Model Compounds for Heavy Crude Oil Components and Tetrameric Acids: Characterization and Interfacial Behaviour

Abstract

The tendency during the past decades in the quality of oil reserves shows that conventional crude oil is gradually being depleted and the demand being replaced by heavy crude oils. These oils contain more of a class high-molecular weight components termed asphaltenes. This class is mainly responsible for stable water-in-crude oil emulsions. Both heavy and lighter crude oils in addition contain substantial amounts of naphthenic acids creating naphthenate deposits in topside facilities.

The asphaltene class is defined by solubility and consists of several thousand different structures which may behave differently in oil-water systems. The nature of possible subfractions of the asphaltene has been received more attention lately, but still the properties and composition of such is not completely understood. In this work, the problem has been addressed by synthesizing model compounds for the asphaltenes, on the basis that an acidic function incorporated could be crucial. Such acidic, polyaromatic surfactants turned out to be highly interfacially active as studied by the pendant drop technique. Langmuir monolayer compressions combined with fluorescence of deposited films indicated that the interfacial activity was a result of an efficient packing of the aromatic cores in the molecules, giving stabilizing interactions at the o/w interface. Droplet size distributions of emulsions studied by PFG NMR and adsorption onto hydrophilic silica particles demonstrated the high affinity to o/w interfaces and that the efficient packing gave higher emulsion stability. Comparing to a model compound lacking the acidic group, it was obvious that subfractions of asphaltenes that contain an acidic, or maybe similar hydrogen bonding functions, could be responsible for stable w/o emulsions

Indigenous tetrameric acids are the main constituent of calcium naphthenate deposits. Several synthetic model tetraacids have been prepared and their properties have been compared to the indigenous analogue. The model compounds display the same high interfacial activity and film behaviour. One of the model compounds, BP10, forms a cross-linked network with Ca2+, crucial for naphthenate deposit formation. On the other hand, no glass transition was found for BP10. Reactions with Ca2+ under emulsified conditions demonstrated the extreme influence of the available interfacial area. Finally, the acid-base properties of several tetraacids were investigated by potentiometric titrations. A high apparent pKa with hysteresis depending on the titration direction was observed for BP10, and could be explained and modelled based on the tetraacid being present as a micellar solution.