Shear and bulk viscosities of natural gas at pre-salt conditions: A comparison between TRAPPE, OPLS and SAFT force fields using Einstein relations

• MD at 10–60 MPa, 323–413 K yields density, shear and bulk viscosities. • OCTP + Einstein approach implemented in LAMMPS for transport coefficients. • TraPPE outperforms OPLS and SAFT for density and shear viscosity. • Bulk viscosity is non-negligible; Stokes’ hypothesis fails in this regime. • New Andrade-type correlation predicts bulk viscosity with R 2 up to 0.98. Molecular dynamics (MD) simulations were performed to determine the density, shear viscosity and bulk viscosity of natural gas mixtures under typical conditions of pre-salt oil reservoirs. The investigated temperature range was 323–413 K, with pressures varying from 10 to 60 MPa, according to experimental data. Three force fields were employed: TraPPE, OPLS, and SAFT. The transport coefficients were determined using the OCTP method and the Einstein relations. The results indicate that the TraPPE force field exhibited the best performance in predicting the viscosity and density values, with minimal deviations from the experimental data and the transport equations. The OPLS force field showed intermediate performance, with greater dispersion at high pressures, while the SAFT force field displayed significant limitations. Additionally, empirical correlations were developed for bulk viscosity using the simulation data, based respectively on Arrhenius and Andrade-type models, showing excellent agreement with the MD results. The compositional variations in natural gas had a negligible impact on the prediction of these transport properties, and it was found that the computed bulk viscosity values are not negligible. Therefore, the Stokes’ hypothesis, which neglects the latter in the viscous stress tensor for all gases, is not valid in the thermodynamic range investigated in this work.