity of the cation is high, and the relative humidity of the supercritical CH4 is low, in agreement with previous publications and predictions made from those results. Comparison of the 13CH4 spectra of laponite, SHCa-1, and fluorohectorite show that 13C MAS NMR is also sensitive to dynamic basal spacing fluctuations, but the wavelength must be at least &gt; ~10 nm.

Probing supercritical methane adsorption and dynamics by 13C MAS NMR: Influence of hydrophobicity and layer charge on methane-smectite interactions at ~1 km depth

2019 Spring National Meeting

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