Quantification of the Tetraprotic Acids Content in Oils for the Prediction of Naphthenate Deposits Risks

Abstract

AbstractDuring oil production and transport, several deposit risks may arise due to varying physical conditions (Pression, Temperature) and chemical nature of oil and water (salinity, pH…). These deposits are mainly hydrates, waxes, asphaltenes, inorganic or organic salts, and their occurrence must be avoided in order to ensure a proper flow assurance during oil production. Particularly, precipitation of calcium naphthenates can occur in the presence of divalent cations (calcium) in basic pH conditions and can lead to dramatic clogging of installations. Compared to some of the other deposits, this one is particularly difficult to predict because it depends on the presence of trace levels of specific species in the crude oil.A decade ago, a particular family of naphthenic acids, called "Arn" or "Tetraprotic Acid" (TPA), has been identified in naphthenate deposits.[7] The occurrence of these acids has been shown to be closely related to naphthenate deposit risks: the tetraprotic nature of these acids favors the formation of polymeric-like calcium precipitates able to imprison oil and solids, leading to impressive deposits. Since then, analytical efforts have been made to quantify the presence of Arn in crude oil, in order to mitigate calcium naphthenate deposits thanks to predictions tools based on chemical equilibriums.In this work, we have put to the test a method of Arn quantification described in the literature, based on SPE (Solid Phase Extraction) and HPLC-UV quantification. A selection of crude oils of varying origin and chemical nature (Total acid content TAN, Total base content TBN, Asphaltenes content) has been chosen to enlighten the effect of oil on the Arn extraction yield. Particularly, we have shown that for crude oils with high TBN values, two consecutive SPE extraction steps are necessary to ensure proper extraction. We have also shown that the optimization of the SPE extraction steps (amount of oil, cartridge capacity, elution volumes) is crucial for proper Arn quantification. Finally, Arn extraction yield is shown to depend on the oil nature and therefore a quantification of spiked samples is necessary.This work gives some guides for proper quantification of the tetraprotic acids content in crude oil in order to get a reliable concentration value that can be further used to predict naphthenate deposit risks.