Solids are removed for better mud control. There are three basic ways of conditioning drilling fluids, all relating to solids:
Removing solids is necessary before a drilling fluid is pumped to the drill bit. The more effective the mechanical solids control, the less dilution and chemical treatment are required. The addition of water does not necessarily dilute a drilling mud in a drilling situation.
In some situations, the dynamic fluid loss can be very high. Failure to replace the filtrate loss would cause a decrease in liquid volume and resultant increase in per cent solids, weight, and viscosity, independent of the drilling rate. If the filtration loss is constantly replaced however, the properties of a mud are continuously restored. Whether this "pass-through water" amounts to most or a minor part of the total water requirement, depends upon the ratio of the dynamic down-hole filtration rate to the solids removal rate. The total amount of solids can be altered only by the addition and removal of drilled and commercial solids regardless of the amount of water being added.
Plastic viscosity is not a function of total solids content by volume, unless it is maintained at an absolute minimum. If a drilling flui does not contain commercial suspended solids it is unweighted and if it does contain them is weighted. The total solids content in weighted muds usually will be high.
The term "shale shaker" is used in drilling mud work to cover all the equipment like shaking screen, vibrating screen, and osillating screen. Vibrating screen devices are most used. The volume of liquid discharge that can pass through a vibrating screen depends upon per cent open area, mesh opening size, type of motion, mud flow properties, solids content of liquid discharge, feed rate of oversize solids, and type of oversized solids. The volumetric rate of solids discharge that can be handled depends on type of screen motion, speed and amplitude, horsepower, type of solids, mesh design, and wire strenght. The size range of solids that can be separated is determined by mesh opening size, type of screen motion, speed and amplitude, and screen mesh weave.
Type of motion is determined both by eccentric weight or reciprocator arrangement and by the suspension system. If either of them is not in good condition, the shale shaker cannot perform properly.
The major operating principles of sand traps are as follows:
Degasser sometimes are essential to the solids control process, even through shale shakers remove a good portion of the gas from a badly gas-cut mud, especially if the yield point is as low as 10lb/100ft2. Slurry-handling centrifugal pumps feeding abrasive oil-field muds to hydrocyclones loss efficiency when pumpimg gas-cut muds. They will "gas lock" when gas content exceeds 3 per cent by volume. Hydrocyclones do not function properly if pumping head is not connstant or if there is gas or air in the feed. Provision for degassing equipment, therefor, should be made between the sand trap and the first hydrocyclones. Hydrocyclone can acoomplish the finest separation of any primary separation equipment normally used on the full-flow circulating rate of an unweighted mud system.
The hydrocyclone has a conical-shaped portion inwhich most of the settling takes place. Usually, a cyclindrical feed chamber is located at the large end of the conical section. At he apex of the conical section is the under-flow opening for the solids discharge. In operation, the under-flow opening is usually facing downward for convenience. Near the top end of the feed chamber, the inlet nozzle is situated tangential to the inside circumference andon a plane perpendicular, or nearly so, to the top-to-bottom central axis of the hydrocyclone. A hollow cyclinder,called the "vortex finder", extends axially from the top into the barrel of the hydrocyclone past the inlet. The inside of the vortex finder forms the over-flow outlet for the liquid discharge or effuent. The "size" of a hydrocylone is determined by the largest inside diameter of the conical portion. The hydrocyclone owes its centrifugal force to the tangential velocity of the slurry entering the feed chamber. An axial velocity component is creat by the axial thrust of the feed stream leaving the blind annular space of the feed chamber.
In a hydrocyclone , the incoming stream spirals down along the wall of the conical section to the under-flow opening, reverses axial direction, and then spirals upward to exit at the over-flow opening. A properly-designed hydrocyclone can be adjusted in such manner that when a clean liquid is fed into it. Special fine shale shaker screens for secondary recovery from primary separation hydrocyclones can be used for unweighted water-base muds. The secondary-separation screening process diminishes the volume of discard by returning the silt with associated free liquid and clays to the system. In a situation where the increased mud weight, total solids content ,and abrasiveness of the returned solids do not harm the drilling operation, the secondary-separation screens can be used to advantage. The under-flow of the primary hydrocyclone removes only a small fraction of the total solids in the cyclone feed. This under-flow contains, selectively, the coarsest fraction in the mud. In order to reduce drilling cost it is better to remove the coarser fraction rather than to attempt to distinguish closely between barite and light solids.
Hydrocylone-fine-screen conbinations are used normally to weighted muds. Application of the secondary centrifuge will probably remain rare in the case of weighted muds. Occasionally reference is made to "double-stage" centrifuging solids weighted muds is quite different from that for unweighted muds.
For example, an ordinary unweighted fresh-water mud will usually contain approximately 50 lb of solids per bbl. If clay constitutes a very high per centage of these solids, viscosity can be easily controlled with a small amount of thinner. On the other hand, a 15 lb/gal mud will contain approximately 380lb of solids per bbl. Obviously, thesolids content can not be signigficantly reduced if specific weight has to be maintained. Higher solids removal capacities are needed in the case of faster drilling rates. The potential drilling benefits justify installation of the best equipment available.