Øystein Brandal

Our current understanding of water-in-crude oil emulsions.: Recent characterization techniques and high pressure performance

Abstract Stable water-in-oil emulsions may form during the production of crude oil, as co-produced water is mixed with the oil from reservoir to separation facilities. Such emulsions introduce technical challenges, as they must be resolved to provide the specified product quality. Asphaltenes and resins indigenous to the oil are acknowledged as the most important components in respect to stabilization of the interface against coalescence. Fine solids may also contribute to the stabilization, as may the presence of naphthenic acids. Combined, this creates a complex picture of several contributing mechanisms, and it is established that the pressure conditions will influence the behavior of active components and the properties of the interface. In order to successfully mitigate the problems of stable emulsions, a thorough knowledge of component properties, behavior, interactions and effect on water/oil interfacial properties must be developed for pressures ranging from ambient to high. This review seeks to bring to light recent findings related to these topics.

Archaeal C80 isoprenoid tetraacids responsible for naphthenate deposition in crude oil processing

The structure of a novel class of octaterpene tetracarboxylic acids which is responsible for naphthenate deposition in crude oil processing has been determined by NMR and mass spectroscopy.

Isolation and Characterization of Naphthenic Acids from a Metal Naphthenate Deposit: Molecular Properties at Oil‐Water and Air‐Water Interfaces

Naphthenic acids from a West African metal naphthenate deposit have been isolated and characterized by infrared (IR), nuclear magnetic resonance (NMR), and Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR MS). The sample has been shown to comprise a narrow group of 4‐protic naphthenic acids of molecular weight ∼1230 Da. The determined mass of 1230.0627 Da suggests a compound with the elemental composition C80H142O8. The NMR data show no sign of carbon‐carbon multiple bonds. Hence, the elemental composition indicates the presence of six saturated hydrocarbon rings. The naphthenic acids have proved to be highly oil‐water (o/w) interfacially active. On elevation of the pH from 5.6 to 9.0, interfacial activity increases gradually due to a higher degree of dissociation of the carboxylic groups. At pH 9.0, the interfacial tension (IFT) between water and toluene‐hexadecane (1–9 vol.) is lowered by ∼40 mN/m at concentrations of only 0.0050–0.010 mM naphthenic acid. The time rate of decrease of the IFT (dγ/dt) is also concentration‐dependent, and a well‐defined IFT is attained at long observation periods. The C80 naphthenic acids form relatively unstable Langmuir monolayers. The stability decreases further with increasing pH as more monomers become dissociated and dissolve into the aqueous phase. The stability is altered upon addition of calcium ions into the subphase due to formation of calcium naphthenate at the surface. In the undissociated state, the acids have a molecular area of ∼160 Å2/molecule in the noninteracting region. The high area reflects an extended molecular structure comprising four carboxylic head groups, which are likely to be separated by hydrocarbon chains.

Interfacial Behavior of Naphthenic Acids and Multivalent Cations in Systems with Oil and Water. II: Formation and Stability of Metal Naphthenate Films at Oil-Water Interfaces

The stability of interfacial naphthenate films of three synthetic naphthenic acids and four different divalent cations is measured by means of the Langmuir technique with a trough for liquid‐liquid systems. The chloride salts of Ca2+, Mg2+, Sr2+, and Ba2+ were dissolved in water at two different pH levels: ultrapure water (pH 5.6) and buffered ultrapure water at pH 8.0. The water phase was covered by the oil phase consisting of n‐decane and dissolved naphthenic acid. The systems were then equilibrated for 45 minutes before the compression of the interfacial films was initiated. The results, plotted as interfacial pressure (IP) versus compression area, showed clear distinctions depending on acid structure, type of metal salt, and pH of the aqueous phase. The differences in film stability by introducing various salts are assumed to be a result of different degrees of hydration of metal cations, which according to the theory increases with decreasing cationic size. Raising the pH from 5.6 to 8.0 generally caused decreasing film stability due a higher ionization of the interfacial layer and a higher water solubility. Furthermore, introducing naphthenic acids with bulky structures increases the molecular distance, which in turn may hinder the monomers to be bound in 2:1 ratio with the divalent cations.

