BAKER HUGHES - Drilling Fluids Reference Manual

Formation Mechanics
in the outer silica tetrahedrons of the clays. Since the potassium cation fits nicely in the molecular
structure of the clays, it is able to hold two platelets tightly together.
The phenomenon of base exchange is used in the design of wellbore fluid systems. The basis for
calcium and potassium fluid systems is to prevent the hydration and swelling of formation drill solids
through the mechanism of base exchange.
States of Clays
In general clays exist in one of two states, either that of aggregation or dispersion. Clay particle
association is shown in Figure 2-1.
Figure 2-6
Cays and Clay Minerals Particle Association
The aggregated state occurs when the clay platelets are stacked parallel to each other, similar to a
stacked deck of cards. Unhydrated clay exists in this state. When the clay contacts water, the nature
of the cations holding the clay platelets together may or may not allow dispersion to take place. The
dispersed state occurs when the clay platelets separate. Aggregated or dispersed clays can undergo
flocculation or deflocculation. Flocculation occurs when clay platelets are electrically attracted to
each other. The predominant area of attraction is thought to be on the edges of the clay platelets.
The edges of the clays are thought to be predominately positively charged, as opposed to the
negatively charged clay basal plane area. When flocculated, the individual clays can clump together in
particles large enough to be seen by the naked eye. They are often large enough to separate and settle
out of solution.
Several mechanisms exist by which flocculation can occur. It is generally caused by changes in the
electrolyte concentration, temperature, and solids crowding. Deflocculation is the reverse of the
flocculation process. Deflocculation occurs when the clay particles remain geometrically independent
and unassociated with adjacent particles. It is achieved by the addition of a deflocculant, a
temperature stabilizer, or a dilution fluid.
Chemical deflocculants are thought to adsorb on the edges of the clays, thereby neutralizing the
positive charges and allowing the clay platelets to separate.
Clay Identification
The most common analysis at present to determine the type of clays in cuttings or cavings is X-ray
diffraction. Table 2-3 is an example of an X-ray diffraction analysis of a Norway shale. It is based on
the reflection of monochromatic radiation of known wavelength. Each atom produces a characteristic
wave reflection which depends on its type and its position in the molecular structure. Each
Reference Manual
Baker Hughes Drilling Fluids
2-8 Revised 2006