Sébastien Simon

Solution properties of asphaltenes.

Ultracentrifugation has been used to produce asphaltene fractions of reduced polydispersity. The structure of these asphaltene fraction solutions has been investigated using viscosity and X-ray scattering (SAXS) measurements as a function of concentration. The relative viscosities of the solutions were found to be fraction-dependent: intrinsic viscosities, radii of gyration, and second viriel coefficients followed a power law with molar mass Mw. A flat disc model succeeded in describing scattering data but failed to take viscosity data into account. By contrast, a fractal model has been found to be consistent with dependence of all measured parameters. Asphaltene-in-toluene solutions were found to form nanometric mass fractal aggregates of fractal dimension 2.1, which in consequence trapped solvent. When, instead of concentration, effective volume fractions are used, the relative viscosities of fractions merge on a master curve which can be fitted by a hard sphere model. In addition, the reduced osmotic moduli deduced from scattering measurements of the different solutions, when expressed as a function of a concentration adimensional parameter, merge again on a master curve which is in accordance with the hard sphere behavior. The viscosities of solutions can be fully predicted from structure considerations if the ratio of hydrodynamic to gyration radius is taken as 0.6. This ratio is found consistent with the fractal description of the aggregates.

Determination of C80 tetra-acid content in calcium naphthenate deposits

A method is described which allows to determine the content of the so-called C(80) tetra-acid molecules (TA) in calcium naphthenate deposits. The method consists of four steps. Molecules present in the deposit are dissolved in a mixture of toluene and 2-butanol after an acidic treatment. All acid molecules are then selectively extracted and concentrated by a solid-phase extraction (SPE) method. After derivatization of acids into their naphthacyl esters to increase the sensitivity of the detection, TA is separated and detected by reversed-phase HPLC with UV detection. We have checked that all the steps are quantitative and the method appears selective. The TA content can be determined in presence of other naphthenic acids. Using this methodology we have determined the TA content in three calcium naphthenate deposits from different oil fields. It appears that these deposits have a similar TA concentration between 28 and 41% (w/w).

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.

Influence of Nonionic Surfactants on the Surface and Interfacial Film Properties of Asphaltenes Investigated by Langmuir Balance and Brewster Angle Microscopy

The interfacial film properties of asphaltenes and their mixtures with nonionic surfactants (polyoxyethylene nonylphenols) have been investigated using a Langmuir trough and a Brewster angle microscope (BAM). The effects of asphaltene concentration, surfactant/asphaltene ratio, and surfactant HLB (hydrophilic-lipophilic balance) have been studied at the air-water interface. The BAM image for asphaltenes show irregular domains with various structures even before compression, indicating preaggregation of asphaltenes in the spreading solution. The film morphology depends on both concentration and total amount of asphaltenes in the spreading solution. Lower proportions of surfactant (5 wt %) compared to asphaltenes increases the film compressibility and disperses the asphaltene domains; however, the behavior of the surface film is still dominated by asphaltenes. When the proportion of surfactant is increased to 50 wt %, surfactant molecules can occupy the interface top layer with multilayer formation by asphaltenes beneath this layer, and a relatively homogeneous film is observed by BAM. At the oil-water interface, surfactant was examined as both an inhibitor and a demulsifier for water-in-oil emulsions. Surfactants with intermediate HLB = 14.2 are most efficient in both cases preventing asphaltene adsorption at the interface by competitive adsorption and breaking the existing asphaltene film by displacement of asphaltenes from the interface.

Rheological Properties of Particle-Stabilized Emulsions

We have studied the rheological properties of fumed silica particle-stabilized emulsions. Two particles of different polarity were considered, the first more hydrophilic “Aerosil R7200,” the second more hydrophobic “Aerosil R972.” These particles flocculate and probably form a network at the investigated concentration. The flow curves of emulsions stabilized by a single type of particles exhibit yield stress, shear-thinning behavior and thixotropy. Moreover they display rheological features typical of gels. These features are attributed to strengthening of the particle network by droplets. Moreover the rheological properties of w/o emulsions stabilized by hydrophobic are similar to the ones of o/w emulsions stabilized by hydrophilic particles. The rheological properties of o/w emulsions stabilized by mixtures of hydrophilic and hydrophobic particles have then been studied by keeping the total particle concentration constant and varying the mass ratio between particles. The results show that when the hydrophobic particle concentration increases, the viscosity and stability of emulsions decrease establishing evidence that the network is weakened due to preferential orientation of hydrophobic particles towards the oil phase.

Sorption and Interfacial Rheology Study of Model Asphaltene Compounds

The sorption and rheological properties of an acidic polyaromatic compound (C5PeC11), which can be used to further our understanding of the behavior of asphaltenes, are determined experimentally. The results show that C5PeC11 exhibits the type of pH-dependent surface activity and interfacial shear rheology observed in C6-asphaltenes with a decrease in the interfacial tension concomitant with the elastic modulus when the pH increases. Surface pressure-area (Π-A) isotherms show evidence of aggregation behavior and π-π stacking at both the air/water and oil/water interfaces. Similarly, interactions between adsorbed C5PeC11 compounds are evidenced through desorption experiments at the oil/water interface. Contrary to indigenous asphaltenes, adsorption is reversible, but desorption is slower than for noninteracting species. The reversibility enables us to create layers reproducibly, whereas the presence of interactions between the compounds enables us to mimic the key aspects of interfacial activity in asphaltenes. Shear and dilatational rheology show that C5PeC11 forms a predominantly elastic film both at the liquid/air and the liquid/liquid interfaces. Furthermore, a soft glassy rheology model (SGR) fits the data obtained at the liquid/liquid interface. However, it is shown that the effective noise temperature determined from the SGR model for C5PeC11 is higher than for indigenous asphaltenes measured under similar conditions. Finally, from a colloidal and rheological standpoint, the results highlight the importance of adequately addressing the distinction between the material functions and true elasticity extracted from a shear measurement and the apparent elasticity measured in dilatational-pendant drop setups.

