Cementing plugs are semirigid barriers used to separate cement slurry from drilling fluids, to wipe the casing, and to indicate when cement placement is complete. Plugs were once made of gunnysacks, wood, and leather. Present designs include top and bottom wiper cementing plugs constructed of nitrile or polyurethane molded over PDC drillable high-density plastic cores (Fig. 1). Most plugs are designed to be nonrotating; as a result, they are easier to drill out (Fig. 2).
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Application Of Top & Bottom Wiper Plugs
Although similar in external appearance, top and bottom plugs differ considerably in internal design and operation (Fig. 3).
Bottom Cementing Plugs
Bottom plugs were developed to precede the cement slurry, requiring an internal bypass or flow-through feature. The bottom plug features a thin membrane that is designed to rupture and permit flow once the bottom plug has seated (usually on the float collar). Bottom plugs also provide a seat for landing top plugs and sealing off displacement. To ensure compatibility, top and bottom plugs and float equipment should be from a common manufacturer. The use of bottom plugs with high LCM concentrations in the slurry can be risky, because the LCM may tend to ball up ahead of the wiper plug and bridge the float valve.
Third Wiper Plugs
More recently, using a third wiper plug has become more common. The extra plug separates chemical washes or spacers from the drilling mud, keeping the preflushes clean before they enter the annulus. A second use of the third plug is to measure the displacement efficiency of the mud pumps. Because of casing-ID variances and pump-liner inefficiencies, the displacement volume sometimes varies from job to job.
With a three plug system, the exact number of pump strokes necessary to bump the plug can be measured before pumping the cement slurry. This allows operators to pressure test the casing while the cement slurry is still fluid without damaging the cement-to-pipe bond.
Top Cementing Plugs
Top plugs are occasionally used alone. They are designed to withstand the pressures and forces generated when landed abruptly. When both top and bottom plugs are used, it is vital that they not be launched out of sequence. Because of the exterior similarity, top and bottom plugs are generally color-coded. The consequence of pumping the top plug first is that it will land and not rupture, leaving the casing full of cement. If this happens, the only recourse is to drill out the casing.
Other Cementing Plugs Types
Other common plugs include the following:
- Tapered plugs, used in multiple-ID strings
- Subsea plugs, used with subsea completions
- Latch-in plugs, used with latch-in equipment
- Flexible-fin plugs, used when passing through stage equipment.
Figure 4 shows one of the newest large-bore subsea wiper-plug-set designs. The new sets come with built-in swivel equalizers and are released with darts instead of balls. The reason for dart release is to provide wiping of the drill pipe strings on deep casings or casing liners and to avoid having to wait for the gravity settling of balls. The operational sequence is illustrated in Fig. 5.
- Well Cementing Second Edition – Erik B. Nelson and Dominique Guillot