In the present paper, the development of a low cost multiphase flow meter, designed for an oil field where it seemed advisable to avoid the use of radioactive sources, is illustrated. This meter combines the measurements of the pressure differential through a multiphase orifice and in a vertical tube. When required, the composition of the liquid-liquid mixture can be determined with a semi-continuous sampling method. Before installation, a set of tests, performed in a laboratory where it is possible to operate at actual field conditions, allowed the development of two semi-empirical equations that enable an accurate evaluation of the gas and liquid flow rates.
Over the last 25 years, the Oil Industry strongly supported the development of Multiphase Flow Meters (MFM), with significant investments in particular oriented towards subsea applications of this technology. Notwithstanding these efforts, the accuracy of MFMs has been and still is disappointing. To some extent, this is due to the complexity of the flow system and to a number of mechanical or physicochemical effects, such as solid deposition, erosion, corrosion, that alter the flow conditions inside the meter. On the other hand, the complexity of the flow system would suggest the development of "as simple as possible" MFMs, but often this is not the case. To give an example, the use of a radioactive source to measure the mixture density, requires a careful analysis of gamma ray attenuation through the pipe wall and the multiphase stream, introduces a number of potential errors and definitely represents a cumbersome device . At the same time, it only provides an indirect measurement of the no-slip liquid volume fraction, based on some type of empirical equation of limited validity.
A second example is the measurement of the water-cut. Quite often, in a MFM the watercut is determined from the electrical impedance or the dielectric constant of the multiphase mixture. The electrical properties of a multiphase stream depend on the gas volume fraction and the composition of the liquid mixture. In particular, they depend on the transition from an oil continuous to a water continuous flow pattern and, when the water is the continuous phase, on the water salinity. However, the identification of this transition and the knowledge of the water salinity may represent additional problems in this type of measurements .