Could Naphthenic Acids be Responsible for Severe Emulsion Tightness for a Low TAN Value Oil? Part 3: Analytical Chemistry. (ABSTRACT)

摘要

The detailed composition of a low TAN crude oil causing severe emulsion problems (see related papers part 1 and 2) was studied by means of Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS). The detailed compositional analysis of the whole crude revealed unusually high amounts of oxygen containing ringed hydrocarbons. Then the characterization of the organic acids as extracted by Ion Exchange Resin Chromatography revealed two classes of acids. Firstly, the classical acyclic fatty acids. Secondly, mono- and bi-protic acids containing numerous naphthenic and aromatic rings, with a molecular weight ranging from 250 to 750 Da. Finally, the whole crude oil and the extracted naphthenic acids were compared to the interfacial material as extracted from an indigenous water/oil emulsion collected in the production stream. A first extraction step used toluene as a solvent to remove the species physically adsorbed on the interfacial layer in contrast to the interfacial material, which is chemically bonded to the water surface. The physically adsorbed material was composed primarily of the common set of acids present in the oil (both fatty and cyclic) and secondarily by asphaltenes. The second extraction method was designed for naphthenate deposits: a sample of emulsion was put in contact with both strong acid and toluene to protonize naphthenic salts into free acids. The result was similar to the sets of fatty acids found elsewhere in sodium naphthenate deposits and differed from the ARN acids found in calcium naphthenate deposits. These findings may explain the observations presented in the two associated papers. It is believed that the synergistic effects between fatty acids and multiply ringed acids in association with an aromatic structure in some acids create a multi-layered interface with particular properties. The fatty acids are probably bound to cations at the water surface and are acting as anchors for physical entanglements with other acids that are present either under the free acid or the salt form. A low molecular mobility of these other acids (due to their multiply ringed structure) offers an explanation for the efficiency of the entanglement. The overall result is a gel-like interface acting as a solid shield against droplet coalescence.