Asphaltenes are typically defined by their solubility in benzene and insolubility in pentane or heptane. They are believed to exist in petroleum crude oil as a colloidal suspension, stabilized by surface-adsorbed resins. Their normal equilibrium under reservoir conditions may be disrupted during production by pressure reduction, crude oil chemical composition changes, introduction of miscible gases and liquids, and mixing with diluents and other oils, as well as by acid stimulation, hot oiling, and other oilfield operations. Electrospray ionization preferentially ionizes polar N-, S-, and O-containing compounds, and its combination with ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry makes a powerful tool for the compositional analysis of petroleum-derived materials such as asphaltenes. In this work, we compare the compositional differences between heptane-precipitated asphaltenes and asphaltenes collected by live oil depressurization. Negative-and positive-ion electrospray yield the acidic and basic species, respectively. We find that the heptaneprecipitated asphaltenes contain higher double bond equivalents (number of rings plus double bonds) compared to the asphaltenes induced by pressure drop. On the other hand, the pressure-drop product exhibits a higher abundance of species containing sulfur. Thus, the solubility criterion for asphaltenes defines a significantly different chemical composition than the (more field-relevant) pressure-drop criterion.

Asphaltenes are known to deposit in both the petroleum recovery and topside refining processes. The asphaltenes are normally in equilibrium under reservoir conditions. As crude oil is produced, that equilibrium may be disrupted by a number of factors including pressure reductions, crude oil chemical composition changes, introduction of miscible gases and liquids, mixing with diluents and other oils, and, during acid stimulation, hot oiling and other oilfield operations. Electrospray ionization preferentially ionizes polar components of a sample matrix without prechromatographic separation, and its coupling with Fourier transform ion cyclotron resonance mass spectrometry makes a powerful analytical tool for the detailed compositional analysis of petroleum-derived materials such as deposit asphaltenes. In this work, we compare two geographically different crude oils to their corresponding asphaltene deposits. Negative-ion electrospray is preferred because of the acidic nature of asphaltenes. We find that the crude oil deposits contain higher aromatic character (more unsaturated) and are enriched in oxygenated species as well as multiple heteroatom classes compared to their crude oil counterpart. Such a detailed compositional comparison of the deposit asphaltenes to its crude oil counterpart should help to develop more cost-effective methods to control the deposition of asphaltenes and to increase the overall efficiency of the processing fields with asphaltene problems.

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Structural and compositional characterization of asphaltenes that were extracted from unstable crude oils, stable crude oils, and organic solid deposits was performed to elucidate their similarities and differences. A fractionation technique that divided the asphaltenes into different subfractions, based on polarity, was used to characterize these asphaltene samples. The parameters affecting the stability of these asphaltene subfractions were elucidated. The asphaltenes that were extracted from unstable crude oils and from solid deposits contained substantially greater portions of the higher polar fractions and have a higher polarity, compared to the asphaltenes obtained from crude oils with no asphaltene stability problems in the field. The dielectric constant, solubility, and flocculation experiments showed that these higher-polarity fractions have a greater tendency to aggregate and are more difficult to remediate. These results suggested that the presence of a certain type of asphalteneparticularly, a high-polarity asphaltenehas a key role in the stability of asphaltene in crude oils.

Calcium and sodium naphthenates are solid deposits and emulsions formed by the interaction of naphthenic acids with divalent (Ca2+, Mg2+) or monovalent (Na+, K+) ions in produced waters. Calcium naphthenate formation, an interfacial phenomenon, is thought to depend largely on tetraprotic naphthenic acids known as “ARN” acids (∼C80) in the crude oil, whereas sodium naphthenates originate from lower molecular weight (C15 to C35) monoprotic naphthenic acids. Here we present detailed chemical heteroatom class composition analyses of calcium and sodium naphthenates from the field based on high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). In all cases, calcium naphthenate deposits consist predominately of tetraprotic acids with a C80 hydrocarbon skeleton whereas sodium naphthenate emulsions consist mainly of specific monoprotic saturated carboxylic acids. Furthermore, low molecular weight tetraprotic (ARN) acids with C60−77 hydrocarbon skeletons were identified in the calcium naphthenate deposit. The high resolution and mass accuracy of FT-ICR MS provide detailed acidic speciation for the analyzed deposits and emulsions.