Paper: Intermittent Gas Lift, Plunger-Lift Assisted

Many of the thousands of wells produced by intermittent gas-lift experience similar problems. One problem is that the system is attempting to lift a static slug from the bottom of the well to the surface, which is quite inefficient. As the slug moves toward the surface, there is a frictional drag on the liquid slug along the inner walls of the tubing. That drag, coupled with the fact that the gas is travelling faster than the liquid, means that there is a tendency for the gas to outrun the liquid. The liquid that is outrun never reaches the surface, it falls back to the bottom of the well. By creating a fluid column in the bottom of the well, this "fallback" increases the back pressure on the formation, thereby reducing the effective average flowing bottom hole pressure. The net result is less production. Another common problem experienced in intermittent gas-lift is over-injection. In some operations the injection gas is intentionally over injected to help "sweep" out the falling liquid. While this could help, it is highly inefficient. On wells where the injection gas is controlled by a choke or orifice, over injecting in nearly impossible to avoid. Over injecting also keeps the tubing pressure of the well higher for a longer period of time. This has a negative effect on the flowing bottom hole pressure, and on liquid production. This report will address these common problems, and offer a sensible solution to both. The applications for this system are widespread, but cannot be automatically applied to every gas-lift well. As with all other artificial lift systems, each well must be studied individually to determine it's suitability. Plunger-lift offers the producer options, but cannot be viewed as a cure-all.