The extraction of crude oils rich in tetraprotic naphthenic acids along with formation water under pH conditions above 6.5 favors the formation of naphthenic acid salts that can generate deposits with high consistency and very low solubility in the oil and water phases. There is still much uncertainty regarding the behavior of naphthenates, and only a small number of tests exist to evaluate it. The aim of this study was to obtain the parameters necessary to assess the formation of calcium naphthenate film through shear rheology (Du Noüy ring method) and to assess the rheological behavior of this film at the water/oil interface as a function of the ARN acid concentration, calcium concentration, presence of magnesium ions, and addition of commercial chemical inhibitors. We observed an increase in the elastic modulus (G′) and a reduction of the viscous modulus (G″) with the formation of the calcium naphthenate film and that the viscous deformation is restricted, which is coincident with the formation of a cross-linked network between the calcium ions and ARN acids. We also found that G′ increases with an increasing calcium ion concentration and salinity. The addition of the commercial inhibitors tested caused a reduction of G′ in relation to the control formulation (no inhibitor), with it being possible to distinguish their different performances in film formation prevention. However, we did not observe a correlation between the results of the biphasic mixture test and rheological assay throughout the concentration range tested.

The tendency of asphaltenes to aggregate and form clusters in solvents was studied by fluorescence spectroscopy. This was done by evaluating the relative fluorescence quantum yield of asphaltenes diluted at several concentrations in toluene and by studying the changes in the fluorescence spectra of asphaltene solutions as the composition of the solvent, toluene and cyclohexane, is changed. The asphaltene fraction (heptane insoluble) was collected from a Brazilian heavy crude oil, and solutions of this material varying from 0.016 g/L up to 10 g/L were prepared in toluene. Front-face emission spectra were obtained in two wavelength ranges, from 310 to 710 nm, excited at 300 nm (short range), and from 410 to 710 nm, excited at 400 nm (long range). Severe quenching was observed at concentrations above about 0.1 g/L. Stern-Volmer plots (reciprocal of quantum yield against concentration) exhibited nonlinear, downward-curved behavior, indicating that a more complex suppression mechanism, probably influenced by the association of the asphaltene molecules, is taking place. The same asphaltenes were dissolved (0.1 g/L) in binary mixtures of toluene and cyclohexane, and emission spectra in both the short range and long range were obtained. Fluorescence was progressively quenched at longer wavelengths of the spectra as the proportion of cyclohexane in the solvent grew. Cyclohexane, a poor asphaltene solvent, is probably inducing static quenching through association of asphaltenes.

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