Experimental and molecular dynamic simulation study on calcite-organic molecule-water interaction mechanism

The mechanism of calcite-organic molecule-water interaction mechanism in oil-wet formation and alteration of carbonates is still ambiguous. This paper combines atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS) and molecular dynamic (MD) simulation to investigate the regularity and mechanism of calciteorganic molecule-water interaction at the molecular scale. First, the calcite-organic molecule-water interaction regularities, i.e., wettability alteration process, were studied using AFM, XPS and MD. AFM, XPS and MD results all demonstrated that acetic acid adsorbs stably on the calcite surface and can-not be displaced by water molecules, while the resistance to water displacement of the other three organic molecules decreased with decreasing molecular polarity. Subsequently, MD was used to analyze the calcite-organic molecule-water interaction mechanism from three perspectives, including the organic molecule-calcite interaction, the organic molecule-organic/water molecule interaction, and the effect of polar molecules. In the organic molecule-calcite interaction, the adsorption intensity of organic molecules on calcite surface decreases as their molecular polarity decreases. Stronger molecular polarity results in tighter adsorption layer and lower adsorption energy. In the organic molecule-organic/water molecule interaction, the interaction intensity increases as the polarity difference between the two molecules decreases. Larger polarity difference causes stronger miscibility and lower interaction energy. For polar molecules effect, the presence of polar molecules stabilizes the non-polar molecules adsorption and diminishes the displacement efficiency of water molecules. Consequently, this paper proposes the bulk adsorption mechanism to reasonably explain the wettability formation and alteration mechanism of calcite surface, which is of great importance for targeted enhanced oil recovery studies.