Plunger velocity is the key to safe and optimized plunger lift operation. Historically, plunger lift control systems have recorded the time it takes the plunger to travel from the bottomhole bumper spring to the lubricator, where a plunger arrival sensor is installed. Using this time and the well’s depth, the average velocity is calculated. This velocity is used to shut in the well on one or more high-speed impacts to protect the lubricator from damage. Extremely high-velocity impacts can compromise the spring and lead to potential lubricator breaches.
Many operators use the average velocity to reactively adjust for fast- or slow-moving plungers. Sales time or shut-in buildup times are adjusted to modify the amount of fluid brought to surface in each trip to maximize production. Using average velocity has become an accepted practice based on the technology available. However, the available technology means a lack of visibility when it comes to plunger lift operation. Historically, there has been no way of detecting when or if the plunger reached bottom, how much the velocity changed as it ascended, and its velocity when it impacted the spring inside the lubricator.
Instances where the plunger rapidly accelerates at surface can be ignored by the control system because average velocity over the entire depth of the well is used. If the plunger’s surface velocity is much faster, it puts undue wear on the surface equipment. This leads to lubricator springs and plungers being prematurely replaced, resulting in increased operating costs.
Conversely, if the surface velocity is much lower than the average velocity, the control system may have unnecessarily shut in the well. Extended shut-ins reduce overall production and create volatility in the production profile. This cost can be amplified because of remote location access restrictions, multiple plunger lifts going into the same line, distance to the compressor or facility restrictions. One well’s inconsistent production profile can cause increased pressure fluctuations, which in turn destabilize entire fields. This single issue can cause lost revenue and production for hours, days, weeks or even months in remote areas.
Plunger Velocity Sensor
The most recent advancement in plunger lift technology is the introduction of the plunger velocity sensor. This new type of sensor not only detects the plunger’s arrival, but also its velocity as it passes by the sensor. It allows operators to finally see the plunger’s velocity at the surface as it impacts the lubricator spring, leading to increased safety, lower maintenance costs and increased production.
Early test results with one plunger velocity sensor have shown that it is common for a plunger to arrive at surface at a velocity much higher than the average calculated velocity. The data confirms that the plunger rapidly accelerates at the surface, often due to gas expansion. The results also showed cases where the surface velocity was well above or below the anticipated velocity. In these cases, the operators believed the plunger lift system was running safely and consistently because they were monitoring the average velocity.
In the example shown in Figure 1, the plunger’s surface velocity was typically slower than the average velocity because it was being slowed when the fluid hit the lubricator and could not exit down the sales line quick enough. However, on occasion the surface pressure began to rise. This rise would lead to slower plunger travel speeds downhole.
The control system would react by venting to bring the plunger to the surface. The longer duration for the plunger to arrive caused the average velocity to be reported lower, when in fact the venting caused a rapid acceleration of the plunger resulting in a dangerous spike in the surface velocity. In some cases, the surface velocity was fast enough that it exceeded the sensor’s maximum velocity, which led to it not being recorded by the control system.
In a second case shown in Figure 2, the velocity sensor was consistently reporting a surface velocity 50 percent higher than the average calculated velocity. On occasion, the surface velocity would dip well below the average velocity. In this case, most plunger trips were coming up with very little fluid. Occasionally, when line pressure dipped 5 to 10 pounds per square inch, the flowing bottomhole pressure dropped, bringing more fluid into the tubing. This caused plunger trips with a much larger slug size, which cushioned the plunger arrival at surface.
In the third case, a well in Canada had a chaotic production profile and was breaking plungers on a monthly basis. When the plunger velocity sensor was first installed, the plunger did not appear to be detected consistently. In reality, the plunger was not making it to the surface in time and was not being detected by the existing sensor. The controller would shut the well in longer in an attempt to build pressure. The result was an exceptionally fast plunger at the surface the next cycle.
Once again, the controller was seeing a slower average velocity, and the operator was not aware of the problem. Once the operators could see what was actually happening, they were able to lengthen the close cycle to build up enough pressure to bring the plunger to surface each cycle. This small change reduced the wear on the equipment and increased production.
As these cases show, monitoring surface velocity and using it for safety and optimization is a better way to operate a plunger lift well. From a safety and maintenance perspective, the velocity at which the plunger enters the lubricator is much more important than average velocity over the entire travel of the plunger. This velocity contributes to the wear of the lubricator spring, union threads, flanges and body.
A new American Petroleum Institute (API) standard for lubricators and their springs is currently being written by a consortium of producers and service companies. The focus has been around how to engineer the lubricator and spring to take a high-speed impact from plungers. The challenge they are facing is the magnitude of the impact at surface is not solely dependent on the plunger’s velocity. A lubricator manufacturer has to account for plungers that are traveling much faster than what has been calculated as well as the large variations in plunger mass. Simply designing a stronger lubricator with a more robust spring to account for faster velocities would ignore the fact that a plunger can range from 3 pounds to nearly 20 pounds.
What really matters is the amount of energy that has to be absorbed by the spring to avoid transferring this energy to the lubricator. By using the mass of the plunger and the surface velocity measured by the plunger velocity sensor, one can calculate the kinetic energy of the plunger as it arrives at surface.
Ideally, lubricator and spring combinations will have a rating that specifies the amount of energy that can be safely absorbed on each arrival. The control system can then compare the kinetic energy of the plunger’s arrival at the surface against this threshold to determine if it is safe and take any action as necessary.
In addition, the kinetic energy of each arrival can be summed over time to quantify the amount of wear on the spring. If manufacturers are able to specify the amount of energy that can be absorbed in the lifetime of a spring, then the control system can predict a failure before it occurs. Ideally, springs that absorb less energy could be left in place longer, reducing maintenance costs, while springs that took more frequent violent impacts would be replaced more often to avoid failure.
Using plunger surface velocity can also help producers maximize production and profits. Historical time-based operation of a plunger lift well was designed around a desired average rise velocity. This average velocity would ensure there was enough gas velocity to lift the weight of the plunger and the hydrostatic column, accounting for the friction between the plunger and the wall of the tubing.
To ensure maximum production and profit, the goal of any plunger lift optimization is to reduce the flowing bottomhole pressure to a minimum throughout the production cycle, while ensuring safe arrivals to mitigate against critical events and minimize equipment wear. Using a velocity sensor at the surface allows the fluid to be optimized on each run to keep the lowest flowing bottomhole pressure while simultaneously ensuring that there is enough fluid to safely cushion the arrival of the plunger at surface.
Now that the technology exists to both detect the surface velocity and use it to increase safety, reduce maintenance costs and increase production, a fundamental shift is under way. Some producers across North America are quickly moving to adopt surface velocity and integrate it into their well site operations. As surface velocity becomes the standard, there will undoubtedly be a wave of new innovations from this change in technology.