Abstract

Wax deposition in pipelines is a constant concern in Flow Assurance studies. When the temperature along a pipeline falls below the so-called Wax Appearance Temperature (WAT) of the oil, various problems due to wax precipitation and deposition may happen: wax can deposit on pipewalls, wax particles can increase the viscosity, and, due to plant shut-down, wax can cause the crude oil to gel. To counter-act wax problems, two approaches are possible: prevention (like e.g. insulating the pipe or adding chemicals to the oil) or removal (like e.g. pigging the line).

In order to correctly design pipelines and to implement proper strategies for prevention and removal, a reliable description of the phenomenon plays a key role: without such a model, every intervention has to be oversized, dramatically increasing costs and risks.

A number of commercial codes have been developed to describe the deposition process based on a number of mechanisms (molecular diffusion, shear dispersion, Brownian diffusion, gravity settling …) that have been proposed to elucidate the evolution of wax deposits on cold pipe walls. The comparison of these different commercial codes to each other, and with field data, represents a first result of this study. The models still present limits, many of which relate to the practical reliability of model predictions and their impact on operations:

  • The relative importance of the different and/or concomitant mechanisms proposed for wax deposition modelling is still not completely clarified.

  • The models present a number of arbitrary or not well defined values of parameters that are often used to adjust predictions with respect to a specific experimental result.

  • The models have been developed using gel deposition rates observed in the laboratory flow loops and the scale-up of the deposition usually leads to an over-prediction of the wax deposition in field pipelines, due to the unavailability of a reliable upscale criterion.

In addition, the lack of detailed field data makes the qualification of the different models difficult.

Due to the failure of available models, the consolidated proposed mechanisms of wax deposition have been critically analysed. It resulted that there is need for alternatives: molecular diffusion controls wax radial transport, but this is not the only process that counts. Convective axial transport brings enough waxes to form a gel on the wall, and this gel formation is the fast process responsible of the deposit geometry.

On these bases, a new approach has been developed founded on this physical picture of the process; the comparison of its results with field data is by far better than the other ones.

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