A new vapor-liquid-asphaltene three-phase equilibrium computation algorithm based on the free-asphaltene assumption

•A so-called free-asphaltene assumption is proposed.•A new algorithm is developed based on the free-asphaltene assumption.•The new algorithm is shown to be effective and robust. Asphaltene precipitation is one of the major flow-assurance problems in petroleum engineering and can occur at different stages of oil production due to pressure/temperature changes or compositional changes. It can cause formation damage in the reservoir and choking of oil tubing in the wellbore, thus leading to the reduction of production rates. Accurately predicting under what conditions asphaltene precipitation is a prerequisite to the effective mitigation of the flow assurance problems related to asphaltene precipitation. In this work, a new three-phase vapor-liquid-asphaltene precipitation algorithm is developed based on Li and Li's work (Li and Li, 2019) using the free-asphaltene assumption, in which the asphaltene phase is considered as a liquid phase that only contains asphaltene component (Nascimento et al., 2018). The algorithm is validated by comparing the calculation results with the asphaltene precipitation data of five different oil samples. The tuned parameters for oil sample 3 and oil sample 4 are then used to predict asphaltene precipitation with different gas injections. The calculation results agree well with the experimental data. It indicates that the three-phase VLA equilibrium calculation algorithm can make reliable predictions of the PT and Px phase diagrams with different gas injections as well as the asphaltene precipitation amount. The calculation results yielded by the free-asphaltene assumption are also compared with the results yielded by the solid assumption and the liquid assumption. The algorithm developed using the free-asphaltene assumption is shown to be able to perform slightly better than the algorithm developed using the solid assumption, and perform as well as the algorithm developed using the liquid assumption. [Display omitted]