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.

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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.

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