The presence of nanopores in tight and shale rocks has been confirmed by numerous studies. Due to the pore-proximity effect, the confined behavior of fluids in nanopores differs significantly from that observed in PVT cell. Currently CO2 huff-and-puff has been used to unlock the tight and shale reservoirs. Because of the high adsorption selectivity of CO2, after the injection of CO2, the original fluid density and composition of hydrocarbons in nanopores has been changed. In this paper, the PR-SLD model is applied to investigate the confined behavior of pure CO2/hydrocarbon fluids and their mixtures in nanopores. The Lee’s partially integrated 10-4 potential model is used to represent the solid-fluid interaction. For mixtures, a group contribution method is used to estimate the binary interaction parameters of CO2/hydrocarbon mixture. Thereafter, from the results of density distribution across the nanopore, the adsorption amount of fluids can be derived. Based on this model, a prediction process for the behavior of pure CO2 and hydrocarbon fluids (of methane and ethane) and their mixtures is performed. Results indicate that the adsorption selectivity of CO2 is much higher than CH4 and C2H6. And the density of pure CO2 in nanopores is higher than that of CH4 and C2H6. For binary mixture, because of the difference of interaction energy, the mole fraction of CO2 molecular is gradually increased from pore center to pore surface, and that of the hydrocarbon molecular is reduced from pore center to pore surface. The composition difference between bulk fluids and adsorbed fluids of CO2-C2H6 mixture is lower than that of CO2-CH4 mixture. For ternary mixture, the mole fractions of CO2 and C2H6 are always increasing from pore center to pore surface, and the mole fraction of CH4 is decreased from pore center to pore surface. Compared the original pure hydrocarbon mixtures, the addition of CO2 further increases the density of bulk fluids and adsorbed fluids. This study sheds some important insights for the behavior of confined fluids in nanopores and provides sound guidelines for the application of CO2 huff and puff in tight and shale reservoirs.
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