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Workflow Automation and Intelligent Control 173

Fig. 5.12 plots relative errors between a flow meter with a well test (WT)
and a VFM with a WT. Fig. 5.12A shows the results of a VFM and a flow-
meter, which has a correlation factor of 0.80. These results are because of the
highest points observed in the blue square. Those points are attributed to the
fact that VFM responds quickly to changes in pressure, whereas, the flow
meter measures the flow instabilities during choke changes. Fig. 5.12B
shows the VFM relative error to the WT versus the FM relative error to
the WT. From this plot, we can make the following conclusions:
• VFM and FM relative errors compared to WTs are positive (quadrant I,
Fig. 5.12B). We can infer that the WT is wrong, or both the FM and
VFM need calibration. There are points beyond the 15% error radius;
both the VFM and FM are reading high rates compared with the
WT. These points beyond the error radius are attributed to the rates after
changing the choke; however, to avoid ambiguities, the WT must be
repeated.
• VFM negative and FM positive (quadrant II). In this quadrant, few
points are observed. The VFM needs to calibrate the multiphase flow
equation coefficient or PVT data. The plaque orifice needs to be cali-
brated and the FM needs better tuning.
• VFM and FM are both negative (quadrant III). The WT value is greater
than the FM reading and the VFM calculation. The WT must be
repeated.
• VFM is positive and FM negative compared to the WT (quadrant IV).
Repeat the WT, or calibrate both the FM and VFM. These values are
very common when the gas flow is under critical conditions.

5.3.4 Benefits and Disadvantages of Using VFM
If economics allow, it is better to have flow meters on each well. However,
VFM offers several benefits such as
• Monitors in real-time multiphase flow gas, oil, and water for all wells in a
field.
• Allows operators and engineers to be prepared and react quickly to
events, to reduce production downtime.
• Reduces the cost of installing and maintaining multiphase flow meters.
Well-tuned VFM software with sufficient data can achieve similar or
better results than physical equipment which may drift from calibration.
• Achieves faster and direct production-allocation process compared with
typical back-allocation systems which have larger errors.