Molecular simulation of kerogen-water interaction: Theoretical insights into maturity

The ubiquity of water and kerogen in source rocks at reservoir conditions is a clear indication that the study of kerogen-water system plays a key role and relevant to the current state of knowledge for fundamental and applied shale research. From the early experimental studies, it is acknowledged that kerogen insolubility in a typical organic solvent is due in part to their complex molecular structure. From a theoretical perspective, we aim to understand the molecular interaction of kerogen-water system at the atomic level. The petrophysical implication of this study is to understand the impact of maturity level of kerogen on the water recognition relative to other inorganic components of shale. For this purpose, we have calculated the adsorption free energies of water molecule on multi-configurations of kerogen model in a high mature state using B3LYP/Def2TZVP level of theory. Different models were built to reflect the diversity of kerogen composition (related to the heteroatoms). Furthermore, we have calculated the electrostatic potential and frontiers molecular orbitals to interpret and explain the trend observed in the adsorption free energies. The main finding of our study supported that there are no tight interactions between the water molecules and kerogen. The water-recognition by kerogen surface depends mainly on its constituents of the heteroatoms (N, S, and O). Indeed, nitrogen atoms displayed the highest affinity to ward moisture, followed by oxygen and sulfur. Given that kerogen in its high mature state does not consists much of nitrogen atoms, in that context, the matured kerogen would be less likely to adsorb water molecules in absence of gas.