Bentonite

Sodium bentonite is a natural lubricant & sealant. It is most commonly used as drilling mud. It is a native clay that is suspended in the fluid while drilling. Bentonite is frequently referred to in the oilfield as "gel". Gel likely makes reference to the fact that while the fluid is being pumped, it can be very thin and free-flowing (like chocolate milk), though when pumping is stopped, the static fluid builds a "gel" structure that resists flow. When an adequate pumping force is applied to "break the gel", flow resumes and the fluid returns to its previously free-flowing state.