The presence of nanopores in tight rocks and shales have been confirmed by numerous studies. These pores are also the primary storage space of oil and gas in shales. Due to the effect of nanoscale confinement, the phase behavior of fluids confined in such extra-low permeability formation (nanodarcy scale) differs significantly from those observed in the conventional formation.

In this paper, the cubic PR EOR is coupled with the capillary-pressure equation and adsorption theory to investigate and represent the phase equilibria of pure components and their mixtures in cylindrical nanopores. The pore confinement effects of interaction between fluid molecule and pore wall and the shift of critical properties are all considered. Also due to the effect of adsorption film, an improved Young-Laplace equation is adopted to simulate the capillarity instead of the conventional equation. For the adsorption behavior, the experimental data on the adsorbent of silicalite are used to represent the adsorption behavior of hydrocarbons in nanopores. Then, a prediction process for the behavior of methane, n-butane, n-pentane, n-hexane and their mixtures are performed. And the results are compared against the available experimental data to confirm the accuracy of this scheme. The actual Eagle Ford oil is also used to examine the performance of our scheme.

Results indicate that the presence of adsorption film could further increase the vapor-liquid equilibrium constant (K-value) and capillary pressure of the confined pure-component fluid, especially for the nanopores with few nanometers. The smaller the nanopore radius, the higher the deviation between the actual K-value and bulk value. For methane, when the pore radius is higher than about 20 nm, the K-value is approaching the bulk fluids and the effect of capillary pressure and adsorption film can be neglected. For n-pentane, it is about 18 nm. For binary mixture, it is found the higher the difference between the two components, the stronger the nanopore confinement effects. The capillary pressure will present a bilinear relation with the pore radius in the log-log plot. For multicomponent mixture, as the pore radius decreases, the bubble-point pressure is depressed, the dew-point pressure is increased, and the phase envelop of confined fluids is also shrinked. When the adsorption film is neglected, the bubble-point pressure will be overestimated, and the dew-point pressure is underestimated. For Eagle Ford oil, when the nanopore radius is higher than about 100 nm, the behavior will approach the bulk value and the influence of nanopore confinement can be neglected. This study will shed some important insights for the phase behavior of tight oil and gas condensate in nanopores.

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