Carboxymethyl Cellulose

Sodium CMC and PAC, a CMC which has a degree of substitution usually greater than 1.0, are two of the more widely used anionic polymers which serve to control the viscosity and the filtration rates .

A higher degree of substitution CMC, e.g., in PAC, offers good fluid loss reduction in an electrolyte-containing system with a smectite-type clay such as bentonite. However, the effectiveness of CMC, being a polyelectrolyte, as a viscosity builder has its limitations, as its effectiveness decreases with increasing electrolyte concentration. Thus, regular CMC is mostly suitable for electrolyte-poor drilling fluid compositions, such as freshwater-based drilling fluid compositions. Although CMCs and PACs with a high degree of polymerization are used as viscosifiers, the regular grades do not have the good suspension carrying properties (high low-shear viscosity), which are needed to bring the cuttings efficiently to the surface.

Alternatively, xanthan gum is employed as viscosifier and suspending agent . Xanthan gum has very suitable rheological properties. It forms a gel within a short period of time when drilling circulation is slow or interrupted. This enables immobilization of dispersed solids in the fluid composition. After circulation is resumed, the gel easily transforms into a flowing fluid, thereby maintaining a good dispersion of the solids contained in the fluid composition.

However, xanthan gum is relatively expensive. Moreover, it is only stable at temperatures below about 120 °C, which makes it less suitable for drilling at temperatures exceeding 120 °C. Furthermore, many xanthan grades contain very fine insoluble material, usually residues from the fermentation production process. These insoluble materials are undesired for drilling operations, as they cause, e.g., more difficult hole cleaning. Only the more expensive xanthan grades do not have these insoluble parts .



In most drilling operations, the drilling fluid composition experiences a high shear. This is particularly advantageous if CMC is used in such fluid compositions, as specific CMC formulations are gelling, when exposed to a high shear. Applying a high shear improves the gelling properties of CMC considerably .

The gelling properties of CMC can also be improved by a heat treatment. Preferably, the CMC is treated at 70 °C or higher. A drilling fluid composition has described that can be used up 140 °C. However, above this temperature the CMC decomposes. In comparison to fluid compositions that are using xanthan gum, which deteriorates above 120 °C, deeper drilling operations at higher temperatures can be carried out.