Which plunger is right for my well?
Plunger lift manufacturers offer a variety of plunger options. Plunger variations within the same tubing size include basic plunger style (Fig 1), variations of basic style, specialty plungers, material, diameter and price. Basic plunger styles include 1) bar stock or fish bone plungers 2) brush plungers 3) pad plungers 4) rapid fall plungers and 5) continuous flow plungers. Specialty plunger examples include wobble washer, snake and brush/pad combinations. Plunger material choices include carbon steel (1075), alloy steel (4130 or 4140), 300 series stainless steel and high strength, H2S resistant PH 17-4 stainless steel. Even within a style, there are multiple options (single row pad plunger, dual row pad plunger, triple row pad plunger, spiral vs no spiral bar stock plungers, etc).
How does an operator decide which plunger will optimize well performance?
Plunger selection starts with well conditions. NACE compliant plungers manufacturered from 17-4PH stainless steel are the best choice for wells with corrosive elements (H2S, CO2). Bar stock, fish bone and brush plungers are best suited for wells with significant sand and wells with high gas to liquid ratios. Bar stock or fish bone plungers fall faster that brush plungers, yet are not as efficient. Brush plungers are better suited for low gas to liquid ratio wells.
If a well produces minimal particulates, the most common plunger is the dual row pad plunger. Dual row pad plungers typically have two rows of 3-4 pads in each row. Designs with two springs behind the pad may create a more even wear pattern across the pad. Designs preventing impact forces from traveling through the pads are less likely to fracture pads. In addition, designs restricting flow behind or between the pads are more efficient.
Wells produce the most at the lowest flowing bottom hole pressure. Setting a plunger cycle that operates at the lowest possible casing pressure is generally the best option. Doing so requires many plunger trips a day, lifting a small amount of liquid on each trip. Once these controller settings stabilize, observe the casing pressure build rate vs the plunger fall time. If sufficient pressure exists to open the well before the plunger reaches the bottom (i.e. waiting on the plunger to fall), then a plunger that falls more rapidly could benefit production.
Rapid fall plungers are available in a number of different styles including ball and sleeve, internal shift rod and shift rod in the lubricator. All open an internal valve allowing rapid descents and close the valve to surface liquid. Again, fall time is the key. In a strong well producing close to 1 MMcf/d, plungers which fall against a flow rate (allowing minimal off time) are typically the best choice. If little liquid is in the well, rapid fall/rise velocities could damage equipment – especially if the lubricator and bottom hole spring are not adequate to dampen the high velocity impacts.
When falling against the flow rate is undesirable, match the plunger fall speed with the casing pressure build rate to ensure the plunger is on the bottom when the pressure conditions exist to lift the plunger. This process ensures selection of the most effective seal (slowest falling) plunger for the well conditions.