Synergistic effect between surfactant and monoacid in inhibiting the formation of naphthenates

Many crude oils have high levels of acidity due to the presence of naphthenic acids, which under certain conditions form insoluble salts in water and/or oil. Chemical additives are a good alternative to avoid formation of these deposits, or at least to cause the formation of a weak film. In this work, four commercial molecules based on ethoxylated nonylphenol were investigated, with varying hydrophilic-lipophilic balances (HLB) (12.3, 13.3, 14.1 and 17.1), to evaluate their action on calcium naphthenate formation. Moreover, the influence of the addition of monoacids (C4, C10 and C18) on the additives’ performance was also evaluated. Tetraprotic acid (ARN) was extracted from an industrial deposit and was characterized for use in the tests. The additives were investigated at 100, 500, 1000 and 2000 mg. L⁻¹. The performance was evaluated by the biphasic mixture test and oscillatory interfacial rheology (using a Du Noüy ring), being characterized by the reduction of precipitate amount and the time for the formation of the film, respectively. The performance increased with rising additive HLB and concentration within the range tested. Concerning the monoacid, the best result was obtained using butyric acid. By using the additive with HLB of 17.1 and butyric acid together, a synergistic effect was observed, with better performances at lower additive concentrations. Moreover, the surfactant:monoacid ratio played an important role in the formulations’ performance, with the best result achieved at 75:25 wt/wt.