Interim Report on Pressure Effect on Waxy-Crude Pipeline-Restart Conditions Investigated by a Model System
- Chiedozie K. Ekweribe (Chevron North America E&P) | Faruk Civan (University of Oklahoma) | Hyun Su Lee (ConocoPhillips) | Probjot Singh (ConocoPhillips)
- Document ID
- Society of Petroleum Engineers
- SPE Projects, Facilities & Construction
- Publication Date
- September 2009
- Document Type
- Journal Paper
- 61 - 74
- 2009. Society of Petroleum Engineers
- 4.3.3 Aspaltenes, 5.8.2 Shale Gas, 4.5 Offshore Facilities and Subsea Systems, 5.5 Reservoir Simulation, 4.1.5 Processing Equipment, 5.3.2 Multiphase Flow, 4.2.4 Risers, 4.1.2 Separation and Treating, 4.6 Natural Gas, 4.3.1 Hydrates, 4.2.5 Offshore Pipelines, 4.3.4 Scale, 4.3 Flow Assurance, 4.2 Pipelines, Flowlines and Risers
- thermochemical process, sludge removal, Urucu, storage tanks, wax
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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.
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.
|File Size||1 MB||Number of Pages||14|
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