Molecular characterization of naphthenic acids and naphthenates in the oil sludge: A case study in Bohai Basin, China

Han Li, Shuofan Li, Jianxun Wu, Da Huo, Yahe Zhang, and Quan Shi*

State Key Laboratory of Heavy Oil Processing, Petroleum Molecular Engineering Center (PMEC), China University of Petroleum, Beijing 102249, China

*Corresponding Author: E-mail: sq@cup.edu.cn; Tel: +86 010 89739157

DOI: 10.1021/acs.energyfuels.2c03329

Keywords:  Calcium; Deposition; Lipids; Petrochemicals; Sludges

Abstact: Oil sludge deposition is a common phenomenon during oil transportation, storage, and processing. Detailed molecular characterization of sludges is a prerequisite for understanding the mechanisms of emulsification, naphthenate, and asphaltene deposition. In this study, naphthenic acids and their salts in the oil sludge from an offshore oil platform were characterized by negative ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry and high-temperature chromatography (HTGC). The results show that the oil sludge is a stable emulsion consisting mainly of water and crude oil. An insoluble fraction, which accounted for roughly 3 wt% of the sludge, was found containing a large amount Ca and Fe. The organic matter in solid was extracted by toluene/formate acid extraction and was identified as large-molecular binary, ternary, and quarternary naphthenic acids, including the C80 tetraacids (so-called “ARN”). HTGC results showed that the ARN accounted for more than half of the polybasic acids. It was considered that the Ca²⁺ in the formation water and large molecular polyacids in the crude oil is the key factor for the sludge formation; small molecular naphthenic acids and other polar components in the crude oil stabilize the emulsion of the sludge.