The Influence of Compressibility and Shrinkage in the Start-up of a Flow of Gelled Waxy Crudes in Pipelines

The procedure of shutdown and restart of an oil production unit due to equipment maintenance or emergency occurs regularly during its operational lifetime. In some cases, the concentration of high molecular weight linear alkanes is very high, leading to the appearance of solids when the oil is cooled below a threshold temperature termed WAT (Wax Appearance Temperature). In such cases, the crude is typically called Waxy Oil, which requires a minimum temperature-dependent yield stress to start flowing. When designing the pipelines and the pump facilities engineers usually do a simplified force balance. The assumption is that when the pressure is enough to overcome the yield stress, flow restart occurs. However, there is considerable evidence that this calculation is overestimated, and therefore, a better understanding of the phenomena involved in this process is required. The present study was performed with two commercial waxy crudes, to better generalize the results. The first part of the study consisted of bench experiments to verify whether the crudes exhibit an yield stress. After we confirmed the presence of this rheological feature, we designed and built an experimental apparatus to verify how much pressure was required and how the pressure propagates through the gelled crude. The experimental data obtained confirms the evidence that the rheology might not be the dominant phenomenon in the restart. It actually indicates that compressibility and shrinkage can influence the necessary pressure to restart the flow as much as rheology or even more. We ran three different experimental procedures to evaluate the influence of shrinkage and compressibility separately, and in all of them we did not observed a real influence of the oil's rheology.