Blockage of a Waxy Gelled Crude Oil in Pipeline: Prediction of the Restart Pressure

Abstract The paper concerns a waxy crude oil from a North Sea field, where the Wax Appearance Temperature (WAT) and Pour Point are both high, they are equal respectively to 34°C and 30°C. A risk of wax deposition exists if operation temperatures are below the WAT. There is also a risk of formation of gel after shutdown in case of cool down to seabed conditions. Production of waxy fluids often encounters difficulties when restarting a pipeline following a shut-in period during which the fluid has developed significant wax structure and exhibits very high resistance to flow. Predicting the conditions necessary to restart flow in gelled lines has long been recognized as a problem. Discrepancies in gel-strength determination have demonstrated among different laboratory techniques. Laboratory measurements and laboratory model pipelines tests give systematically overestimated restart pressures. The limitation of current practice commonly leads to the adoption of wide margins of safety that engender considerable increase in both CAPEX and OPEX. This paper discusses the yield stress measurements performed with three types of test equipment: rheometer, two sizes of coils and two sizes of flow loops, using both dead crude and gas pressurized crude. Restart pressures evaluated by rheometer data were about 2 to 3 times greater than restart pressures measured in a model pipeline. Classical rheological approach seems to predict very high restart pressures. The differences in the results obtained from different equipment are explained by parameters such as the pressure propagation in pipeline; these parameters are not considered in the laboratory tests. The shrinkage flow is also discussed in the paper. The flow assurance strategy for this challenging crude oil is also presented.