Comparative Performance Assessment Study of a High-Speed Gamma-Densitometer in Horizontal and Vertical Multiphase Flows
Authors: Arubi Tesi Isaac Marcus , Yeung Hoi Cheong 
Published: October 2011

Abstract
Multiphase flow meters are increasingly replacing test separators but existing meters remains bulky, intrusive and expensive with a market penetration of approximately 0.3%. The ultimate target remains a low cost measurement solution to facilitate per well installation. Clamp-on gamma-densitometry could be exploited to facilitate cost-effective non-intrusive multiphase flow metering. High-speed (250 Hz) gamma densitometer units were installed at the top of the 50.8mm diameter, 11m high vertical riser as well as horizontally close to the riser base in the Cranfield University multiphase flow test facility. A comprehensive experimental data comprising of air-liquid two-phase and air-liquid-liquid three-phase flows were collected for caesium-137 radioisotope-based densitometers. Investigation of the gamma densitometer signal response identified the presence of quasi-periodic waveforms in the time-varying multiphase flow densities. A comparative performance assessment study of the density measurement for gamma densitometers was undertaken for both two and three-phase flows in horizontal as well as vertical pipe orientation. The mixture densities obtained from the gamma densitometers correlate quite well in both horizontal and vertical orientation with a percentage difference of ±0.5. However, discrepancies in the density measurement between the high and low energy levels of the Gamma densitometers were noted at 70% gas volume fraction (GVF) and above.

Introduction
Multiphase metering is an exciting solution to the growing production measurement issues in the petroleum industry. Oil and gas production operations is occurring in more remote locations and deeper water depths (e.g. BP's PSVM Block-31, offshore Angola is located in water depth of 2000m; also the Great White, Silvertip and Tobago developments in the Gulf of Mexico are in water depths ranging from 2360 to 2940m, [Letton et al, 2010]), and with increasing tieback distances, calling traditional measurement employing three phase separator well testing into question. Moreover, new oil and gas developments commingled with existing infrastructures leads to various royalty payment requirements and further complicate the allocation process. These issues, coupled with widening operating envelope and improve measurement quality, is driving the development of multiphase meters to realize their full potential for reservoir monitoring, flow assurance calculations, production optimization, and reservoir engineering analysis [Kelner, 2009], in addition to their traditional areas of application such as well surveillance or monitoring, well testing, production allocation metering and fiscal or custody transfer measurements.
Multiphase meters today are vital to oil companies' field development and production plans. This is because over the past decade multiphase measurement technology has undergone a significant transformation such that the number of multiphase flow meters (MPFMs) installed globally has continued to increase [Joshi and Joshi, 2007]. Industry analysts predict that there will be 1,000 additional subsea multiphase meters deployed by 2015 [Ruden, 2010]. A number of factors are responsible for this rapid uptake of multiphase measurement technology. These are improved meter performances, decreases in meter costs, more compact meters enabling deployment of mobile systems, increases in oil prices, a wider assortment of operators [Blaney, 2008] and the development of compact subsea meters.