Effect of water presence on asphaltene-precipitation behavior of CO2 and crude oil mixtures during CCUS processes

• First study to draw PT and Px phase diagrams involving VLAS four-phase equilibria. • Reveal the effect of water on asphaltene precipitation during CO 2 injection. • Water poses a minor effect on asphaltene precipitation behaviour. Water can extensively exist in reservoirs. Since the solubility of CO 2 in water is not negligible under reservoir conditions, the aqueous phase should also be considered during the phase-behavior simulations of the CO 2 flooding process. With the presence of an asphaltene phase and an aqueous phase, four phases (i.e., a vapor phase, a hydrocarbon phase, an asphaltene phase, and an aqueous phase) can coexist at a given thermodynamic equilibrium. In this study, a robust and efficient four-phase vapor–liquid-liquid-aqueous equilibrium calculation algorithm is applied to conduct four-phase vapor–liquid-asphaltene-aqueous equilibrium calculations by assuming asphaltene as a dense liquid phase. The algorithm is first validated by comparing the calculated asphaltene precipitation data with the presence of water against the experimental asphaltene precipitation data documented in the literature. The validation shows that the calculation results agree well with the experimental data, indicating that such algorithm can make reliable predictions of asphaltene precipitation with the presence of water. The validated algorithm is subsequently used to construct pressure–temperature ( PT ) and pressure-composition ( Px ) phase diagrams to study the effect of water on asphaltene precipitation under different pressure/temperature conditions and under the injection of CO 2 . We can conclude from the calculated results that although adding water to the reservoir fluid can cause obvious changes in the PT phase diagrams, it does not have a noticeable influence on the asphaltene precipitation onsets. However, it can be seen from the calculated Px phase diagrams that the presence of water during the CO 2 flooding process can make asphaltene precipitated more easily. The maximum amount of asphaltene precipitation is decreased by the presence of water due to the fact that a considerable amount of CO 2 is dissolved in the aqueous phase instead of the liquid hydrocarbon phase.