Determination of asphaltene precipitation conditions during natural depletion of oil reservoirs: A robust compositional approach Forough Ameli a, b , Abdolhossein Hemmati-Sarapardeh c , Bahram Dabir a, c, d, ** , Amir H. Mohammadi e, f, g, * a Department of Chemical Engineering, Amirkabir University of Technology, Tehran, Iran b Department of Chemical Engineering, Islamic Azad University, North Tehran Branch, Tehran, Iran c Department of Petroleum Engineering, Amirkabir University of Technology, Tehran, Iran d Energy Research Center, Amirkabir University of Technology, Tehran, Iran e Institut de Recherche en G enie Chimique et P etrolier (IRGCP), Paris Cedex, France f Thermodynamics Research Unit, School of Engineering, University of KwaZulu-Natal, Howard College Campus, King George VAvenue, Durban 4041, South Africa g D epartement de G enie des Mines, de la M etallurgie et des Mat eriaux, Facult e des Sciences et de G enie, Universit e Laval, Qu ebec, (QC) G1V 0A6, Canada article info Article history: Received 10 August 2015 Received in revised form 13 November 2015 Accepted 14 November 2015 Available online 8 December 2015 Keywords: Asphaltene precipitation Onset pressure Saturation pressure Constrained multivariable search method Correlation abstract 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 pre- cipitation 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. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Asphaltene precipitation alters the fluid flow rate in the reser- voir and causes several problems. As this phenomenon alters the wettability and causes permeability reduction and operational problems, studying the conditions at which the asphaltenes precipitate out of the solution, and determining the amount of precipitated/deposited asphaltene are of vital importance [1,2]. Experimental determination of asphaltene phase behavior is expensive, cumbersome, and time consuming. Therefore, many researchers have focused on developing predictive models for asphaltene phase behavior [3e9]. These models are mainly classi- fied into five main groups which include, polymer solubility models, equation of state (EOS) models, colloidal techniques, thermodynamic micellization approaches, and scaling law models [10,11]. Solubility model was introduced by Hirschberg et al. [4], to predict the heat of solution. The liquideliquid equilibrium is computed based on the FloryeHuggins polymer solution model * Corresponding author. Institut de Recherche en G enie Chimique et P etrolier (IRGCP), Paris Cedex, France. ** Corresponding author. Department of Chemical Engineering, Amirkabir Uni- versity of Technology, Tehran, Iran. E-mail addresses: drbdabir@aut.ac.ir (B. Dabir), a.h.m@irgcp.fr, amir_h_ mohammadi@yahoo.com (A.H. Mohammadi). Contents lists available at ScienceDirect Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid http://dx.doi.org/10.1016/j.fluid.2015.11.013 0378-3812/© 2015 Elsevier B.V. All rights reserved. Fluid Phase Equilibria 412 (2016) 235e248