Modelling of Partially Miscible, Multiphase Hydrocarbon Mixtures Using a Fugacity-Based Lattice Boltzmann Method

Abstract

Summary A thermodynamically consistent, fugacity-based lattice Boltzmann method (LBM) is used to carry out simulations of multicomponent, multiphase hydrocarbon mixtures. The mixtures are simulated as partially miscible and a multicomponent equation of state (EOS) is incorporated into LBM through the fugacity. Vapor Liquid Equilibrium (VLE) cases and dynamic cases are presented for different hydrocarbon mixtures. The free energy LBM model, applicable to multiphase systems, makes use of a functional of the (Helmholtz) free energy. Traditionally, this free energy has been limited to empirical "double-well" models and very limited use of EOSs. A new approach utilizing component fugacity bypasses the need for a free energy functional. The fugacity provides a link to incorporate multicomponent EOSs into LBM, resulting in an accurate modelling of hydrocarbon mixtures. We use this fugacity-based LBM to carry out VLE simulations for a binary hydrocarbon mixture to generate pressure-composition and temperature-composition plots. Next, we extend VLE simulations to cases of ternary hydrocarbon mixtures. In all cases of VLE, results from LBM are compared with the results obtained from a flash calculation and are shown to be in excellent agreement. This is significant as the LBM has a legacy of thermodynamic inconsistency due to which such agreement is hard to achieve. Next, we simulate far from equilibrium systems by considering the case of spinodal decomposition for a binary mixture. A homogeneous system is observed to decompose into separate phases, consistent with thermodynamic principles, when initialized in the unstable region of the phase envelope. Our results show the fugacity-based LBM to be a highly accurate model for hydrocarbon mixtures when it comes to capturing thermodynamic equilibrium and fluid dynamics. This study presents a comprehensive analysis of multiphase, partially miscible hydrocarbon mixtures using a lattice Boltzmann model which is consistent with thermodynamics and offers predictions in line with accurate equations of state. This opens the door for realistic pore-scale simulations of hydrocarbons which can be upscaled to develop better reservoir simulators.