Here are the primary considerations that need to be top of mind when dealing with cement plugs. We think that all the most relevant ones are here, and can serve as a revision checklist. We’d welcome any additional suggestions to add to the page.
- Use a cementing stinger and cover both the cement and spacer lengths. Doing this will minimize the risk of contamination and the fluids intermixing.
- Always set a base for off-bottom cement plugs. This can preferentially be a bridge plug (if cased hole) or a viscous pill. In the cases where you use a viscous pill, add a side jet diverter sub at the end of the stinger to avoid jetting the flow downwards.
- Displace at the maximum rate possible, ECD permitting. The cementing contractor should provide a computer simulation for optimized placement.
- The maximum recommended cement plug length that can be placed in a single attempt is 200 meters. (As a ‘general rule’).
Slurry Contamination & Cement Plugs
- The main reason for failed cement plugs is cement slurry contamination:
- Reverse circulation is ‘preferred’ to normal direct circulation. The reason for this is because conventional (bottom-up) circulation takes a longer interval of time.
- Do not circulate out at the top of the cement plug. The drill pipe should be pulled back at least 20 to 50 meters above the expected top of cement before attempting to reverse or circulate out.
- Beware of the ECD when reversing out if there are open perforations at the top of the liner or an exposed open-hole. Reverse circulation applies more pressure on the formation than direct circulation.
- Whenever we employ reverse circulation, both the circulating pressure and the slurry thickening time must be considered and carefully monitored.
- In the use case of open hole cement plugs, the preferred method is direct circulation to avoid high ECD.
Please help me expand these guidelines. Are there any suggestions that should be added to the list based on your experience?