BAKER HUGHES - Drilling Fluids Reference Manual

Formation Mechanics
Attapulgite
Figure 2-5
Structure of Palygorskite
The attapulgite, sepiolite and polygorskite clay minerals are completely different in structure and
shape from the clay minerals discussed so far. Chemically, attapulgite is a hydrous magnesium
alumina silicate. The structure of the attapulgite crystals consist of bundles of laths. These are hollow
needle-like structures. When mixed vigorously with water the bundles separate into individual laths.
The individual laths then clump together in “brush heap” type arrangements. This provides a
viscosifying effect to the water. Since the viscosity in attapulgite suspensions is dependent on
mechanical interference rather than electrostatic forces, it makes an excellent suspending agent in
saltwater.
Sepiolite is similar to attapulgite in structure and chemistry. Sepiolite-base fluids have hightemperature
stability and have been used on many high-temperature wells.
Mixed-Layer Clays
Mixed-layer clays are formed by the alteration of different clay minerals. The mixed-layer clays are
usually a combination of illite and smectite type clays. The proportion of swelling to non-swelling
layers determines the expandability of these clay minerals.
Base Exchange
Base exchange in clays occurs when certain cations in solution are exchanged for others which are
adsorbed on the crystalline surfaces of the clay. In many clays, the adsorbed cations control the
tendency of the clay to swell and/or disperse in the presence of water. Those cations that are most
frequently involved in the base exchange of clays are sodium, potassium, magnesium, and calcium.
These cations are adsorbed on the clay surfaces between individual platelets and around the edges of
the structural units. The crystalline structure of the clay mineral is not altered in the base exchange
progress. Several factors affect base exchange in clays.
• The relative replacement power of the available cations
• The type of clay (see Table 2-1)
Reference Manual
Baker Hughes Drilling Fluids
2-6 Revised 2006