Insights into the nano-structure of oil-brine-kaolinite interfaces: Molecular dynamics and implications for enhanced oil recovery

Oil-brine-clay minerals interactions play an important role in in-situ reservoir wettability, which govern the fluid flow during enhanced oil recovery, CO2 geosequestion and water remediation at subsurface. Kaolinite clay minerals are widely existed in subsurface reservoirs. Therefore, it is of vital importance to understand how kaolinite controls the in-situ wettability through the nanostructure of oil-brine-kaolinite interfaces. In this context, molecular dynamics (MD) simulation was therefore performed to compare the nanostructures at kaolinite surfaces as a function of brine salinity in the presence of organic materials. Acid component (decanoic acid) and base component (quinoline) were incorporated to evaluate their adsorption on kaolinite surface as a function of salinity. The density distribution and radial distribution function (g(r)) were calculated in high salinity water and low salinity water. The density distribution profile showed that the aluminol layer was decanoic acid wet while the siloxane layer was insensitive to both decanoic acid and quinoline regardless of salinity. The g(r) of Al and Si atoms in clay structure revealed that more water accumulated adjacent to the aluminol layer in low salinity water and screened out the ions and functional groups adsorption on the surface. However, less water accumulated around Si atoms in low salinity water than in high salinity water. The more accumulated water on aluminol layer than siloxane layer can explain the wettability (adsorption) differences between the aluminol layer and the siloxane layer. Furthermore, the MD results revealed that the decanoic acid was more responsive to salintiy variations than quinoline, which confirmed that acid component was the primary element in oil to determine wettability alteration. This work sheds light on the significant influence of mineral surface chemical species on oil-minerals adhesion as a function of ionic strength at molecular scale. This work also provides an overall practical framework, which predicts the wettability alteration with applications in enhanced oil recovery, CO2 geosequestration and water remediation. •Brine salinity determines organic-kaolinite adsorption (or wettability alteration).•Lowering salinity leads to a more hydrophilic aluminol layer.•Lowering salinity leads to a more hydrophobic siloxane layer.•The aluminol layer adsorbs the anions while the siloxane layer adsorbs cations.•The acid adsorption is more salinity sensitive than th