Effect of Carbon Number Distribution of Wax on the Yield Stress of Waxy Oil Gels

Abstract

Wax deposition is one of the most important problems in flow assurance of petroleum pipelines. Pigs are commonly employed for the removal of wax deposit (actually a wax–oil gel consisting of liquid oil and solid wax particles) on the pipe wall. Understanding of the wax deposit strength, which may be a function of the carbon number distribution of wax, assists in preventing the pig from getting stuck in the pipeline. This study focuses on the effect of the carbon number distribution of wax on the yield stress of waxy oil gels. Waxes with different carbon number distribution were dissolved into a crude oil to prepare the model oils. The vane method was used to determine the yield stress of waxy oil gels formed under quiescent or shear conditions, in which an applied shear stress was maintained during the process of cooling and isothermal holding. The results showed that the yield stresses dramatically decrease with increase of average carbon number of wax regardless of the quiescent or shear conditions. However, the applied shear stress has little effect on the yield stress of the wax–oil gel 12.5% W1 + oil-A and no effect on the yield stresses of 12.5% W2 + oil-A and 12.5% W3 + oil-A. Under quiescent conditions, the changing rate of the yield stress with respect to the solid wax content reduces as the average carbon number of wax increases. The morphology and structure of the wax crystals were also observed using optical microscopy. Microscopic observation indicated that the average size and boundary fractal dimension of the wax crystals decrease but the aspect ratio increases with the increase of the average carbon number of wax