Low-Cost Measurement Option for Wet-Gas Flows to Optimise Reservoir/Production Management
Authors: Graham E. M. , Reader-Harris M. , Forsyth C. , Pinguet B. 
Published: March 2018

Venturi tubes are one of the most common types of device used worldwide for wet-gas flow measurement as they are a simple, robust and cost-effective flow meter. They also form the main component in the majority of commercial wet-gas and multiphase flow meters. Major operators acknowledge that more accurate measurement of wet-gas and multiphase flows can be used to optimise reservoir conditions and increase production by 5%. Hence there is a drive to improve the accuracy and increase the use of this technology. In the southern North Sea in the UK there are over a hundred Venturi tubes installed to measure wet-gas flows from wells to enable effective reservoir monitoring and eliminate the need for tests separators. The number of Venturi tubes installed in this one area is set to increase over the next few years with new field developments planned.

There are standards available for using Venturi tubes in wet-gas conditions; ISO/TR 11583 and ISO/TR 12748. However, these only cover limited conditions. The standards and past research has been directed at topside applications where adequate upstream piping lengths are available to ensure optimum performance and high accuracy. However, for more challenging offshore and subsea environments the piping configurations and pressure constraint to reduce space, cost and weight of equipment have limited the deployment of these meters with the recommended configurations and meant that installed meters have significantly larger errors. The UK Regulator for the oil and gas sector has produced flow measurement guidelines which recommend the use of the equations from ISO/TR 11583 when using Venturi tubes to meters wet-gas flows.

NEL has conducted tests to investigate the effect of upstream installations on a Venturi to mimic offshore and subsea conditions. The results indicated that the required upstream lengths were heavily dependent on the flow conditions: in some cases, the upstream lengths can be significantly reduced and in other cases the measurement errors were up to 9%. This is three times the uncertainty limits in ISO/TR 11583 and obviously a large impact on the reservoir management, allocation, and tax.

Most research has been for horizontal installations; this has limited the use of Venturis in vertical installation, which can reduce the footprint on platforms. Additionally, many of the commercial multiphase meters are installed in a vertical installation, and, using the same hardware but different models, could enable cost-effective wet-gas metering. NEL has collected new data and has been able to review the impact of upstream effects on the vertical installation of Venturis.

The data presented in this paper quantifies the impact of upstream installation effects on Venturis in wet-gas flows and when they are installed in a vertical orientation. It is anticipated that this will lead to new research and form the basis on which the current wet-gas standards/best practice will be updated to cover a wider range of installations for not only offshore and subsea installations but also onshore. This should enable use in more restricted installations with guidance available on the impact. This will be a leap forward in offering low-cost options for flow measurement offshore and subsea to ultimately enable increased production, and reservoir management knowledge, and reducing dispute on allocation.