Multiphase Flowmeter Working Principle

What are Multiphase Flow Meters ?

Multiphase Flow Meters (MPFM) are devices used to measure the individual oil, water and gas flow rates in a multiphase flow. The term MPFM is used to define also the metering of wet gas streams (i.e. multiphase flow where the gas content is very high).

A multiphase flow meter is a device used to measure the individual phase flow rates of constituent phases in a given flow (for example in oil and gas industry) where oil, water and gas mixtures are initially co-mingled together during the oil production processes.

Principle:

Magnetic resonance makes use of a fundamental property of atoms and in effect makes it possible to “count” hydrogen atoms. Since oil, gas and water all contain hydrogen atoms, multiphase flow can be measured using magnetic resonance.

For this purpose the fluids are magnetised and subsequently excited by radio frequency pulses. The hydrogen atoms respond to the pulses and send back echoes which are recorded. The amplitude of the echoes and the rate at which they decay is used to calculate the flow rates. The difference in magnetic resonance properties allows making a distinction between the flowrates of oil, gas and water.

Conceptually, a magnetic resonance multiphase flowmeter consists of three sequential steps:

1. Creating a net magnetization by aligning the magnetic moments of hydrogen atoms in an applied, constant magnetic field.

2. Perturbing the alignment of hydrogen atoms by employing electro-magnetic radio frequency (RF) pulses.

3. Detecting the radio-frequency signal emitted by the hydrogen atoms during their return to equilibrium alignment in the external magnetic field

This measurement principle also forms the basis of magnetic resonance based flow metering.

Advantages

Multiphase Flow Meters (MPFM)s are recognized to provide a number of additional benefits to O&G production:

1. Production allocation metering: MPFM can be employed to reconcile oil, gas and water measurement at all entry and exit points of a production network, minimizing potential serious economic consequences in terms of taxes and custody transfer due to incorrect allocation;
2. Fiscal or Custody transfer: when a single processing facility is shared by different producers, MPFMs can be used to meter the production from each license owner;
3. Production Optimization: multiphase flowmeters are able to deliver real-time, continuous data concerning production of the different phases; therefore they can be very useful for reservoir management and for optimization of the production;
4. Flow Assurance: The continuous measurement from MPFMs provide further information to detect potential issues in the production system.

Applications

Multiphase streams are typical of the O&G fields, since the producing wells normally generate mixture of liquid and gaseous hydrocarbons with a variable quantity of water. Knowing single rates of gas liquid and water is crucial for the production industry in order to monitor the reservoir, to improve the well performances and to optimize the production.

The traditional solution is represented by the test separators, large vessels where the different phases are divided and then measured through standard singlephase techniques. However, MPFM are becoming more and more appealing to the O&G Upstream industry, proving as a valuable alternative to the test separators.

Article Source: krohne & Abb

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