Plug Cementing | What is Plug Cementing?
General Cement Plug Job Design Considerations
Cement plugging essentially consists of placing a cement column in an open or cased hole. It is a relatively simple process, although problems may be encountered such as fluid migration and cement contamination.
Cement plug failures can be minimized by:
- Using a caliper log to accurately determine the hole gauge
- Using sufficient cement slurry to cover the plug volume
- Using a spacer or preflush ahead of and behind the cement slurry volume to minimize contamination
- Maintaining drill pipe or tubing movement during cement placement
- Moving the pipe slowly when lifting it out of the cement to minimize contamination
- Using diverter tools and viscous pills to minimize contamination at the base of the plug
Other important factors to consider include slurry composition and volume, statutory requirements, placement technique, and well conditioning.
Cement volume needed for a specific plugging operation depends on plug length and hole/casing diameter. Government regulations and safety considerations may also dictate plug length for well abandonment.
For sidetracking, sufficient volume is required to minimize drilling-fluid contamination that occurs at the plug top. Additionally, allowances are usually made for dressing off the drilling-fluid-contaminated top of the cement plug before attempting to sidetrack.
Fluid spacers should be used both ahead of and behind the cement slurry to minimize mixing of cement and displacement fluid. Also, spotting a viscous pill spacer at the intended plug bottom can improve cement-plug stability. Use of a diverter tool, which forces fluid to flow directly at the wellbore face, provides a more uniform placement of both spacers and cement slurry.
There are three common methods for setting cement plugs in a well:
- Balanced plug method
- Dump bailer method
- Two-plug method
Each of these is described in the following sections.
