Ann: Yangir-2 core-hole spudded, page-220

Drilling mud has two main purposes - to lubricate and cool the drill bit and to flush the rock chips to the surface. The mud is usually either water-based or oil-based and can contain a variety of natural or synthetic additives, depending on what the engineer is trying to control. Two of the most common additives are finely ground bentonite and barite.

Bentonite clay keeps the drilling fluid viscous, hence the term "mud".

The density of the mud has to be kept above the density of the rock that is being drilled, otherwise the rock fragments will sink through the lubricating fluid and stay at the bottom of the hole. Barite has a high SG (specific gravity) and is used to keep the density of the mud above the density of the rock, so that the rock chips then float to the surface with the circulating mud.

Sometimes the ground being drilled is highly fractured, or contains cavities, causing the drilling fluid to be lost. Materials can then be added to the mud that help to seal the fractures or cavities, to minimize or eliminate mud loss.

Controlling the properties of the mud is the responsibility of the mud engineer.