Applying the PR-EOS to Predict the Onset of Asphaltene Precipitation from n-Alkane Diluted Bitumens

Asphaltenes are the n-pentane or n-heptane insoluble fractions of crude oil that remain in solution under reservoir temperature and pressure conditions. They are destabilized and start to precipitate when the pressure, temperature and/or composition changes occur during primary production. The precipitated asphaltene particles will then grow in size and may start to deposit onto the production string and/or flowlines, causing operational problems. In this paper, a combination of the modified Peng-Robinson (PR) EOS and regular solution theory is used to estimate the onset of asphaltene precipitation of some oils. The PR EOS was modified using contribution methods for the asphaltenes which are assumed to be polymeric-like compounds consisting of aggregates of monodisperse asphaltene monomers. The modified EOS with the Peneloux correction was used to estimate the molar volumes and solubility parameter of the four solubility classes (Saturates, Aromatics, Resins, Asphaltenes) of bitumens. The predicted parameter of EOS was used to determine the onset of asphaltene precipitation from bitumen upon the addition of heptane and predictions were compared with measured onsets. Liquid-Liquid equilibrium was assumed between an oil phase and an asphaltene-rich phase. The asphaltene were divided into several pseudo components based on the Schultz-Zimm distribution function. Application of this model will help operators to better forecast the onset of asphaltene precipitation as it relates to flow rate history of the well and the pace of plugging induced by asphaltenes, and thus better plan remedial measures. The agreement between the predicted and measured onsets is very good.