Asphaltene fractionation based on adsorption onto calcium carbonate: Part 3. Effect of asphaltenes on wax crystallization

Interfacial Dilational Rheology of Molecular Films in DC Electric Fields

We present a novel technique to measure the interfacial dilational rheology of a molecular film adsorbed at a water-oil interface in a direct-current (dc) electric field. A film of a highly polar subfraction of asphaltenes was allowed to adsorb at a water drop interface, surrounded by an organic phase and subjected to a dc electric field. The measurements involved calculations of the dynamic interfacial tension (IFT), while the drop was sinusoidally oscillated, using our in-house axisymmetric drop shape analysis (ADSA) algorithm adapted for electric fields. The amplitude of the IFT waveform over equilibrium IFT and the phase difference from the applied area oscillations were used in the estimation of surface moduli. The asphaltene films were found to become more elastic on increasing bulk concentrations and electric field strengths. However, the effect was not monotonous and observed to be governed by combinations of these parameters. The Lucassen-van den Tempel (LVDT) model was used to further elucidate the experimentally obtained interfacial dilational moduli.

Toward molecular characterization of asphaltene from different origins under different conditions by means of FT-IR spectroscopy

Asphaltene is one of the polar and heavy fractions of crude oil that is complex from a molecular perspective. For this reason, the interaction between asphaltene molecules and the surface, as well as the interaction of asphaltene with chemicals such as amphiphile, are not well identified. Fourier-transform infrared spectroscopy (FTIR) is a useful tool for identifying the functional groups of molecules, as well as intra-molecular and inter-molecular bonds. Through reviewing previous studies, here the peaks in an FTIR spectrum of an asphaltene molecule were divided into polar, aromatic and aliphatic groups and discussed using quantitative indices. Then, the difference in the FTIR spectrum of asphaltene with wax and resin was addressed according to molecular structure. The effect of common impurities such as moisture, CO₂ and saturated and aromatic compounds of crude oil in asphaltene on the FTIR spectrum is assessed. Moreover, the application of the FTIR spectrum of asphaltene is used to determine the API value of crude oil, the asphaltene onset is given. In addition, possible changes in the FTIR spectra of asphaltene are investigated by various processes such as pyrolysis, microwave and ultrasonic radiation. Also, asphaltene subfractions is also one of the best methods to better understand asphaltene components. This study examines the FTIR spectrum of asphaltene subfractions from conventional methods and examines the spectral properties, which in many cases can be useful to researchers working in this field.

In situ observation of microscopic motions and the structure dynamic transformation of wax crystals in waxy crude oil subjected to shear

The high-efficient development, storage and transportation of waxy crude oil has a significant meaning for stable supply of petroleum energy.

Assessing the Interfacial Activity of Insoluble Asphaltene Layers: Interfacial Rheology versus Interfacial Tension

Asphaltenes have been suggested to play an important role in the remarkable stability of some water-in-crude oil emulsions, although the precise mechanisms by which they act are not yet fully understood. Being one of the more polar fractions in crude oils, asphaltenes are surface active and strongly adsorb at the oil/water interface, and as the interface becomes densely packed, solid-like mechanical properties emerge, which influence many typical interfacial experiments. The present work focuses on purposefully measuring the rheology in the limit of an insoluble, spread Langmuir monolayer in the absence of adsorption/desorption phenomena. Moreover, the changes in surface tension are deconvoluted from the purely mechanical contribution to the surface stress by experiments with precise interfacial kinematics. Compression "isotherms" are combined with the measurement of both shear and dilatational rheological properties to evaluate the relative contributions of mechanical versus thermodynamic aspects, i.e., to evaluate the "interfacial rheological" versus the standard interfacial activity. The experimental results suggest that asphaltene nanoaggregates are not very efficient in lowering interfacial tension but rather impart significant mechanical stresses. Interestingly, physical aging effects are not observed in the spread layers, contrary to results for adsorbed layers. By further studying asphaltene fractions of different polarity, we investigate whether mere packing effects or strong interactions determine the mechanical response of the dense asphaltene systems as either soft glassy or gel-like responses have been reported. The compressional and rheological data reflect the dense packing, and the behavior is captured well by the soft glassy rheology model, but a more complicated multilayer structure may develop as coverage is increased. Potential implications of the experimental observations on these model and insoluble interfaces for water-in-crude oil emulsion stability are briefly discussed.

