STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Muds and Completion Fluids 673
b. Inject the solution at the top of the flowline or below the shale shaker.
The injection rate depends upon the hole size and the polymer efficiency
(lb/lOO ft of hole).
c. Let the mud circulate through all pits (tanks) available to increase the
settling time; do not agitate the mud.
d. If additional flocculation is required, use lime or calcium chloride. The
water at suction should be as clean as possible.
e. Slug the drill string prior to tripping with a high viscosity bentonite slurry
(about 30 bbl) to remove excessive cuttings from the annulus.
Extended Bentonlte Systems
To obtain a high viscosity at a much lower clay concentration, certain watersoluble
vinyl polymers called chy extenders can be used. In addition to increasing
the yield of sodium montmorillonite, clay extenders serve as flocculants for
other clay solids. The flocculated solids are much easier to separate using solids
control equipment.
The vinyl polymers increase viscosity by adsorbing on the clay particles and
linking them together. The performance of commercially available polymers
varies greatly as a result of differences in molecular weight and degree of
hydrolysis. However, it is not uncommon to double the yield of commercial clays
such as Wyoming bentonite using clay extenders in fresh water.
For low solids muds with bentonite extenders the API filtration rate is
approximately twice that which would be obtained using a conventional clay/
water mud having the same apparent viscosity.
However, good filtration characteristics often are not required when drilling
hard, consolidated, low-permeability formations. In these formations the only
concern is the effective viscosity in the annulus to improve the carrying capacity
of the drilling fluid. The use of common grades of commercial clay to increase
viscosity can cause a large decrease in the drilling rate. That is where bentonite
extenders are mostly applicable. In addition to its viscosifying property bentonite
extenders flocculate formation solids. A typical formulation of extended
bentonite system is shown in Table 4-49 [28].
The bentonite should be specially selected for this type of system as being
an untreated high yield Wyoming bentonite. The fluid has poor tolerance to
calcium and salt, so the makeup water should be of good quality and pretreated
with sodium carbonate, if any hardness exists. To increase viscosity bentonite
extender is added through the hopper at the rate of one pound for every five
sacks of bentonite. The extender is dissolved in water in the chemical barrel
and added at a rate dependent on the drilling rate. Excessively high viscosities
and gel strengths are normally the result of too high a solids content, which
should be kept in the range 2-5% by dilution. Dispersants should not be added
Table 4-49
Extended Bentonite Mud System [28]
Fresh water
1 barrel
Bentonite extender
0.05 Ib
Bentonite
11 Ibs
Soda ash
0.25-0.5 Ibs
Caustic soda pH 8.5-9.0