The removal of rock chips from the cutting face of the bit and carrying these cuttings to the surface is dependant on the flow properties (viscosity) of the drilling fluid. Viscosity is defined as the resistance of a fluid to flow.

The drillers recognises this as ‘thicknes’. Polymers are employed to build viscosity for drilling fluids while improving borehole stability and maximizing core recovery through inhibition of water sensitive clays and shales. Bentonitic clays are also used to provide viscosity and to develop ‘gel strength’. Gel strength develops when the mud stops moving and can be advantageous when drilling loose sand or gravel formations.

Bentonitic clays swell when mixed in fresh water, imparting viscosity and gel strength to thefluid. In additional, they assist in the movement of drill pipe and casing by the developing of athin filter cake on the walls of the borehole.