Prediction of Critical Gas Velocity of Liquid Unloading for Entire Well Deviation
Authors: Carpenter Chris 
Published: October 2016

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 17846, “Prediction of Critical Gas Velocity of Liquid Unloading for Entire Well Deviation,” by J. Li, Trican Well Service, and F. Almudairis and H. Zhang, The University of Tulsa, prepared for the 2014 International Petroleum Technology Conference, Kuala Lumpur, 10–12 December. The paper has not been peer reviewed.
Liquid loading is a common production problem in mature gas wells. Correct prediction of the critical gas velocity at which the well starts to load is very important to the operators because it may allow them to take appropriate measures to prevent liquid loading and extend well production life. In this paper, a database related to the critical gas velocity is compiled with data from published literature. A new model is developed and evaluated with the experimental data.
Introduction
Gas wells often produce water and other condensate. In the early production stage, the gas-flow rate is high enough to carry the produced liquids to the surface. As the reservoir pressure is depleted, the produced-gas-flow rate decreases until the gas reaches a critical condition, at which time liquid loading is initiated. At the inception of liquid loading, the gas-flow rate is not sufficient to carry the liquid completely to the surface and the liquids start to accumulate at the bottom of the well. A backpressure then builds up, and the increased reservoir pressure will eventually be sufficient to lift the liquid to the surface. Once the liquid slug is pushed out by the gas, the liquid starts to reload the well again and the cycles are repeated until the well is eventually loaded completely and gas production stops. The most common liquid-loading symptoms observed in the field are pressure (or pressure-gradient) fluctuations and reduction of gas production.
In general, there are two different liquid-loading principles proposed in the literature: liquid-droplet fallback and liquid-film reversal. The complete paper contains a literature review that follows the development of these principles.
Data Compilation
Liquid-loading-initiation mechanisms have been investigated in 3-in.-innerdiameter pipes with deviations of 0, 15, 30, and 45° from vertical. Others have used the same flow loop to study the liquid loading at deviations of 60, 70, 80, 85, and 88° from vertical. The superficial gas velocity ranged between 2 and 40 m/s, and the superficial liquid velocity varied from 0.01 to 0.1 m/s. Approximately 450 tests were performed. Pressure gradient, liquid holdup, and liquid-film thickness at the bottom of the tubing were also measured. Highspeed video cameras were used to observe flow pattern and film reversal. Complete film-reversal transition was applied to determine the critical gas velocity for the liquid-loading initiation. In the vertical case, the liquid film around the pipe periphery is almost uniform, and in the deviated case, it is nonuniform. In the latter, there is more liquid accumulated at the bottom of the pipe than at the top. This difference in liquid-film distribution around the pipe yields a difference in flow-pattern transitions and film reversal.