Do fast falling plungers improve production?
Dual row pad plungers are one of the best-selling plungers on the market. The spring-loaded pads press against the wall of the tubing, creating an effective seal. This seal allows efficient use of the available gas energy to surface the liquid column. Padded plungers offer excellent efficiency, second only to the brush plunger. However, the pads contact with the tubing slows the plunger as it falls. Most dual row padded plungers fall around 180 feet per minute. For an 8,000 feet deep well with no liquid in the tubing, 45 minutes is required for the plunger to reach bottom. Liquid in the tubing slows the fall rate, requiring an even longer “off” period for the plunger to descend.
Fast falling plungers are typically pad less with an outer diameter slightly less than the tubing’s inner diameter. An internal passageway opens when the plunger falls and closes when the plunger ascends. The internal passageway and undersized OD creates a greater open cross sectional area promoting faster fall velocities. Some rapid fall plungers fall against a flow rate when the well is open, while others perform best by descending when the well is closed.
Free cycling plungers designed to fall against a flow rate continuously cycle in the tubing, surfacing small amounts of liquid many times a day. When the plunger triggers the arrival sensor, the controller closes the well for a short period, allowing the plunger to initiate its descent. The plunger continues to fall against a flow rate after the well is open. When the plunger reaches bottom, the internal passageway is closed and the plunger begins to rise. Continuous flow plungers produce significant production improvements in some wells. Continuous flow plunger velocities can be extreme with little or no fluid to cushion the plunger at the top or bottom of the well.
Optimizing well performance ultimately strives to flow the well for the longest period in clear tubing. When tubing liquid accumulates, the well closes until it gains enough lift pressure to surface the liquid column. Lift pressure is the numerical difference between casing pressure and line pressure.
Using a conventional plunger requires two events to occur before the well opens:
- The plunger completely descends to the bottom hole spring
- Sufficient lift pressure is available to surface the plunger
If lift pressure is sufficient to surface the liquid column before the plunger reaches the bottom, then a fast falling plunger may improve production.
Consider these two examples using an 8,000 feet deep well:
Even though the fast falling plungers have a less efficient seal, the significant increase in daily production time can add significant uplift. Many cycles a day allow increased production time in clear tubing while producing at the lowest possible flowing bottom-hole pressure.