Bentonite

Bentonite forms a thin, low-permeability filter cake on the borehole wall when subjected to differential pressure. This cake prevents excessive drilling fluid loss into permeable formations, maintains wellbore pressure balance, and reduces the risk of formation damage or differential sticking of the drill string.

API 13A grade bentonite must produce ≥16 cP viscosity per API yield test and meet specific fluid loss and sand content criteria. OCMA grade meets equivalent European specifications. Both are suitable for oil and gas drilling, with selection based on operator preference and regional standards.

Bentonite swelling is suppressed by monovalent (Na⁺, K⁺) and particularly divalent (Ca²⁺, Mg²⁺) cations in salt water, which exchange with sodium on the clay surface, reducing electrostatic repulsion and interlayer swelling. In saline conditions, attapulgite (Saltgel) is used instead of bentonite.

Yes. Bentonite drilling mud is widely used in HDD (trenchless pipeline installation), foundation drilling (slurry walls, bored piles), and geotechnical investigation boreholes. In these civil applications, bentonite provides borehole wall stabilisation, cuttings transport, and lubrication — the same functions as in oil and gas drilling.

Bentonite filter cake on the borehole wall must be removed during well completion to restore formation permeability. Chemical breakers (oxidisers, enzymes, acids) are used to degrade the filter cake. Unlike CaCO₃ bridges, bentonite filter cake is not acid-soluble and requires specific breaker treatments.