Cementing Plug

Cementing plugs are pumped into the casing string through the cementing head. They isolate the cement slurry from the drilling fluid, wipe the casing wall clean, provide a bump-pressure indication of slurry placement, and prevent cement contamination. Selecting the right cementing plug is therefore critical to the success of the cementing operation.

Two-Plug Cementing Technique

In most cases, cementing operations adopt the two-plug cementing technique, which includes a top plug and a bottom plug. Cementing plugs come in either standard or anti-rotation types.

Inner String Cementing Technique

This technique is suitable for cementing operations on large-diameter conductor or surface casing strings (16" – 30"). In such cases, a specialized cementing plug – drill pipe self-locking plug – is required.

Multi-Stage Cementing Technique

For complex well conditions or specific operational needs, an intermediate plug may be added. Specific applications include multi-stage cementing, prevention of cement slurry contamination, or special wellbore structures.

Other Cases

There are rare cases where only the top plug is used:

• Simple well conditions: In wells with simple structures, shallow depths, or uncomplicated formations, a single cementing operation may be sufficient. In such cases, only the top plug is used to separate the displacement fluid from the cement slurry.
• Liner cementing: In liner cementing operations, where the liner is typically hung from the main casing, only the top plug is needed to isolate the displacement fluid from the cement slurry, and the bottom plug may not be necessary.
• Bottom plug-free operations: Some cementing tools or methods – such as self-sealing float collars – can perform the function of a bottom plug, allowing the operation to proceed with only the top plug.
• Remedial cementing: In remedial jobs where cement is injected into a specific section of the wellbore, only the top plug may be required to isolate the displacement fluid from the cement slurry.

In addition, the cementing plug must be accurately sized to match the casing. Its body is made from nitrile rubber, offering excellent resistance to abrasion and high temperatures. The core is typically constructed from either aluminum alloy or polymer materials, depending on downhole conditions. Aluminum alloy cores are particularly well-suited for deep and ultra-deep well applications due to their superior strength and durability.