Adsorption behavior of n-hexane and its mixtures with CO2, CH4, H2O and SDBS in hydrophobic silica nanopores

Different solvents including CO2, CH4, H2O and chemical agents are used to enhance oil recovery (EOR) in shale oil reservoirs. However, due to the complex pore structure and multi-phase fluids, the adsorption behavior of hydrocarbons in nanopores becomes an important effect for the shale oil stripping. A comprehensive comparison of n-hexane (n-C6H14) and its mixtures of CO2, CH4, H2O and sodium dodecyl benzene (SDBS) in hydrophobic silica nanopores is studied using molecular dynamics (MD) simulation in this paper, and the adsorption configuration, the adsorption density and the adsorption energy are discussed successively to reveal the adsorption mechanism of different injection solvents. The configuration results show that n-hexane forms a relatively denser adsorption structure on the hydrophobic silica surface. After solvents of CO2, CH4 or SDBS is injected with the same injection volume, CO2 or CH4 will be scattered around the system, H2O will be aggregated on surface as a group, while SDBS will be aggregated in the adsorbed oil layer. Six symmetrical adsorption layers of n-hexane are formed in nanopores with different solvent/n-hexane, and the density increases gradually from center to surface. The density peak of the first n-hexane adsorption layer (near the surface) is in the order of CO2/ n-hexane system > CH4/n-hexane system > H2O/n-hexane system > SDBS/n-hexane system. The van der Waals interaction is the main interaction between fluids (n-hexane and different solvents) and the silica surface. SDBS has the strongest ability to reduce binding energy, followed by H2O, CH4 and CO2. The density distribution and the calculation of C atoms show that n-hexane molecules can be effectively desorbed from the silica surface. With the same injection volume, SDBS has the best ability of stripping shale oil, followed by H2O, CH4 and CO2. The results obtained in this paper provide important insights into the mechanism of oil adsorption and stripping using different solvents in shale oil reservoirs.

Automated summary

This study compares the adsorption behavior of different solvents in hydrophobic silica nanopores using molecular dynamics simulation, and discusses the adsorption mechanism and stripping ability of different solvents for shale oil. The results provide important insights into the mechanism of oil adsorption and stripping using different solvents in shale oil reservoirs.