Plug Cementing | What is Plug Cementing?

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Plug Cementing, What is a plug cementing, Remedial cementing, plug cementing, plug cementing process, bump plug cementing

Balanced Plug Method

The balanced plug method is the most common placement method. A tubing or drill pipe work string is run into the hole to the desired depth of the base of the plug, preceded by a spacer and followed by a spacer. The volumes and densities are calculated and displaced in such a manner that they align at the same levels in both the annulus and the casing.

Animation 1 illustrates the sequence of the balanced plug method:

Work string is positioned above viscous fluid plug base
Spacer is circulated ahead of cement slurry
Slurry is followed by tail spacer
Spacers and cement in hydrostatic balance inside and outside work string
Pipe is pulled up out of cement to circulate out excess

It is common practice to pump the displacement fluid 2 or 3 barrels shy of full displacement, so that the plug can reach hydrostatic balance without any mud flowing back on the rig floor.

After the plug is balanced, the pipe is slowly pulled out of the cement to a depth above the plug, and excess cement is reversed out.

A risk of cement contamination during the displacement process is the primary drawback to the balanced plug method. If a high gel strength fluid is placed in the hole below the plug depth as a base, it can help to prevent migration of the cement slurry downward.

For example, Animation 2 illustrates how 18 $\tfrac{lbm}{gal}$ cement slurry can fall through a 9 $\tfrac{lbm}{gal}$ drilling fluid but remain intact if a viscous bentonite pill spacer is used as a base when pumping a balanced plug.