• Vinay, Guillaume

Abstract

<jats:sec><jats:title>Abstract</jats:title><jats:p>In this paper, we investigate the problem of the start up of gelled waxy crude oil flows in a pipeline. Waxy crude oils are deemed to be weakly compressible, thixotropic and viscoplastic material. In severe situations, the waxy crude oil is fully gelled in the pipeline and the main issue concerning the restart of waxy crude oils relies on the estimation of the minimum pressure required at the pipe inlet. The objective of this study is to examine the possibility that the flow can restart for a pressure drop below the value predicted by the conservative relation Dp=4tyL/D, where ty denotes the yield stress, L the pipe length and D the pipe diameter, thanks to the combined effects of compressibility and thixotropy.</jats:p><jats:p>Using a 1.5D numerical model, we evidence that there exists situations where, though the pressure drop is below the theoretical minimum pressure drop 4tyL/D, the flow restarts thanks to the combined effects of thixotropic and compressibility. From a parametric survey, we derived a new analytical relation able to predict more accurately the minimum pressure required to restart pipelines filled with a gelled waxy crude oil. This relation accounts for not only viscoplastic effects but also for compressibility and thixotropic effects.</jats:p><jats:sec><jats:title>Introduction</jats:title><jats:p>In the oil and gas industry, the use of pipelines to convey large amounts of crude oil over short or long distances has been extensive. Transportation of conventional (Newtonian, low viscosity, steady physical properties, single-phase. . .) crude oils is a relatively easy-to-handle task; however, pipelining crude oils that contain large proportions of high molecular weight compounds such as paraffins can cause many specific difficulties. Most of the complexity is related to the paraffin crystals forming an interlocking gel-like structure that changes some of the crude oils rheological features. The crystallization mechanism is mainly controlled by temperature. These oils, known as waxy crude oils, usually exhibit high "wax appearance temperature" (WAT) and high "pour point". Using a standardized test (ASTMD97), the pour point corresponds to an experimentally measured temperature at which oil has a tendency to gel or not to pour while it is being cooled. The word "high" describes situations where this temperature is higher than the external temperature conditions surrounding the pipeline. Below the pour point, the rheological behavior is characterized by thixotropic, temperature-dependent and shear dependent yield stress and viscosity.</jats:p><jats:p>The primary interest with waxy crude oils transportation is the issue of restarting [Perkins (1971), Sestak (1978)]. From an operational viewpoint, transporting waxy crude oil under steady flowing conditions is not a too complex operation. However, for maintenance, emergency or other reasons, the pipeline can be temporarily shut down. During shutdown, the temperature decreases in the pipeline, the gel-like structure builds up and the waxy crude oil undergoes thermal shrinkage. Gas voids appear and the fluid becomes unusually compressible, i.e. significantly more so than the base crude oil. Gas voids in the range of 4-8% of the total pipe volume, according to the shutdown conditions, are not unusual. Finally, the waxy crude oil restart issue consists of resuming the flow of a compressible flow of a gel-like viscoplastic and thixotropic material, usually by injecting some fresh warm oil (expected to be Newtonian and incompressible) at the pipe entry [Cawkwell (1987), Chang (1999)].</jats:p></jats:sec></jats:sec>

Topics

• impedance spectroscopy
• compound
• phase
• viscosity
• forming
• void
• molecular weight
• size-exclusion chromatography
• crystallization