Molecular dynamics study of the desorption mechanism for shale oil in supercritical water within reservoir nano-slits

Supercritical water (SCW) is an effective medium for in-situ shale oil recovery. However, comparatively limited systematic research has been conducted on the desorption mechanisms of shale oil with SCW flooding. In the current study, molecular dynamics simulations were performed to investigate the desorption of two- and multi-component hydrocarbons (aromatics, chain alkanes, cycloalkanes, and asphaltenes) by SCW within various shale nano-slits (calcite, montmorillonite, quartz, organic), thereby systematically demonstrating the key desorption behaviors and revealing the underlying mechanisms governing desorption performance. The results show that in hydrophilic/ionic nano-slits (calcite and montmorillonite), hydrocarbon desorption characteristics mainly were determined by molecular polarity and involved a homogeneous desorption mechanism. It was characterized by the hydrocarbon concentration decreasing exponentially with time, and a remarkably high desorption ratio of over 90% was achieved through SCW film replacement. In hydrophobic/non-ionic nano-slits (quartz and organic), the two-component hydrocarbons followed a phase-change desorption mechanism with three stages: firstly, an exponential desorption driven by SCW, secondly a transient phase-change, and lastly linear desorption driven by liquid water. For the multi-component hydrocarbons, light components were desorbed initially with competitive and synergistic effects enhancing the overall extraction performance. Finally, the desorption kinetic models for shale oil in SCW were established for the first time.