Summary
Production shutdowns occur often throughout the life cycle of an oil field. In offshore fields, shutdown situations are accompanied by an intense heat exchange between pipeline and cold water, which exponentially increases oil viscosity. Such an event may lead to serious difficulty to restart the production, or even render it unfeasible, especially for heavy oil fields. Therefore, a preventive procedure is required to remove the ultraviscous oil from pipelines and risers; for example, by pumping diesel or methanol in a flush procedure. Designing an efficient cleanup procedure is therefore essential in terms of time, amount of fluid injected, and pumping system requirements. However, the amount of research published in this area is limited. In this paper, we propose a comprehensive analysis on how the displacement of a viscous liquid by a less-viscous liquid occurs in a pipeline through footages in different segments, varying the injection velocity. Two mineral oils with different viscosities and tap water were used as working fluids for this study. The experimental setup was built with a horizontal 10-m-long acrylic pipe with 19-mm internal diameter. Two high-speed cameras were placed both in the inlet and outlet segments. Our results demonstrate how water displaces viscous oil in a pipeline, showing different flow configurations as superficial water velocity increases, depending on the oil viscosity and distance from the inlet. A dimensionless analysis was performed by a combination of the forces that govern the flow and dimensionless groups found in literature. The results show an expected area of optimum values regarding cleaning time according to flow configuration. A unidimensional model using a logistic function was proposed and showed a good agreement with the experimental data. The model itself proven to be an easy tool for industry and academic purposes, supporting even more robust and elaborated models in the future.
