BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
Barite Plugs
One special situation that will require the supervision of the drilling fluids engineer is the
mixing of a barite plug. If during a well kick situation, the fracture gradient of the formation is
exceeded, an underground flow will occur. This condition is signaled by a loss of gauge
response and loss of returns. If the drill string is above both the loss zone and flowing zone, it
should be stripped to bottom, if possible. If this is not practical, a lost circulation squeeze may
plug the loss zone and allow circulation of fluid of required density.
If pipe is below fractured zones, it is sometimes possible to spot heavy fluid on bottom through
drillpipe and pump light fluid down the annulus. In some instances, a balance point can be
reached by this method and flow can be stopped. One method for killing underground blowouts
is to bridge off the flowing zone with a barite plug.
Barite plugs seal the wellbore in four ways.
1. Due to low viscosities and yield points, barite may settle to form a solid plug in the
hole.
2. Their high density increases the hydrostatic head on the active zone and helps
prevent additional influx of formation fluid.
3. Due to their high fluid loss, they may dehydrate to form a solid plug of barite in the
hole.
4. Their high fluid loss may also cause the hole to slough and bridge itself.
Procedure to Set Barite Plugs
The barite-water-phosphate slurry is usually mixed with the cementing equipment, pumped
through the drillpipe and spotted on bottom. The bit jets do not have to be removed. Fresh
water should be used, because barite does not settle as desired in sea water. Add SAPP to the
mixing water before adding barite. This keeps the slurry thin to promote settling. The optimum
amount of phosphate ranges from 0.2 to 0.7 lb m /bbl in the mixing water. A one-quart fluid cup
of powdered SAPP weighs 2.5 lb. Use CAUSTIC SODA with SAPP to adjust pH to 8 to 10
range (approximately 0.25 lb m /bbl). If TSPP is used, CAUSTIC SODA is not required. In high
temperatures, phosphates will not be effective and lignosulfonates can be used. Up to 6 lb m /bbl
may be required.
Slurry
Density
(lb m /gal)
18.0
19.0
20.0
21.0
22.0
Water
(gal/bbl
26.9
25.3
23.7
22.2
20.6
Barite
(sacks/bbl)
5.30
5.94
6.43
6.95
7.50
Slurry Volume/
Sack of Barite
(bbl/sack) (ft 3 /sack)
0.189
0.168
0.156
0.144
0.133
1.060
0.945
0.873
0.807
0.748
Table 12 - 4 Barite Slurry Weight / Volume Relationship (Barite Specific Gravity
= 4.2)
Follow these steps in mixing and placing a barite plug (see Table 12-4).
1. Choose a slurry weight between 18 and 22 lb m /gal.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-35