Naphthenic acid solubility in produced water and their interactions with divalent cations
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To keep up with the growing energy demands, the production of high acid crudes is increasing worldwide. This acidity is mainly caused by a subgroup of resins called naphthenic acids. With the increased production and normalization of new technology like seabed liquid-liquid separation, more knowledge about these acids is necessary to maintain a predictable production. One of the most important factors in understanding the physical and chemical behavior of crude oils has been found to be their content of polar components. Multiple flow assurance problems can occur during crude oil production, as the temperature and pressure changes from the reservoir to the platform and over the lifetime of the field. The role of naphthenic acids in flow assurance relates mainly to issues like emulsion stability and naphthenate formation. Acids are prone to partition from the oil phase to the water phase, as the release of CO2 during depressurization increases the water phase pH. The goal of this thesis is to study the solubility and partitioning of naphthenic acids in oil-water systems as a function of the composition of the aqueous phase. The strategy adopted was the following. First, model acid and base compounds were studied experimentally and through modelling by dissipative particle dynamics (DPD). The partitioning of these acids and bases were determined. The thermodynamic characteristics determining this partitioning, the partition ratio, the dissociation constant and the solubility constant were determined. In order to be close to a real crude oil system, two mixtures of naphthenic acids were then studied, one commercial naphthenic acid mixture from Fluka and one extracted crude oil acid mixture. In order to analyze these mixtures, a GC/MS method was improved to provide a reliable structure distribution by considering fragmentation. The developed GC/MS techniques then allowed the study of the oil-water partitioning of these mixtures with pH in 3.5 wt. % NaCl water. The cologarithm of the partition ratio between the oil and water was found to increase linearly with the molecular weight of the acids. The relationship found for the commercial naphthenic acid mixture and crude oil extracted acids are similar, which seems to point to universal properties for naphthenic acids. Finally, the effect of the presence of calcium in the water phase on the partitioning of naphthenic acids was determined. Contrary to the precipitation observed for the single model acid 4-heptylbenzoic acid, the results obtained with naphthenic acid mixtures points to the formation of oil soluble calcium naphthenate at high pH. The last part of the thesis is dedicated to determining the influence of solubilized naphthenic acids on oil-in-water emulsions. A microfluidic method was applied, and it was found that naphthenic acids in the water phase influences the coalescence of oil droplets in water. Moreover, the coalescence results with the commercial naphthenic acid mixture was found to be similar to the results obtained with dissolved crude oil components for three different crude oils.
Består avPaper 1: Bertheussen, Are; Simon, Sebastien Charles; Sjøblom, Johan. Equilibrium partitioning of naphthenic acids and bases and their consequences on interfacial properties. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2017 ;Volum 529. s. 45-56.
Paper 2: Skartlien, Roar; Bertheussen, Are; Simon, Sebastien Charles; Sjøblom, Johan. Development of electrochemical DPD molecular simulations for oil/water partitioning of organic acids at varying pH. Journal of Dispersion Science and Technology 2018 ;Volum 39.(9) s. 1367-1375. Not included due to copyright, available at http://doi.org/10.1080/01932691.2017.1404471
Paper 3: Bertheussen, Are; Simon, Sebastien Charles; Sjøblom, Johan. Equilibrium Partitioning of Naphthenic Acid Mixture, Part 1: Commercial Naphthenic Acid Mixture. Energy & Fuels 2018 ;Volum 32.(7) s. 7519-7538
Paper 4: Bertheussen, Are; Simon, Sebastien Charles; Sjøblom, Johan. Equilibrium Partitioning of Naphthenic Acid Mixture Part 2: Crude Oil-Extracted Naphthenic Acids. Energy & Fuels 2018. Final published version available at http://doi.org/10.1021/acs.energyfuels.8b01494. Copyright © 2018 American Chemical Society
Paper 5: Dudek, Marcin; Bertheussen, Are; Dumaire, Thomas; Øye, Gisle. Microfluidic tools for studying coalescence of crude oil droplets in produced water. Chemical Engineering Science 2018 ;Volum 191. s. 448-458.