Interaction mechanism of calcite and four representative organic molecules: Experiments and DFT study

Calcite crystal-organic molecule interaction governs the wettability formation and alteration of carbonate reservoirs, and influences the oil recovery. However, its in-depth interaction mechanism in atomic scale have not been classified clearly. Regarding this, this study combines the atomic force microscopy (AFM), fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) to systemically investigate the adsorption regularities and mechanisms of four representative organic molecules of crude oil on calcite surface. AFM results illustrated that the adsorption amount of the four organic molecules on calcite surface is in the order of benzoic acid>pyridine>thiophene>toluene. FTIR and XPS results explicated that the -COOR and -OH functional groups are formed in the benzoic acid adsorption; the Ca of calcite surface and N of pyridine are involved in the interaction, and -OH functional groups are also formed in pyridine adsorption; only the -OH bond is formed in thiophene adsorption, and no bond is formed in toluene adsorption. DFT results verified the experiments results and further revealed the interaction mechanism in atomic scale. Firstly, DFT results showed that the adsorption energies of benzoic acid, pyridine, thiophene and toluene on calcite surface are -1.39 eV, -0.97 eV, -0.49 eV and -0.31 eV, respectively. Secondly, DFT results explicated that the Ca-O ionic bond and H-O covalent bond are formed in benzoic acid-calcite interaction; the Ca-N ionic bond and H-O hydrogen bond are formed in calcite-pyridine interaction; only the H-O hydrogen bond is formed in calcite-thiophene interaction; No chemical bond is formed in calcite-toluene interaction.