Molecular dynamics simulation of CO2 dissolution in heavy oil resin-asphaltene

Based on the molecular dynamics method, association systems for resin, resin-asphaltene and asphaltene were constructed respectively. Under different temperatures and pressures, the density distribution and adsorption capacity of CO2 in each association system were investigated. According to the free volume of each association system and the interaction between CO2 and the association system, the solubility parameters of CO2 and the mechanism of CO2 dissolution were analyzed. The results demonstrate that many voids exist in the three association systems. The voids in the resin association system and the resin-asphaltene association system are relatively close and continuous, while dispersed in the asphaltene association system. The dissolved gases are mainly concentrated in the asphaltene association system. The larger the free volume fraction, the larger the space of dissolved gas molecules. The CO2 interaction strength in a descent order is resin, resin-asphaltene and asphaltene. In addition, the dependence of CO2 dissolution on mechanical interaction energy was also revealed, which provides a direct explanation for the swelling of dissolved crude oils. The solubility parameter increases with the molecular weight of the association system in the ascent order of resin, resin-asphaltene and asphaltene. Accordingly, the order of the CO2 solubility in the three association systems from high to low is resin, resin asphaltene and asphaltene.