Rheology and Pipeline Start up Pressure of Raguba Crude Oil

Waxy crude oil is a valuable soft that undergoes long journeys from the well to the terminals through thousand of miles of pipeline in various parts of the globe that include deserts, arctic areas and sub-sea beds. Under such conditions, pipeline cease pumping with serious consequences both in cost of interruption of production and damages to the pipeline. This paper describes research carried out on Libyan waxy crude oil namely (Raguba) to study (1) the rheology of this crude oil under various conditions and (2) the start-up pressure of this oil when temperatures decrease below the gel point. The rheological data to be presented are extensive in that they cover a range of conditions of temperature below and above gel and pour point and cooling histories (from very low to rapid cooling, 0.1 to 6.5°C/min representative of conditions under pipeline operations). Steady and unsteady states rheological tests were carried out under controlled shear stress and shear rate using an MCR-501 Anton Paar rheometer. Experiments were also carried out to measure the extent of visco-elasticity. As for the start-up pressure measurement, the paper describes a laboratory pipeline rig developed for the purpose of conditioning the oil (setting a measured cooling history) and measuring the corresponding start-up pressures after a range of stoppage time of 1, 2, 4 and 6 hours. The data obtained shows a good agreement between the yield stresses measured in the rheological tests and the start-up pressure tests but only when conditioning of the oil in the pipeline rig is duplicated in the rheological tests. The data also give practical guidelines on the minimum pressures required after certain time of pipeline stoppage. The data are useful in practice as they reduce excessive waste of power during pumping the oil.