STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

674 Drilling and Well Completions
as they compete too effectively with the extender for the adsorption sites on
the clay.
A small excess of soda ash, of 0.57 kg/mq (0.2 lb/bbl), should be maintained to
ensure the calcium level remains below 80 mg/l and to improve the efficiency of
the extender. This level of soda ash will produce the required pH in most cases.
The system can be weighted to a maximum of 11 lb/gal provided the ratio
of drill solids to clay solids is maintained at less than 21, by correct use of the
solids removal equipment and careful dilution and makeup with bentonite from
a premix tank.
Bentonite Substitute Systems
In this system, the high molecular weight polysaccharide polymer, is used to
extend the rheological properties of bentonite.
A biopolymer produced by a particular strain of bacteria is becoming widely
used as a substitute for clay in low-solids muds. Since the polymer is attacked
readily by bacteria, a bactericide such as paraformaldehyde or a chlorinated phenol
also must be used with the biopolymer. The system has more stable properties
than the extended bentonite system, because biopolymer exhibits good rheological
properties in its own right, and has a better tolerance to salt and calcium. The
system can be formulated to include salt, such as potassium chloride. Such a
system, however, would then be classed as a nondispersed inhibitive fluid.
Nondispersed inhibited Systems. In these systems, the nondispersed character
of the fluids is reinforced by some inhibition system, or combination of systems,
such as (1) calcium ions, lime or gypsum; (2) salt-sodium chloride or potassium
chloride; (3) polymers such as Polysaccharides, polyanionic cellulose, hydrolyzed
polyacrylamide.
In these systems, particularly systems such as potassium chloride polymer, the
role of bentonite is diminished because the chemical environment is designed
to collapse and encapsulate the clays since this reaction is required to stabilize
water-sensitive formations. The clay may have a role in the initial formulation
of an inhibited fluid to provide the solids to create a filter cake.
Potassium Chloride-Polymer Muds
KC1-polymer (potassium chloride-polymer) muds can be classified as low
solids-polymer muds or as inhibitive muds, due to their application to drilling
in water-sensitive, sloughing shales. The use of polymers and the concentration
of potassium chloride provide inhibition of shales and clays for maximum hole
stability. The inverted flow properties (high yield point, low plastic viscosity)
achieved with polymers and prehydrated bentonite provide good hole cleaning
with minimum hole erosion.
The KC1-polymer muds are prepared by mixing potassium chloride (KCl) with
fresh or saltwater. The desired KCl concentration depends upon the instability
of the borehole and ranges from 3.5% by weight for drilling in shales containing
illites and kaolinites to 10% by weight for drilling in bentonite shales. The
polymer is then mixed in slowly through the hopper to the desired concentration
(0.1 to 0.8 lb/gal depending upon the type of polymer). For additional viscosity,
prehydrated bentonite (salt makeup water) can be added (0 to 12 lb/bbl) until
satisfactory hole cleaning is achieved. The mud is adjusted to a pH of 9 to 10
with KOH or caustic soda. For filtration control, an organic filtration control
agent should be used as recommended by the manufacturer.