Startup flow of yield-stress non-thixotropic and thixotropic materials in a tube

An experimental study of the flow of yield stress materials was conducted. The main goal was to understand the relationship between the minimum axial pressure gradient required for the onset of motion and the rheological properties. The sought-for application is the startup flow of waxy crude oils in subsea pipelines. We examined two types of yield stress materials, namely a non-thixotropic acrylic polymer solution and a layered silicate suspension which presents thixotropic properties. We observed that for both materials the measured minimum axial pressure gradient was in good agreement with the prediction of a simple force balance. On the other hand, a puzzling fact is observed in the oil industry, namely that the just mentioned force balance result grossly overestimates the minimum axial pressure gradient for the case of gelled waxy crude oils. One often evoked explanation is the thixotropic behavior of the gelled crude, but the present results serve to falsify it.