A mechanistic analysis of naphthenate and carboxylate soap-forming systems in oilfield exploration and production
Abstract
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
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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.