Video: Phase Equilibrium of Hydrocarbons Confined in Nanopores from a Modified Peng-Robinson Equation of State

Cui, Xiaona (Northeast Petroleum University and Texas A&M University) | Yang, Erlong (Northeast Petroleum University) | Song, Kaoping (Northeast Petroleum University) | Huang, Jingwei (Texas A&M University) | Killough, John (Texas A&M University) | Dong, Chi (Northeast Petroleum University) | Liu, Yikun (Northeast Petroleum University) | Wang, Keliang (Northeast Petroleum University)


Phase behavior of hydrocarbons in confined nanopores is quite different from that of the bulk. In confined space, the high capillary pressure between vapor phase and liquid phase, and depressed critical properties under confinement will all affect the in-situ phase behavior. According to the theory of adsorption-induced structural phase transformation in nanopores, we modify the molar volume term of the Peng-Robinson equation of state (PR-EOS) by considering the reduced mole number of fluids caused by absorption to describe the phase behavior of fluids under confinement. Then capillary pressure is coupled with phase equilibrium equations, and the resulting system of nonlinear fugacity equations based on the modified PR-EOS is solved to present a comprehensive examination of the effect of capillary pressure and confinement on saturation pressures. Binary mixtures of methane with heavier hydrocarbons and a real reservoir fluid from the Eagle Ford confined at different pore sizes are considered. The effect of capillary pressure and confinement on the phase envelop shifts are compared.

The modified PR-EOS show that there exists a linear relationship between critical temperature shift and pore size reductions, a quadratic relationship between critical pressure shift and pore size reductions which are consistent with the experimental and molecular simulation results. The shift in the phase envelop of binary mixtures and Eagle Ford fluids show that both the capillary pressure and confinement decrease the bubble point pressures, while they oppositely influence dew point pressures. It is worthy to be noted that the effect of capillary pressure on phase envelop shifts will be suppressed when taking the critical point shifts caused by confinement into consideration. For Eagle Ford fluids, the effect of confinement on phase envelop shift is dominant compared with that of capillary pressure, and the capillary pressure cannot be overlooked when pore radius decreases to 50 nm. While the confinement begins to play an important role on the saturation pressures when pore radius decreases to 100 nm.

In addition, the methodology presented in this study can be extended to the phase equilibrium calculations of multiple pores since the modified PR-EOS can provide a consistent phase behavior description of fluid molecules over the whole range of pore sizes.