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Abstract

Deep well deposits are solid or semi-solid components of a crude oil that are deposited during exploitation deep in the well. They may precipitate and obstruct the flow of crude oil, thus causing large economic losses. A crude oil deep well deposit was investigated using solvent extraction and electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in both positive and negative modes. Six extraction solvents were used, covering a wide range of polarity, namely, toluene, dichloromethane, chloroform, acetonitrile, dimethylformamide, and dimethyl sulfoxide. The highest extraction efficiency (about 56%) was obtained with chloroform, whereas dimethyl sulfoxide offered the lowest efficiency (about 21%), among the six solvents. With our analytical protocol using both solvent extractions and ultrahigh-resolution mass spectrometry with electrospray ionization (ESI–MS), we were able to detect the N1 class, containing most likely pyridinic and carbazolic species, as the dominant one in the deposit, although each solvent extracted a wide variety of heterocyclic compounds of varying intensities. Although toluene, by definition, should dissolve the asphaltene content here, only a part of the sample was soluble, including mostly the N-containing classes, indicating that the deep well deposit does not consist entirely of asphaltenes. Dimethyl sulfoxide showed the lowest mass balance but one of the broadest class distributions.