Plug Cementing | What is Plug Cementing?
Testing Cement Plugs
After placing the plug and waiting on cement, the first step in testing the plug is to “tag” the top of the cement with the drill pipe or tubing. This step is very important, because the actual top of cement may be different from the pre-job calculation. The second test is usually a performance test. For example, if a cement plug has been drilled for lost circulation and full returns are observed during further drilling, the operation is considered to be successful.
It is very important to allow adequate setting time before tagging a cement plug. Waiting on cement (WOC) time is determined using tables.
With primary cementing, drilling operations may resume after the cement has attained a compressive strength of approximately 500 psi. But when sidetracking off of a cement plug, the plug compressive strength must be substantially higher before drilling is initiated. In general, for a successful sidetrack, the cement plug must be harder than the formation. Past experience in a given location should indicate the compressive strength requirement for a whipstock plug.
Reasons for Failure of Cement Plugs
Cement plug failure can occur for any of the following reasons:
- Lack of adequate compressive strength (sidetracking plugs)
- Poor isolation (plug back, well abandonment plugs)
- Improper setting depth (all types of plugs)
- Plug either sinks in the wellbore fluid (all types of plugs) or it is lost to a thief zone (lost circulation plug)
Reasons for such failure can generally be traced to the following input or operating errors:
- Equipment failure
- Poor slurry design
- Inadequate WOC time
- Drilling-fluid contamination
