Sreedhar Subramanian

Asphaltene Precipitation Models: A Review

Asphaltenes are the heaviest and the most polar fractions 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 are major challenges faced by the oil industry. Over the last 30 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 the 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. GRAPHICAL ABSTRACT

Microcalorimetry Study of the Adsorption of Asphaltenes and Asphaltene Model Compounds at the Liquid-Solid Surface.

The adsorption of an acidic polyaromatic asphaltene model compound (C5PeC11) and indigenous C6-asphaltenes onto the liquid-solid surface is studied. Model compound C5PeC11 exhibits a similar type of adsorption with a plateau adsorbed amount as C6-asphaltenes onto three surfaces (silica, calcite, and stainless steel). Model compound BisAC11, with aliphatic end groups and no acidic functionality, does not adsorb at the liquid-silica surface, indicating the importance of polar interactions on adsorption. The values of the adsorption enthalpy characterized by the ΔHz parameter (the enthalpy at zero coverage) indicate that the type of adsorption and the driving force depend on the surface, a key feature when discussing asphaltene deposition. The adsorption of C5PeC11 onto silica is shown to be driven primarily by H bonding (ΔHz = -34.9 kJ/mol), unlike adsorption onto calcite where polar van der Waals and acidic/basic interactions are thought to be predominant (ΔHz = -23.5 kJ/mol). Interactions between C5PeC11 and stainless steel are found to be weak (ΔHz = -7.7 kJ/mol). Comparing C6-asphaltenes and their esterified counterpart shows that adsorption at the liquid-solid surface is not influenced by the formation of H bonds. This was evidenced by the similar adsorbed amounts obtained. Finally, C5PeC11 captures, to a certain extent, the adsorption interactions of asphaltenes present at the calcite-oil and stainless steel-oil surfaces.

Solvent desorption of asphaltenes from solid surfaces

ABSTRACT The main goal of this paper is to compare the ability of different organic solvents to desorb asphaltenes from stainless steel surfaces. The asphaltenes were extracted from a North Sea crude oil by precipitation. The organic solvents are characterized based on their Hansen solubility parameters (HSPs). The adsorption of asphaltenes was followed by means of a Quartz Crystal Microbalance with Dissipation (QCM-D). The asphaltene desorption efficiency of the solvents tested varied between 20% and 70%, with pyridine as the most efficient solvent. Carbon disulfide was found to be a poor desorption solvent, indicating the importance of solvent polarity. A simple model based on the HSPs seemed to give a good quantitative explanation of experimental desorption experiments. GRAPHICAL ABSTRACT

Interfacial dilational rheology properties of films formed at the oil/water interface by reaction between tetrameric acid and calcium ion

ABSTRACT This article aims to determine the applicability of interfacial dilational rheology to study the formation of viscoelastic film at the oil/water interface by reaction between tetrameric acids ARN and calcium ions, and to determine the influence of asphaltenes and naphthenic acids (NA) on this film. It was first found that the formation of viscoelastic film by reaction between ARN and calcium ions is easily observed by dilational rheology: Significantly high values of E′ (130 mN/m) were measured for this system at low ARN concentration (10 µM). These values are at least 5 to 10 times higher than values obtained for ARN without Ca2+ or other crude oil components such as asphaltenes and naphthenic acids. The influence of asphaltenes and NA on the viscoelastic film formation has been studied. When asphaltenes or NA are present, the interfacial viscoelastic film is weakened: There is a gradual decrease of E′ and E″ when the asphaltenes or NA concentration increases. These two components can therefore inhibit the ARN/Ca2+ film formation. This decrease is similar to the one previously observed by shear rheology. Several explanations are proposed. GRAPHICAL ABSTRACT

Interaction between asphaltenes and fatty-alkylamine inhibitor in bulk solution

ABSTRACT In the present work, the mechanism of interaction between asphaltenes and a commercial fatty-alkylamine inhibitor was investigated by a combination of techniques. The “macro” properties, like the asphaltene precipitation onset and the amount of asphaltenes precipitated, were measured by near-infrared (NIR) and UV-vis spectroscopy, respectively, while the interaction enthalpy between asphaltenes and inhibitor was measured by isothermal titration calorimetry (ITC). Asphaltenes subfractions and derivatives were also used to identify the mechanism. ITC indicated that only a small fraction (∼6%) of asphaltenes interacts strongly with the inhibitor. The proportion of interacting species was found to be higher in irreversibly adsorbed asphaltenes subfraction. These 6% are mostly composed of acidic asphaltenes, as indicated by measurements involving ester asphaltenes. However, the measurement of precipitation onset and amounts precipitated for whole and ester asphaltenes indicated that the acid–base interaction was not the main interaction responsible for the inhibitory action. Other type(s) of interaction is/are responsible for the inhibition properties of the inhibitor, which are not detected by ITC. The nature of other interactions is not known for the moment, but it was shown that irreversibly adsorbed asphaltene fraction contains a higher concentration of the functionality (ies) responsible for the “other” type of interaction. GRAPHICAL ABSTRACT