Critical velocity calculations in the form of charts or simple equations are frequently used by field personnel to evaluate a gas well's flowing conditions to determine if the well is experiencing liquid loading problems. Literature detailing the critical velocity necessary to keep a gas well unloaded suggests using the conditions at the top of the well as an evaluation point. This is convenient for personnel conducting the evaluation as wellhead pressure and temperature data are readily available. A number of situations exist where the use of the wellhead as the evaluation point can lead to erroneous conclusions. The most obvious situation occurs with a change in geometry downhole when a tapered tubing string is run in a well or when the tubing is set above the perforations. In these instances a more robust evaluation results from using conditions at the bottom of the well and the downhole tubing geometry. Other conditions exist where the use of downhole conditions provide a better evaluation point. The assumptions used in the development of the standard, simplified form of the critical velocity equations and charts may not be appropriate for downhole application. In these cases the fundamental equations must be used. The calculation of critical velocity requires knowledge of pressure, temperature, produced fluids and PVT properties. The determination of critical rate requires the same properties with the addition of pipe diameter. The required PVT properties including surface tension and density for both the gas and liquid phases are reviewed. Correlations to calculate water-gas surface tension were found to have excessive error so a new, more accurate method is presented. This paper provides recommendations when the use of a surface or downhole evaluation point is more appropriate in the determination of the minimum critical gas velocity for a well.
The calculation of critical velocity is frequently used by the operators of natural gas wells to determine the gas production rate required to prevent liquids from accumulating in the well. Turner 24,25 developed a method for calculating critical velocity which has gained wide acceptance and use within the industry. In order to efficiently lift water to the surface, gas wells should produce in the mist flow region where liquid exists as a film on the wall of the pipe or as droplets within the flow stream. The basis for Turner's method is the determination of the gas rate necessary to overcome the terminal fall velocity of a liquid droplet which Turner determined to be the phenomena controlling liquid accumulation in a well. For liquid droplets that are roughly spheroidally shaped, Turner presented the following equation for calculating the terminal fall velocity of the droplet. The required gas flow velocity to keep the well unloaded then equates to this terminal fall velocity.