BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Smectite
Figure 2-1
Structure of Montmorillonite
Of the clay minerals, smectite is the least stable and the most susceptible to hydration and diagenetic
alteration. Two other terms often associated with the smectites are montmorillonite and Bentonite.
Montmorillonite is the name given to the clay mineral found near Montmorillon, France. The term
was used until recently to refer to the now known smectite group of clays. Bentonite is the term
applied to a clay type found near Fort Benton, Montana. Bentonite was formed by the alteration of
volcanic ash by water. For this discussion, the terms smectite, montmorillonite, and Bentonite will be
used interchangeably.
The basic structure of Bentonite is a thin sheet-like form of alternating layers of silica and alumina.
The figure contains diagrams of the montmorillonite structure. Each sheet would resemble a
honeycomb three layers thick. The two outside layers would be silica tetrahedrons and the central
layer alumina octahedrons. The layers are held together in a very intricate lattice of aluminum, silicon,
oxygen, and hydrogen atoms.
This clay “sheet” is a rigid crystal-like salt or diamond. If the clay sheet were pure aluminum, silicon,
oxygen, and hydrogen, all of the charges would be equal and the sheet would be electrically neutral
and inert. Mica is a common mineral having this characteristic.
Since Bentonite has been exposed to seawater and other sources of cations, some of the silicon and
aluminum cations in the structure have been replaced. Iron and magnesium typically replace
aluminum and aluminum typically replaces silicon. This replacement causes an imbalance in charges
in the structure. This causes the bentonite sheet to be negatively charged. This charge occurs on the
flat surface of the sheet or the area called the basal plane. Cations (positive charged particles) are
attracted to this negatively charged surface. The predominant cation type determines the type of
bentonite and the degree of hydration that it will undergo. The rank of cations from those which
promote the most amount of swelling to those which promote the least amount of swelling is lithium,
sodium, potassium, magnesium, calcium, aluminum, and hydrogen.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 2-3