Rafael Mendes - Petrobras/IFP
12 mai 2015
Transporting waxy crude oils through long pipelines at low temperatures is challenging due to the solidification of paraffinic components, which leads to a complex yield stress fluid behavior with time-dependent characteristics. This rheological behavior is strongly dependent on the flow and thermal histories, which has a critical impact for predicting flow restart after pipeline shut-in.
This work is focused on the analysis of the waxy oils rheological behavior, with the objective of improving predictions of flow properties of waxy oil transport in long subsea pipelines, especially the minimum required pressure for restarting the flow and the subsequent flow development.
The qualitative rheological behavior of a model waxy oil with macroscopic flow properties analogous to those of waxy crude oils is first analyzed using Magnetic Resonance Imaging velocimetry associated to stress measurements. The rheological behavior of the model fluid is evaluated for cooling under different shear conditions and for various flow histories at constant temperature. It is then possible to distinguish low intensity thixotropic effects, which happen when the fluid flows under a shear rate below its cooling shear rate or below the maximum shear rate it has ever experienced, and irreversible structure breakdown, which occur each time the fluid is sheared at a new maximum shear rate.
An additional set of rheometrical tests with two crude oils for various flow and temperature histories provides a general picture of the rheological behavior of waxy crude oils. These tests include flow restart at different shear rates, creep tests at different shear stress levels, abrupt changes of shear rate and steady flow, after cooling under static or flowing conditions. It was observed that the evolution from strong gel state to a less structured state does not directly depend on the shear rate, but on the total deformation. After reaching steady state flow, the analyzed crude oils mostly behave as a Newtonian fluids for shear rates below the maximum experienced shear rate. For a new maximum shear rate level, a progressive viscosity decrease towards the new equilibrium state is observed.
A new rheological model that represents the experimentally observed trends is proposed here. Additionally, a comprehensive study of the yield stress as a function of flow and temperature histories is presented. It provides an approach for describing the yield stress field inside the pipeline at the flow restart moment. Finally, all the knowledge developed in this work is applied in a complete flow restart simulation of a real scale pipeline. This allows translating conclusions from laboratory tests to a real pipeline scale.
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