Page 320 - Drilling Technology in Nontechnical Language
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Chapter 13 – DRILLING PROBLEMS AND SOLUTIONS 311
About 10 bbl of diesel is pumped as a spacer ahead, followed by the DOB
slurry, and then another 10 bbl of diesel behind. Fresh water is pumped
behind the diesel. The plug is pumped down the drillstring to the loss zone.
As the water behind exits the drill bit, the larger annular capacity allows
the water to mix with the DOB slurry. Any water already present in the
loss zone will also mix with the slurry, hydrating the bentonite. Water can
be pumped ahead of the first diesel spacer if desired (e.g., if oil mud is in
use), in which case the diesel spacer ahead might be increased in volume.
Curing total losses with cement
The best lost circulation material for severe to total losses is cement.
There are two keys to success that the procedure must aim for:
1. Sufficient cement must be placed in the loss zone immediately
around the wellbore.
2. The cement must not move away from the near-wellbore zone
while it sets.
Many times, a small volume of cement is used, and the subsequent
actions almost guarantee that the cement will move away from the
wellbore after placing. Here is an outline procedure that has worked well
in the field:
1. Drill right through the loss zone so that it is completely
penetrated first, if it is possible and if it can be done safely.
2. Position drillpipe just above the top of the loss zone.
3. Pump a large quantity of lightweight extended cement slurry
(about 200 bbl).
4. Pump a large quantity of neat cement, which ideally incorporates
polypropylene fibers in the slurry (about 100 bbl or so). The
slurry should be designed so that the compressive strength when
set will be less than the formation compressive strength. This is
to avoid the drill bit from drilling an unintended sidetrack away
from the wellbore. If the fibers plug off against the formation
face, that is fine—it will prevent the earlier slurry from moving
away from the wellbore.
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