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Fluids in Nano- and Meso-Pores: Insights into CH4-CO2-H2O Behavior in Unconventional Reservoirs


Geochemical interactions of non-native fluids in unconventional hydrocarbon reservoirs are complex and create dynamic chemical environments that can interfere with gas desorption and transport in the pore network and engineered fracture system. In this study, we explore fluid-rock interactions with a novel combination of in situ experimental 13C NMR spectroscopy and computational molecular dynamics (MD) modeling of CH4, CO2, and H2O at reservoir conditions to better understand the molecular scale structure, dynamics and energetics of the fluid species in these systems, which are otherwise very difficult to probe. For the first time, we directly identify multiple pore size environments in a natural shale sample occupied by CH4. This CH4 was subsequently partially replaced by a CO2 flood. As for synthetic clay and compressed clay (synthetic shale) samples, the natural shale sample shows that both CH4 and CO2 enter the pore system, with both occurring in bulk fluid and adsorbed in the pores. In parallel, MD modeling provides a molecular scale picture of the origin of the interactions among CH4, CO2, H2O, and the mineral surfaces. The results show that for slit-like pores with thicknesses between 0.3 and 70 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 the clay surface. These results are fully consistent with the 13C NMR results and provide a molecular scale structural and energetic basis for interpreting them. The 13C NMR spectra were obtained at T = 50°C and Pfluid = 90 bars using a unique magic angle spinning sample loading system (WHiMS) at the Environmental Molecular Sciences Laboratory of the Pacific Northwest National Laboratory. The spectra for the natural and synthetic shales show that CH4 occurs in both nano-pores in the < 1 nm range and meso-pores in the >10 nm range. Partial replacement of CH4 by CO2 or H2O results in the CH4 occupying larger nano- and meso-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. This combined experimental and computational approach improves our understanding of non-native fluids that can further advance development of injection strategies for unconventional reservoirs.