Compositional Characterization of Oil Sludge in a Crude Oil Storage Tank

The formation of sediment in crude oil storage tanks poses flow assurance challenges during transportation. Evaluating the flow stability of crude oil is challenging due to the difficulties in real-time monitoring and the complex, dynamic nature of the pipeline system. At present, there are some understandings of the composition of depositions in oil and gas systems, but the molecular-scale transformations during prolonged static storage remain poorly understood. This study systematically characterized both the bulk crude oil and the bottom sediment (sludge) collected from an industrial storage tank, investigating the molecular compositional redistribution and self-association processes that drive sludge formation under static storage conditions. Analytical characterization reveals that the sludge comprises both organic and inorganic constituents. Within the organic fraction, waxes and asphaltenes are significantly enriched, accounting for 33 and 8 wt %, respectively, representing 11-fold and 7-fold increases relative to their concentrations in the overlying crude oil phase. Molecular characterization indicates that the deposited wax fraction is mainly composed of n -alkanes in the C 30 –C 50 carbon number range. Complementary Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis revealed distinct heteroatom-class distributions: the sediment was rich in oxidative species such as sulfoxides, while the asphaltenes were specifically enriched in O2-class compounds (naphthenic acids) with low double bond equivalent (DBE) values of 3–5. These findings provide direct experimental evidence for oxidative transformation and selective deposition of polar compounds during static crude oil storage. This comprehensive molecular characterization provides a well-documented case study, delivering compositional data essential for mechanistic interpretation of static storage deposits, and thereby establishing a foundational basis for future systematic investigations into their formation pathways.