Asphaltene phase behavior modeling in crude oil systems: Effects of SARA consistency and polydispersity

  • Saoud Hareb Al-Darmaki

Student thesis: Master's Thesis


Asphaltene deposition is one of the existing flow assurance problems faced nowadays in some of the Oil producing wells in Abu Dhabi. This problem has motivated the development of numerous theoretical models and experimental methods in order to understand the mechanisms of Asphaltene precipitation and deposition. Even though deposition is the actual problem, it only happens after precipitation occurs. Hence, the inspiration of this work is to revise the current models for Asphaltene precipitation prediction by including the effect of two aspects that had not been successfully addressed until now: 1) the effect of the inconsistency of the Saturates, Aromatics, Resins, and Asphaltenes (SARA) analysis of the Stock Tank Oil (STO), and 2) the effect of Asphaltene Polydispersity. The SARA analysis is a routine test performed by several Service Companies around the world and provides an indication of the composition of the STO. Although this method is subject to controversy, because it is well-known that different methods to measure SARA produce different results, it is widely used in the Oil industry and thus it is the starting point of several simulation models. Therefore, it is of utmost importance to verify the Consistency of the SARA analysis, by comparing the composition of the STO with other physical properties. A simple model has been proposed that verifies the Consistency of the SARA analysis against the density of the STO and it has been shown that at least one laboratory is reporting inconsistent experimental results. Actions have been taken to correct this problem. The Consistency test is of great importance to assure a good quality of the experimental data produced, which is the input of any simulation tool. The content of Asphaltenes in the STO is the minimum information that is required to model their Phase Behavior. However, the complex mixture that conform the Asphaltenes in the Crude Oil is oversimplified when people attempt to predict the stability at reservoir conditions. For instance, although it is well-known that Asphaltenes are a polydisperse distribution with a wide range of molecular weight aggregates, for practical purposes all the current simulation methods assume that Asphaltenes are mono-disperse with one single molecular weight. The reason is that following the traditional simulation methods the addition of more Asphaltene fractions would substantially increase the number of simulation parameters. Due to the very limited data available the inclusion of an arbitrary number of simulation parameters does not help in understanding the nature of Asphaltene behavior, and transform this challenging task into a simple fitting exercise. However, recent results obtained at the Petroleum Institute in collaboration with Rice University, suggest that the chemical nature of Asphaltenes within the Abu Dhabi region is very similar, and it has been possible to reduce substantially the number of simulation parameters. In this work, the addition of Asphaltene Polydispersity is done without increasing the number of adjustable parameters. A log-normal distribution is proposed to understand the effect of Asphaltene Polydispersity and a sensitivity analysis is performed to study the effect of the parameters on the Phase Behavior. The results obtained suggest that the addition of Polydispersity improves the agreement between simulation results and available data at reservoir conditions.
Date of AwardDec 2011
Original languageAmerican English
SupervisorFrancisco H. Vargas (Supervisor)


  • Applied sciences
  • Crude oil systems
  • Oil field chemicals
  • Chemical engineering
  • Petroleum engineering
  • 0765:Petroleum engineering
  • 0542:Chemical engineering

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