The effect of crude oil acid fractions on wettability as studied by interfacial tension and contact angles

Naphthenic acids have been extracted from three North Sea crude oils with different acid numbers. The acid fractions were diluted with toluene to their original crude oil concentrations, and also further diluted and compared at equal concentrations (milligram acid/gram oil). The toluene phases were then equilibrated with an aqueous phase, and the interfacial tension (IFT) and contact angles were measured as a function of equilibrium pH and acid concentrations. The aqueous phase used was 0.5 M NaCl(aq), and the contact angle experiments were performed on silicate glass cover slips. The results show a correspondence between the decrease in interfacial tension as a function of pH and the acid number of the original crude oils. Generally, low contact angles were observed at high and low pH values, whereas angles up to 47 degrees were observed in the intermediate pH range. The contact angles did not show a direct correspondence with the acid numbers but gave the same trend as the corresponding whole crude oils (presented in a previous work). Observations from both interfacial tension and contact angle experiments emphasize the importance of acid structures, or acid types, present in the fractions compared to the acid concentrations. Acid characterization by gas chromatography with a mass sensitive detector (GC-MS) showed significant differences in the molecular structures for the acids dominating each of the three fractions. Fractions enriched in alkyl acids, phenols and cyclic acids were found to have impact on the wetting properties of the silicate surface, whereas fractions enriched in more complicated aromatic ring structures with high degree of carboxylic compounds did not affect the contact angle. FT-IR spectra of the acid extracts confirmed the results from the GC-MS analysis and showed correspondence to the acid numbers of the three crude oils. (C) 2001 Elsevier Science B.V. All rights reserved.