ASPHALTENES ADSORPTION BY QUARTZ AND FELDSPAR

Model molecules mimicking asphaltenes

Asphalthenes are typically defined as the fraction of petroleum insoluble in n-alkanes (typically heptane, but also hexane or pentane) but soluble in toluene. This fraction causes problems of emulsion formation and deposition/precipitation during crude oil production, processing and transport. From the definition it follows that asphaltenes are not a homogeneous fraction but is composed of molecules polydisperse in molecular weight, structure and functionalities. Their complexity makes the understanding of their properties difficult. Proper model molecules with well-defined structures which can resemble the properties of real asphaltenes can help to improve this understanding. Over the last ten years different research groups have proposed different asphaltene model molecules and studied them to determine how well they can mimic the properties of asphaltenes and determine the mechanisms behind the properties of asphaltenes. This article reviews the properties of the different classes of model compounds proposed and present their properties by comparison with fractionated asphaltenes. After presenting the interest of developing model asphaltenes, the composition and properties of asphaltenes are presented, followed by the presentation of approaches and accomplishments of different schools working on asphaltene model compounds. The presentation of bulk and interfacial properties of perylene-based model asphaltene compounds developed by Sjöblom et al. is the subject of the next part. Finally the emulsion-stabilization properties of fractionated asphaltenes and model asphaltene compounds is presented and discussed.

Understanding mechanisms of asphaltene adsorption from organic solvent on mica

The adsorption process of asphaltene onto molecularly smooth mica surfaces from toluene solutions of various concentrations (0.01-1 wt %) was studied using a surface forces apparatus (SFA). Adsorption of asphaltenes onto mica was found to be highly dependent on adsorption time and asphaltene concentration of the solution. The adsorption of asphaltenes led to an attractive bridging force between the mica surfaces in asphaltene solution. The adsorption process was identified as being controlled by the diffusion of asphaltenes from the bulk solution to the mica surface with a diffusion coefficient on the order of 10(-10) m(2)/s at room temperature, depending on the asphaltene bulk concentration. This diffusion coefficient corresponds to a hydrodynamic molecular radius of approximately 0.5 nm, indicating that asphaltene diffuses to mica surfaces as individual molecules at very low concentration (e.g., 0.01 wt %). Atomic force microscopy images of the adsorbed asphaltenes on mica support the results of the SFA force measurements. The results from the SFA force measurements provide valuable insights into the molecular interactions (e.g., steric repulsion and bridging attraction as a function of distance) of asphaltenes in organic media and hence their roles in crude oil and bitumen production.

Asphaltene adsorption mechanisms on the local scale probed by neutron reflectivity: transition from monolayer to multilayer growth above the flocculation threshold

We present here a study of the adsorption of asphaltenes on hydrophilic and hydrophobic solid surfaces by coupling measurements of adsorption isotherms on the macroscopic scale on silica powder with measurements of the structure of the adsorbed asphaltene layer on the microscopic scale obtained by neutron reflectivity on flat silicon wafers. Under good-solvent conditions, if adsorption isotherms reveal that the interaction potential between asphaltenes and the surface is slightly higher for the hydrophilic surface than for the hydrophobic one, then the mechanism of adsorption is similar in both cases because all samples exhibit the same local structure of the adsorbed asphaltene layer: it is a solvated monolayer with thickness of the same order of magnitude as the size of the asphaltene aggregates in the bulk. The surface excess, gamma, is thus always of the same order (approximately 3 mg/m2). The adsorption process induces a densification of the aggregates at the interface because the adsorbed monolayer is much less solvated than aggregates in bulk solution. When a bad solvent is progressively added, the asphaltene adsorbed layer keeps its monolayer structure as long as the bulk flocculation threshold is not reached. Above the threshold, the size of the asphaltene adsorbed layer grows and forms a multilayer structure.

Adsorption of Asphaltenes and Its Effect on Oil Production

Summary

Asphaltenes show moderate capacity to reduce oil/water interfacial tension (IFT) and/or to stabilize emulsions. However, in the presence of other surfactants and fine particles presenting intermediate presence of other surfactants and fine particles presenting intermediate wettability, these compounds enhance emulsion formation and stability, At the solid/solution interface, asphaltenes adsorb up to a saturation value of 2 to 3 mg/g and modify the mineral wettability. At the oil/gas interface, the asphaltic fractions adsorb as single molecules or as liquid crystalline phases, causing foaming problems.

Introduction

Asphaltenes and resins are the polar fractions of crude oil and bitumen that can be precipitated by the addition of low-molecular-weight alkanes. These fractions are formed by molecules containing paraffinic, naphtenic, and aromatic hydrocarbons and functional groups containing oxygen, nitrogen, and sulfur. Owing to this amphiphilic character, these compounds, also denominated asphaltic material, possess the ability to aggregate and to adsorb to interfaces. These processes normally are associated with such production problems as sludge formation, foaming in the primary treatment, and wettability changes that may be detrimental to oil production. In this paper, we investigate the behavior of asphaltic materials at the liquid/liquid, solid/liquid and liquid/gas interfaces. We isolated asphaltenes and resins from the Carmopolis field, an onshore field in the Sergipe-Alagoas basin, and from the Cherne field, an offshore field in the Campos basin. Carmopolis crude presents a basic sediments and water (BS&W) of about 80% and forms very stable emulsion. Cherne crude, however, has a BS&W of 10% but precipitates asphaltic sludge during acid stimulation operations. Some preliminary studies also were carried out on crude oil foam with samples from the Pampo field (offshore in the Campos basin) that displays a natural tendency to foaming.