Molecular Modeling of the Volumetric and Thermodynamic Properties of Kerogen: Influence of Organic Type and Maturity

Molecular modeling is applied to a representative array of kerogens for the purpose of obtaining quantitative predictions of thermodynamic properties from quantum mechanics and volumetric properties from molecular dynamics. The kerogen model units (175–260 carbon atoms) have been built in the MedeA environment from the sole consideration of the elemental analysis and functional group analysis documented in the work of Exxon and IFP-EN scientists [Kelemen, S. R., et al., Energy Fuels, 2007, 21 (3), pp 1548–1561]. The density results are in good agreement with the well-documented trends of kerogen density with thermal maturity and organic type. The heat capacity in the ideal gas state is predicted to increase as a function of temperature, as obtained from quantum mechanics at the semiempirical level (MOPAC-PM7). This result is in quantitative agreement with experimental heat capacity data on type I kerogen and on coal. This behavior appears clearly as a nonclassical feature, because of the quantization of energy levels in molecular vibrations. Also, the residual heat capacity estimated from molecular dynamics appears subordinate, compared with the ideal heat capacity evaluated from quantum mechanics. The change from negative to positive standard enthalpy of formation when changing from low-maturity kerogen to high-maturity kerogen is also predicted in agreement with correlative methods based on numerous experimental data from coals and fossil fuels. Kerogen model units are available for download free of charge in .xyz or .sci formats from www.materialsdesign.com/science/structures/kerogens_and_coals.