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The aggregation of asphaltenes by pressure depletion both in a live crude oil and model systems of asphaltenes in toluene/pentane solvents is studied. Near-infrared spectroscopy utilizing a high-pressure NIR cell with a maximum operating pressure of 325 bar is used to study the onset of asphaltene aggregation. The NIR spectra are subject to a principal component analysis (PCA) in order to detect the asphaltene aggregation onset pressure. The effect of fluid compressibility on the NIR spectra is also demonstrated. The aggregation behavior of asphaltenes in model systems is shown to resemble the aggregation behavior for the crude oil. However, while the asphaltene aggregation in the crude oil is more or less completely reversible with repressurization, indications of only a partial redissolution are seen in the model systems. The kinetics of the redissolution is quite slow. A time of 72 h to equilibrate at the original pressure of 300 bar was required to redissolve the asphaltene aggregates formed within the crude oil by depressurization of the sample. Near-infrared spectroscopy in combination with principal component analysis is shown to be an efficient tool in detecting both bubble points and asphaltene aggregation onset pressures in high-pressure systems.

Asphaltenes were precipitated into two fractions using a two-step precipitation procedure. The first fraction was obtained by mixing 3:1volumes of n-pentane/crude oil followed by filtration. In the following step the second fraction was precipitated out from the filtrate using 18:1 volumes of n-pentane/crude oil. Whole asphaltenes were precipitated using 40:1 pentane-to-crude oil ratio. Three crude oils were used and the asphaltene fractions obtained were characterized with regard to onset of precipitation, interfacial tension and radius of gyration. The second fraction was more soluble at increasing heptane-to-toluene ratios, more interfacially active and showed different organization properties at the interface between oil and water. Small angle neutron scattering showed that the second fraction formed aggregates with lower radius of gyration. The results show that asphaltenes consist of fractions with different solvent properties and indicates that asphaltenes should be looked at as more than one solubility class.

In a previous study (10.1021/ef060311g), a two-step precipitation procedure for asphaltenes from three crude oils (WA, NS-A, and NS-B) was reported. Crude oils were diluted 3:1 with n-pentane, and precipitated asphaltenes were filtrated off (first fraction). A second fraction consisting of asphaltenes still present in the crude oil was precipitated by further dilution of 18:1 n-pentane/crude oil. In the previous work, interfacial tension, aggregation size, and onset of precipitation were investigated and shown. In the current work, elemental analysis indicated that the first fractions contain relatively more heteroatoms than the second fractions and whole asphaltenes. Fourier transform infrared (FTIR) spectroscopy, proton and carbon nuclear magnetic resonance (NMR) spectroscopy, and NMR–distortionless enhancement by polarization transfer (DEPT) indicated that the less soluble fractions WA and NS-B were more aromatic and had a more polar aromatic core and a larger aromatic core consisting of more rings. Furthermore, there were indications that the more soluble fractions contained more branched aliphatic side chains with a larger degree of hydroxylic and carboxylic groups. Laser desorption ionization–mass spectroscopy (LDI–MS) molecular-weight determination indicated that the less soluble asphaltene samples had a higher average molecular weight compared to the more soluble fractions and the whole asphaltenes. The results could help explain the differences in interfacial tension and solvent properties that were reported previously.

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A near-infrared (NIR) scattering technique is used to measure the wax appearance temperature of several petroleum fluids under nonquiescent conditions. Within the Rayleigh scattering limit, NIR attenuation measurements at a wavelength of 1100 nm can theoretically detect wax crystallites <55 nm in size. In comparison, commonly used cross-polarized microscopy (CPM) observations are limited by a resolution of ∼0.5 μm. The NIR scattering technique readily allows for application of nonquiescent and thermal equilibrium conditions, effectively accelerating the crystal growth process and overcoming subcooling effects. Wax appearance temperature measurements are demonstrated using a waxy crude oil, a waxy gas condensate fluid, and model fluids consisting of macrocrystalline or microcrystalline paraffin wax dissolved in dodecane. Light scattering by wax crystals is evidenced by a baseline elevation in the measured NIR attenuation spectra, with higher shifts observed at lower wavelengths. For opaque crude oils, WAT determination requires delineation of the radiation attenuation originating from the precipitated wax crystals and the mother crude oil. The NIR scattering technique yields WAT values similar to the classical CPM technique. In addition, NIR scattering is shown to be an appropriate technique for measuring the time necessary to melt paraffin wax solids from waxy petroleum fluids at warm temperatures and under nonquiescent processing conditions.