Handling of High Pour Point West African Crude Oils
- C. Irani (Gulf Research & Development Co.) | J. Zajac (Gulf Research & Development Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- February 1982
- Document Type
- Journal Paper
- 289 - 298
- 1982. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 4.1.9 Tanks and storage systems, 4.5 Offshore Facilities and Subsea Systems, 4.1.5 Processing Equipment, 5.3.2 Multiphase Flow, 4.2 Pipelines, Flowlines and Risers, 4.3.3 Aspaltenes
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Problems related with the transportation of two high pour point west Africa (Zaire and Cabinda) crudes are discussed. Relevant rheological parameters of untreated and treated (chemical additives for pour point depression) crude were determined in the laboratory with a rotational viscometer and these results are presented. In both cases, low concentrations (30 to 50 ppmw) of the chemical additives improved the mobility and reduced the restarting pressure requirements for the crude.
On the basis of the successful laboratory program, a field test for Zaire was undertaken in June, 1977. The treatment was implemented by preparing a batch solution of the additive in crude oil and injecting the solution into the transfer line at the offshore platform. Field test results are presented. The experimental batch mixing equipment has been in use since then to treat crude to be left in the tanker loading line. Field implementation of the Cabinda treatment represents a more difficult situation requiring injection of the additive at the wellhead. On the basis of the rheological evaluation of Cabinda crude, continuous circulation of the crude in the platform storage tank was suggested to field personnel as a means to partially alleviate the transportation problems.
Continued uncertainties in the availability of existing crude supplies has placed additional significance on the production and transportation of the difficult-to-handle waxy, high poor point crudes. The low sulfur content of many waxy crudes makes them even more desirable from an environmental standpoint. However, numerous flow problems attend the normal handling of waxy crudes: (1) temperature must be maintained substantially above the pour point to permit crude handling, (2) transportation costs tend to be much higher because of special pumping and heating requirements, and (3) waxy components are deposited in pipelines and storage tanks, which then must be pigged or scraped. If pump failure occurs during pipeline transportation, the results could be potentially disastrous. In such a situation, the crude could undergo static cooling to a temperature below its pour point, and, in the process, form a three-dimensional cohesive structure or gel over part of or all the pipeline. Depending on the final temperature, the externally applied pressure required to destroy the gel structure and initiate flow in the pipeline could well exceed the burst pressure of the pipeline.
Given these considerations, it is not surprising that a large number of methods have been proposed for transporting waxy crudes.1 These include heating the crude with or without the use of insulated pipe, dilution with lighter oils, transporting the crude as an aqueous emulsion, injecting a water layer to line the inside walls of the pipe, using chemical additives for pour point depression, and using a very specific heating and cooling cycle to modify wax crystal growth and thus lower the pour point.
In the past, heating the crude and insulating the pipeline has represented the most frequently used and most successful approach for moving waxy, high pour point crude. Certainly, heating the crude above its pour point represents the single panacea for improving the pumpability of high pour point crudes. No matter how high the pour point, how high the wax content, or how intricate the interaction between the high molecular weight species responsible for the final gelling phenomenon, heating the crude above its pour point guarantees successful pumpability. However, sharply rising energy costs and the impracticality of heating in certain environments (offshore or in long, buried pipelines) has resulted in substantial effort being devoted to develop alternate schemes for improving pumpability.
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