BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling Fluids
bacteria will attack the starch and fermentation will proceed at a rapid rate. This will be observed
by a rapid loss of fluid loss control.
Prehydrated MILGEL and Salt Fluids
Pre-hydrated MILGEL salt fluids are used in top hole drilling for hole cleaning and suspension.
The pre-hydrated MILGEL slurry increases the yield point and gel strengths due to flocculation
of the bentonite with salt compounds. These properties give the fluid excellent carrying capacity.
Colloidal Protected MILGEL may be pre-hydrated as follows.
• 1.0 bbl Drill Water
• Soda Ash (as needed)
• 0.5 lbm/bbl Caustic Soda
• 20 to 25 lbm/bbl MILGEL
• 0.50 to 2.0 lbm/bbl UNI-CAL
• 15 to 20 lbm/bbl additional MILGEL
This slurry should be mixed thoroughly before it is added to the salty drilling fluid.
Since pre-hydrated bentonite contributes very little to fluid loss control, normal salt water fluid
loss control materials such as MILSTARCH should be used as required.
The high gel strengths of these fluids contribute to the retention of air or gas. Adding small
quantities of MIL-PAC and UNI-CAL will aid in controlling gel strengths and allow gases to
break out of the fluid. LD-8 or the alcohol-base defoamers may also be required to control gas or
air entrainment and foaming.
Avoid using surface guns for fluid agitation because they tend to inject air into the system,
foaming problems are inherent to saturated salt systems.
High-Salt or Salt-Saturated Polymer Fluids
High-molecular-weight polysaccharides and XAN-PLEX ® (xanthan gum) are unique because
they perform in saturated salt water as well as freshwater. XAN-PLEX has shear thinning
characteristics which result in very low bit viscosities with good annular viscosity recovery as
shear rate decreases. This characteristic contributes to increased penetration rates and good hole
cleaning.
Under laboratory conditions these polymers start to degrade at temperatures above 250°F.
However, this does not prevent their practical use at temperatures greater than this range.
Polymers tend to exhibit lower viscosities as temperature increases. Viscosity may decrease to
one-half or less when temperature is increased from 125°F to 250°F. For this reason,
supplementing the material with small quantities of pre-hydrated MILGEL (5 to 10 lbm/bbl)
would be advantageous. Bentonite tends to flocculate at elevated temperatures, which helps
counteract the temperature thinning action on the polymers.
If an API filtrate below 20 to 25 cc/30 min is desired, it is necessary to add a supplementary
filtration control material such as BIO-LOSE ® or MIL-PAC. Small additions of MIL-PAC (0.1
to 0.25 lbm/bbl) may have a thinning effect on these fluids if they contain flocculated drill solids.
Although deflocculants are not normally added to this system, pre-solubilized UNI-CAL can be
added. The pH of these fluids is usually maintained on the basic side (10.0) to minimize
hardness. Caustic Soda is used, but should be added slowly to prevent momentary pH humps
Reference Manual
Baker Hughes Drilling Fluids
3-36 Revised 2006