BAKER HUGHES - Drilling Fluids Reference Manual

Formation Mechanics
Hydration Mechanics
If a particle of dried bentonite could be enlarged so that it would be visible to the naked eye, it would
look like a fan of cards. Each card would be composed of alternating layers of silica and alumina.
Between the individual platelets, there would be a thin layer or layering of partially hydrated cations.
These cations would be composed mainly of sodium, calcium, magnesium, and potassium. The origin
of these cations was the environment into which the clay was deposited. These positive particles are
attracted to the negative charge that exists on the surface of the clay. They loosely hold the clay
platelets together. Even in a dehydrated state, each of the cations is surrounded by a thin layer of
water and the entire clay platelet is surrounded by a thin layer of water. If this group of clay platelets
is dropped in water, the outer exposed surfaces of the platelets and attached ions immediately become
hydrated.
As time goes on, water molecules begin to seep in between the platelets. Some of the molecules of
water adhere to the clay platelets and some of them go to the individual cations on the platelet surface.
In freshwater, bentonite will swell 8 to 10 times its original dry volume. This process is a form of
osmosis and hydrogen attraction into the negative clay surfaces. The net effect of these processes is
that the clay platelets are pried apart. This is the swelling effect that is noticed when dried bentonite is
placed in freshwater. This process of hydration also causes the individual clay platelets to disperse
(separate). Even in pure water, complete dispersion of all clay platelets is unlikely. The amount of
dispersion that occurs will be dependent upon the type and number of cations that are on the clay
surface. If the cations are divalent (two positive charges) dispersion will be less complete. Other
factors which contribute to the amount of dispersion are temperature, mechanical agitation, and the
purity of the water.
When a clay platelet is fully hydrated, it is surrounded by a cloud of water and hydrated ions. The
greatest concentration of water molecules and hydrated cations are near the surface of the clay.
Other Clay Types
Illite
Figure 2-2
Structure Illite/ Mica
Illite is essentially a group name for non-expanding, clay-sized, dioctahedral, micaceous minerals. It
is structurally similar to muscovite, but substitution of aluminum for silicon in the silica tetrahedron
Reference Manual
Baker Hughes Drilling Fluids
2-4 Revised 2006