Mechanisms of supercritical water conversion of shale kerogen: Insights from ReaxFF molecular dynamics simulations

• Non-isothermal and isothermal degradation of typical lacustrine kerogen in SCW and PY were simulated using ReaxFF MD. • The influence of SCW on kerogen activation energy, heteroatom distribution, and product saturation was analysed. • Interactions between hydrogen and oxygen in SCW with organic atoms were examined. • Typical reaction pathways of kerogen in SCW are identified. Supercritical water (SCW) conversion technology is a clean and efficient approach for the development of low-maturity shale oil. However, the conversion mechanisms of kerogen within SCW remain unclear. This study employs ReaxFF MD simulations to systematically investigate the SCW conversion mechanisms of kerogen. First, a comparative analysis was performed to investigate the isothermal and non-isothermal degradation of three representative lacustrine kerogen models under pyrolysis (PY) and SCW conditions. Second, the effect of SCW on the activation energy of kerogen, the distribution of heteroatoms, and the saturation of products was explored. Third, the interactions between H and O atoms in SCW with organic atoms and the typical reaction pathways of kerogen in SCW were elucidated. The findings suggest that type I kerogen exhibited a 760 K lower degradation temperature and a 31 % higher yield at 5 K/ps compared with 100 K/ps, and degraded 170 ps faster with a 27 % higher yield at 2400 K than at 1800 K. Type I kerogen, characterized by long aliphatic chains and low condensation, exhibits a peak activation energy 2–4 kcal/mol lower and a secondary cracking temperature 200 K lower than those of Type II 1 kerogen. The yield and light oil content of low-maturity type II 1 kerogen are 8.9 % and 18 % higher, respectively, than those of medium-maturity kerogen. SCW and its OH and H radicals promote bond cleavage and ring opening in kerogen, reducing its peak activation energy by 2 to 18 kal/mol and degradation temperature by 60 to 160 K while facilitating the removal of N and S heteroatoms. The introduction of H and OH radicals enhance the saturation of gases and light oils, suppresses coke formation. This study provides theoretical support for the innovation of SCW in situ conversion technology for low-maturity shale oil.