Selection of Artificial Lift Systems for Deliquifying Gas Wells Page 2
Annex 7 --- Calculation of Tuner and Coleman Critical Velocity
This annex presents methods proposed by Turner and Coleman to calculate Critical Velocity. The Turner Equation is correlated to flowing gas well performance data having surface pressures generally much more than 1000 psi. The Coleman Equation (Exxon) is correlated to flowing gas well performance data having surface pressures generally less than 1000 psi. Critical Velocity corresponds to the in situ gas flow rate that generates the velocity needed to lift liquid droplets out of the wellbore.
The Turner and Coleman equations and their associated graphs can be seen by opening the corresponding documents from the same web site where this page is displayed. Critical Velocity charts are used by field personnel to evaluate a gas well’s flowing conditions to determine if the well is experiencing liquid loading problems. Use of wellhead pressure and gas production rate is a quick way for the operator to determine if the well may be experiencing liquid loading problems.
For a more rigorous analysis of the well either the wellhead or bottom hole conditions should be used as the point in determining Critical Velocity. In general use wellhead conditions when determining Critical Velocity when the wellhead pressure is greater than 1000 psia and use bottom hole conditions when wellhead pressure is less than 100 psia. If the well is producing free water, for most wells when the wellhead pressure is less than 1000 psia use the bottom hole conditions for determining Critical Velocity.
In gas wells where the gas flow rate is above the Critical Velocity then liquid level is usually at the surface. Any liquid being produced with the gas or condensing due to temperature and pressure changes is usually uniformly distributed in the tubing. The gas velocity carries all the liquid as a fine mist or small droplets to the surface and a relatively light-uniform flowing pressure gradient is established in the tubing.
In flowing gas wells where the gas velocity is below Critical Velocity then liquid accumulates in the bottom of the tubing. A fluid level survey down the tubing will usually show a liquid level echo below the surface of the well. The flowing pressure gradient will show two values, a very light gas gradient above the gas/liquid interface and a heavier gaseous liquid gradient below the gas/liquid interface. Below the liquid level the flow is characterized as zero net liquid flow with gas bubbles or slugs holding up the liquid and upon exiting the gaseous liquid surface the gas flows the remaining distance up the tubing to the surface. Failure to flow gas in the well above the Critical Velocity can result in the accumulation of liquids in the bottom of the well and can result in a reduced gas production rate from the well.