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).
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.
If only a top plug is used, any debris in the casing ID will be wiped off by the top cementing plug. If the accumulation of debris is greater than the length of the shoe track (distance between float collar and bottom of the casing), this debris will be pumped into the annulus between the casing and the wellbore, contaminating the cement opposite the shoe track. This contaminated cement may result in a poor quality of cement around the shoe joint. A top and bottom cementing plug should always be run to help ensure a successful cement job.
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.
Halliburton High Wiping Efficiency Cementing Plugs
The HWE top and bottom cementing plugs are designed to help improve wiping efficiency during cementing operations. The more efficiently the wiper plugs wipe the casing ID, the less fill will be left in the casing above the top cementing plug. The wipers of the HWE plug are a deep-cup design, which provides greater wiping efficiency. The bottom plug is supplied with a 750-psi rupture disc that helps ensure that the plug reaches the float collar. Because of its rubber molding, the HWE plug can be run in water-based mud, oil-based mud, and synthetic mud systems.
Halliburton Standard Five-Wiper Cementing Plugs
The five-wiper cementing plug is Halliburton’s primary cementing plug. Five-wiper plugs are available in 4 ½- through 20-in. casing sizes with standard plastic or aluminum inserts and 24-tooth NR plastic inserts in many sizes. In all types of five-wiper cementing plugs, the top cup of the top cementing plug is a deep-cup design that provides maximum wiping efficiency. The top plug is black, while the bottom plug is orange or red. When landed on the float collar, the bottom wiper of the bottom plug provides the seal for pressure shutoff. When landed on a bottom plug, the top plug seals on the bottom plug.
Halliburton Plastic Insert Standard FiveWiper Plugs
Five-wiper plastic insert plugs are available molded with standard (water-based) rubber for use with water-based mud compounds as well as synthetic (oil-resistant) rubber for oil-based or synthetic mud systems. Proper cementing plug selection is essential to a successful cement job. Ensure that the proper plug is selected for the type of mud system in which it is to be run.
The proper plug should be supplied for the mud system in which it is to be run. If a new synthetic (usually oil or mineral oil) base is to be used, contact the Casing Equipment Group in Duncan, Oklahoma. After cementing operations are completed, the plastic insert plug can be drilled out with either a standard roller-cone rock bit or a PDC fixed cutter bit.
Halliburton Aluminum Insert Standard FiveWiper Plugs
Aluminum plugs are made with aluminum core inserts and are available as top and bottom plugs. Aluminum insert plugs are supplied for water-based mud systems and synthetic (oil-resistant) mud systems. The proper plug should be supplied for the mud system in which it is to be run. After cementing operations have been completed, the aluminum-inserted plugs can be drilled out with a standard roller-cone rock bit. These plugs should not be drilled out with PDC bits.
Halliburton Nonrotating (NR) Cementing Plugs
Designed for use with Super Seal II™ NR float collars, top and bottom NR plugs have locking teeth that help prevent the plugs from spinning during drill out, which reduces drill out times and associated rig costs (check also drilling cost per foot). The high-strength plastic inserts used in NR plugs increase plug-landing pressures and allow easy drill out with both PDC and roller-cone rock bits. NR plugs are available for 7- to 20-in. casing sizes in surface release and SSR plug systems. NR plugs are made of a synthetic service compound that is compatible with water-based and synthetic mud systems. Five-wiper high-strength NR cementing plugs can be used with high-strength float collars to pressure-test casing immediately after the cement has been pumped and is still in the fluid stage. However, when NR plugs are used with wiper cups to make combination plug sets, rubber compatibility should be considered.
- Well Cementing Second Edition – Erik B. Nelson and Dominique Guillot