BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Table 3-11
Composition of a Typical Seawater Sample
Constituent
mg/L
Sodium
Potassium
Magnesium
Calcium
Chloride
Sulfate
Carbon dioxide
10,400
375
1,270
410
18,970
2,720
90
(Density above - 8.56
lbm/gal)
Hydration and subsequent swelling of clays and reactive shale is minimized by the sodium
chloride, magnesium, and calcium in seawater.
These inhibitive properties are particularly useful when drilling the surface interval of a hole with
a non-dispersed, low pH fluid system.
A dispersed system may be required for drilling other intervals, however. These same inhibitive
properties make it more difficult and costly to control and maintain the rheological and filtration
characteristics of the fluid.
For non-dispersed seawater fluids, use a viscosifier (SALT WATER GEL, pre-hydrated
MILGEL) and, if filtration control is necessary, use materials such as BIO-PAQ ® , CMC or MIL-
STARCH ® and pre-hydrated MILGEL. Pre-hydrated MILGEL contributes to viscosity and filter
cake quality and gives a slight fluid loss reduction. The advantage of pre-hydrating bentonite
prior to its addition to salt water is illustrated below.
Figure 3-15
Fluid Loss and Viscosity with Bentonite Prehydration
Pre-hydrated bentonite mixtures can be prepared by mixing 25 to 35 lbm/bbl of MILGEL in
freshwater. To this slurry is added 0.5 to 1 lbm/bbl UNI-CAL ® and 0.2 to 0.4 lbm/bbl of caustic
soda. To this slurry is added 10 to 15 lbm/bbl additional MILGEL. This mixture is generally
added at an initial concentration of 25% to 30% of circulating volume, and then added as required
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-33