Interfacial Behavior of Naphthenic Acids and Multivalent Cations in Systems with Oil and Water. I. A Pendant Drop Study of Interactions Between n‐Dodecyl Benzoic Acid and Divalent Cations

Abstract Dynamic interfacial tension (IFT) measurements were used to investigate the interactions between a dissociated model naphthenic acid (p‐(n‐dodecyl) benzoic acid) and various divalent metallic cations (Mg2+, Ca2+, Sr2+, and Ba2+) across a toluene/hexadecane–water interface. The measurements were performed by using the pendant drop technique. The results obtained, plotted as IFT vs. time gave curves with similar shapes but different slopes and levels of the equilibrium IFT, depending upon the acid and salt concentrations and the type of cation added. Due to differences in degree of hydration of the various cations, the products of the reaction between dissociated acid monomers and the cations showed differences in solubility, which, in turn, affected the IFT. Based on the shapes of the curves, the mechanisms of the reactions involved in the process are discussed.

Compression Isotherms and Morphological Characteristics of Pure and Mixed Langmuir Monolayers of C80 Isoprenoid Tetraacids and a C18 Monoacid

Monolayers of indigenous isoprenoid tetraacids (TA) isolated from an oilfield naphthenate deposit sample, and mixed monolayers of TA and a synthetic monoacid (MA), have been investigated under different experimental conditions by means of the Langmuir balance equipped with a Brewster angle microscope (BAM). For aqueous subphases at pH 2.3 and 5.6, the surface pressure area isotherms of pure TA show a broad plateau region. This shape is ascribed to a phase transition in which two of the carboxylic groups in the folded conformation are lifted to an upright position, leading to tightly packed molecules in a bilayer‐like arrangement and formation of TA domains at the water surface. Upon compression beyond the plateau region, the domains gradually fuse and a uniform morphology appears. At pH 7.0 and 8.0, the compression isotherms increase steeply toward the point of monolayer collapse without reaching any well‐defined phase transition region. This behavior is associated with dissociation of the carboxylic groups, which due to electrostatic repulsion hinder the molecules to pack densely at the surface and further to adopt a bilayer‐like arrangement. In presence of CaCl2 in the subphase at pH 8.0, a Ca‐TA network is formed at the water surface. This is clearly demonstrated by the morphology upon film decompression during which the network breaks up into fragments. Miscibility of mixed monolayers of TA and MA has been examined on the basis on excess area calculations. Negative deviations from ideality suggest interactions between TA and MA for all compositions. The match in length of the extended MA molecule and the longest TA chains allows MA to occupy the gap in between the TA chains to form mixed domains of high density. BAM images of high contrast and domain brightness support this assumption.

Formation, Growth, and Inhibition of Calcium Naphthenate Particles in Oil/Water Systems as Monitored by Means of Near Infrared Spectroscopy

A new experimental setup based on near infrared (NIR) spectroscopy has been utilized to monitor the formation, growth, and inhibition of calcium naphthenate particles in oil‐water (o/w) systems under different experimental conditions. The naphthenic acids were dissolved in toluene and brought in contact with an aqueous solution. The reaction between the dissociated acid monomers and Ca2+ at the o/w interface was initiated by adding dissolved CaCl2 to the water phase. By using stirrers in both bulk phases, the formed particles were dispersed into the oil phase and the changes in optical density (OD) of the naphthenate solution were continuously monitored using a fiber optic NIR probe. Due to the particles present in the solution, the baseline of the NIR spectra was shifted upwards compared to the pure toluene phase, depending on the size and the number of particles detected by the probe. The formation and growth of naphthenate particles have been shown to depend on the naphthenic acid structure, concentrations, and pH of the aqueous phase. In addition, the presence of oil‐soluble surfactants has been shown to cause a lowering of the particle volume. Possible mechanisms behind this effect are briefly discussed.