BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Chemical Equilibria
Many of the reactions that have been considered are examples of chemical equilibria. The
ionization of water, written as:
H 2 O ↔ H + + OH –
is a description of an equilibrium. Both the forward reaction, H2O → H+ + OH– and
the back reaction, H + + OH – → H 2 O, occur all the time at identical rates such that at one time:
Where K is a constant, known as the equilibrium constant.
+ −
( H )( OH )
( H O)
2
= K
In the case of water, where the amount of ionization that occurs is very small, the concentration
of un-dissociated water remains constant so that:
Therefore {H + }{OH – } = KC = 10 –14 moles 2 /liter 2
Any one water molecule will never remain un-ionized for very long. It will be continuously
changing from the un-ionized to the ionized state and back again. When an H + ion combines with
an OH – ion, there is no special reason for it to combine with the same OH – ion with which it was
formerly associated in a water molecule. Thus, the H + and OH – ions are continually changing
partners with ions from different water molecules in an unending game of “musical chairs”.
When something acts to change an equilibrium, it always does so such that the equilibrium
constant remains constant. For example, when an acid is added to water, {H + } increases. Then in
order to preserve the equilibrium, some of the hydrogen ions react with the hydroxide ions so that
{OH – } decreases and {H + }{OH – } stays at 10 –14 moles 2 /liter 2 .
Solubility of Lime
The solution of slightly soluble salts is an important example of equilibria. The solution of lime
(calcium hydroxide) in water is written:-
And at equilibrium
Ca(OH)2 ↔ Ca2+ + 2OH–
2+
−
( Ca )( OH )
{ Ca( OH ) }
2
2
= K
The {OH – } term is squared, since two OH – ions appear in the equation. For this reason the
equilibrium is more sensitive to {OH – }, than to {Ca 2+ }.
The un-dissociated lime remains as a solid (it is not molecularly dispersed) and has effective
concentration of unity as long as it is present.
If calcium ions or hydroxide ions are added to the system from an independent source, then the
solubility of the lime will be suppressed. Thus, if caustic soda is added, {OH – } increases and
{Ca 2+ } must decrease, so that the equilibrium is maintained. Thus the concentration of calcium in
drilling fluids can be controlled by the addition of caustic. This is shown in Figure 3-13 (the
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-21