Analytical detection of ARN-type naphthenic acids in crudes

Benjamin Brocart, Total Petrochemicals France, Lacq, France

C. Hurtevent and J-L Volle, Total E&P, Pau, France

B. Escoffier, Arkema, Lacq, France

Calcium naphthenates deposits recovered on site have been shown to contain massive proportions of acids belonging to a specific class of high molecular weight tetra acid, so-called ARN. The detection of ARN acids in crudes is therefore of paramount importance, especially for designing future installations and allowing for mitigation means.

The ARN content in crudes is suspected to be very low, and concentrating of these molecules is required before analysis. We have taken advantage of its main characteristics to concentrate it from crudes. Specifically, ARN being an acid, it can be extracted from crudes by using classical ion exchange resins, or is precipitated from crudes by contacting with alkali solutions to form naphthenates sludges. It is also precipitated by dilution in polar solvents like ketones, along with other paraffinic components.

Extracts are then analyzed by high-pressure liquid chromatography / mass spectrometry (HPLC/MS).The presence of ARN is guaranteed when simultaneously detecting the monocharged anion [M-H]^- and the corresponding discharged anion [M-2H]^2-. In most cases, the latter is to be carefully distinguished from a monocharged ion of equivalent m/z. To prevent overlapping of these two ions at same m/z value, an HPLC separation was achieved using particular reverse phase conditions.

Using this methodology, ARN has been detected in a large number of crude oils, especially we found ARN in many paraffinic oils with a very low TAN, even in condensate. Most of these oils don’t show any operational difficulties.