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Accumulation of oil sludge poses operational challenges in petroleum storage facilities. Sludge formation in underground crude oil storage using water-sealed rock-cavern tanks remains poorly understood. To understand the mechanism of sludge formation in the rock-cavern tank, this study comprehensively characterized oil sludge collected from a rock-cavern tank in the Kikuma stockpiling base (Ehime, Japan). The sludge exhibited overall composition similar to sludge samples from other storage facilities with high total acid number values and wax contents. X-ray fluorescence analysis indicated calcium salts as the dominant inorganic constituents. Thermogravimetry-differential thermal analysis, Fourier transform infrared spectroscopy and pyrolysis-gas chromatography/mass spectrometry revealed that the primary organic components were naphthenic acids, likely present as calcium naphthenates. Biomarker analyses indicated that crude-oil biodegradation is negligible in the rock-cavern tank. Analysis of microbial community in the tank excluded in situ biosynthesis of precursors of naphthenic acids within the tank. These results suggest that naphthenic acids originally present in the stored crude oil reacted with calcium ions in groundwater to induce the sludge formation in the rock-cavern tank. The findings provide insights into sludge formation risks in underground rock cavern storage and offer guidance for crude oil selection and sludge mitigation strategies.
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Goto et al. (2026) studied this question.
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