Plug Cementing | What is Plug Cementing?
Two-plug Method
The two-plug method (Figure 1) refers to dual wiper darts or foam balls that displace cement down a work string and place it in the casing in a manner similar to the balanced plug method but with less likelihood of cement contamination.
A spacer is pumped down the work string followed by the first of two wiper darts/balls to isolate the cement from the spacer. The cement slurry is pumped after the dart. When the dart reaches a locator sub near the bottom of the string, pressure is increased until pins shear, rupturing the dart and allowing the cement slurry to move through it.
The cement is followed by a top dart to isolate it from the displacement fluid. When this plug arrives at the seat an increase in pressure provides a positive indication that the entire cement slurry volume has been displaced without contamination ahead of or behind it.
The work string is then pulled up until the lower end of the tailpipe reaches the calculated depth of the top of the cement plug. Excess cement is circulated out. The tailpipe may be constructed of aluminum, making it easy for the pipe to be broken off and the work string retrieved if the tailpipe becomes stuck in the cement plug.
Some advantages of the two-plug method are:
- It provides isolation ahead of and behind the slurry.
- The pipe is cleaned to the bottom of the tailpipe.
- The breakable tailpipe can be readily abandoned if stuck.
- Reverse-circulation is accomplished through the bottom of the tailpipe.
- It allows accurate placement of the cement plug.
- It provides positive surface pressure indications of proper displacement.
