Plunger Lift: The Looming Liability of Fast Plungers

Plunger Lift, an economical form of artificial lift, uses a reservoir’s own energy to lift fluids from the well bore. The plunger acts as a mechanical interface between gas and fluids, traveling to the bottom when a surface valve is closed and returns to the surface carrying fluid sometime after the valve is opened again. The principle is simple and the system is economical because external energy is not required to lift the fluid.

The drawback is that the velocity of the plunger at surface is not measured, nor is it directly controllable. Plunger travel times are currently recorded using a plunger arrival sensor mounted at the surface. An average velocity can be calculated if it is assumed that the plunger was at the bottom when the valve was opened.

The velocity of the plunger is an important metric that can be used to ensure safe operation and optimal production of a plunger lift well. A properly set target velocity will ensure that the plunger travels fast enough to bring the fluid to surface, but slow enough to avoid damage to the well head. The problem however is that using an average velocity ignores the fact that the plunger does not travel at a constant velocity over the entire depth of the well.

As the plunger arrives at the surface, the fluid column unloads and the gas decompresses. This can lead to a sharp acceleration and a much more forceful than expected impact. Other factors such as fluid escaping around the plunger, instances where the plunger does not reach the bottom, and holes in the tubing can lead to reduced fluid coming to surface, or even dry plungers. Each of these issues can have catastrophic consequences.

Fast plungers (20 km/h) eventually cause damage to wellhead equipment and the plunger itself. Repetitive fast plungers or just a couple plunger runs at dangerous speeds (40 km/h) can cause enough damage to break open the well head resulting in spills, injury, or potentially death if there is an operator at site. There are several incidents of breached well heads each year.

Some of these incidents can be eliminated with better training, smarter optimization, and more robust equipment. In order to truly track dangerously fast plungers and react to them, there needs to be a fundamental shift in the plunger lift industry. Instead of calculating an average velocity and relying on this for safety, the actual plunger velocity at surface must be measured, recorded and used to change the behaviour of the system.

The technology in plunger arrival sensors has moved from electromechanical sensors that are unreliable and inconsistent to digital magnetic sensors, such as Cyclops, that are much more accurate. This technology is now being embedded in a new generation of plunger velocity sensors, such as Sasquatch from Extreme Telematics Corp., that not only detect the arrival of a plunger, but that measure and report the surface velocity. Adding this capability will allow the industry to finally shift to operating plunger lift wells off actual velocity.