Analysis of a Thermodynamic Micellization Model for Asphaltene Precipitation
Authors: Tejeda P. , Lira-Galeana C. , Rodriguez F. 
Published: December 2007

Asphaltene precipitation and plugging has shown to be a major problem in petroleum production and processing. As part of our research aimed at studying the calculation capabilities of some thermodynamic models, the predictive features of the micellization model proposed by Victorov and Firoozabadi are investigated. While this model provides a good representation of asphaltene titration data with a few number of fitting parameters, our calculations show that the model is highly sensitive to the chosen value of the resins interaction energy parameter, Ur and to the interfacial tension between the asphaltene micellar core and the crude, 0. To relax this dependence, asphaltene titration data of two paraffin-oil mixtures are used to obtain a linear correlation for the dependence of above two parameters with the carbon-number of the paraffinic solvent. The results from the new correlation show an improved representation of asphaltene solubility data of the crude oils studied.

Introduction

Petroleum reservoir fluids are complex mixtures of hydrocarbons having both identifiable and non-identifiable classes of compounds. The high-molecular weight and ill-defined pat of these fluids is often referred to as asphaltenes and resins. Under certain thermodynamic conditions, both asphaltenes and resins can precipitate in the form of large aggregates. Deposition of these aggregates may plug the pipelines and reduce the oil flow in some production and transportation processes.

The molecular and physicochemical properties of asphaltenes and resins are no well understood. For this reason, a conceptual rather than a formal scientific definition for these two fractions of crude oil has been widely utilized. Asphaltenes are defined as the fraction of crude oil insoluble in normal alkanes such as n-pentane and hot n-heptane Petroleum resins, on the other hand, are insoluble in liquid propane but soluble in n-pentane. In 1940, it was recognized that asphaltenes exist in the crudes in a colloidal state. Both asphaltenes and resins are thought to associate with each other in the form of compound complexes called micelles. Electron-donor-acceptor or hydrogen bonding mechanisms are thought to play a role in the formation of the micelle. Currently, the micellar nature of these systems is a well-established fact.

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 theoretical investigations have been reported in literature. However, quantitative representation is still far from satisfactory.

Since asphaltenes have a broad distribution of molecular weights, most models proposed in the literature are based on polymer and regular solution theories for mixtures which have been able to describe only a few experimental results. Most of the proposed theories have not taken into account the various physical issues related to the observed micellar behavior. To the best of the author's knowledge, the only thermodynamiccolloid model that explicitly deals with the role of resins on the stability of asphaltenes is the model proposed by Leontaritis. However, this model does not explicitly address the concept of micelle formation behavior in crude oils and its stability to asphaltene precipitation.