BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
creates a greater charge deficiency. This deficiency is frequently satisfied by potassium ions located
between the successive clay units. Illites are essentially inert to hydration. It is the most abundant clay
mineral found in sedimentary rocks.
Kaolinite
Figure 2-3
Structure of a Kaolnite Layer
The kaolinite group includes dioctahedral minerals and trioctahedral minerals. These minerals are
composed of one silica (octahedral) sheet and one alumina (tetrahedral) sheet. The atoms making up
its structure combine almost completely, resulting in a neutrally charged surface. This provides for
very little expansion or hydration of the structure.
Chlorite
Figure 2-4
Structure of Chlorites
Chlorite is similar to the smectite minerals except for a layer of magnesium hydroxide between the
clay sheets. This layer neutralizes the surface charge so that the chlorite is non-expandable. Chlorites
are common constituents of argillaceous sedimentary rocks where these minerals occur in both detrital
and authigenic forms. The basic structure of chlorites consists of negatively charged mica-like layers
regularly alternating with positively charged brucite-like octahedral sheets. The various members of
this group of clays are differentiated by the kind and amount of substitutions within the brucite-like
layer and the tetrahedral and octahedral positions of the mica-like layer.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 2-5