Liquid loading is a common production problem in mature gas wells. As the formation pressure decreases and gas flow rate also decreases, the gas kinetic energy is unable to lift the liquid phase completely to the surface. Liquid starts to accumulate at the bottom of the well, which increases the back pressure of the reservoir. This is reflected in a fluctuated production of liquid and gas rates. At the same time, it significantly reduces the production rate and eventually kills the well. Correct prediction of critical gas velocity at which the well starts to load is very important to the operators since they could take appropriate measures to prevent liquid loading and extend the well production life.
In the past, most studies on liquid loading focused on vertical wells. Only recently have a few studies been conducted to deal with the liquid loading for deviated and horizontal wells. To date, there are few published studies discussing how to predict the critical gas velocity across a wide deviation angle range. In fact, there are contradictory conclusions made by different researchers. 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. The gaps in the previous studies and modeling are also discussed.
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 the liquid loading is initiated. At the inception of liquid loading, the gas flow rate is not enough to carry the liquid completely to the surface and the liquids start to accumulate at the bottom of the well. Then, the back pressure 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 again and again until the well is eventually loaded completely and gas production stops. Accurate prediction of liquid loading in the well is very important to the operators since this will allow them to take the necessary measures to void liquid loading and extend the well production life. 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 fall back and liquid film reversal. The liquid droplet mechanism is based on the falling back of liquid droplets while the liquid film concept is related to the partial or full reversal flow of the liquid film.