Mud is the common name for drilling fluid.  While it is outside the scope of this handbook to offer a detailed discussion of drilling fluids, a brief outline of the general characteristics of drilling fluids is included to establish the basic relationships between drilling mud and solids control.  Similarly, any discussion of solids control would be incomplete without establishing an understanding of the nature of mud solids — their size, shape and composition.

Functions of a Drilling Fluid:
The mud system in a drilling operation performs many important functions. These include:

-Clean beneath the bit: To maximize drilling efficiency, the drilling fluid must translate hydraulic horsepower from the main mud pumps into enough velocity to remove drill cuttings away from the face of the bit and allow the cutters to be in contact with the formation. If the cuttings are not removed, they will be ground into smaller particles and adversely affect drilling rate, mud properties, and project costs.

-Carry drilled solids from the bottom of the hole to the surface: Once cuttings are removed from beneath the bit, the fluid must transport them toward the surface faster than they can settle back toward the bottom of the hole.  Factors which influence cuttings movement are velocity, cuttings size and shape, and the fluid properties.

-Suspend cuttings when circulation is stopped: Circulation of the drilling fluid stops periodically during operations, such as when making a bit trip.  The drilling fluids must be able to suspend cuttings and weighting material while circulation is stopped, but should begin to flow easily when the pumps are switched back on.  Properties which affect cuttings suspension are the density and viscosity of the mud and the density of the solids in the mud.

-Allow removal of cuttings by the surface system.
-Control formation pressures.
-Promote borehole stability.
-Cool the bit and lubricate the drill string.
-Help support the weight of the drill string.
-Allow accurate information from the well.