Abstract: To clarify the dynamic wettability changes during CO₂ injection in unconventional reservoirs,molecular dynamics simulations were performed of water droplets wetting hydroxylated quartz and methylated quartz surfaces at various CO₂ pressures.The study shows that hydroxylated quartz is initially strongly hydrophilic,while methylated quartz is initially weakly CO₂-affine;as CO₂ pressure increased,the water wettability of both decreased,with smaller changes for hydroxylated quartz and larger changes for methylated quartz.With higher CO₂ pressure,CO₂ and water molecules co-adsorbed on the methylated quartz surface,forming a discontinuous CO₂ adsorption film that reduced non-bonded interactions and led to lower hydrophilicity until at 44.96 MPa the water contact angle reaches to 180.00°.As CO₂ pressure increases,the number of water molecules in the contact line region decreased and their motion space is compressed,reducing water molecular jump frequency and increasing jump distance.Compared with methylated quartz,hydroxylated quartz remains more hydrophilic with higher water molecule jump frequency and shorter jump distance.This study revealed the dynamic mechanisms of contact angle change for hydroxylated vs. methylated quartz under varying CO₂ pressures,established relationships between contact angle,relative permeability and CO₂ pressure and then a more accurate description of capillary pressure is proposed.These results provide new theoretical support for nanoscale fluid flow and a more reliable basis for reservoir evaluation,productivity prediction, and development strategy.

Key words: molecular dynamics, CO₂, hydroxylated quartz, methylated quartz, dynamic wettability