This paper focuses on the problem of gellation, one of the key problems associated with the pipeline transportation of waxy crude oils. The impact of thermal cycling on the rheological properties, such as viscosity and yield stress, of waxy crude oils is investigated in detail with the help of a rheometer. At a given temperature, the yield stress and viscosity of crude oils were found to differ by more than one order of magnitude depending on the temperature limits imposed during thermal cycling; these findings have direct implications for pipeline restartability after gellation. Based on these findings, a method to define the heat treatment of waxy crude oils prior to pipeline transport is discussed.
Pipeline transportation of waxy crude oils often encounters difficulties in reinstating flow following a shut-in period during which the crude oil gels, i.e. develops significant wax structure that exhibits a very high resistance to flow. Reinstating flow in a gelled pipeline requires the application of a certain minimum pressure drop across the gelled section – referred to as the restart pressure, which can be significantly higher than the normal operating pressure drop. Predicting the restart pressure required to restore flow in such gelled pipelines has long been recognized as a problem (8); this limitation often brings additional cost and complexity to the design and operation in the form of high system design pressures, heat retention/addition requirements, chemical consumption and production deferment (5, 6,12).
In the early years, the pour point temperature of the crude oil was often taken as an indication of its pumpability. A systematic investigation of the variables affecting the yield stress of waxy crude oil gels was started in the 1970's after it was gradually realized that the yield stress was a better measure of pumpability than the pour point. Since then, several researchers have reported the impact of parameters that affect the yield stress of crude oil gels such as the crude oil composition, the cooling process (starting and final temperatures, cooling rate), isothermal holding time before restart, and the shear history (1–4, 9–11).
In this paper, we present the impact of thermal cycling, that a crude oil may undergo as it is travels from the reservoir to an export pipeline, on its rheological properties. The effect is demonstrated by subjecting a crude oil sample to a well defined temperature cycle in a rheometer cell while measuring its rheological properties such as viscosity and yield stress. The experiments were repeated for a variety of crude oils with pour points ranging from 18°C to 39°C.