The strength of paraffin gels formed under static and flow conditions

When the temperature of a solution of paraffins in oil is dropped below the ‘cloud point’ temperature, the high molecular weight nn-paraffin molecules precipitate out of solution, crystallize and form a gel. The gelation and concomitant deposition of these paraffin gels in crude oil pipelines poses a major transportation problem by reducing the flow efficiency. Implementation of mechanical methods of remediation of paraffin deposition requires knowledge of the gel strength. Rheometric studies were performed on a model system to study the yield strength of paraffin–oil gels formed under various shear and thermal histories. It was observed that when the gel was formed under quiescent (shut-in) conditions, the yield stress of the gel decreased with an increasing cooling rate. However, when a shear stress was exerted on the gel during cooling (as would be experienced in a flow line), the trend of the yield stress vs. cooling rate curve was strongly influenced by the magnitude of this shear stress. Additionally, experimentation over a range of applied shear stresses revealed that the yield stress of the gel reaches a maximum at a moderate value of the applied shear. These rheometric results are explained with the help of 3-D polarized light microscopy observations of the paraffin crystal structure formed under various shear and cooling conditions using static and flow cell systems. The effects of crystal size on the gel properties are enunciated.