Vapor-Liquid Equilibria and Diffusion of CO2/n-Decane Mixture in theNanopores of Shale Reservoirs

摘要

Numerous laboratory tests on the Northern American shale plays have observed a large number ofnanopores.Because of the pore-proximity effect,the vapor-liquid phase equilibrium and transportperformance of fluids in nanopores differ significantly from that observed in PVT cell.In recent years,CO2 huff-and-puff has been widely applied to unlock the shale reservoirs.But on account of the highadsorption selectivity of CO2,after the injection of CO2,the original vapor-liquid equilibria of hydrocarbonsis changed.The purpose of this study is to predict the phase behavior and diffusion of the CO2/n-decanemixtures in the nanopores.The Peng-Robinson(PR)equation of state is combined with Young-Laplaceequation to calculate the phase-composition diagram at the presence of capillary pressure.The equilibriummolecular dynamics simulations(MDS)are also conducted to study the phase behavior,and the numberdensity profiles of different molecules are calculated.Then,based on the discussion of phase behavior,aseries of equilibrium MDS runs are carried out to calculate the self-diffusion coefficients of CO2,n-decane,and all fluid molecules.For each MDS with a different CO2 mass fraction,the two types of fluid moleculesare thoroughly mixed,the conditions of pore size and temperature are consistent with those in the phasebehavior studies.Results indicate that considering the capillary pressure,when the mass fraction of CO2 is less than 40%,the bubble point suppression is more clearly shown in the phase envelope.The number density profiles ofn-decane molecules show the apparent characteristics of adsorption layers.As the mass fraction of CO2molecules increases,the self-diffusion coefficients of CO2,n-decane,and their mixtures all increase.Theself-diffusion coefficients of CO2 molecules are higher than that of the n-decane molecules,and the diffusioncoefficients of the entire fluid system are somewhere in between.Appropriate CO2 injection into shale oilreservoirs can not only reduce the confinement-induced bubble point suppression but also improve the flowbehavior of oil in nanopores.This study can shed some critical insights for the vapor-liquid phase equilibriaof confined fluids in nanopores and provide sound guidelines for the application of CO2 huff and puff inshale reservoirs.