Regional Variations in Physicochemical Properties and Oil–Solid Interaction Mechanisms of Oily Sludge from Three Chinese Oilfields

Oily sludge is one of the most challenging solid wastes generated during petroleum production and wastewater treatment, posing long-term environmental risks and demanding effective resource-recovery strategies. This study systematically investigated the physicochemical characteristics, compositional differences, and oil–solid interaction mechanisms of oily sludge (OS) from three representative Chinese oilfields, Panjin, Daqing and Xinjiang, through integrated analyses of elemental composition, oil composition, X-ray diffractometer (XRD), Fourier-transform infrared (FT-IR), Gas chromatograph (GC), and Confocal laser scanning microscope (CLSM). The results revealed pronounced regional variations in oxidation degree, hydrocarbon composition, and mineralogy that critically influenced oil occurrence and removal behavior. The Panjin OS sample (PJ-OS) exhibited a high oxidation degree, enriched resins and asphaltenes, and compact film-like oil–solid structures, resulting in the lowest oil mobility and recovery potential. The Daqing OS (DQ-OS) was dominated by light saturates and showed the weakest oil–solid bonding, while the Xinjiang OS (XJ-OS) displayed moderate oxidation and intermediate properties. A novel room-temperature high-speed stirring cleaning method was applied to evaluate oil removal performance under ambient conditions. The residual oil contents after treatment were 4.43% (PJ-OS), 1.65% (DQ-OS), and 1.22% (XJ-OS), corresponding to removal efficiencies of 80.86%, 86.74%, and 90.33%, respectively. The cleaning efficiency was strongly governed by the sludge composition and oxidation state: higher O/C ratios and enrichment of polar heavy fractions enhanced oil–solid adhesion and hindered oil detachment, whereas higher saturate contents and lower oxidation degrees facilitated rapid oil separation. Overall, the findings demonstrate that the treatability of oily sludge is controlled by its intrinsic physicochemical properties. The proposed high-speed stirring technique provides a promising, energy-efficient, and environmentally sustainable approach for oily sludge remediation and resource recovery, offering valuable insights for optimizing treatment parameters and scaling up green petroleum waste management technologies.