Spiral Plunger
The spiral plunger, the bar-stock plunger and the fishbone plunger are all the same item. It is the least expensive design consisting of a series of grooves cut into a length of steel bar stock. The grooves cause turbulence when the up-flowing gas slips past the rising plunger. While this turbulence is not a seal per se, it reduces the free flow of gas past the spiral plunger causing a differential that lifts the plunger to the surface. This plunger is preferred under two circumstances; when very little fluid is to be moved and because it requires less shut-in time in comparison to other solid plunger-types.
Because the diameter of the spiral plunger is undersized, the slippage rate is typically 20% per 1000’ of lift, meaning that it arrives at the surface with a lot less liquid than it began with. However, the efficiency of the spiral plunger increases when it is carrying less fluid, therefore, near the surface, the slippage rate decreases considerably. This is because of the blow-by effect. The blow-by gas aerates and lightens the fluid column that is above the spiral plunger into mist and droplets that stay suspended without falling past the spiral plunger. This phenomenon cannot be accomplished under a tall column of liquid, however.
The inefficiency of a spiral plunger can be an advantage in a well that lacks sufficient bottom-hole pressure to lift a tall column of liquid. In this example, a portion of the gas below the plunger migrates above the plunger while a portion of the liquid above the spiral plunger migrates (leaks, sinks) below the plunger. This phenomenon allows the hydrostatic head above the plunger to be reduced to a point at which the plunger can make a successful trip to the surface with at least a little liquid. Moving only a small portion of the liquid to the surface is not the preferred operation, but it may be the best a low bottom-hole pressure well can achieve at lowest cost. See Ball and Sleeve Plunger for other options.
Brush Plunger
The brush plunger equals the spiral plunger in efficiency (poor) but is preferred in those wells that make sand. Like the spiral plunger, the surface area of the brush plunger is only 90% of the area of the tubing bore (2.83” vs. 3.14”). Because it is lighter than the spiral plunger, the brush plunger does not fall as fast, increasing the shut-in time requirement. The lighter weight of the brush plunger adds no appreciable advantage.
Pad Plunger
The pad plunger comes in a single or dual metallic pad arrangement. The single pad can be used when the tubing drift is too tight for a double pad. A dual pad arrangement is more efficient and more expensive, compared to the spiral plunger. The sealing pads extend outward from the plunger body and contact the inner tubing wall. Manufacturers differ on the number and shape of the pads used. All pad plungers have gaps in the seal between the individual pads. Although these gaps reduce the efficiency, they allow the plunger to fall through the liquid column at the bottom of the well and still bring up more liquid, using less gas than a spiral plunger. The slippage rate of a pad plunger is typically 5% per 1000’ of lift; however the slippage rate will vary considerably among manufacturers. The pads will wear out after 6 to 12 months of use. As the pads wear, efficiency will drop off. The operator is advised to remove the plunger and check it for size and worn condition instead of relying on well performance indications. It is simply too expensive to wireline the pieces of a fall-apart plunger from the bottom-hole spring area.
Casing Plunger with dual cup seals
The casing plunger is used in 4.5″ and 5.5″ casing only. Larger sizes are deemed too dangerous due to the cross-sectional area dynamics. The condition of the casing material wall must be appropriate for the use of the elastomeric seal material. The advantage of the casing plunger over the tubing plunger is that the liquid to be moved does not present as a large hydrostatic head due to the dynamics of the larger diameter of the casing. The seal can be either the parachute-cup shape shown here or the bellows type expandable seal. Both are positive seals. The advantage of the expanding bellows over the cup-type seal is that the bellows seal won’t invert. Occasionally, the cup-style seal will invert and lodge the casing plunger in the production string, necessitating a wireline trip. Because of the large diameter of the plunger and lubricator system, the casing plunger is generally the most expensive of the various plunger lift installations. The price advantage of a casing plunger is the savings realized from not having to run tubing in a low productivity location. With their positive seals, casing plunger won’t fall without a method of bypassing fluid. The casing plunger pictured here includes a bypass valve located in the lower section.