Page 403 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 379

3.3.10
Drilling Problems [ ]

3.3.10. Diesel Oil Bentonite Plugs (Gunk Plug)

Diesel Oil Bentonite plugs are also known as “gunk plugs.” They
work by holding bentonite in suspension in diesel until the plug is
placed and then arranging for water to hydrate the bentonite. The ben-
tonite yields rapidly, becoming extremely viscous.
They can be very successful in shutting off flow in an underground
blowout, especially if the flow is water. A DOB plug will not maintain
strength indefinitely. Cement should be spotted to give a permanent
seal once the plug has worked.
The main potential problem with the DOB plug is that it will set up
inside the drillstring if it contacts any water. A good diesel spacer ahead
and behind are essential to prevent this.
To make the plug slurry, mix three sacks of bentonite per barrel of
diesel for an 11.0 ppg slurry. Mica can be added at 15 ppb to increase
the final plug strength if desired; use fine mica otherwise the nozzles
may get plugged. Volume of slurry will vary between 30 and 150 bbls;
more slurry for higher flowing rates and/or more open hole.

Procedure.
1. Line up both rig and cement pumps on the drillstring so that either
can be used for displacement if the other fails.
2. Pump 5-10 bbls diesel ahead.
3. Either batch mix or mix/pump the DOB plug on the fly and dis-
place it into the drillstring.
4. Pump 15-20 bbls of diesel behind. Displace with water-based mud
with a reasonable rate down the string.
5. Once the slurry reaches the bit, start to pump slowly (1 -1
/4 /2
bbl/min) into the annulus while displacing the slurry and diesel
behind out of the string; over-displace by 5 bbls.
6. Wait about 6 hours for temperatures to stabilize. Run a tempera-
ture survey through the drillstring (pressure equipment will be
needed on the string). The loss zone should appear hotter than
normal.
7. Wait another 4-6 hours and run another temperature survey. If the
underground blowout has stopped, the temperature in the loss
zone will have decreased. If the blowout is still going, set another
(larger) plug.

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