The combination of experimental 13C NMR spectroscopy and computational molecular dynamics (MD) modeling of CH4, CO2, and H2O in the nano- and meso-pores in shales and clays provides an otherwise unobtainable, molecular scale picture of the structure, dynamics and energetics of the fluid species these systems. The 13C NMR chemical shift of CH4 is a direct probe of the size of the pore space occupied by that component, and the 13C NMR chemical shift and chemical shift anisotropy of CO2 are direct probes of its interaction with mineral surfaces. In parallel, MD modeling provides a molecular scale picture of the origin of the interactions among CH4, CO2, H2O, and the mineral surfaces. The 13C NMR spectra were obtained at insitu conditions of T = 50o and Pfluid = 90 bars using a unique magic angle spinning (MAS) sample loading system (WHiMS) at the Environmental Molecular Sciences Laboratory of the Pacific Northwest National Laboratory. Results for synthetic and natural shale samples show that both CH4 and CO2 enter the pore systems, with both showing separately resolvable resonances for the bulk and the pores. The spectra for both the natural and synthetic shales show that CH4 occurs in both nano-pores in the &lt; 1 nm range and meso-pores in the &gt; 5 nm range. Partial replacement of CH4 by CO2 results in the CH4 occupying larger pores. These behaviors directly parallel those of pure smectite clays, in which the CH4 and CO2 both enter the interlayer galleries and pores between clay particles. The MD simulations were undertaken using a novel approach (constant reservoir composition molecular dynamics, CRC-MD), which provides for calculation of the equilibrium partitioning of fluid or gas species between a reservoir and pores of variable size. The results show that for slit-like pores with thicknesses between 0.3 and 7 nm, the CO2/CH4 ratio in pore volume between clay basal surfaces increases dramatically with decreasing pore size due to preferential attraction of the CO2 for both the bare and hydrated clay surface. H2O molecules on the hydrated surface do not significantly enter the mixed CO2/CH4 fluid either in the bulk or the pores. These results are fully consistent with the 13C NMR results and provide a molecular scale structural and energetic basis for interpreting them.

13C NMR spectroscopy and computational molecular dynamics modeling of fluids in shales and clays

2019 Spring National Meeting

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