Evaluation of Dry-Gas Meters in Wet-Gas Conditions: Turbine and Venturi Meters

Abstract The accurate and reliable measurement of gas flowrate in wet gas conditions continues to be important to the Oil and Gas Industry. The ability to measure wet gas flows is particularly crucial in small, marginal developments that would otherwise be uneconomic to exploit due to the requirement for expensive separation equipment. This paper presents the results of two independent studies conducted on the new NEL high pressure wet gas test facility that uses nitrogen and a kerosene substitute (Exxsol D80) as test fluids to simulate a natural gas/ condensate system. The first study involved the the testing in wet gas of a 6- inch turbine meter in liquid loadings of up to 2% by mass at a pressure of 60 barg and pipe Reynolds numbers from 1 to 9 million. The maximum shift in the turbine meter factor found due to the presence of the kerosene was -0.75% at a Reynolds number of 2.7 million. Mechanisms that could cause the observed shifts are provided. These results presented here differ from published data taken for a similar test arrangement. Possible explanations for the disagreement are given. The second study was performed on a 6-inch (0.55 diameter ratio) Venturi meter manufactured to the ISO 5167 standard. Liquid contents up to 5% by volume, gas velocities from 6-15 m/s and line pressures of 20, 40 and 60 barg were used in the test programme. The calculated overreadings in gas flowrate are consistent with existing data. Introduction The metering of multiphase flows in the field is becoming more common due to requirements for the reduction of costs through removal of expensive separation equipment and the improvement of well management via sufficiently accurate and reliable real-time data. Wet gas flow measurement (as a subset of multiphase flow) is receiving particular attention by the industry due to limitations of existing multiphase flow measurement devices in the required high GVF range. The continued study of the response of current metering technologies using modern experimental test facilities that simulate field conditions is vital to the development of reliable techniques that can be used for determining gas flowrate in high GVF systems. Just such a facility has been available to the Oil and Gas Industry at the National Engineering Laboratory (NEL) since the summer of 1999. The results of two separate wet gas studies using this facility are reported in this paper. Typical examples of a turbine meter and Venturi meter have been exposed to high pressure wet gas flows and the responses obtained compared with their normal dry gas behaviour. The aims of each test programme were slightly different due to the nature of each meter type and their likely applications. For the turbine meter installation in a wet gas line is not advisable due to the likely damage that the meter would sustain. However, there are occasions when liquid can enter a dry gas metering run and therefore it is desirable to know the effect that a small quantity of liquid may have on the calibrated meter factor.