Page 300 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 276

[ ] Well Programming
2.7.5

Kickoff plugs. Particular reference and acknowledgment is made
to the article Process Implementation Improves Cement Plug Success in
the Petroleum Engineer International cementing supplement.
When an open hole plug is set for directional kickoff (perhaps after
abandoning a fish or plugging back to sidetrack to a new target), the
objective is a correctly placed plug of high-compressive strength. If the
cement is weaker than the formation, because the native strength is
low or due to contamination, it will be hard or impossible to get away
from the old hole. Other open hole plugs may not need high strength
but require as much planning to assure first-time success.
Successful open hole plugs need to address the following design
criteria:

1. Temperature at the setting depth
2. Mud removal
3. Slurry properties
4. Slurry volume
5. Slurry stability after placement

Setting depth temperature. If the temperature that the slurry will be
subjected to during setting is not known, a temperature log should be
run. Over-retarding the slurry will be detrimental, both for plug stabil-
ity after placement and for wasting rig time waiting on cement.
Mud removal. In some ways, mud removal requires more thought
than with a casing job. In washed out zones, the annular capacity
around the cement stinger is much higher than if casing was in
the hole.
If a traditional, muleshoe type cement stinger is used, cement exits
the stinger jetting downwards where it will mix with mud before start-
ing back up the hole. In addition, the only mechanism in this case for
moving gelled mud in enlarged areas is pure speed/annular velocity.
This also tends to contaminate a lot of the slurry since mud is slowly
moved into the fluid stream as the job progresses.
The answer to these problems is a jetting tool. This is a joint of tub-
ing or drillpipe, closed at the bottom and with holes or nozzles point-
ing outward. The best results are reported with ports placed at a tan-
gent to the inside diameter and tilted slightly upwards, so that a flow
pattern spiraling upwards is initiated. (See Fig. 2-10.)

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