Interfacial Shear Rheology of Gels Formed at the Oil/Water Interface by Tetrameric Acid and Calcium Ion: Influence of Tetrameric Acid Structure and Oil Composition

The tetrameric acid ARN is present in crude oil and can form deposits that can cause plugging in oil production facilities during oil production in the presence of calcium ions. It has been previously shown that BP-10, a compound designed to mimic the properties of ARN, can form gel at the oil/water interface by reacting with Ca2+ if the pH is high enough to ionize BP10s carboxylic acid functions. In this study, the BP-10 and ARN are compared. Although BP-10 forms a gel at the interface, ARN creates rather a viscoelastic material perhaps due to the presence of impurities like naphthenic acids in the sample analyzed. The amount of BP-10 required to form a gel at the interface was found to be ca. 0.45-0.76 mg.m(-2). The influence of the presence of asphaltenes on the gel formation has then been studied. It has been found that the gel is weakened in the presence of asphaltenes and G' is not measurable at high enough concentrations. This inhibition could result from either a competition between ARN and asphaltenes to adsorb at interface or complexation of ARN and asphaltenes in the oil bulk phase. The influence of naphthenic acid (NA) on the gel formation was also studied by considering a commercial mixture of NA from Fluka. It is shown that NA can inhibit the gel formation of ARN or BP-10 at the oil/water interface most likely by acting as a cross-linking termination point agent. Finally, the effect of commercial calcium naphthenate inhibitors on gel strength has been studied. It is found that the inhibitors delay the formation of tetrameric acid gels and reduce their elastic modulus G'. However, results show that the order of inhibitor efficiency depends on the tetrameric acid used (ARN or BP-10). This could be linked to different specific inhibitor/TA interactions between ARN and BP-10. In conclusion, interfacial shear rheology appears to be a promising method to screen calcium naphthenate inhibitor.