Confined Behavior of CO₂/Hydrocarbon System in Nanopores of Tight and Shale Rocks

Abstract

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.