BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling Fluids
dispersion does not occur. The pH obtained is limited by the solubility, which about 1.6 grams of
lime per liter at 20 º C, giving a pH of 12.5.
Carbonic Acid and Carbonates
Pure water at pH 7 is very difficult to obtain, because carbon dioxide from the air readily
dissolves in it, forming a solution of carbonic acid.
CO 2 + H 2 O ↔ H 2 CO 3
Carbon dioxide + water ↔ carbonic acid
Carbonic acid is a weak dibasic acid and the hydrogen carbonate ion is commonly referred to as
the bicarbonate ion ionized to a small extent producing small numbers of hydrogen carbonate and
carbonate ions.
H 2 CO 3 ↔ H + + HCO 3 – ↔ 2H + + CO 3
2–
Carbonic acid ↔ Hydrogen ion + Hydrogen carbonate ion ↔ Hydrogen ion + Carbonate ion
Thus distilled water frequently has a pH in the range of 4 – 6.
If an alkali is added to water containing dissolved carbon dioxide, then the hydrogen ions
produced by the dissociation of carbonic acid react with the alkali. The effect of this is to push all
the reactions over to the right hand side, thus producing mainly carbonate ions at high pH (see
Figure 3-12). This is one of the reasons for running drilling fluids at alkaline pH. Hydrogen
carbonate ions can have a detrimental effect on the viscosity of clay based drilling fluids. By
adding caustic soda they are converted into carbonate ions, which are less of a problem.
100
80
60
CO 2
Mainly
CO 2
at low pH
Pure Water
Sea Water
HCO 3
Mainly HCO 3
-
ions at neutral
pH
CO 3
Mainly
CO 3
2- ions
at high pH
40
20
0
4 5 6 7 8 9 10 11 12
pH
Sea Water
2¯ Figure 3-12 Distribution of the CO 3 - HCO 3¯- CO 3 System in Pure Water and Sea-water at 1
atm. as a Function of pH
Carbonates, which are soluble (only those of the Group IA metals, such as sodium), produce
alkaline solutions by the reverse of the reactions given for carbonic acid.
Na 2 CO 3 → 2Na + + CO 3
2–
Sodium carbonate → Sodium ions + Carbonate ion
The hydrogen ions are provided by the ionization of water. Since hydrogen ions are removed, the
pH increases. Thus sodium carbonate is occasionally used to increase pH in a drilling fluid. It is
not as effective as caustic soda in this respect and the maximum pH that can be achieved in pure
water is only 10.3.
Reference Manual
Baker Hughes Drilling Fluids
3-20 Revised 2006