Influence of the initial cooling temperature on the gelation and yield stress of waxy crude oils

One of the major problems in waxy crude oil production and transportation is oil gelation that takes place within pipelines as a result of wax crystallization at low temperatures. In such cases, the pressure needed to restart the oil flow in subsea pipelines can be much larger than the usual steady-state pressure, as the temperature in such environment can be as low as 4 °C. The literature has shown that not only the temperature itself but also the fluid shear and thermal histories have significant influence on the yield stress of waxy crude oils. This paper investigates the effect of the initial cooling temperature on the waxy crude oil viscosity, gelation temperature, and yield stress. In order to accomplish that, rheological tests were carried out under static and dynamic cooling conditions. The results show that there is a critical range for the initial cooling temperature that provides maximum values for viscosity, gelation temperature, and yield stress. In other words, the highest values of those properties are observed when the cooling started within this temperature range. The effect of a thermal pretreatment usually used to remove light ends was also investigated. In spite of not changing the initial temperature critical range, the yield stress was slightly affected by the thermal treatment. It is worth noting that the yield stress varies from approximately zero to hundreds after dynamic cooling or to thousands after static cooling within the tested range of the initial cooling temperatures.