Clay minerals are important components of shales and other tight formations, and because the pores in these rocks are so small, interaction of the pore fluids (e.g., H2O, CO2 and CH4) with the surfaces of these clays plays an important role in controlling the structural environments of fluid species, their partitioning between pores of different sizes, and their transport properties. The clay mineral illite is an important component of many shales and other fine grained sedimentary rocks, and its interaction with fluid species in the pore systems of these rocks is likely to play a significant role in controlling their structural and dynamical behavior during oil and gas production from tight reservoirs, in cap rocks of traditional petroleum reservoirs, and in geological C-sequestration. Recent advancements from experimental and simulation studies have indicated that the partitioning of H2O and CO2 is greatly affected by clay composition, location of structural layer charge, solvation properties of the charge compensating cations, and composition of fluid phase. Mixed CO2/CH4 fluids are of significant potential importance, especially near injection wells, but little is known about their mutual behavior in the nano- and meso-pores of shales. We present the results of a molecular modeling study of the partitioning of CO2 and CH4 on the basal (001) and protonated (010) surfaces of illite. The simulations were performed at 323K and a total fluid pressure of 124 bar using a constant reservoir composition molecular dynamics (CRC-MD) simulations. The results clearly show that the both basal and protonated surfaces show a strong preference for CO2 over CH4 and that both charge compensating Na and K show this preference. The effects of the surfaces on the fluid composition to ~2.0 nm from them. The orientation of CO2 molecules near the basal surfaces vary greatly with distance from the surface. The results are in reasonable agreement with recent single-phase adsorption simulation studies of illite at elevated temperature and pressure conditions.

Partitioning of methane and carbon dioxide in nano- and meso-pores of illite using constant reservoir composition molecular dynamics simulations

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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