Well Integrity Technical Section

Dear Colleagues,

If cements plugs calculations have been made with 6 Arm Caliper data and 10 % excess for 8-1/2" deviated hole and balanced spotted across Carbonates for 500' of column with no backflow thru the drill pipe while pulling stinger thru the plug, how come it has been tagged 300' shallower.

On the other hand, even if the hole size is 8-1/2" and considering no excess, the plug should be tagged ONLY 80' above with the pumped quantity of the slurry. Also there are no clays present across the zone of plug. No gain was noticed while WOC.

Just getting curious about the route cause?

Good question, Sajid:

You have an obstacle in your casing 250' above the expected top of cement (500'+10% excess). I would think of three possibilities:

• The well is deviated so the top of your plug is slanted. Unfortunately the deviation required for 250' interface "length" is 89 degrees, which may not be the case.
• A more likely cause could be contamination: you have 45% more slurry volume than expected, which would imply a 70/30 mixing ratio between slurry and spacer or mud. Depending on your fluid types (e.g. OBM vs. WBM) and original slurry density, the mixture may set fairly quickly and be taggable. Contamination may have happened as the slurry climbs the (deviated) annulus, or because your pipe diameter is smaller than expected (say, cement deposits) so you end up accidentally over-displacing. Most contamination happens on the way down the pipe, and this is reduced by the small diameter, so I would tend to opt for the second explanation.
• You may have sagging spacer or drilling fluid, but you will need very thin fluids with lots of solids to get almost 3 m3 of junk in a relatively short time.
• There are more esoteric explanation. If the carbonates are very permeable you may have a thick filter cake: 1.33" would do the trick. Trouble is: carbonates are not "permeable" in general, and you would see some filter cake on your caliper. Or you could have a subsurface fluid flow that disturbs the plug while setting without affecting the top of cement, but brine flow during setting tends to leave a very small channel instead of mixing with cement (unless brine was flowing during placement and stopped after the plug was in place).

There are a few thing that can help you home in on the cause: how high did you pull your pipe before circulating out? Is it above the depth you tagged solids at? Did you try to rotate/drill and recover some of the cuttings? If yes it should be able to see if it's solids, barite or cement. Density can tell you whether contamination is an issue (and it's trivial to measure). If contamination is suspected, a little fingerprinting by the mud engineer can find what the fluid was (spacer, mud, brine), and the cementing company will be glad to check whether the fluid mixture sets easily.

Keep us posted,