BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Table 2-1 Clay and Clay Minerals Base Exchange Capacity
Clay Material
Meg/100 g of Dry Clay
Montmorillonite 50 - 130
Illite 10 - 40
Kaolinite 3 - 15
Attapulgite-Sepiolite 10 - 35
• The amount of charge deficiency on the clay surface
• The concentration of the cations in solution
• The size and type of the replacing cations
The replacement power of various cations is controlled by two major factors:
• The number of charges on the cation
• The size of the hydrated cation
In general, cations with more positive charges (polyvalent) will have greater replacement power. An
exception to this is the hydrogen ion which often acts as a polyvalent ion.
The most commonly occurring cations may be ranked according to their replacement power as shown
in Table 2-2.
Table 2-2 Order of Cation Replacibility
Easiest
Lithium
Sodium
Potassium
Magnesium
Calcium
Hydrogen
Hardest
For example, if the concentration of calcium ions in a sodium bentonite suspension were increased, the
calcium ions would tend to replace the sodium cations.
The relative concentration of the various cations present has a significant effect on base exchange.
When cations of a lower replacement power are present in greater concentration than those of a higher
replacement power, the desired base exchange may not occur. In this situation, the lower replacement
power cations are occupying a large number of the base exchange sites.
Cation size is determined by the atomic radius of the cations and the amount of water required to
hydrate the cations. Smaller cations are easier to exchange and result in less hydration. Most
monovalent cations are smaller and allow less hydration. An exception to this is the potassium cation.
The potassium cation is thought to prevent hydration of clays due mainly to its size. The potassium
cation hydrates very little and has a diameter almost the same size as the spacing between the oxygen
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 2-7