Multiphase Flow Measurement Using Multiple Energy Gamma Ray Absorption (MEGRA) Composition Measurement
Authors: Scheers A.M. , Slijkerman W.F.J. 
Published: December 2007

Some multiphase flowmeters use the principle of Dual Energy Gamma Ray Absorption (DEGRA) composition measurement to determine the individual water, oil and gas fractions. Under homogenous flow conditions the ultimate uncertainty in phase fractions achievable with this technique depends strongly on the choice of hardware. The meter presented in this paper uses unique components that have been optimized for the water, oil and gas fraction measurement with theoretical uncertainties of 2% in the fractions over a 1 second measurement period. Generally, composition meters are sensitive to a change in production water salinity and this will cause significant systematic errors in the fraction and watercut measurements. A new measurement concept is presented that is not sensitive to salinity variations and based on Multiple Energy Gamma Ray Absorption (MEGRA) composition measurement. A multiphase flowmeter equipped with the MEGRA concept does not require field-calibration, a decisive advantage in subsea or marginal field developments.

Introduction

A multiphase flowmeter is a device, or a combination of devices, to measure the individual flow rates of water, oil and gas in a multiphase flow environment. The ultimate aim for multiphase meter technology is to replace the measurement function of the large and expensive test separator. Multiphase metering should be considered as a means of providing measurements where conventional test separators would be uneconomic or impractical. All measurement concepts used in multiphase meters have their specific areas of application. Considerations such as flow rate, Gas Volume Fractions (GVF), watercut, flow regime and the uncertainty achievable under all prevailing conditions should be carefully considered in the selection process (Ref. 8). In addition, operational aspects (e.g. calibration, application of radioactive sources) and costs (both capital and operating expenditures) should be given careful thought.

Most multiphase flowmeters consist of a total fluid (water, oil and gas) flow rate measurement combined with a composition measurement. In homogeneous flow, these two measurements can be directly integrated resulting in individual water, oil and gas flow rates. In non-homogeneous flow regimes, either advanced flow models are required, or conditioning devices, such as mixers or compact (in-line) separators upstream of the multiphase flowmeter, should be used.

Various types of multiphase composition meters are currently being developed, the majority is based on the measurement of impedance or on gamma ray absorption. Applying gamma ray absorption techniques has the advantage that it is, in contrast with the impedance principle, able to cope with any oil-water ratio. Also the non-intrusiveness of the method should be considered as an advantage.

Composition measurement based on DEGRA. As a further introduction the basics of Dual Energy Gamma Ray Absorption (DEGRA) measurement are explained. The principle is based on the absorption of a narrow beam of two - or X-rays of energy e1 and e2. In a pipe, with inner diameter d, containing a water, oil and gas mixture, the measured count rate Im(e) is:

(1)

Iv(e) is the countrate when the pipe is vacuum and i, represents the linear absorption coefficients for the water, oil and gas phases.

P. 203