Definition - What does Drilling Fluid mean?
Drilling fluid is a mix of water and other components, such as bentonite, that helps a bore maintain its shape throughout the boring process while loosening the material you’re drilling through. It's also called mud or barite. Whatever you call it, its whole purpose, according to the North American Society for Trenchless Technology, is to “enhance stability and excavatability.”
Drilling fluid is used in conjunction with a kelly to bore.
Trenchlesspedia explains Drilling Fluid
Drilling fluid can be a liquid, known as drilling mud, or it can be a gas, such as compressed air. It can even be a mix of water and compressed air blown into the bore. Drilling fluid also helps to cool the auger or drill bit. Because drilling fluid helps ease the process of boring, it also reduces wear and tear on equipment.
Drilling mud is mostly water. It can have additives that help stabilize the bore further than water can, such as bentonite, an absorbent clay or soda ash. It’s a mixture; its additives won’t remain suspended in the water base. This means it must be agitated or stirred (much like concrete) before use. During agitation, either by a mixer or a drill, it’s relatively thin. When the drilling or agitation ceases, it thickens quickly.
Drilling mud is heavier than water, but lighter than the material through which the drill moves. This means it dilutes the spoil materials cohesion, breaking it up and allowing the drill or auger to move the spoil out of the bore.
Because drilling mud is water with additives, its viscosity (its resistance to flow) is greater than that of water. This increased viscosity means that, as the drilling fluid is forced into the wall of the bore by the drilling process, it begins to form a “mud cake” that clings to the inside of the bore walls. When drilling ceases, the mud cake thickens to a consistency similar to that of compacted mud that clings to shoes or tires. This mud cake preserves the structure of the bore.