Well Integrity Technical Section

Dear All,

It is common method to use a smaller diameter stinger at the bottom of the drill pipe for setting the balanced cement plug.

Usually, 2-3/8", 2-7/8" or 3-1/2" tubing is considered as a stinger / tail pipe for that purpose. However, it is possible that using such small size tubing for the plug displacement may not remove mud efficiently (result in more contamination) as a larger pipe say 4" could do. So, is there any guideline mentioning the minimum tail pipe size needed for a specific hole size in order to have an efficient balanced cement plug placement.

Dear Sajid,

For mud removal, the pros and cons are not so clear-cut:

• A lot of contamination happens on the way down the pipe, where you have heavier fluids chasing lighter ones. Smaller drill pipe and tubing ensures that the contamination length (due to the fluids swapping) represents a smaller percentage of the cement volume.
• Eccentered, larger pipe is not much better than eccentered, smaller pipe. At any rate, mud removal during plug placement is ensured by heavier slurry displacing lighter mud. As long as you use neat G in thin and light mud, 2⅞" is not necessarily worse than 4". If, on the other hand, you want to push the envelope by using light slurry and heavy mud, then you should worry about centralising the tubing and serious mud removal design.
• A larger pipe will swab the annular part of your balanced plug as you pull out of the hole. This is potentially worse than mud displacement issues.

My suggestion is to keep whatever (small) tubing size you have on the rig and give equal attention to all aspects of plug placement, e.g.:

• Do not let the plug jet or slump: use a diverter plug (mule shoe with holes) at the bottom of your stinger and have a base, either solid (bridge plug, Parabow, ...) or a viscous pill.
• Don't over-retard your plug: ensure you know the circulating temperature and design slurry setting accordingly.

Hope it helps,