Virtual Flowmeters: A Low-Cost Alternative

The use of virtual flowmeter (VFM) systems can help lower the costs and improve the reliability of subsea multiphase metering efforts. However, despite the maturation of VFM technology and the widespread use of VFM models as backups to physical meters, the industry has still not accepted it as an alternative to the physical meter or even as a tool to supplement physical meter measurements, an expert said. Speaking at a technical section on Tuesday at the Offshore Technology Conference in Houston, Amin Amin, senior technical adviser at the Letton-Hall Group, discussed the study his group conducted into the viability of VFM systems. Amin compared five commercially available VFM software packages by using production data from a subsea well with a subsea multiphase flowmeter. He ran three rounds of evaluation. The first was an “out of the box” test, where the models were tested without any modifications. In the second round, the meters were tuned using elemental computing parameters for subsea multiphase meters such as total mass flow rate, gas volume fraction, and water/liquid ratio. In the third round, Amin factored fluid property errors into his measurements. “We really wanted to have a complete vote of confidence of VFMs,” Amin said. “Here was a real test case for us.” After the first round of testing, Amin noted that the real production data were valuable in comparing the methodologies used by each flow model to arrive at the given flow rates measured by the subsea multiphase flowmeter. In the second round, he found that the gas volume fraction could be estimated to a sufficient degree of accuracy by tuning the total mass flow rate in each model. Though the gas flow rate predictions grew increasingly uncertain over time, each package was consistent with regards to the anchoring role that the total mass flow rate parameter played in the VFM tuning process. The flow rate predictions from all participating models showed less than a 5% deviation from the measured oil flow rate and less than a 10% deviation from the measured gas flow rate. Adjusting the models to account for fluid properties did not change the results much. The deviation of individual phase densities had little effect on the flow rate projection, provided that the total mass flow rate used in the tuning process was accurate. Amin said the dependency of VFM models on the mixture density measurement at a given point for the purpose of defining the total mass flow rate is greater than it would be on the individual phase densities. Amin found that the presence of anomalous input data can impair the flow rate predictions of VFM systems, but with only one case study of note, it is hard to determine if this is a serious concern. He said more work is needed to establish a systematic approach to understand and interpret VFM systems in order to identify anomalous devices. “The systematic errors can be compromising, so the detection of these errors may not be as easy as one might expect from this system,” he said.