A heavy challenge

Drill floor operations on the Rowan Norway. Image from Xcite Energy.

Work on the Bentley heavy oil field in the North Sea showed the significance of experimental data when unconventional fluids are expected to be transported through pipelines. Christian Chauvet explains. 

Wood Group Kenny's (WGK) flow assurance team was involved in the preparation for the extended well test (EWT) of wells 9/3b-7 and 7Z in the North Sea Bentley field, performed between July and September 2012.

The scope of work was to design the export pipeline to ensure that the processed oil would be exported safely and efficiently from the Rowan Norway jackup drilling rig topside facilities to the tanker, Scott Spirit.

The Bentley field, operated by Xcite Energy Resources, is in North Sea Block 9/3b, 160km east of the Shetland Islands. The entire field is estimated to contain about 900MM stock tank barrels of oil with an American Petroleum Institute (API) gravity between 10 ̊ and 12 ̊. The crude remains mobile under planned operating conditions, however, if allowed to reach ambient pressure and tempera- ture, the viscosity of the oil would be too high to allow practical flow.

The challenges of transporting such a fluid through a pipeline are obvious, but, in addition, the lack of information from analogues, prior to the EWT, made the modeling and predictions very difficult. Viscous fluids, like the oil from the Bentley field, do not behave like conventional oil durng transport, especially when they are colling down.

In a cross section of fluid, a steep gradient of temperature can be observed. This change in temperature is accompanied with a significant change in the fluid viscosity, hence the transport properties. Such fluctuations of behavior make the modeling of the transportation with the standard one-dimensional or more complex computer simulation tools very challenging. It is possible to take such properties into account, but the model- ing in this case relied heavily on accurate laboratory-derived empirical data in order to calibrate the numerical model.

The Bentley field. Image from Wood Group Kenny.

For operational reasons, a 2.1-km, 6-in. Manuli flexible rubber pipe was selected to transport the fluid. The initial flow assurance analysis focused on the heat conservation through the system in order to keep the fluid temperature as high as possible and a minimum level of flow, to retain heat, thus reducing the pressure drop. It was anticipated that it would be possible to export the processed oil with- out the help of a carrier fluid. Although the analysis showed that it would be possible, operating reasons indicated that it would be more efficient to export the oil with a carrier fluid, such as filtered seawater. The main reason for this was that in case of an unplanned shutdown, it would be very difficult to recover flowing conditions in the pipe. If the carrier fluid was not used, there was a danger that on cooling of the pipeline, the viscosity of the fluid would increase significantly and the pressure required to restart the pipeline would be higher than the design specifications of the pipe.

The presence of the carrier fluid changed the modeling approach. The flow pattern generated in the pipe by mixing the seawater with the processed oil through the export would have a strong effect on the behavior of the fluid throughout the pipe: Would both fluids stay separated; would they form a fully- mixed fluid or a very tight emulsion? Each possibility was investigated prior to the flow test. The flow pattern would significantly influence the pressure drop and heat conservation. The modeling relied exclusively on the laboratory data as, there was limited information avail- able in the literature. A full operating envelope was defined prior to the EWT.

WGK also assisted Xcite Energy during the EWT. The first data coming from the drilling rig and tanker indicated that there was a poor match between the model and the measured parameters. Even if the expected arrival fluid temperature was within the range of accuracy of the model, the pressure drop observed was significantly lower than predicted by the model. Therefore the modeling approach had to be re-evaluated.

Comparisons of the fluid clarity in samples taken at both ends of the pipe indicated that the pipeline was acting as a separator. The inlet topside samples showed a fully-mixed emulsified brown fluid, while the tanker samples showed clear separation between the phases, with clear water and black oil. However, oil and water flow measurements showed there was no differential hold up of any crude oil in the pipeline.

In-depth analysis of the data provided by the flow test showed interesting results— the pressure drop observed was slightly above that if the fluid was pure water, but much lower than if dry oil or oil emulsions were flowing down the pipeline.

An initial hypothesis was based on the fact that the shear generated by the export pump was not sufficient to create a stable emulsion. Therefore, the flow pattern could be anything between a tight stable emulsion and two fluids being fully separated. It would be possible that the pump would disperse the oil into small droplets that would be carried by the seawater. Unless enough water was in the pipe to fully surround the oil droplets, the overall viscosity of the fluid would be high. However, when a mini- mum volume fraction of water is reached, the pressure drop would decrease significantly. But, this is inconsistent with the low pressure drop experienced for water cut points above 20% and therefore an alternative hypothesis was sought.

The extended well test setup. Image from WGK.

The second hypothesis is that the pipeline, acting as a separator, would be at the origin of a core annular flow. A core annular flow is characterized by the viscous fluid forming a core in the middle of the pipe surrounded by the annular of the less viscous fluid, in this case the seawater. By assuming a core annular flow in the horizontal section and a mixed flow in the vertical section, this could explain the significantly lower than anticipated pressure drop. Although this type of flow has been observed in the laboratory with viscous oil, it has never been reported in a 6in. pipe. At the end of the test, the pipeline was flushed easily, indicating that the pipe surface was water wet. The combination of evidence, therefore, seems to support this hypothesis.

The project showed the criticality of reliable experimental data when unconventional fluids are expected to be trans- ported through pipeline.

The flow test and the data recorded prove that the heavy oil from the Bentley field can be transported in a controlled manner and based on the information gathered to date, the prediction tools would be able to reproduce the behavior of the fluid within the pipe.

Christian Chauvet is a graduate of the University of Poitiers in France. Chauvet has worked for different industries including the aerospace and automotive industry before moving into the oil and gas industry 10 years ago. He joined Wood Group Kenny (WGK) in 2010 and he is now the North Sea regional manager for flow assurance. In addition, he is the global head of CFD for WGK.