During water flooding, the crude oil-water-rock interaction constantly changes as the composition of produced oil    

keeps changing, and the changing rules and mechanisms are of great importance for the efficient development of oil reservoirs.    

Experiments and molecular dynamic simulations are combined in this study to investigate the composition changes of produced    crude oil during water-flooding and reveal their in-depth mechanisms. Firstly, long core displacement experiments together with    compound-grouped fraction experiments, and Fourier transform infrared spectroscopy experiments shed light on the change rules    of crude oil composition during water-flooding. Secondly, molecular dynamic simulation is used to study the change mechanisms    of crude oil compositions at a molecular scale. Experiments results showed that the change of the produced oil composition in    the water-free stage is small, while this change is much larger at the water breakthrough stage. Saturated hydrocarbons keep    decreasing as verified by the continuous decrease of -CH3 and -CH2- bands. Aromatic hydrocarbon increases significantly with    the enhancement of -CH- off-plane vibration and benzene symmetrical stretching vibrations. Meanwhile, small increases of resin    and asphaltene are also observed with a slight increase of the oxygen/nitrogen functional groups. Molecular dynamic simulation    results indicated that the saturated hydrocarbon-aromatic hydrocarbon-resin-asphaltene adsorption sequence is adsorbed on    calcite surfaces. In the water flooding process, water molecules successively contact with saturated hydrocarbon, aromatic hydro   carbon, and free resin and asphaltene, consequently displacing them away from the calcite surfaces. Finally, the adsorbed resin    and asphaltenes remain stable, one end is anchored on calcite surfaces, and the other side is dragging a small amount of aromatic    and saturate hydrocarbons that still have not been driven away. The stronger the polarity of the crude oil components, the stronger    the interaction between crude oil components and calcite surfaces, and the greater the contribution of electrostatic force and the    smaller the influence of van der Waals force. The closer the polarity of crude oil components, the stronger the intermolecular in   teraction. π bond interaction exists among aromatic compounds. Saturated hydrocarbons interact with other components through    van der Waals force. The intermolecular interactions among the crude oil components integrate them together on the calcite    surfaces, and causes the retention of non-polar crude oil components. The change rules and mechanisms of crude oil components    during the water-flooding process are systematically investigated in this study from a molecular scale by combining experiments    and molecular dynamic simulation, which provide great support for the optimization and application of oil recovery enhancement    technology.