(559n) Determination of Confined Fluid Phase Behavior Using Modified Peng-Robinson Equation of State

Confinement effect of nanopores in shale reservoirs causes the fluids behaving significantly different from bulk fluids. The unique structural and thermodynamic properties of the confined fluids, such as oscillatory density profile and critical property shift, disable the commonly used equations of state (EOS) to be applied in such reservoirs. The objective of this work is to modify the existing Peng-Robinson EOS (PR EOS) to make it applicable for unconventional reservoir fluids.

In this work, techniques have been developed to compute the phase behavior and thermodynamic properties of the confined fluids by coupling critical property shift and capillary pressure in the PR EOS, where a new term representing the molecule-wall interaction resulted from the pore proximity 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 modified PR EOS to correct the shifted critical properties. The capillary pressure effect is also 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 modified PR EOS has been 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 impact factors on the phase behavior of confined fluids. The critical property shift is found to cause a significant shrinkage of the phase envelope, whereas the capillary pressure leads to a mild expansion of vapor-liquid two-phase region. The combined effect has been incorporated in the modified PR EOS which has been 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.