Summary
A comprehensive review of literature concerning wax crystallization,
deposition, and gelation, as well as the yielding behavior of waxy crude gels
during pipeline restart is presented. Detailed experimental investigation of
the temperature and pressure effects on gel strength is carried out. Influence
of testing temperature on gelation kinetics is studied using a controlled
stress rheometer (CSR), and the data is analyzed by applying a modified Avrami
model for isothermal crystallization. The effect of system pressure on restart
conditions is studied using a model pipeline system. It is observed that this
system provides the best reproducibility of yield pressures. The model pipeline
results are compared with the yield point tests obtained on a CSR. The results
of the experimental work reported in this paper suggest that weaker gels may be
formed at higher shut-in pressures, which is a favorable condition for pipeline
restarting operations.
Introduction
A critical operational problem involving subsea pipeline transport of waxy
crude is the safe restarting of flow after shut-in for a period of time. The
impact of pressure during gelation of waxy crude is of great practical
interest, especially for a subsea pipeline connected to long vertical risers as
shown in Fig. 1. This is an issue of utmost practical importance because most
crude oil found in many parts of the world, including the North Sea, Middle
East, Australasia, North Africa, West Africa, Alaska, Indonesia, and China, are
of waxy types. Earlier concepts of waxy crude oils assumed that they were
derived from terrestrial or higher plant source materials. However, these oils
can also be originated from lacustrine and marine sources. These crudes contain
high molecular weight n-paraffin (waxes), mostly in the range from
C18– C65. Wax content of crude oil has been reported to
be as low as 1% in south Louisiana and as high as 50% in Altamont, Utah. Under
hot reservoir conditions, waxy crudes behave like Newtonian fluids. However,
upon experiencing cold temperatures for some time on the sea floor lower than
the wax appearance temperature (WAT), the heavy paraffin (mainly straight
chains) may precipitate from the oil and render the crude a non-Newtonian flow
behavior. The wax molecules precipitated within the oil may subsequently
deposit over the pipe wall as a layer of a gel-type material. This, in turn,
reduces the effective cross-sectional area available for flow of crude oil and,
in worst cases, can completely clog the pipeline. The detrimental consequence
of such incidences is that these pipelines may have to be abandoned eventually
if proper wax mitigation measures are not taken or are not economically
feasible under given conditions.
This paper investigates the consequences of quiescent cooling process of
waxy crude to temperatures below the gel-point at conditions encountered in
subsea environments. Obviously, this is actually the worst case scenario in
waxy oil flow assurance issues where different phases of wax transformation
during cooling are inherent, namely wax precipitation, deposition, and
gelation. In addition, the yielding behavior of the gel becomes a matter of
interest during the restart of such gelled pipelines. Hence, for a
comprehensive understanding of the overall problem, the different stages of
waxy crude evolution during cooling are considered, highlighting key efforts
made to elucidate each process. In addition, the necessity to incorporate
system pressure effects on gelation into restart models is argued upon and the
experimental results supporting this viewpoint are discussed and presented.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
14 July 2008
- Meeting paper published:
21 November 2008
- Revised manuscript received:
11 December 2008
- Manuscript approved:
13 December 2008
- Published online:
17 September 2009
- Version of record:
21 September 2009
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