Page 400 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 376

[ ] Practical Wellsite Operations
3.3.9

not necessarily true that the wiper trip was necessary, except in mobile
“plastic” salts and shales, as previously noted. Apply wiper trips intel-
ligently to minimize cost and problems associated with wiper trips.

3.3.9. Barite Plugs

Barite plugs work by allowing barite to settle out rapidly after spot-
ting the pill, forming a solid mass that is capable of holding some pres-
sure. They can be pumped through a drill bit if necessary. Mixed at 22
ppg, the hydrostatic head of the plug will inhibit flow from a lower
kicking zone. Barite plugs can be used to seal off a high pressure/low
permeability kicking zone while casing is set, for some lost circulation
problems and also for isolating a loss zone from a kicking zone during
an underground blowout.
The plug slurry is usually mixed with barite, fresh water, sodium
acid pyrophosphate (SAPP) and caustic soda. The SAPP lowers the
slurry rheology, increasing the settling rate and the caustic soda raises
the pH (to 10) to increase thinner effectiveness. Desco can also be used
instead of SAPP, which does not need caustic soda to work.
A one liter pilot test should be done first since some supplies of
barite do not settle well. This may be due to trace impurities. Mix in
the correct proportions, stir well, and pour into a clear glass or plastic
container. Half of the barite should settle out within 15 minutes (i.e.,
the bottom quarter of the container will have settled barite).
For one barrel of a 22 ppg slurry, mix either /2 lb SAPP and /4 lb
1
1
caustic soda, or /2 lb Desco in 21 gallons of fresh water. Add 750 lbs
1
barite using a high pressure jet mixer. Mix the slurry using the cement
unit. The volume may vary from a minimum of 40 up to 450 barrels,
depending on the hole capacity and problem. If the treatment has to be
repeated, use a larger volume than the previous.
Displace as soon as possible after mixing, at 5 bbl/min or more to
prevent settling out in the drillstring. U-tubing will tend to occur;
pump fast enough to maintain pressure on the drillstring until dis-
placement is complete. Have cement and rig pumps lined up during
displacing; if one pump fails switch immediately to the other or else
you may end up with a plugged drillstring.

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