Application of Wet Gas Metering in the Gulf of Mexico

Abstract

Abstract The application of a new Multi Phase Flow Meter (MPFM) for monitoring the production of three gas wells in the Gulf of Mexico, Allegheny TLP, is presented. The meter is based on a sampling method, does not make use of ?-sources and is selfcalibrating. In the present application, it has been possible to compare the measurement of the MPFM with production data obtained from a conventional well testing system certified for production allocation. The accuracy of present measurements of the gas and total liquid flow rates is similar (or better) than the accuracy of the well testing system (2% of the actual readings). A special feature of this MPFM is the direct measurement of the condensate and water phase densities. This allows a very accurate measurement of the Water Volume Fraction, with an absolute uncertainty of WVF measurements equal to 0.001%. This value is two orders of magnitude better that the accuracy of other MPFMs under development. The measurement of the water phase density also permits an estimate of the methanol flow rate injected in the subsea pipeline to prevent hydrate formation. Introduction In recent years a number of multiphase flow meters have been developed mainly for oil field applications. These instruments are characterized by a large uncertainty unless they are properly calibrated by in situ methods. Errors become relevant when any of the commercial multiphase flow meters are used for wet gas metering (Gas Volume Fractions, GVF, = 90%). Besides the low accuracy, these flowmeters often adopt a ?- source to determine the mixture density. This makes their use troublesome in respects to regulatory requirements and potential risks to health, safety and environment. The problems encountered in the application of multiphase metering to wet gas led to the development of the flow meter presented in this paper, named VEGA, which is based on the isokinetic sampling of the gas-liquid mixture, followed by separation and metering of the individual phases. With respect to other MPFMs, the main feature of the meter is the advantage of self-calibration, [1], [2]. Moreover, sampling allows the water cut to be determined in the separated liquid with good accuracy rather than in the gas-liquid mixture. The development phase of the meter and the preliminary tests carried out at Trecate field, in Italy, with a 2" prototype are described in [1] and [2]. In these experiments the self dcalibration features of the VEGA were fully tested and appear to be a characteristic unique to this flowmeter. From the tests it has been found that the expected accuracy of both liquid and gas flowrate measurements was about equal to 2-3% of the actual readings. However, the accuracy of these measurements was affected by the presence of appreciable flow oscillations induced by the particular collocation of the meter and by the control system of the separator from which the gas and liquid streams fed to VEGA were drawn. Field tests conducted with VEGA essentially covered the case of wet gas (high GVF).