Heavy Petroleum Composition. 3. Asphaltene Aggregation

Abstract

Molecular characterization of asphaltenes by conventional analytical techniques is a challenge because of their compositional complexity, high heteroatom content, and asphaltene aggregate formation at low concentrations. Thus, most common characterization techniques rely on bulk properties or solution-phase behavior (solubility). Proposed over 20 years ago, the Boduszynski model proposes a continuous progression in petroleum composition (molecular weight, structure, and heteroatom content) as a function of the atmospheric equivalent boiling point. Although exhaustive detailed compositional analysis of petroleum distillates validates the continuum model, the available compositional data from asphaltene fractions supports the extension of the continuum model into the nondistillables only indirectly. Asphaltenes, defined by their insolubility in alkane solvents, accumulate in high-boiling fractions and form stable aggregate structures at low parts per billion (ppb) concentrations, far below the concentration required for most mass analyzers. Here, we present direct mass spectral detection of stable asphaltene aggregates at lower concentrations than previously published and observe the onset of asphaltene nanoaggregate formation by time-of-flight mass spectrometry (TOF−MS). We conclude that a fraction of asphaltenes must be present as nanoaggregates (not monomers) in all atmospheric pressure and laser-based ionization methods. Thus, those methods access a subset of the asphaltene continuum.

Diffusion Model Coupled with the Flory–Huggins–Zuo Equation of State and Yen–Mullins Model Accounts for Large Viscosity and Asphaltene Variations in a Reservoir Undergoing Active Biodegradation

“…Mass fraction of asphaltenes is computed by Eq. (11). The calculated asphaltene mass percentage with depth and time is shown in Figure 14.…”

Molecular Weight and Density Distributions of Asphaltenes from Crude Oils

“…15,22 Recent time-of-flight data are consistent with this range of molecular weights. 24 On the other hand, Eyssautier et al 25 observed a range of aggregates an order of magnitude larger than the VPO data based on a combination of ultracentrifugation and X-ray scattering measurements. This difference can only partially be attributed to the difference between molar averages (VPO) and mass averages (scattering) but could be explained by flocculation of the aggregates, which would not be detected by VPO measurements.…”

Structural Comparison of Asphaltenes of Different Origins Using Multi-stage Tandem Mass Spectrometry

“…1,[16][17][18][19][20][21][22][23][24][25][26][27] The disparity of these measurements has been suggested to result from the tendency of asphaltenes to aggregate. 27,28 In the past decades, two structural models have been debated for asphaltene molecules: the island model and the archipelago model. 16,[19][20][21][22]29 The island model (sometimes referred to as the continental model 30 ) has only one aromatic core with peripheral alkyl chains, whereas the archipelago model has multiple aromatic cores that are bridged by alkyl chains and may also contain peripheral alkyl chains.…”