Drilling fluid - the lifeblood of drilling

Mud rotary drilling is a method of drilling a borehole into the subsurface by rotating a string of drill pipe and bit against the formation. By circulating water based drilling fluid, the drilled material (cuttings) is carried to the surface. This drilling method is used in the environmental or water wells, mining, geotechnical and oil/gas drilling industries. A complete drilling fluid system must be properly designed in order to efficiently construct a well. The two main parts of the fluid system consist of the actual drilling fluid, and the solids separation equipment designed to remove the cuttings from the mud at the surface.

What does drilling fluid do?

Removes material (cuttings) from the borehole
The drilling fluid carries the drilled material to the ground surface either by viscosity or velocity.

Cools and lubricates the bit
The drill bit becomes hot due to friction generated during the drilling process. As the drilling fluid passes through the bit and exits the jets/nozzles, the excess heat is removed and carried up the borehole.

Cleans the drill bit
When the drilling fluid exits the bit jets, the fluid’s velocity removes material from the bit teeth and the cuttings from the bit formation interface. This prevents the cuttings from being re-cut or re-ground.

Controls fluid loss
As the fluid moves from the borehole into the formation, clay particles are deposited on the borehole wall. The clay particles form a barrier limiting the amount of drilling fluid penetrating the formation. This barrier, called a filter cake, is important for the stability of the borehole. Additionally, well development time is reduced if fluid loss to the formation is limited.

Stabilizes the borehole
The drilling fluid’s weight in the borehole must overcome the formation pressure to pre¬vent the borehole from collapsing. Also, the fluid prevents formation swelling by “coating” the formation with an impermeable barrier.

Lubricates the drill pipe
The drilling fluid reduces friction between the drill pipe and the rising cuttings, and also between the drill pipe and the formation.

Suspends cuttings
When the mud pump stops, the drilling fluid velocity stops. The fluid must have enough gel strength to keep the drilled material (cuttings) in suspension until the mud pump activates.