Objective Vortex precession flowmeters or differential pressure flowmeters are prevalently used for metering wet gas flow at wellheads in Yan'an Gas Field. However, single-well gas production metering often involves significant errors, and it is challenging to accurately determine liquid output for individual wells. To tackle these challenges, an experimental study was conducted to investigate metering rules of gas-liquid two-phase flow and a metering model was developed. This study aims to provide valuable reference for gas-liquid two-phase metering practices within Yan'an Gas Field.
Methods Leveraging the characteristics of “under-reading” in vortex precession flowmeters and “over-reading” in Venturi flowmeters, these two kinds of flowmeters were combined in series. This combined metering approach was used to conduct a multiphase flow loop experiment using an air-water medium. This experiment focused on examining the variations in precession frequency and Venturi pressure drop with different liquid volume fractions (LVF) under varying operating pressures and superficial gas velocities. Furthermore, based on the principle of single-phase flow measurement, dimensionless parameters that affect frequency signals and pressure drop signals were introduced through a dimensional method. A combined frequency-pressure drop model for wet gas metering was rationally established, incorporating exponential correction terms for these dimensionless parameters.
Results With the increase of LVF, the precession frequency decreased gradually while the pressure drop of the Venturi flowmeter increased, embodying their respective tendencies of “under-reading” and “over-reading”. The “frequency-pressure drop” signals became distorted at an LVF exceeding 1%. Through field experiments, the efficacy of the devised combined metering model was verified for wellhead metering at the on-site gas wells. The resultant gas-phase volumetric flows, attained through a solving process, displayed a maximum error of 5.2% and an average relative error of 2.5%. Concerning liquid-phase volumetric flows, the maximum error reached 46.8%, with an average relative error of 15.0%. LVF calculations exhibited a maximum error of 0.15% and an average absolute error of 0.05%.
Conclusion The proposed metering model that combines vortex precession flowmeters and Venturi flowmeters demonstrates controllable measurement accuracy when dealing with LVFs below 1%. This range aligns well with the LVF distribution typically observed in the natural gas from the gas-producing wells in the Yan'an Gas Field. The study outcomes offer valuable technical insights to support the meticulous management of gas wells.
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