BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling Fluids
O δ- δ + H δ+ H δ+ O δ-
H
H δ+
δ + Cl -
H
H δ+
H δ+ H δ+
O δ- O δ-
Weak attraction.
No permanent structure of water molecules
H δ+ H δ+
H δ+ O δ- O δ-
H δ+
H δ+ Na +
O O δ-
δ - H δ+
H δ+ H δ+
Strong attraction producing an order
structure of water melecules around the ion
Figure 3-9
Hydration of Sodium Chloride in Water
This ability to dissolve ionic compounds is solely due to the polar nature of water. Non-polar
solvents, such as diesel oil, do not hydrate the ions and ionic compounds are not soluble in them.
The salts that are soluble and insoluble are summarized in Table 3-6 The insoluble salts are
generally those where both anion and cation are multivalent (i.e. their valency is greater than
one). The attraction between multivalent ions is much stronger than that between univalent ions
(with only one charge). Salts such as CaCO 3 have much higher lattice energies than salts such as
NaCl. The hydration energies of the calcium and carbonate ions are not high enough to
compensate for this high lattice energy and thus CaCO 3 is insoluble. In contrast, the energy
released by hydrating sodium and chloride ions is sufficient to overcome the lattice energy of
sodium chloride and thus NaCl is soluble.
Solutions of a salt, or salts, in water are frequently termed brines and are a common base from
which drilling fluids are often made up.
Table 3-6 Solubility of Salts
Anions
* Soluble
** Slightly Soluble
*** Insoluble
Cation OH¯ Cl¯ HCO 3¯
2–
CO 3
2–
SO 4 S 2–
Na + * * * * * *
K + * * * * * *
Mg 2+ *** * ** *** * –
Ca 2+ ** * ** *** ** **
Ba 2+ * * ** *** *** *
Reference Manual
Baker Hughes Drilling Fluids
3-14 Revised 2006