Why is Maintaining Critical Velocity Important?
Critical velocity is the rate of gas needed to keep fluids entrained in the gas stream and lifted to the surface. The higher the line pressure, the higher the flow rate needed. The larger the pipe or tubing size, the higher the flow rate needed. Density and volume of fluids to be lifted are also factors that can usually be overcome with “higher flow rates.” The pattern emerges, extra gas flow rate is needed to keep a well unloaded. But as wells age and lose bottom hole pressure, their gas to fluid ratio gets out of balance and they become more likely to start having loading issues. Add to this, new wells being brought online or surrounding wells beginning to use various production strategies that temporally raise line pressures and you can greatly affect a well’s ability to stay unloaded.
You can attack the line pressure issue by setting a compressor and reducing the head pressure that the well has to flow against. Now the 200 mcfd rate that was not keeping the well unloaded at 50-60 psi line pressure with 2-3/8” tubing can keep it unloaded and keep fluids entrained in the gas at 10 psi suction pressure of the compressor. Another strategy is reducing the tubing size or running a velocity string inside the well’s tubing can get a well’s flow rate back above the critical velocity need to stay unloaded. You don’t increase the flow rate, you reduce the rate needed to stay unloaded by reducing the production string size.
Soap sticks, capillary strings or injection of surfactants down the casing are designed to lighten fluid density and help it flow to the surface. The soap helps increase surface area of the liquid and makes it easier for the fluids to stay entrained in the gas stream. Sometimes it takes a combination of artificial lift options to keep a well unloaded. Usually in older wells, producing fluids is a must and depending on what stage of life cycle a well is in can help determine what strategy will be the most economical and produce the best results. Reviewing production data, fluid volumes produced, casing pressure and line pressure changes are all important to the artificial lift community and can lead to faster solutions to loading issues. Challenging, yes, as there is no one right answer and sometimes is comes down to “trial and error” and you’ll find out what works!