A mechanistic analysis of naphthenate and carboxylate soap-forming systems in oilfield exploration and production
Shepherd, Andrew G.
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This project entailed mechanistic aspects of the formation of oilfield soaps. An integrated approach to the study of field deposits was developed leading to an optimised analytical protocol which is one of the major contributions of this thesis. The philosophy behind the choice of techniques was to integrate measurements suitable for bulk and surface properties. The selected and optimised techniques were electrospray mass spectrometry (ES), energy dispersive X-ray (EDAX) and solid state 13C nuclear magnetic resonance (NMR) as well as thermal analysis (TGA/DSC) and interfacial tension (IFT). These allowed for the differentiation of the two end-member types of soaps, namely, calcium naphthenate soap scales and sodium carboxylate soap emulsions, as well as for the identification of chemically-treated deposits and asphaltenes. It was concluded that the analysis of naphthenic acids from field soap deposits in mass spectrometry was a function of: ionisation source, solvent and instrument settings (e.g. voltages). These parameters had a direct effect on the relative detection of particular naphthenic acid species such as the Arn. Though the electrospray (ES) source was observed to lead to a more realistic fingerprint for naphthenic acid extracts, it was also suggested that the atmospheric pressure chemical ionisation (APCI) source could be used in conditions where identification of Arn was the ultimate objective. A series of static bottle tests were devised to simulate the pH changes associated with the occurrence of deposits in the field. The procedures focused on a number of model naphthenic acid systems, as well as acids extracted from field deposits and soap-forming crude oils. Soap formation was found to be a function of the precise aqueous phase (e.g. cations and pH) in addition to the oil phase (e.g. acyclic vs. cyclic/aromatic naphthenic acid content). It was possible to form soap deposits in the laboratory from both indigenous acids, as well as crude oils. Detailed speciation of certain indigenous acids allowed for the identification of Arn and the special properties of this species namely: four carboxylic acid groups by tandem mass spectrometry (MS/MS) and surface properties given by interfacial tension (IFT). Previous literature claims stated that Arn acid presence were solely responsible for the precipitation of calcium naphthenate soap scales. The results in this thesis show that although Arn acids have predominant surface properties, they compete with lower molecular weight acids for aqueous phase cations at high pH values. This was observed in static bottle tests as well as results from field precipitation samples. Fourier-Transform infrared (FTIR) spectroscopy showed potential as a technique for the prediction of soap deposition onset in the laboratory. Supporting experiments were designed to validate a simple thermodynamic model to predict the phase iii behaviour of oil-water-naphthenic acid systems. A sensitivity study showed that the dissociation constants (pKa) of the naphthenic acids were the most important model parameters and could affect predicted output pH values. For indigenous naphthenic acids, alternative procedures for both dissociation constant and partition coefficient were introduced. A comprehensive suite of crude oil analysis and water properties were employed for correlating field soap-forming systems. It was possible to obtain some trends which relate geochemical parameters with bulk crude oil properties, as well as naphthenic acid speciation. Based on this information, a preliminary attempt to establish prediction guidelines for soaps, one of the major contributions in this thesis to the current knowledge of soap-forming systems, is presented.