Waxy oil gel breaking mechanisms: Adhesive versus cohesive failure

Highly paraffinic (or waxy) crude oil can cause significant problems in subsea pipelines due to wax-oil gel blockage resulting from the precipitation of the wax components. Once blockage of the pipeline occurs and flow ceases, the pipeline flow cannot be restarted with the original steady state operating pressure but instead requires significantly higher pressures to restart the flow. In order to understand the wax-oil gel restarting phenomenon in the field pipelines, we have investigated the gel breaking mechanism at various temperatures and cooling rates using a model pipeline and also a controlled stress rheometer. This study has revealed that the controlled stress rheometer can successfully predict the required gel breaking pressure of a gelled pipeline when the cooling rate is low and breakage occurs between the gel and the pipe wall (adhesive failure). Furthermore, we have experimentally shown that there exists a delineation point between cohesive and adhesive failures when the gel strength is measured as a function of cooling rate. Using the controlled stress rheometer and the cross-polarized microscope, the existence of the delineation point between cohesive and adhesive failures can be elucidated. Finally, we developed a restart model based on the results of model pipeline experiments and compressibility of the wax-oil gel system and confirmed that the restart model matches well with the model pipeline experimental results. A field implication regarding restart pump design is also presented using the results of the restart model.