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 hydrocarbon, 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 interaction. 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.
