Impact of surface choking on gas-lift stability and flow behaviour in oil producing wells

In this work, we investigate the effect of tubing head choking on gas-lift stability and flow regime transition in the vertically upward cylindrical pipe during gas-lift operation. For this purpose, a laboratory scale multiphase flow rig was designed, fabricated and assembled. lift gas injection rates of 1.13e−4 m3/s and 1.87e−4 m3/s were imposed in a 1 mm internal diameter (ID) single nozzle gas-lift injector connected to a rotameter and a liquid flow rate of 7.7e−6 m3
/s was set in a speed potentiometer connected to a positive displacement gear pump. Tubing head choking is implemented by setting the system to 0% closed (fully opened) to partially closed – 15%, 25% and 50% opened (85%, 75% and 50% choking). For each choking condition, the transient pressure at four measurement points located at length to diameter ratios L/D of 0.0;18.3;36.7, and 53.3 was recorded. Fluid flow structure and regime transition are distinguished and characterised based on the visual observations, photographic and slow motion videos, recorded for every single cycle of experiment run in the production test sections from L/D = 18.3 to L/D = 53.3. It was found that
increasing the choke up to 85%, a transition from random peak-to-peak oscillation of pressure to a cyclic periodic oscillation is observed and this behaviour is accompanied with increase in pressure along the production tubing. The effect of topside choking on bubble size showed a drastic reduction of the size of the leading bubble as it flows upwardly with a maximum perimeter of 35.65 mm the flow is characterised by low vorticity and turbulence of the liquid film when the choke is increased up to 85%. Based on the experimental results, the topside coking technique as flow stabiliser during gas-lift operations has to be conjugated with knowledge and understanding of fluid flow structure transition along the production tubing