The viscosity properties of oil sludge from tank bottom and different methods for its viscosity reduction were investigated using HAKKE VT550 rotational viscometer. The viscosity reduction methods included heating range of 20—60℃

adding surfactant Triton X-100 aqueous solution with the concentration of 0.5% (by mass)

and adding the organic solvents of 1-pentanol and 120# solvent oil. The results indicated that the oil sludge exhibited the characteristics of both plastic fluid and pseudo-plastic fluid in the range of 0—600 s-1 for the examined shear rate. The significant shear-thinning behavior can be attributed to the high content of solid particles in the oil sludge. The modeling analysis showed that the Casson model fitted well with the viscosity characteristic curve of the oil sludge with the highest determination coefficient R2=0.986. In comparison with heating to 50℃

the similar viscosity reduction performance can be achieved by adding 10% Triton X-100 solution (by mass) at 20℃ with the viscosity reduction of 50%. The rheological properties of sludge had a tendency to pseudo-plastic fluid behavior. Better performance can be achieved by blending the sludge with a certain amount of organic solvent. When 10% of 120# solvent oil was added to the sample at 20℃ and 300 s-1

the viscosity was reduced by 90% owing to the spatial structure of both the oil and the solid phase was destroyed. The sludge gradually exhibited Newtonian flow behavior.