Molecular Dynamics Simulation and Experimental Studies of the Wettability Behaviors of Shales

The wettability behaviors of nanoscale pores in shales from Longmaxi Formation in the Sichuan Basin of South China, which contain quartz pores, illite pores, and organic matter pores, were investigated using the molecular dynamics simulation and experimental methods, and the organic matter surface was approximated by grafting oxygenated functional groups onto a graphite surface. The distribution characteristics of a water-methane system in organic matter pores were studied. The results indicate that the organic matter pore network and the inorganic pore network in shales show cross-distribution characteristics. The speed of spontaneous imbibition of oil is less than that of water, and the spontaneous imbibition mass of oil increases with increasing the total organic carbon (TOC) contents. The surfaces of illite and quartz mineral are hydrophilic. With the increase in oxygen-containing functional groups, the interaction energy between the organic matter surface and water molecules decreases, resulting in an increase in the wetting contact angle of the organic matter surface. With increasing temperature, the interaction energy between the organic matter surface and water molecules increases, resulting in a decrease in the wetting contact angle. In symmetrical graphite pore models with different C/O ratios, water molecules are symmetrically spread near the oxygen-functionalized graphite wall, and with a decrease in the C/O ratio, the relative concentration of water molecules increases and the diffusion coefficient decreases. In contrast, methane molecules are gathered and distributed in the center of the pore. In an asymmetric graphite pore model with different C/O ratios, water molecules are asymmetrically spread near the oxygen-functionalized graphite wall, whereas methane molecules are concentrated and spread in the pore center. The side with a lower C/O ratio has stronger hydrophilicity and a higher relative concentration of water molecules, whereas the side with a higher C/O ratio has stronger hydrophobicity and a lower relative concentration of water molecules.