Determination of asphaltene precipitation conditions during natural depletion of oil reservoirs: A robust compositional approach

• Three expressions are presented for lower and upper onset pressures of asphaltene precipitation and oil saturation pressure.
• Two constrained multivariable search methods, namely GRG and SLP were employed for this purpose.
• Comparative studies were conducted between the developed equations and equations of state and empirical correlations.
• The developed equations are accurate, reliable and superior to all other published models.

Asphaltene precipitation causes rigorous problems in petroleum industry such as: relative permeability reduction, wettability alteration, blockage of the flow, etc. Therefore, accurate determination of onset pressures of asphaltene precipitation is necessary. These pressures can be obtained by experimental measurements on representative samples of the crude oils; however, laboratory analysis of crude oil samples is costly, time consuming and cumbersome. In this communication, three simple and accurate expressions have been proposed for prediction of lower and upper onset pressures of asphaltene precipitation as well as saturation pressures. To this end, 33 crude oil samples were collected from open literature sources. Afterward, two constrained multivariable search methods, namely generalized reduced gradient (GRG) and successive linear programming (SLP), were employed for modeling and expediting the process of achieving a good feasible solution. Then, comparative studies were conducted between the developed equations and equations of state as well as empirical correlations. The results illustrate that the developed equations are accurate, reliable and superior to all other published models. The results show that the proposed equations can predict lower onset pressure, upper onset pressure and saturation pressure with average absolute percent relative errors of 5.04%, 3.93%, and 3.81%, respectively. Besides, it is found that molecular weight of heptane-plus fraction has the greatest impact on the lower onset pressure, while methane has the most significant effect on both of the saturation and upper onset pressures.