Clay-Based Viscosifiers

Bentonite, a name given for commercially mined sodium montmorillonite (a form of smectite clay) is the most common clay additive used in both oil and water based drilling fluids.  When a bentonite clay particle is hydrated, the interlayers expand as water is absorbed.  This expansion is what creates viscosity in a clay-based drilling fluid system.  The distance at which the particle expands is related to the type and amount of cations present

Another type of commercial clay often used is Attapulgite, a needle-shaped non-swelling particle primarily used in saltwater muds.  The inherit shape of this material is responsible for the clay’s behavior and ability to build viscosity.  Because of its non-swelling and crystalline structure, Attapulgite alone does not provide sufficient filtration control.

Polymer Viscosifiers

Polymers, defined as large molecules comprised of small, identical recurring units, are widely used in water-based muds. Their structures can usually be classified as linear, branched, or cross-linked.  However, polymers can also be categorized by their origin:  natural, modified, or synthetic.

One of the most often used natural polymers is known as xanthan gum. When hydrated, xanthan gum arranges itself as a long branching structure with weak hydrogen bonding – this provides it’s viscosifying characteristics.

The effectiveness of polymers/starch additives rely on many factors including the concentration of the material and the fluids inherent pH/hardness/salinity. Starch additives, typically added for filtration control, also can provide viscosity as a secondary function.