PIPELINE FLOW OF WAXY CRUDE OILS

Abstract

The various methods for handling waxy crude oils at temperatures below the pour point have been difficult to assess quantitatively owing to the lack of reliable measurement techniques for properties such as the non-Newtonian viscosity and yield stress. Research undertaken at The University of Melbourne has been aimed at the development of reproducible measurement techniques for laboratory scale rheometers and, in so doing, has provided an understanding of the rheology of waxy oils that is applicable to the design and operation of waxy oil pipelines and handling systems and in understanding the startup behaviour of pipelines.

The equilibrium flow properties of waxy oils are determined by the shear and thermal history applied to the oil. In particular, the very strong shear history dependence influences the behaviour of pipelines servicing declining fields, leads to an over-estimation of the flowrate when conventional design methods are used, and provides a mechanism for wall deposition of wax that depends on the oil rheology rather than mass transfer mechanisms. Modified design methods are outlined for both laminar and turbulent flow which account for the effect of shear history and enable a quantifiable measure, under steady conditions, of the return on investment of alternative handling techniques such as the use of flow improver additives.

Waxy crude oil that has been statically cooled develops solid-like character at temperatures below the pour point. The complex yielding process exhibits three distinct behaviours-yield, creep and fracture, each of which influences the startup behaviour of a gelled pipeline.