In nanoscale pores, the adsorption of gas is a multilayer adsorption process, and the conventional Langmuir model is no longer valid. In particular for the unconventional gas condensate reservoir, the adsorbed gas will become condensate once the pressure is above the critical condensate pressure at pore scale. In this paper, considering the effect of adsorption (wetting) film, the multicomponent Kelvin equation (Shapiro and Stenby, 1997) is modified to computing the isotherm of capillary condensation. Then it is coupled with the multispace adsorption model (MSAM) and Peng-Robinson equation of state (EOS) to investigate and represent the phase behavior of hydrocarbons in organic nanopores. Then, a prediction process for the behavior of methane, n-butane, n-pentane, n-hexane and their mixtures are performed. And the actual Marcellus shale gas is also used to examine the performance of this model.
Results indicate as the pore size decreases, the dew point pressure is increased and the P-T envelop is depressed, especially for the nanopores with only several nanometers. The existence of pore confinement effect will reduce the critical temperature of capillary condensation. For the four-component mixtures studied in this paper, the capillary condensation will occur at about 0.80 P/Pd. It is lower than the dew point pressure. Furthermore, the Gamma distribution function is used in this paper to generate different pore size distributions to investigate the effect of pore size on the adsorbed gas content. It is observed that the smaller the pore size, the higher the adsorbed gas content. The adsorption behavior of hydrocarbon fluids in nanopores is pore-size dependent. Also, as the temperature increases, the adsorbed gas content is slightly decreased. For the behavior of Marcellus shale gas, the dew point pressure and P-T phase diagram are calculated. Marcellus shale gas belongs to the unconventional gas condensate reservoir. As the pore size decreases, the dew point pressure of Marcellus shale gas is increased. This paper sheds some important insights on the phase behavior of unconventional gas condensate reservoirs. And it could be used as a tool to effectively analyze the confined behavior of fluids in nanopores.
