Computational and Rheological Study of Wax Deposition and Gelation in Subsea Pipelines.

Abstract

Highly waxy crude oils can cause significant problems such as blockage of a pipeline because of the precipitation and deposition of select wax components during the production and transportation of the crude oil. The cost of wax management is enormous and rapidly increasing because of increased oil production in deep sea areas. Wax management costs can be significantly reduced if wax deposition and gelation in pipeline can be accurately predicted. In this research, a rigorous wax deposition model combined with the wax precipitation kinetics in the boundary layer was developed using a computational heat and mass transfer analysis. This model accurately predicted the deposition and aging rates for lab scale and pilot plant scale flow loop tests under laminar and turbulent flows. The model was also extended to make prediction in subsea field pipelines. Studies of wax deposition under turbulent flow conditions showed that the deposition rate is significantly reduced by the precipitation of waxes in the thermal boundary layer. Furthermore, this analysis proved that the convective mass flux is bounded by the Venkatesan-Fogler solubility method as the lower bound and the Chilton-Colburn analogy method as the upper bound. The challenging issue of the restart of a gelled subsea pipeline after shut-in period was also studied experimentally and theoretically. The gel inside the pipeline formed during a stoppage of oil flow must be broken to restart the flow. The gel breaking mechanisms during the restart of a pipeline were investigated and were found to be a function of cooling rate. The existence of a delineation point between cohesive and adhesive failures was found by measuring the gel strengths using various cooling rates. Using a controlled stress rheometer and a cross-polarized microscope, we elucidated the phenomena behind the existence of a delineation point between cohesive and adhesive failures. This study has shown that the controlled stress rheometer can predict the restart pressure of a gelled pipeline when the cooling rate is low and breakage occurs adhesively. Finally, we developed a restart model that can predict the relationship between the amount of injection fluid and the pressure applied to the pipeline.