Continuous Run Plunger Lift (April 19, 2016)
Continuous plunger lift is the next in a series of articles related to plunger lift written by Shale Tec LLC (www.ShaleTec.com). Plunger lift is an artificial lift method used to remove liquids from oil and gas wells. The prior published articles include:
- Why Plunger Lift (Why is plunger lift needed?)
- When Plunger Lift (When is plunger lift needed?)
- Which Algorithm (Which algorithm is best?)
This article takes a closer look at continuous run plungers and the algorithms used to cycle these plungers.
Fast falling plungers are known by a variety of names – including rapid fall, dart style, shift rod (or sliding sleeve) and ball and sleeve plungers. I prefer to separate this group into two distinct categories – continuous run and rapid fall. Continuous run plungers are specifically designed to fall against a flow rate (ie fall while the well is open and flowing). Continuous run plungers have a large internal passageway which is closed while the plunger is rising, and opens as the plunger descends. Rapid fall plungers may either have a fixed opening through the center of the plunger, or a restricted passageway limiting the plunger from falling against a significant flow rate. Using this distinction, dart and ball and sleeve style plungers fall into the continuous run category. Venturi and shift rod style plungers fall into the rapid fall category. Additionally, a trigger or separator rod is required in the lubricator to operate ball and sleeve and most dart style plungers, yet not required for venturi or shift rod style plungers.
It’s important within an organization to use common terminology when referring to a specific plunger. Referring to a plunger by its specific name, as opposed to a generic name, can aide in avoiding confusion among operators.
Within these generic categories, the exterior of the plunger may have various configurations. While alternating ridges and grooves are most common, the exterior can also contain a number of rows of pads. When referring to a fast falling plunger to also describe the external sealing surface, the appropriate description becomes:
- Padded dart style plunger
- Bar stock dart style plunger
- Padded ball and sleeve plunger
- Bar stock shift rod plunger
The term “bar stock” simply means the plunger was machined from a single bar of steel. The sealing surface of bar stock plungers is typically an alternating series of ridges and grooves – which may vary in thickness, diameter and depth within a plunger and/or between suppliers.
Continuous run plungers are used early in the life of a well, before the flow rate becomes insufficient to lift all liquid in the well to the surface. The Artificial Lift Research and Development Council in “Guidelines and Recommendations for Tubing Plungers” shows the life of a well as shown below:
When the flow rate of the well approaches the critical flow rate, it’s time to consider continuous run plungers. The critical flow rate is the gas flow rate that carries all liquid in the well bore to the surface. When the well’s actual flow rate drops beneath the critical flow rate, some liquid begins to fall to the bottom of the tubing. The liquid impedes the flow of gas to the surface. SPE paper 120625 “Guidelines for the Proper Application of Critical Velocity Calculations” and SPE paper 100663 “Acoustic Liquid-Level Determination of Liquid Loading in Gas Wells” contain excellent insight in the use of bottom hole pressure and well geometry to correctly identify the well’s critical flow rate.
For most dart style and all ball and sleeve plungers, a separator or trigger rod extends downward from the top of the lubricator. When the plunger engages the separator rod, the rod opens an internal passageway in the plunger by pushing the dart to the open position, or knocking the ball off the sleeve. Once the plunger’s internal passageway is open, the rod still partially blocks this passageway. Some rods used with ball and sleeve plungers are configured with a tear drop shaped bulge near the end of the rod, designed to block a large portion of the plunger’s internal passageway. When the internal passageway is blocked, less gas flow is required to hold the plunger in the lubricator – thus allowing the well to flow for a longer time period before the plunger drops out of the lubricator.
The lubricator’s lower flow outlet also plays an important role in determining how long the plunger is held at the surface by the upward flow of gas. When the lower outlet is fully open, gas flow is diverted through this outlet – as it provides the path of least resistance. When the lower flow outlet is partially or fully closed, gas flows past the plunger and through the upper flow outlet, thus applying additional upward pressure on the plunger. In this method, the afterflow time can be modified within a limited range.
An electronic controller may be used to provide a more precise end to the afterflow time. By using an algorithm within the controller that closes the well based on time, pressure, flow rate or a combination thereof, the point at which the plunger falls out of the lubricator is controlled.
If the well’s flow rate is insufficient to hold the plunger in the lubricator for the desired time period, an autocatcher may be used. The autocatcher engages when the well is open and disengages when the well is closed. Thus, when the production or afterflow period is ended by the controller, the autocatcher releases the plunger. Once the plunger falls out of the lubricator, the well is re-opened, allowing the plunger to descend against a flow rate.
Some operators keep the well closed until continuous run plungers reach the bottom of the well. Since these plungers fall rapidly (1500 to 3000+ fpm), the close time is very short. Rapid fall plungers are operated in like manner, yet fall slower (around 800 to 1400 fpm). The key advantage of operating the well in this manner is that the time required for the plunger to surface is known. A timer starts when the well is opened (plunger is now at the bottom) and stops when the plunger arrives at the surface. Since the depth of the well is known, the plunger velocity can be determined – allowing the operator to ensure the rise velocity remains within a safe range.
Recently, Extreme Telematics Corporation introduced a new sensor which records the plunger impact velocity at the surface. This new sensor eliminates the need to know when continuous run plungers reach the bottom of the well in order to record plunger rise velocity.
Most continuous run plungers are controlled based on time. Actual round trip time is compared to expected round trip time. Round trip time (RTT) is the time for the plunger to fall from the surface to the bottom hole spring location and return back to the surface. RTT is the Fall Time plus the Rise Time. Starting with the plunger at the bottom of the well on the first run, the rise time is recorder. On the next full cycle, the RTT is recorded. Subtracting rise time from round trip time defines the fall time. The actual fall time should be compared to the expected fall time as provided by the plunger lift manufacturer. Significant variations in actual RTT vs expected RTT should be carefully reviewed to determine root cause – and adjustments made if warranted.
As discussed above, extending the production or afterflow stage with continuous run plungers is common. When desired, the afterflow stage can be ended on the same algorithms as conventional plungers. Since critical velocity is an indicator of when liquid will start to fall back into the well, ending the afterflow stage on a multiplier times critical velocity will often ensure a consistent volume of liquid in the tubing. For wells in which liquid follows the plunger to the surface, a minimum afterflow period is beneficial to allow this liquid to clear the well head prior to ending the afterflow period.
When using continuous run plungers, use the trigger rod designed for that specific plunger. Doing otherwise can cause damage to the plunger and possibly the lubricator. Also, ensure the bottom hole spring will absorb the impact of the plunger. The plungers fall velocity contributes exponentially more to the impact force of the plunger than its weight. Since these plungers can reach fall velocities in excess of 3,000 fpm, it’s critical the bottom hole spring be of appropriate design to absorb the energy at impact.
And, consider joining the Linkedin Group “Plunger Lifted Gas Wells”!
The above article was written by David Cosby P.E. of Shale Tec LLC
Copyrighted April 2016