Determination of confined fluid phase behavior using extended Peng-Robinson equation of state

Phase behavior of confined fluids deviate significantly from that of bulk fluids because of the strong confinement effect in nanopores. The classical equation of state (EOS) based on bulk fluids is inapplicable for fluids in shale reservoirs because it cannot reflect the unique thermodynamic properties of confined fluids, such as the suppressed bubble-point pressure. The objective of this work is to extend the existing Peng-Robinson EOS (PR EOS) for unconventional reservoir fluids. In this work, techniques have been developed to compute the phase behavior of confined fluids by incorporating critical property shift and capillary pressure in the PR EOS, where a new term representing the molecule-wall interaction has been introduced. On the basis of data collected from both experiments and molecular simulations, a correlation which is a function of dimensionless pore size has been developed and incorporated in the extended PR EOS to correct the shifted critical properties. The capillary pressure effect is considered in phase equilibrium computation to account for the strong capillary force in nanopores, where the pressure in vapor phase is larger than that in liquid phase. The newly extended PR EOS is validated by reproducing the molecular simulation data from a previous literature, yielding an overall error of 7.71%. Both critical property shift and capillary pressure are found to be important factors influencing the phase behavior of confined fluids. The combined effect has been incorporated in the extended PR EOS which is successfully used to compute the phase behavior of three unconventional reservoir fluids. The bubble-point pressures under reservoir temperatures for Bakken, Eagle Ford, and Wolfcamp fluids at 10 nm are found to be reduced by 31.98%, 23.15%, and 37.77%, respectively.