Molecular dynamics simulations of interactions of organic molecules found in oil with smectite: Influence of brine chemistry on oil recovery
M Szczerba and DK McCarty and A Derkowski and M Kowalik, JOURNAL OF PETROLEUM SCIENCE AND ENGINEERING, 191, 107148 (2020).
The study is aimed at employing molecular dynamics simulations to study the interactions between smectite and major oil constituents, along with the impact of different brine chemistry (type of cation in a solution) and ionic strength on the compound mobility in fine pores. Clay minerals with smectitic type of mineral outer surface (smectite, illite-smectite, illite) are major constituents in many unconventional oil and gas reservoirs. Therefore, smectite was used as a model compound to test interactions of oil compounds with mineral surfaces. Natural crude oil is a complex mixture, which - for the purpose of simulating an influence of surrounding fluid on its interactions with smectite surfaces - needs to be simplified. The following molecules were selected as organic model compounds: decane, quinolone, nonanoic acid, benzamide, and heptylamine. Various protonation states were considered. The first two sets of simulations showed remarkably different interactions of the selected organic molecules with the smectite surface. In the first one, three water/oil coverage models of the surface were considered. In the second one, two different pore sizes were taken into account: 3 nm and 5 nm. Significant differences were readably visible between polar and non- polar organic molecules. Also, the degree of protonation was found to be a crucial factor affecting the molecules' affinity to the mineral surface. The molecules that are well soluble in cyclohexane (here representing crude hydrocarbons phase) formed droplets in the water phase, whereas organic compounds miscible in water were concentrated at the water-monolayer film on the smectite surface. Neutral organic molecules in the interlayer influences hydration spheres of the cations and their distance from the surface, while organic cations tend to substitute for inorganic ones. The third set of calculations aimed at testing mobility of organic model compounds within a wide-open smectite interlayer space (acting as a narrow pore model), under one-dimensional pressure. The results show significant differences in the pressure that allows expulsion of organic molecules in different systems and different ions from a narrow slit. The mobility of oil compounds depends mainly on differences in the type of molecules and the size of effective hydration sphere of the cations adsorbed on a mineral surface, which is clearly visible in the simulations.
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