Molecular Simulation Of The Effect Of SC-CO₂ On Formation Water Wetting Behavior On Rock Surface

Molecular dynamics（MD） simulation is performed to investigate the microscopic wetting behavior of water droplets on the sandstone in the presence of NaCl. The simulation results show that NaCl can transform the hydrophilic SiO2 surface into weak hydrophilicity. The wettability of water droplet on SiO2 surface weakens with increasing brine concentration. The contact angle increases with the increase of the NaCl concentration until the density rises to 19.6%. The potential of mean force（PMF） between water molecules and Na+ or Cl- have indicated that the binding energy barrier is higher than dissociation energy barrier which causes water molecules gathering around Na+ or Cl-, and therefore leading to the increase of contact angle. The adsorption energy between the sandstone surface and the water molecules have decreased with increase of the NaCl concentration. However, when the concentration is beyond 19.6% the adsorption energy changes little. This is consistent with the trend of the change in contact angle.The contact angle of CO₂-water-quartz system and microscopic mechanism of the changes in wetting behavior are studied using molecular dynamics simulation,which are at typical formation temperature with CO₂ injected. Our results show that the contact angle increases along with the CO₂ density raised and finally tends to be a constant. The wettability of quartz surface changes from strong hydrophily to weak hydrophily.In the interaction process of CO₂ and the water, CO₂ break the water layer into some water clusters, which leads to the increasing of the CO₂ adsorption amount and reduces the risk of the leak of CO₂. Moreover CO₂ reduce the wettability of the local water, and it is better for the water displacement and expanding CO₂ storage space.Then, we add NaCl into the CO₂-water-quartz system and use the molecular dynamics simulation method to investigate the contact angle of CO₂-brine-quartz system and microscopic mechanism of the changes in wetting behavior induced by the injection of different density CO₂. Our results show that the contact angle increases along with the CO₂ density raised and finally tends to be a constant, and the figure is greater than it is found in pure water. Cohesion ability of the water cluster and adsorption ability of CO₂ enhance after the addition of NaCl, which leads to the reduction of the water spreading degree and the increase of the contact angle. When CO₂ density increases, the adsorption ability of CO₂ and the interaction of CO₂ and the water cluster enhance, the number of hydrogen bonds increases and then decreases,as a result, the contact angle tends to be a constant with the increasing of CO₂ density.