Extraction, quantification and study of interfacially active petroleum components”

Abstract

The recovery, transport and production of crude oil include challenges related to pipeline flow, separation and pressure drops occurring in the process, due to a vast array of indigenous molecules present in petroleum including bases, complex naphthenic acids and asphaltenes. The issues related to these molecules involve formation of kinetically stable emulsions, corrosion, naphthenate deposits, etc. Gaining a better understanding of their behaviour at the interface is therefore of interest in order to manage and minimize their negative influence.

Calcium naphthenate deposits are produced due to the increase of pH induced by the depressurization from the reservoir to topside facilities. A substantial amount of the deposit is caused by a tetrameric acid (TA) called ARN, which forms a cross linked network at the interface with calcium ions from the water phase. A four-step method has been presented for determining the concentration of TA in crude oil by using an anion exchange cartridge and analysing by HPLC. The measured concentrations spanned from 0.1 to 30 ppm.

A method for extracting basic nitrogen compounds from real crude oil samples has been developed. The bases are extracted using a cation-exchange sorbent and recovered by methyl amine. The selectivity was checked with a model system of known basic and non basic compounds. The method gives an extraction yield of 80-90 % from the majority of crude oils employed, although only around 70 % was recovered from an extra heavy sample.

The surface and interfacial properties of the separated basic fraction was investigated with a Du Noüy ring and a Langmuir trough. The oil/water interfacial tension is governed by the basic molecules below pH 5, where the basic nitrogen is protonated. The bases are able to form a strong Langmuir film which appeared to be unaffected by pH, but highly dependent on salinity of the subphase. The behaviour of the basic fraction was compared with that of asphaltenes and maltenes. The similarities with the latter suggest that the bases may be a subfraction of the maltenes.

Numerous problems are related to crude oil emulsions that are kinetically stable, due to indigenous surfactants in petroleum. The ability to stabilize emulsions was investigated for a model acidic asphaltene, C5Pe, hydrophobic silica particles and extracted asphaltenes. C5Pe described the asphaltene behaviour quite well both in similar solvent systems and at high pH. The results seemed to indicate that only a fraction of asphaltenes are responsible for the film forming properties. The particles displayed a behaviour different from that of the two other systems, showing no droplet coalescence after emulsification.

The interfacial rheology of extracted asphaltenes was investigated by the oscillating droplet technique. The correlation with emulsion stability was not straight-forward. The elastic properties of the film appeared to be affected by diffusion and aggregation of asphaltenes at high bulk concentrations (≥ 0.1 g/l), causing a decrease in elasticity, while the emulsion stability increased in the same region. Similar trends in interfacial rheology and emulsion stability were observed when altering the pH or the solvent composition, which demonstrated the correlation between the macroscopic observable emulsion stability and the microscopic film properties.