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Asphaltenes are the heaviest and the most polar fraction of crude oil and are defined as a solubility class (typically soluble in toluene but insoluble in n-alkanes like n-heptane).Precipitation and deposition of asphaltenes during production, processing and transportation of oil is a major challenge faced by oil industry. Over the last thirty years, a number of different models have been proposed for predicting the onset of asphaltene precipitation and also the amount of precipitated asphaltenes. This article reviews the different models that have been proposed for predicting asphaltene precipitation either at atmospheric pressure or under depressurization. A brief summary of the different modeling approaches is presented followed by description of work done by different research groups. Our focus will be on the description of the basic assumptions underlying different models and also the ability/performance of the model to match the experimental data. Finally, a comparison of models is presented and discussed along with suggestions for improvement. Keywords:Asphaltene precipitation, Thermodynamic models, Solubility models, Equation of state models, Colloidal models INTRODUCTIONAsphaltenes are considered to represent the most polar and the heaviest components in crude oil. 1 They are a solubility class and defined as the fraction that is insoluble in n-alkane (like n-heptane) but soluble in aromatic solvents (like toluene). [2][3] The precipitation and deposition of asphaltenes during production and transport of crude oil is recognized as a serious problem in the oil industry since it can lead to formation damage, plugging of wellbores and production facilities. [4][5] Asphaltene precipitation can be encountered during primary recovery of oil as well as during enhanced oil recovery operations like miscible flooding with CO 2 or natural gas. 6The precipitation of asphaltenes occurs due to a number of reasons. These include changes in pressure, temperature, chemical composition of crude oils, acid stimulation, mixing of oil with diluents, other oil and gas components like CO 2 . 4 Heavy crudes usually have a lower tendency to give asphaltene deposition problems inspite of their higher asphaltene content.This observation is normally attributed to their higher resin content and lower saturate content. 7Asphaltenes are generally assumed to be composed of fused ring aromaticity, small aliphatic side chains and heteroatom functional groups. 8 The molecular weight of asphaltenes has been a source of controversy for a very long time. 9 It is now recognized to be close to 750 g/mol with a factor of 2 in the width of the molecular weight distribution. Solubility ApproachThe solubility models are most commonly used for predicting asphaltene precipitation. These models make use of the concept of solubility parameter and assume that petroleum crude Models based on Solution theory and Flory-Huggins theory assume that asphaltenes have homogenous structure and properties while models based on Scott-Magat theory assum...

The self-association and aggregation properties of asphaltene sub-fractions obtained by adsorption onto CaCO3

The influence of micro-crystalline wax addition upon the rheological properties of model wax-oil gels is investigated. Addition of less than 1 wt% micro-crystalline wax to a model oil consisting of 5 wt% macro-crystalline wax in dodecane shows no significant impact on the WAT and gelation temperature. Beyond 1 wt% added micro-crystalline wax, increases in WAT and gelation temperature are observed, and are attributable to the higher crystallization temperature of the micro-crystalline wax. The effect of micro-crystalline wax addition upon the WAT and gelation temperature are shown to be attributed to merely overlapping compositions of macro-and microcrystalline wax. However, a substantive effect of micro-crystalline wax addition is observed on the yield stress. Addition of 0.13 wt% micro-crystalline wax reduces the yield stress of waxy oil model from 238.0 to 22.5 Pa. Addition of 0.5 wt% micro-crystalline wax decreases the yield stress to 5.4 Pa, which is close in value to the yield stress of neat 5 wt% micro-crystalline wax gel.2 Microscopic images reveal two mechanisms leading to formation of a weak mixed wax gels. At low to medium addition of micro-crystalline wax, micro-crystalline crystallites formed during cooling provide nucleation sites for subsequent precipitation of macro-crystalline wax. Macrocrystalline wax crystals formed in contact with micro-crystalline crystallites are smaller in size and the growth is localized, in comparison to neat macro-crystalline wax. Modified macro-crystalline wax precipitation leads to uneven dispersion of the macro-crystalline crystals in the liquid phase.At high concentration, micro-crystals form throughout the sample prior to precipitation of macrocrystalline wax. Hence, only small and discrete space remains for macro-crystalline crystals to grow, forming small crystals. Interlocking among macro-crystals is spatially hindered by the presence of micro-crystalline crystallites. In this condition, the system completely behaves as a micro-crystalline gel. This investigation provides a plausible mechanistic account for the known gel weakening activity of micro-crystalline wax.

Adsorption of an acidic polyaromatic asphaltene model compound (C5PeC11) and indigenous C 6 -asphaltenes onto the liquid-solid surface is studied. The model compound C5PeC11 exhibits a similar type of adsorption with a plateau adsorbed amount as C 6 -asphaltenes onto three surfaces (silica, calcite and stainless steel). The model compound