Liquid transport during gas flow transients applied to liquid loading in long vertical pipes

Abstract

An experimental study was carried out to investigate the liquid distribution and transport in vertical gas–liquid flows in a 42m long, 0.048m ID tube system. Liquid loading in gas wells is generally defined as the inability of the produced gas to lift the co-produced liquid up the tubing, resulting in liquid accumulation in the wellbore. The characterization of the liquid loading phenomenon is often based on field monitoring, with limited measurements of pressure and liquid holdup profiles, and usually without visual observations of the phenomenon.The experimental observations obtained in this work show the liquid re-distribution during changes in gas flow rate. These observations revealed that, after a change in gas rate, the liquid film starts to flow downwards in the pipe, and subsequently, the liquid begins to steadily flow upward at a constant velocity. The experimental data were compared with a numerical model, showing an agreement of ±25%. A pseudo-steady state approach was used to model the vertical gas–liquid flows in this long pipe to describe liquid loading in vertical wellbores. The liquid droplets entrained in the gas core were observed and estimated to be flowing upwards in order to obtain a good agreement between experimental observations and model results, even though the gas velocities were lower than Turner critical velocities. The widely accepted droplet model of Turner says that below the critical velocity, the liquid droplets should be flowing downward and not upwards.