Development of Effective Equations of States of H2, CH4, and CO2 for Subsurface Storage Conditions Using Data Generated by Molecular Simulation

Abstract The wide range of data generated by molecular simulation are facilitated for rapid and accurate development of the practical equations of state (EOS) for the hydrogen, methane, and carbon dioxide gases. The density and its derivative with respect to pressure are required to satisfy the conditions necessary for the model EOS to reduce to the ideal gas EOS as the pressure approaches zero. The modified power-law equation is applied for the hydrogen gas and the two-exponentials equation is applied for the methane and carbon dioxide gases. The new EOS developed for the hydrogen gas by the modified power-law equation is much simpler and practical than the previous virial EOS containing thirteen constants. The comparison of the results obtained using the hydrogen gas molecular simulation data with the results obtained using the experimental data demonstrates that the molecular simulation data can be used for developing effective correlations of the gas density. However, the correlations using the molecular simulation data of the methane and carbon dioxide gases are developed by the two-exponentials EOS because the modified power-law equation does not perform well for these gases. Further, the accurate correlations of the carbon dioxide gas density with pressure at various temperatures required the shifting of the pressure by 6.1 MPa. These practical approaches demonstrate that effective correlation of experimental data for development of EOS is as important as obtaining quality data and the averaging nature of the data correlation process may reduce the errors due to the uncertainties involving the experimental data but at some unknown extent. The advantage of the practical EOS for the hydrogen, methane, and carbon dioxide gases containing only a few constants developed in this paper is proven by comparison with the complicated virial EOS containing many empirically determined constants proposed in the literature.