Flow, level & control
standard conditions. This leaves the end-user to translate any numbers to the expected parameters and associated values themselves, if indeed the manufacturer has provided them with enough relevant data to achieve this. As this information is left to the manufacturers’ discretion, there are no standard requirements. Numerous combinations of technologies exist
on the market, and none is addressing the entire spectrum of parameters possible in multiphase or being systematically better than the others. When the end-user has selected an MFM, the fluid behaviour should also to be taken into account as this establishes the true performance at standard conditions. This step is especially important because it will highlight the level of uncertainty that should be achieved in the different fluid properties faced in field conditions. It should be remembered that it is the
combination of the performance of the MFMs under well-established flow and process conditions, and the estimation of the uncertainty of the relevant PVT package, from line to standard conditions, that will provide the overall uncertainty of the system in field conditions. This does not have to be physically in the field to establish the overall uncertainty for two reasons: (1) the uncertainty in field conditions is low due to a lack of accurate reference measurement. Test separators are rarely better than 5-10 per cent following the conditions, this can be highlighted by the fact that the allocation factor is in general smaller than one and in the range of 0.85, and (2) the capability to do the test in a full range of conditions, versus GVF (Gas Volume Fraction) and WLR (Water Liquid Ratio), is not possible most of the time. Therefore, the best option is a third-party laboratory, using the lowest possible uncertainty with a two-step analysis that uses (1) pure flow meter performance and (2) compatible equations of state with the associated uncertainty.
How do we address tHe in-situ flow meter performance?
There are two methods that can be used to address this. The first is to take the manufacturer’s statement, literature and the laboratory’s knowledge to establish the performance of the water, oil, gas at line conditions. To ensure estimates are correct, a unique analysis based on the Monte Carlo simulation is developed and the uncertainty established on the total mass flow rate and total volumetric flow rate, so that the performance of the liquid, gas, water and oil is established clearly. This innovative algorithmic computation gives a unique vision on the response of the MPMs based on a small set of industry-available recorded data. This performance analysis should then be
coupled with the PVT uncertainty performance; using PVT software developed by the UK’s Designated Institute for Flow Measurement over the last 40 years. This is based on a huge database of composition that
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is unique, as it is based on physical and real measurements collected over time, the opposite of many PVT simulators which aggregate the equations of state (EOS) with little consideration of the validity. This is the benefit of the Designated Institute for Flow Measurement developing a commercial solution for end-users and leading the development of industry standards. The third step is then to combine both
uncertainties from the MFM performance and from the EOS and propagate this to the standard conditions. This allows us to establish without any ambiguity the performance and how the meter will behave in field conditions. The second MPM performance review
method is at the well site, either by remote or physical witnessing. This is usually done if there is some doubt about the performance that requires secondary equipment to verify it, orwhen advice is required on the best metering solution to be defined as a reference. As indicated earlier, in field conditions, the uncertainty is way higher than what can be delivered in well-controlled
conditions such as in third-party facilities. After meter selection, a test programme is
established and a specific procedure defined to validate the response of the MFM, using engineering expertise and some statistical evaluation. It is then possible to understand the typical response of the MFM in the specific field conditions, identify the sweet spot and what should be avoided. This could result in a new manufacturer maintenance programme, the MFM’s replacement, or the installation of a complementary device following the end- user's expectation. Overall, the work to be done to state the
uncertainty accurately, and therefore the performance of MFMs, requires expertise and precise calculations. Thorough mapping of MFM performance against its in-situ application should be established by either oil and gas operators, or third-party multiphase flow meter experts - and validated when possible at a calibration facility.
TÜV SÜD National Engineering Laboratory
www.tuvsud.com/en-gb/nel
April 2021 Instrumentation Monthly
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