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Virtual flowmeters

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A virtual flowmeter (VFM) can be used as a backup or an alternative to physical meters. Currently, VFMs are mainly used in a backup role but recent developments could change the future of these meters, thus changing the future of the industry.

Applications

Virtual flowmeters work by calculating flow based on existing instrumentation, knowledge of a facility and fluid properties, which is made possible with the use of correlations that relate the flow rate to the pressure and temperature drop through the system.[1]

Complementing physical flowmeters

In certain cases, virtual flow metering can be considered an alternative to physical metering, thus far it has been more successfully applied as a complementary technology. When applied in this manner, the different measurement principles of both methods can be used to cross-correlate actual and predicted flows to improve accuracy. Virtual flow measurement has an attraction in that the prediction solution is relatively insensitive to the loss of one or two physical measurements across a multi-well field.

Replacing physical flowmeters

While the industry widely uses VFMs as backups to physical meters, it has still not accepted them as alternatives to physical meters, or even as tools to supplement physical meters.[2]

The Letton-Hall Group conducted a recent study into the viability of VFM systems, comparing five VFM software packages that are commercially available. To gain a full picture of the reliability of VFMs, the group ran three rounds of evaluation on production data from a subsea well with a subsea multiphase flowmeter. First, they ran a test where models were not modified in any way. Then, they tuned the meters with elemental computing parameters for subsea multiphase meters (i.e. total mass flow rate, gas volume fraction, and water-liquid ratio). Lastly, they factored fluid property errors into their measurements.

The first round of testing showed that the production data used were valuable in comparing the methodologies of each flow model in arriving at the given flow rates measured by the flowmeter. In the second round, the team found that the gas volume fraction could be estimated accurately if the total mass flow rate of each model was tuned. Gas flow rate predictions grew more uncertain over time, however each package was consistent regarding the anchoring role that the total mass flow rate parameter played in the VFM tuning process. All of the flow rate predictions showed a less than 5% deviation from the measured oil flow rate and less than a 10% deviation from the measured gas flow rate.

Though the presence of anomalous input data showed an impairment in the flow rate predictions of VFM, it is not conclusive as to whether it is a serious concern. More studies need to be conducted, but VFM has potential as more than a backup.

References

  1. Offshore Engineer. 2013. Virtual flow metering enters reality. http://www.oedigital.com/pipelines/flow-assurance/item/2631-virtual-flow-metering-enters-reality.
  2. Whitfield, S. 2015. Virtual Flowmeters: A Low-Cost Alternative. SPE News. http://www.spe.org/news/article/virtual-flowmeters-a-low-cost-alternative.