Fractionation and Characterization of Petroleum Asphaltene: Focus on Metalopetroleomics

Asphaltenes, as the heaviest and most polar fraction of petroleum, have been characterized by various analytical techniques. A variety of fractionation methods have been carried out to separate asphaltenes into multiple subfractions for further investigation, and some of them have important reference significance. The goal of the current review article is to offer insight into the multitudinous analytical techniques and fractionation methods of asphaltene analysis, following an introduction with regard to the morphologies of metals and heteroatoms in asphaltenes, as well their functions on asphaltene aggregation. Learned lessons and suggestions on possible future work conclude the present review article.

Experimental Evidence of Enhanced Adsorption Dynamics at Liquid-Liquid Interfaces under an Electric Field

In this work, we investigate the adsorption of surface-active compounds at the water-oil interface subjected to an electric field. A fluid system comprising a pendent water drop surrounded by an asphaltene-rich organic phase is exposed to a DC uniform electric field. Two subfractions of asphaltenes having contrasting affinities to the water-oil interface are used as surface-active compounds. The microscopic changes in the drop shape, as a result of asphaltene adsorption, are captured and the drop profiles are analyzed using our in-house code for axisymmetric drop shape analysis (ADSA) under an electric field. The estimates of dynamic interfacial tension under different strengths of the field (E₀) and concentrations of the asphaltene subfractions (C) are used to calculate adsorption dynamics and surface excess. The experimental observations and careful analyses of the data suggest that the externally applied electric field significantly stimulates the mass-transfer rate at the liquid-liquid interface. The enhancement in mass transport at the water-oil interface can be attributed to the axisymmetric electrohydrodynamic fluid flows generated on either side of the interface. The boost in mass transport is evident from the growing decay in equilibrium interfacial tension (γ_eq) and increased surface excess (Γ_eq) upon increasing strength of the applied electric field. The mass-transfer intensification does not increase monotonously with the electric field strength above an optimum E₀, which is in agreement with the previous theoretical studies in the literature. However, these first explicit experimental measurements of adsorption at an interface under an electric field suggest that the optimum E₀ is determined by characteristics of the surface-active molecules.

New Approach for the In Situ Microscopic Observation of Wax Crystals in Waxy Crude Oil during Quiescent and Dynamic Cooling

The purpose of this attempt is to present a new investigation approach to achieve the in situ observation of the microscopic structure and morphology of wax crystals under quiescent and shear conditions. The rheo-microscopy simultaneous measurement system of a rheometer is employed to ensure in situ observation. A multi-angle composite light source is created to obtain a high-quality image. It is demonstrated that the new approach can achieve a better identification and distinction of wax crystals, as well as the outstanding wax boundary delineation. Based on this, some new findings related to the microscopic structure and morphology of wax crystals are elaborated. Additionally, the in situ observations of wax crystals under dynamic cooling at different shear rates are performed. It is noticed from the obtained results that wax crystals and their aggregates exhibit significant stereoscopic structural characters, because of growth of wax crystals and their overlap in 3-D space. Shear can change the morphology of single wax crystals, but hardly destroy the structure or growth. The increase of the shear rate can induce the deformation propensity of wax crystals to flow field. The effect of shear on aggregation of wax crystals depends on the chemical composition and inherent structural properties. Normally, a low rate of shear can promote aggregation, and result in a complicated stereo-structure. Upon increasing the shear rate, two opposite effects simultaneously occur including promotion and inhibition of aggregation. As the shear rate further increases, the destructive effect increases and gradually plays a leading role, causing the wax crystal aggregates exhibit a smaller size and a weaker stereo structure.

A critical update of experimental techniques of bulk and interfacial components for fluid characterization with relevance to well fluid processing and transport

The present article reviews techniques to address central flow assurance and separation issues. It is our purpose to update the need for extended information in order to draw adequate conclusions about the reason for irregularities in production and how this is related to individual components or fractions in the crude oil. Our intention is to show that the mass related analysis (such as SARA, MS etc.) are insufficient for a validation of the early stage predictions concerning irregularities. The review introduces a set of new characterization and fractionation techniques such as interfacial rheology, SANS, and NMR, where the central theme is the functionality of the components and not just their mass. Two crude oil-related issues are addressed: Wax precipitation and deposition, and crude oil/water resolution. First, bulk techniques to characterize wax precipitation are reviewed. The influence of the chemistry of other crude oil components (asphaltenes) and wax inhibitor on the precipitation is highlighted. Secondly, in aqueous systems, interfacial w/o conditions are important for the stability of dispersed systems. Asphaltenes have a crucial and important role in the stability of crude oil emulsions. Here special attention is directed to properties like interfacial viscosity and elasticity as well as the adsorbed layer structure determination. Small molecular changes in these properties will have dramatic influence on the stability of the heterogeneous systems. A good example is inhibitor functionality.