Highlights• Partitioning of acids and bases to determine water quality of produced water • Equilibrium partitioning of acids and bases vs pH, with or without calcium • No interfacial interactions were detected between acids and bases

Crude oil contains naphthenic acids that can partition into water. This phenomenon is a function of several parameters, such as the naphthenic acid composition, pH, and water phase salinity. This article is a continuation of previous work regarding the partitioning between oil and water of model acids and bases in model systems, with regard to pH and salinity. The present work will focus on a commercial naphthenic acid mixture from Fluka, while the next work will deal with extracted naphthenic acids from a crude oil. The composition of the acid mixture was determined by GC/MS and it was found that the commercial naphthenic acid mixture is mostly composed of saturated acids with 0–3 ring structures. The partitioning of the commercial naphthenic acid mixture was determined. The equilibrium partitioning of acids with different molecular weight was determined over a large pH interval, using toluene as the oil phase and 3.5 wt % NaCl aqueous buffers as the water phase. The partitioning of the naphthenic acid mixture with pH was successfully modeled by dividing the naphthenic acid mixture into narrow molecular weight range fractions characterized by a single partitioning ratio (pP wo). The variation of the pP wo of the fractions with molecular weight was found to be linear. In the presence of calcium and high pH, the partitioning of higher-molecular-weight acids was reduced, likely because of the formation of oil-soluble calcium naphthenates, since no precipitation was observed. The partitioning of low-molecular-weight acids was not affected by calcium.

The major contaminant targeted during the treatment of the oilfield produced water is dispersed oil.The efficiency of most separation processes highly relies on the size of the droplets, which can be increased through coalescence. Crude oil has a complex and field-dependent composition, which can affect the interfacial properties of the drops, and consequently the merging process in different ways. This study focused on the development of microfluidic techniques for investigating coalescence between crude oil drops. The experiments were performed with six diluted crude oils and three neat oils, the latter in the presence of an oil-soluble surfactant. The composition of the water phase was systematically varied (pH, ionic composition, presence of dissolved components). In general, crude oil droplets coalesced more readily in lower or neutral pH. The addition of dissolved Fluka acids to the water phase had a unique effect on each crude oil, reflecting their composition.What is more, this effect was similar to the presence of water-soluble crude oil components in the aqueous phase. The pressure did not have a significant effect on the coalescence, which was explained by the lack of the lightest components (C1-C4) in the system. In summary, the results revealed several trends, however it was clear that the coalescence highly depended on the oil composition. This underlined the necessity for experimental methods, such as microfluidics, which allow for quick assessment of the stability of crude oil droplets.

Composition of naphthenic acids by GC/MS • Partitioning of acids to determine water quality of produced water • Equilibrium partitioning of crude oil extracted naphthenic acids vs pH

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