High-gas-volume-fraction measurement of wet gas using a gamma-ray sensor at different mounting angles

Abstract

With the development of oil and gas fields in the South China Sea, the focus is on measurement accuracy pertaining to wet gas around subsea wellheads. Owing to limited space and safety with regard to subsea installations, the horizontal installation of a wet gas meter is preferred. However, the phase distribution of wet gas with a high gas volume fraction (GVF) in a horizontal pipe is non-homogeneous and causes uncertainty in the measurement of void fraction using the single-beam gamma-ray method. To resolve this issue, this study aims to determine a sensor position to best estimate the gas void fraction of a wet gas flow. An experiment was designed and conducted to study measurement results by mounting a gamma-ray sensor at different angles. A computational fluid dynamics numerical simulation was performed to investigate the phase distribution at the Venturi throat, and the resulting GVF based on the slip ratio and void fraction was used for comparative analysis considering the input void fraction as reference. The comparative analysis shows that the measurement errors at mounting angles of 0° and 90° are large in numbers, and optimal results are achieved at 45°. Therefore, to obtain accurate measurement results in horizontal pipes, an optimal mounting angle of gamma-ray source and detector relative to horizontal is recommended for the single-beam gamma-ray void fraction measurement method.