A solution to measure the flow rate of wet gas

ABB’s Mark Niblett and Ian Robertson discuss the key benefits that wet gas meters provide in comparison with expensive separators, multiphase metering, and traditional single phase DP producing devices such as orifice plates.

Wet gas metering is becoming increasingly important in the development of marginal oil and gas fields. Wet gas flow measurement is increasingly gaining acceptance in replacing expensive well test separators and related infrastructure with individual wet gas meters at each wellhead. Wet gas metering is at the high gas fraction end of multiphase metering, typically with a gas volume fraction (GVF) above 90%, and mostly above 95%. Standard multiphase meters cannot operate satisfactorily in such conditions. The development of wet gas meters therefore meets a key requirement of the oil and gas industry.

At present there are relatively few wet gas meters established in the market, although several manufacturers are currently promoting or developing such meters. In order to develop a wet gas meter, there must be a good understanding of how the constituent components behave in wet gas.

Although defined in different ways, depending on the type of process in question, ‘wet gas’ is essentially a flowing mixture of gas and liquid, where the gas forms the major part of the mixture. ISO: DIS14532 / ISO TC 193 Natural Gas refers to it as ‘gas containing free water, water vapour and/or liquid hydrocarbons’.

Under ideal circumstances, accurate measurement of wet gas should be straightforward, by using the ratio of gas (the gas volume fraction, or GVF) to liquid (the liquid volume fraction, or LVF) to calculate the total volumetric flow.

However, for reasons that will be outlined, this has been almost impossible to achieve to date. Instead, operators have needed to invest in expensive separation plant to try to achieve not only the best levels of gas measurement but also measurement of the valuable constituents of the liquid phase.

Traditionally the mixture of fluids at the wellhead is separated into its various phases. Proper separation and accurate measurement of each of the component phases is critical in determining production capabilities. Gas is separated from the liquid by passing the emitted well fluids through a separator. The phases are then further separated and measured independently. Using a flowmeter (instead of separators) to determine the gas and liquid phases gives a very significant reduction in CAPEX in the evaluation of new wells.

Multiphase meters are considered exceptional in the industry as true multiphase can be measured with a relative high degree of accuracy. Furthermore, even these technologies have drawbacks for operators, including:

1. These meters are mounted in the vertical plane and quite often the velocity of flow is not enough to lift the gas, especially in the case of low flows at marginal wells

2. These multiphase meters generally do not like condensate

3. Any bends in the pipework before and after the point of measurement may mean the measurement itself may not conform to international standards such as ISO5167

4. As this technology may use a Gamma source (ISOTOPE) some unmanned platforms may also be seen as a security risk

Orifice plates can be used. However, obvious challenges occur with these simple devices, namely:

1. High liquid hold-up

2. Poor self clearing

3. Wear, deposition and damage (slugging) susceptible and no natural mixing as with a venturi

Wet Gas Flowmeters (WGFM) are becoming the economical choice of operators due to their use of Venturi technology, which provides a very low pressure loss and also natural mixing due to the conical leading cone. The Venturi is also very robust when possible slugs / liquid hold up are present and also can handle relative low flows in the horizontal plane.

WGFM technology is also becoming simpler, with manufacturers like ABB able to provide enhanced solutions by using GC (gas chromatography) and FT-IR/NIR (Fourier Transform Infrared / Near Infrared) in its analysis of the gas composition providing operators with increased real-time intelligence of their process.

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