There are different thermodynamic models that have been applied for modeling of asphaltene precipitation due to various reasons, such as CO2 injection, mixing of different oils in oil storage and pressure changes. In this work, two computer codes which are based on two different asphaltene precipitation thermodynamic models, 1-Thermodynamic Micellization Model and 2- Solid Model, have been developed, and, used for predicting of asphaltene precipitation data reported in the literature as well as the experimental data obtained from high pressure high temperature asphaltene precipitation experiments performed on Sarvak reservoir crude, one of potentially problematic Iranian heavy oil reserves, under CO2 injection condition. The model parameters obtained from sensitivity analysis as well as various characterization methods, were applied in the thermodynamic models. The developed computer code of solid model is able to predict the asphaltene precipitation data under gas/solvent injection condition with a close agreement. Especially, for the maximum value of asphaltene precipitation and for the trend of the curve after the peak point, good agreements were observed which could rarely be found in the available literature. It has been observed that the thermodynamic micellization model, which is more complex than solid model, is able to predict the trend of the asphaltene precipitation curve for gas titration condition reasonably well. Also, its predictions matched well with more experimental data points in comparison to the solid model predictions.
Asphaltenes are a solubility class of crude oils and are defined as the fraction that precipitates upon the addition of an n-alkane (usually n-pentane or n-heptane), but dissolves in aromatic solvents such as toluene. Asphaltenes can precipitate upon a change in pressure, temperature, and/or composition. For example, when pressure is drawn down around and within a wellbore, asphaltenes can precipitate and significantly reduce production.
For efficient process design, it is important to know when and how much asphaltene precipitation may affect the production of petroleum in all facets of field development. To predict precipitation, it is useful to develop thermodynamic models for describing the precipitation properties of dead and live oil fluids. Various thermodynamic models such as solubility model, solid model and thermodynamic micellization model have been reported in literature [1, 2, 3]. However quantitative representation is still far from satisfactory.
The simplest model for precipitated asphaltene is the singlecomponent solid model that was tried by Gupta [4] and Thomas et al.[5]. Nghiem et al.[2] used solid model along with a robust flash calculation procedure for vapor/liquid/asphalt systems to predict asphaltene precipitation data from the literature and the industry under pressure depletion, gas or solvent injection conditions. They observed that their proposed model could not exhibit the correct behavior with increasing solvent concentration under gas/solvent injection condition.
Victorov and Firoozabadi [6] proposed a thermodynamic model that considers that asphaltene precipitation from petroleum fluid is a micellization process. This model has shown promising results in explaining most of the experimentally observed results. However, their proposed approach could not predict asphaltene precipitation data under gas titration condition with a close agreement.
