BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling Fluids
addition of sodium carbonate which removes all calcium as insoluble, calcium carbonate). Since
the solubility of calcium is controlled by {OH – }, it can also be expressed as a function of pH.
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Caustic soda (lb/bbl) in water
Figure 3-13
Solubility of Calcium in a Solution Containing 4 lb/bbl Lime vs. Caustic Soda
Addition
Solubility of Other Compounds in Water
The solubilities of many other compounds, other than lime, are also controlled by equilibria
involving {OH – } and therefore, can be expressed as a function of pH. Generally, metal
hydroxides are insoluble, thus the higher the pH, the higher {OH – } and the less soluble the metal
becomes. Some elements with high valences form complex negatively charged anions at high pH
and thus become more soluble. Silica, for example, forms the H 3 SiO 4 – and the H 2 SiO 4 2– ions and
aluminum forms the aluminate ion, AlO 2 – .
The electronegativity of an element can be looked at in terms of the charge and size of its cation.
The group 1A metals, such as sodium and potassium, form singly charged cations which have a
large ionic radius. As the charge on a cation increases, the size of the ion decreases. This is
because, as electrons are lost, the nucleus exerts a greater pull on those electrons that are still left.
Thus, reading along the third row of the periodic table (see Table 3-2), Mg 2+ is a smaller ion than
Na + ; Al 3+ is smaller than Mg 2+ and Si4+, if it ever existed, would be smaller than Al3+ and so on,
across the row. The other effect in determining ionic size is atomic weight. The higher the
atomic weight, the more electrons there are and the more space they occupy. Thus, the K+ ion is
larger than Na+ ion and similarly Ba2+ is larger than Mg 2+ .
Hydration of Ions
Most ions in solution are hydrated, that is they are surrounded by a shell of water molecules that
form an organized structure around the ion. The size of this water shell depends on the intensity
of the ions’ electric field. Thus small highly charged ions are extensively hydrated. The
Reference Manual
Baker Hughes Drilling Fluids
3-22 Revised 2006