Separation profile of model water-in-oil emulsions followed by nuclear magnetic resonance (NMR) measurements: Application range and comparison with a multiple-light scattering based apparatus

The application range and validity of two new NMR sequences (hereafter called sequence 1 and sequence 2) for the study of water-in-oil emulsions (w/o) has been assessed using model emulsions and comparison with results obtained by a commercial apparatus (Turbiscan). These new NMR sequences allow to determine the brine profile i.e. the vertical variations of the dispersed phase content (brine) in the NMR tube. Measuring these parameters as a function of time allows to monitor the separation (sedimentation and coalescence rate) between oil and water. The results obtained on model water-in-oil emulsions with both NMR sequences are consistent and meaningful for both stable and coalescing emulsions and are similar, even if not strictly identical, to the ones obtained with the Turbiscan. It also appears that the second NMR sequence is faster (30s to obtain a profile compared with 3 min for the 1st one in the conditions used in this article) and has a broader application range. Indeed, for these two methods, the oil phase must have a viscosity higher or equal than values which is around 5 mPas for the sequence 2 and 20-25 mPas for the method 1.

Influence of Interfacial Rheological Properties on Stability of Asphaltene-Stabilized Emulsions

A series of oscillating droplet measurements have been performed on asphaltenes at the oil/water interface, in order to correlate the interfacial rheological behavior to their ability to stabilize emulsions. In the concentration sweep, the elastic modulus goes through a maximum around an asphaltene concentration of 0.05–0.10 g/l. This behavior was not in good correspondence with emulsion stability, which increased consistently from low to high concentrations. The decrease above 0.10 g/l was most likely an effect of diffusion of asphaltenes in the bulk to the interface, which became more significant at higher bulk concentrations. The rheology data as a function of concentration has been fitted to Butlers surface equation of state and the Lucassen–van den Tempel model. A decent correlation was found between emulsion stability and elasticity for both the effect of solvent aromaticity and pH. The elastic modulus displayed a gradual increase when xylene was mixed with heptane as the solvent, as was seen with emulsion stability. This was not caused by a significant increase of the adsorbed amount of asphaltene at the interface, as shown by a quartz crystal microbalance (QCM), but a more efficient reorganization of the already adsorbed asphaltenes. The ability asphaltenes displayed in stabilizing emulsions was significantly increased at both low and high pH, according to a previous study. The elastic modulus, on the other hand, only showed a very weak increase at pH 2, but a better correlation with emulsion stability above pH 8. From this it would appear that the dissociation of acid groups in the asphaltene structure at high pH has a bigger impact on the interfacial activity than the protonation of bases at low pH, while their effect on emulsion stability was the same.

The chemistry of tetrameric acids in petroleum

This article reviews the properties of a novel class of molecules: the tetrameric acids. These molecules have brought a large interest in petroleum science since the discovery of the family of molecules named ARN in 2004. ARN, which is naturally present in oil, is responsible, by reaction with calcium ion, of the formation of calcium naphthenate deposits; organic deposits that cause irregularities in crude oil production and processing. In order to study the properties of ARN, a model tetrameric acid molecule mimicking some of its properties named BP-10 has been developed in 2008 by Nordgård and Sjöblom and has been extensively used since then. After presenting the experimental techniques used to study the tetrameric acids, this review describes in detail the structure, preparation, detection and the bulk and interfacial properties of tetrameric acids ARN and BP-10. Finally the prediction of the operational problems with calcium naphthenate precipitation in new fields is discussed.

Interfacial Shear Rheology of Calcium Naphthenate at the Oil/Water Interface and the Influence of pH, Calcium, and in Presence of a Model Monoacid

In this article, the interfacial shear rheological properties of calcium naphthenate with a model tetraacid at the chloroform/xylene-water interface has been investigated as a function of aqueous pH, calcium concentration and monoacid concentration. The experiments are carried out using an interfacial rheology system with an electro commutated motor, direct strain oscillation and a biconical bob geometry. The model tetraacid used, BP10, has previously been shown to have similar bulk and interfacial properties as a narrow group of tetraprotic, so-called Arn acids, and these acids are known to be responsible for formation of hard deposits during oil recovery. A great increase in the elastic modulus was observed around pH 6.2, which is in agreement with observations from oil fields with calcium naphthenate deposition problems. The gel strength and elastic nature is highest around the gelation onset, believed to be due to a bilayer-like conformation of the tetraacid generating a densely packed interface with high cross-linking density and possible film growth. As a function of calcium concentration, both a reduction of the gel strength and slower gel formation was observed when decreasing the calcium concentration from 10 to 4 mM. Myristic acid, a linear C14 fatty acid, was employed as a model for indigenous monoacids and the influence onto the viscoelastic properties of the Ca2+-TA film was studied as a function of myristic acid concentration at pH 8.0 and 6.5. A great reduction of both the gel strength and elasticity was obvious in the range of 100 to 1000 higher monoacid than tetraacid concentration. This is however typical indigenous acid concentrations for an acidic crude oil, and may indicate that indigenous monoacids have the ability to act as indigenous inhibitors towards formation of calcium naphthenate. This could explain why some Arn-containing acidic crude oils have deposition problems while others do not. Moreover, all parameters should be taken into account when predicting the deposition risk for a given crude oil, such as concentrations of Ca2+, Arn, monoacids and other indigenous acids.