In its pristine state, drilling mud may contain two types of solids – bentonite providing viscosity and, optionally, barite for weight. The bentonite particles range in size up to 10 microns. The barite ranges from 5 to 80 microns, with the greatest concentration around 20 microns.
After mud has circulated, it also contains drill cuttings. These range from large chips to the finest colloidal particles (< 1 micron). The job of solids control equipment is to discard the cuttings, retain as much barite as possible(because it’s cost money) and keep enough colloidal material to maintain viscosity.
Shale Shaker Efficiency
Solids control removes the coarsest particles first and then progressively the finer particles. Mud returning to the surface passes first to shale shakers. These machines have one or more vibrating mesh screens that sieve large particles and discard them to the mud pit (firgure 1). The finest mesh, #200, stops particles about 75 microns in diameter, which is just large than the largest barite particles. Liquid mud and smaller particles meanwhile flow through the mesh to a settling tank and are eventually pumped to the next piece of solids control equipment.
A shale shaker, workhorse of solids control. Mud issuing from the well is fed on to the top, usually coarse. Large cuttings are retained by the screen and discarded into mud tank. The lower, finer screen removes smaller solids. Traditionally, the screens agitated in a circular motion. Recently, less plugging of the screens by cuttings has been noted with a new type of shale shaker using linear motion.
Traditionally, screen meshes are square, the size being classified by an API number. A mesh’s ability to catch solids is classified by its cut point, the particle size that has a 50 percent chance of retained – particles smaller than mesh’s cut point will likely pass, particle larger than the cut point will likely retained. The coarsest mesh, #10, has a cut point of 2 millimeter(mm). The finest mesh, #200, has a cut point of 74 microns (1000 microns = 1mm). Recently introduced retangular and other shaped meshes do not fit the API scheme, and their performance must be compared with traditional meshes by experiment. Nonsuqare meshes are designed by a letter and a number in the fluid management report.
Hydrocyclone （mud Cleaner）Efficiency
Particles smaller than 75 to 100 microns are removed by desanders, desilters and mud cleaners, all of which use a divice called a hydrocyclone to separate out solids (figure 2). The mud is propelled at high speed into the top of an inverted cone along tangent.
The centrifugal force caused by the resulting spiralling motion press the larger and heavier solids to the side of the cone where they slowly drift down and are finally discarded. The remaining fluid and smaller particles are forced to the center of the cone and ejected out of the top. These then pass to the next piece of solids control equipment. Hydrocuclone cone diameters range from 4 to 12 inches, the smaller the diameter, the material discarded. In theory, a 4-inch hydrocyclone discarded particles as small as 15 microns.
Schematic of a hydrocyclone used to separate fine solids from drilling mud. The mud is injected at high speed along a tangent at the top of an inverted cone. Cntrifugal force separate the solids that hug the cone’s perimeter and descend by gravity to the bottom ejection port. The mud, meanwhile, stays near the center and is propelled out of the top.
Hydrocyclones come in several sizes from 4 to 12 inches in diameter at the top, the smaller ones discarding finer material. Hydrocyclones are the least reliable of all solids control equipment. Their performance depends on mud rheology and density, and powerful centrifugal pumps required to propel the mud into the hydrocyclone tend to break uo cuttings into smaller pieces making solids control ultimately more difficult.
Hydrocyclones are arranged several ways. A desander comprises one or two large-diameter hydrocyclones working in tandem. A desilter comprises several of 4-inch diameter. In the mud cleaner, 8 or 10 of the smallest diameters are arranged above a bottom shaker where the discarded solids are sieved through a very fine screen designed to pass barite and smaller particles (figure 3). The barite is saved and mixed back into the mud.
A mud cleaner, comprising two 12 inche hydrocyclones and eight 4-inch hydrocyclones stacked about a shale shaker that collects solids. The solids then encounter a very fine screen. This passes small solids such as barite that for econimy are retained and added back into the circulating mud.
Decanting Centrifuges Efficiency
Centrifuge used to remove very fine solids. Mud is fed into the middle of an archimedes’ screen that rotates inside a faster-rotating conical tube. The solids are expelled radially by centrifugal force, forced to the right by the screw and finally expelled. The cleaned mud is ejected at ports on the left. Decanting centrifuges handle only a fraction of the total mud flow and are mainly used to reduce viscosity.
To discard the finest solids, in particular colloidal-size particles, mud engineers use a decanting centrifuge. Not availble on every rig and used selectively when available, this machine removes colloids that are areating too much viscosity. The device comprises an archimedes’ screw that rotates slowly inside a conical tube that rotates at high speed in the same direction (figure 4). As mud is introduced in the middle of the screw, centrifugal forces separate solids from liquids. Decanting centrifuge accommodate small flow rates and can only be used to treat a fraction of the mud flow.
Arrange Solids Control Equipment
In theory, the available equipment offers substantial flexibility – more or less any size particle can be discarded or retained (figure 5). In practice, some equipment are more reliable than ohters, and performance may depend on fluid rheology and flow rate. The experience of solids control have shown that the most reliable equipment is the shale shaker. equipped with very fine screen, shale shaker can control solids down to 75 microns, the top limit of barite. Its operation is relatively uninfluenced by mud properties. The performance of hydrocyclone device (desander, desilter, mud cleaner), however, depends greatly on fluid density and rheology. One 4-inch hydrocyclone , rated to discard particles as small as 15 microns, failed to discard particles 10 times that size. The centrifgual pumps required to pump mud into hydrocyclones tend to break solids into smaller pieces making them harder to discard. The mud engineer found that decanting centrifuges performed best at maximum flow rate.
Typical arrangement of solids control equipment for a mud weighted with barite. The mud first passes through shale shakers into a steel mud tank where sediment settles and is periodicalluy extracted. If necessary, the mud can then be led through a degasser(usually a vacuum degasser), which operates like a conventional gas-liquid separator, into another mud tank. then, the mud proceeds to a mud cleaner. Cleaned mud and barite are returned to a third mud tank, where part of the mud flow can then be cleaned by a centrifuge. The mud finally reaches a fourth mud tank, where reserve mud can be added, and is then pumped downhole.