Drilling mud is used to control subsurface pressures, lubricate the drill bit, stabilize the well bore, and carry the cuttings to the surface, among other functions. Mud is pumped from the surface through the hollow drill string, exits through nozzles in the drill bit, and returns to the surface through the annular space between the drill string and the walls of the hole.
As the drill bit grinds rocks into drill cuttings, these cuttings become entrained in the mud flow and are carried to the surface. In order to return the mud to the recirculating mud system and to make the solids easier to handle, the solids must be separated from the mud. The first step in separating the cuttings from the mud involves circulating the mixture of mud and cuttings over vibrating screens called shale shakers. The liquid mud passes through the screens and is recirculated back to the mud tanks from which mud is withdrawn for pumping downhole. The drill cuttings remain on top of the shale shaker screens; the vibratory action of the shakers moves the cuttings down the screen and off the end of the shakers to a point where they can be collected and stored in a tank or pit for further treatment or management.
Often two series of shale shakers are used. The first series (primary shakers) use coarse screens to remove only the larger cuttings. The second series (secondary shakers) use fine mesh screens to remove much smaller particles. In general, the separated drill cuttings are coated with a large quantity of drilling mud roughly equal in volume to the cuttings.
Additional mechanical processing is often used in the mud pit system to further remove as many fine solids as possible because these particles tend to interfere with drilling performance. This mechanical equipment usually belongs to one of three types: 1) hydrocyclone-type desilters and desanders, 2) mud cleaners (hydrocyclone discharging on a fine screened shaker), and 3) rotary bowl decanting centrifuges. The separated fine solids are combined with the larger drill cuttings removed by the shale shakers.
If the solids collected by the shale shakers are still coated with so much mud that they are unsuitable for the next reuse or disposal step or if the used mud is valuable enough to collect as much of it as possible, the solids can be further treated with drying shakers utilizing high gravitational separation, vertical or horizontal rotary cuttings dryers, screw-type squeeze presses, or centrifuges. The cuttings dryers recover additional mud and produce dry, powdery cuttings.