Molecular dynamics simulation for quantitative characterization of wettability transition on silica surface

Surfactant controlling the wettability of reservoir rocks is one of the important ways to enhance oil recovery in low-permeability tight reservoirs. A large number of scholars have developed a series of excellent wettability control reagents through macro physical simulation experiments, but macro experiments are difficult to analyze the micro mechanism of liquid/liquid and liquid/solid interface action of reagents controlling rock wettability. In this paper, the wetting behavior of water molecules on silica surfaces modified with different hydroxylation, methylation and hydrophobic chain length was studied by molecular dynamics simulation, and the quantitative relationship between different modification degrees and wetting angle was established. The results show that the original wetting angle of silica is 58.5 degrees, and the surface wettability can be changed from hydrophilic to hydrophobic through surface modification, with a wetting angle of 27.25 degrees similar to 115.78 degrees. With the increase of the coverage of hydrophobic methyl or hydrophilic hydroxyl modified groups, the wetting angle increases or decreases respectively. The quantitative relationship between modification degree and wetting angle was established by simulated data. With the increasing of the chain length of methyl group, the distance between water molecule and silica surface increases and the interaction decreases, which can improve the regulation effect of methyl group hydrophobicity. This paper provides a reference basis for the synthesis of rock wettability control reagent and the quantitative characterization of modification degree.