Naphthenic acid extraction and characterization from naphthenate field deposits and crude oils using ESMS and APCI-MS

The formation of different types of naphthenate deposit during production operations is becoming an increasing flow assurance problem for the oil industry. Two "end-member" types of naphthenate salts are observed in the field, viz. the more precipitation-like calcium naphthenates and the more emulsion like sodium naphthenates. In order to characterise these naphthenate deposits, it is important to be able to extract and characterise the different types of individual naphthenic acids that are involved in them. Naphthenic acids may be extracted both from the original crude oil(s) in a given field and also from the actual naphthenate deposits which are found in that field although, as we demonstrate, such results must be interpreted with care. A general synthesis of the theory of how naphthenates form has been presented recently (Sorbie et al. [1]) showing how the calcium and sodium naphthenates arise as the natural end-members referred to above. However, some workers have argued that, for calcium naphthenates formation that a special role is played by a high molecular weight (M.Wt. ~1230) 4-protic acid, initially referred to as "ARN". Since its initial discovery, this high molecular weight naphthenic acid species has been detected by a number of groups, including ourselves but its role is still not fully established and further results are reported in this paper. Sodium naphthenates appear to form with moderate molecular weight naphthenic acids (M.Wt. ~200-500) and do not form a solid deposit but they do tend to induce emulsion problems and lead to difficulties in oil/water separation. Indeed, many previous described "emulsion problems" in water/oil separation in oil production systems may be due to sodium naphthenate formation. In this work, various field naphthenate deposits from both the North Sea and Asia were used for the naphthenic acid extraction using three different methods of increasing naphthenate dissolution severity. Characterization of the extracted naphthenic acids was then carried out using electrospray mass spectrometry (ESMS) and atmospheric pressure chemical ionisation mass spectrometry (APCI-MS) techniques. Our results show that these two techniques (ESMS and APCI-MS) have quite different responses to the detection of higher molecular weight acids. For example, when "ARN" acids are present, ESMS shows some cluster of ion peaks in the region associated with these high molecular weight acids, whilst the APCI technique shows very distinct ion peaks in the region of m/z 1230-1310 in all the spectra. Extracts of the different field naphthenate deposits showed two categories of classifications with some containing both lower and higher molecular weight naphthenic acids in relative proportions as in samples (A and C) whilst samples (B and D) showed more of "ARN" acids compared to lower molecular weight naphthenic acids. When the naphthenic acids were extracted from the corresponding crude oils using an acid-ion exchange resin (IER) technique, somewhat different results from those found for the deposits were seen. The deposit had clearly "concentrated up" the depositing acids. The Acid-IER results for the crude oil revealed similar detection of lower medium molecular weight naphthenic acids but with much weaker or no detection of "ARN" acids due to the very low concentration of these acids in the crude oils. © 2009 Elsevier B.V. All rights reserved.