Effect of the Flow Shutdown Temperature on the Gelation of Slurry Flows in a Waxy Oil Pipeline

When uninsulated pipelines are used to transport crude oils with high wax content (greater than 2 wt %) in cold environments, the pipelines may become plugged with wax gels when a flow Shutdown occurs. The local value of the oil temperature in a pipeline decreases monotonically with axial distance from the well head and, in some pipelines, may decrease significantly below the wax appearance temperature (WAT), in which case the local wax gel forms from a wax slurry precursor state. For a model waxy oil, we show that the restart pressure of a gelled laboratory pipeline at fixed restart temperature decreases with a decrease in the temperature of the slurry flow at the time of flow shutdown. Approximately 90% of this decrease occurs for a flow shutdown temperature only five degrees below the WAT. These laboratory results imply that the strength of the wax gel that forms during the shutdown of an uninsulated pipeline in the field decreases with axial distance from the well head. The dependence of the laboratory flow loop restart pressure on the flow Shutdown temperature is qualitatively similar to the dependence of the wax gel yield stress value on the preshear temperature, as measured by cone plate rheometry.