Multiphase Flow Meter Solutions Defined
ARC defines a multiphase (i.e. multi-component) flow metering solution as being able to measure three or more non-homogenous phase fluids, such as oil, gas – wet and dry, condensate and water. There are several major multiphase flow regimes recognized when describing flow encountered in oil and gas wells: bubble flow, plug flow, slug flow, stratified (smooth) flow, wavy stratified flow, and annular mist flow.
Early developments of multiphase flow metering solutions began back in the early-to-mid 1990s as end users struggled to find more cost effective and efficient methods to conduct well testing, well monitoring, production allocation and much more. Over the last two decades suppliers and related stakeholders have worked together to develop more accurate, more reliable and effective solutions to meet the challenges encountered when measuring multi-component phase flow.
Every known physical multiphase flow metering solution relies on a com-bination of at least three or more different sensing technologies, one of the most common of which relies on the Venturi tube using differential pressure sensors to measure flow rate, combined together with another technol-ogy such as gamma ray attenuation to calculate the density and composition of the fluid and the mass/volume fractions of oil, gas and wa-ter. Some suppliers even rely on incorporating array cross-correlation using capacitance and conductance measurements as capacitance sensors work best in oil-continuous flow, while conductivity sensors work best for water-continuous flow.
Some suppliers have tried relying on RF or microwave technologies through either resonance or absorption since microwave can exploit the difference in permittivity of the multiphase components to determine the individual phase fractions of the mixture. ARC has only found two suppli-ers, Agar and GE Oil & Gas, which employs microwave technology along with advanced Coriolis and dual Venturi. In many cases suppliers also employ pressure and temperature transmitters to help develop the neces-sary PVT correlations (Pressure, Volume and Temperature) that help understand the effects on viscosity and density as downhole pressure makes the base oil more viscous and dense, whereas temperature has the opposite effect.
The combination of these different measurements are complemented with advanced flow software modeling capability sufficient to provide as low a level of uncertainty among the various inputs being measured based on some key parameters, such as gas volume fraction (GVF), water-liquid ratio (WLR), liquid flow rate, and gas flow rate, among other parameters such as oil density, oil viscosity and salinity. The output generated from any MPFM solution must be provided in a format required for production re-porting at standard conditions as they are required for the respective governmental regulatory body of a specific country.
Gamma Ray Attenuation Multiphase Flow Metering Technology
Gamma ray attenuation multiphase flow meters have become the largest growing segment in multiphase flow metering market as a result of the ability to handle GVF above 95 percent and the ability to handle other challenges posed by factors such as slugging and developing a better under-standing of the composition and density. While gamma ray attenuated multiphase flow meter technology has been available for decades, only in recent years has it become relatively more widely adopted. The three most common types of radioactive materials employed include Americium 241, Barium 133 and Caesium 137. The number of gamma rays passing throughout the pipe depends on the composition of the multiphase mixture inside the pipe and therefore indicates the composition of the flow. For ex-ample, gas is a weaker absorber of gamma rays, while water is a stronger absorber.
Users have begun to realize the many benefits gamma ray attenuated multiphase flow meters offer, including higher GVF accuracy, directly measuring density of fluid mixture, a non-intrusive measurement, and low total cost of ownership (TCO) in comparison to established mechanical flow meter technologies. However, since end users are operating in countries or regions in which the use of a radioactive based multiphase flow meter is not allowed and/or condoned, some suppliers have been developing MPFM solutions which can provide reasonably accurate levels of perfor-mance without the use of radioactive based density measurement.
Non-Radioactive Multiphase Flow Metering Technologies
For those suppliers that are deploying multiphase flow metering solutions which do not employ any radioactive technology, there are different combinations of sensing technologies that are suitable for a more limited set of applications, particularly as it relates to GVF less than 90 percent. Some of the non-radioactive technology combinations suppliers have used in MPFM applications include advanced Cor-iolis combined with a multivariable pressure and temperature transmitter, an integrated water-cut meter, microwave and a dynamic flow computer which calcu-lates certain parameters based on physical and/or simulated inputs of net oil flow rates, water flow rates, total liquid flow rates and gas flow rates in standard volumes. More typical is the use of Venturi, cross-correlation using capacitance and conductive measurement and a third technology which typically has been patent pro-tected employing some advanced simulation algorithms or advanced sensing technology, such as computer tomography or geometry sensors as is the case with MPM and Roxar, respectively.
Emerging Technologies - Virtual Flow Metering Solutions
Given the challenges found in some challenging subsea environments and for more mature wells which may not be sufficiently productive to support a more expensive MPFM solution, some users are testing virtual flow metering (VFM) solutions suitable for applications that measure the flow of several types of media, such as liquids, gases, and saturated and superheated steam. In subsea applications, VFM solutions are being tested to determine if they can represent complementary and/or replacement alternatives to physical MPFM solutions as they have not been operationally reliable in some of the more challenging subsea environments.