Isolation of tetrameric acids from naphthenates deposits and characterization by high-resolution analytical techniques

Abstract

Calcium naphthenate deposits consist of an insoluble salt of polymeric structure and high interfacial activity, formed in the oil/water interface due to a specific tetrameric group of naphthenic acids containing high molecular weight (Mw, about 1230 Da), usually named ARN. These acids are made up of four carboxylic acid terminations that allow these species to cross-link with divalent cations (mainly calcium ions) present in the produced water in oil production fields. Herein, we present an alternative method of specific isolation of ARN from calcium naphthenate deposits, which includes cleaning the deposit, followed by converting naphthenate salts into organic acids (naphthenic acids (NAs) and their separation using sequential elution. For this purpose, we used silica-based with aminopropyl groups, a sorbent material previously selected. Thus, they were characterized by high-resolution analytical techniques such as NMR, ESI(–)FT-ICR MS, and ESI(–)Orbitrap MS and complementary such as FTIR. The ESI(–)FT-ICR MS results showed that the strategy provided an optimal separation of acids with a distinct Mw range. The high resolution and mass accuracy indicated that one of the fractions (fourth fraction) concentrated only the ARN species, which they were identified from their characteristic fragmentation patterns by ESI(-)MS/MS experiments, with 94.5% purity. The data from ESI(–)Orbitrap MS corroborated with the FT-ICR MS results, showing consistency in relation to the fractionation provided by the specific isolation of ARN. The integrations of the ¹H and ¹³C NMR spectra of the acidic fractions highlighted the expressive presence of alkyl compounds and the absence of aromatic hydrogens in the subfraction of interest. Orbitrap MS offered high-resolution power and mass accuracy like FT-ICR MS, proving to be advantageous and promising and elucidating complex mixtures of naphthenic acids with an important differential in the economic viability offered by the technique.