Solids Control –Centrifuges

•As with hydroclones, decanting-type centrifuges increase the forces causing separation of the solids by increasing centrifugal force.
•The decanting centrifuge consists of a conical, horizontal steel bowl rotating at a high speed, with a screw-shaped conveyor inside.
–This conveyor rotates in the same direction as the outer bowl, but at a slightly slower speed.
–The high rotating speed forces the solids to the inside wall of the bowl and the conveyor pushes them to the end for discharge.
•Centrifuges are capable of making a sharp cut point.
–The ideal cut point is the particle size at which all larger particles are separated and all finer particles are retained.
–This is, however, not possible, so the actual stated percent of cutpoint(D number) should be included when comparing centrifuge performance characteristics.

Solids Control –Centrifuges Discharge




Solids Control –Centrifuges -Applications

•Weighted mud
–Dual centrifuges can be rigged up for barite recovery.
–The first centrifuge recovers barite and send it back to the mud system.
–The overflow is sent to the second machine to remove LGS and send clean mud back to the mud system
•Un-Weighted mud
–Centrifuges set up in a parallel configuration, both remove low gravity solids and send cleaned fluid back to the mud system
•De-watering
–Whole mud is treated to form dry solids for disposal and clear water for recycling.
–For this application, the solids content of the mud is brought to a very low level.
–Then, chemicals are added to encourage the particles to coagulate and flocculate.
–Once the fluid is properly treated, it can be processed through a centrifuge, with mostly dry solids and water being recovered
•Reduction of mud costs, without sacrificing control of essential mud properties, is the main purpose of, and justification for, using a decanting centrifuge.
•Although it helps control undesirable fine solids, the centrifuge’s principal function is to minimize dilution and maintain acceptable properties in the mud system

Solids Control –Centrifuges –Internal Parts

Solids Control –Hydrocyclones

Hydrocyclones are cone shaped designed to take out smaller particles–Desanders 74-45 microns–Desilters 35-15 microns–Micro Cones 7-9 microns

•A centrifugal pump feeds a high-volume mud through a tangential opening into the large end of the funnel-shaped hydroclone.
•When the proper amount of head (pressure) is used, this results in a whirling of the fluid much like the motion of a water spout, tornado or cyclone, expelling wet, higher mass solids out the open bottom while returning the liquid through the top of the hydroclone.
•Thus, all hydroclones operate in a similar manner, whether they are used as desanders, desilters or clay ejectors.
•Head is related to pressure as follows: –Head (ft) = Pressure (psi)/[.052 x mud weight (lb/gal)] –Most hydrocyclones designed for about 75 ft of head at the inlet manifold
•Capacity is related to hydroclone size, so more smaller hydroclones are required for a given volume than larger ones.
•An example of hydroclone removal efficiency, showing the cut and D10-D50-D90values for typical 3-, 4-and 6-in. hydroclones, is depicted in the graph opposite.
•Desanders usually are 6-in. hydroclones or larger, with two 12-in. hydroclones being common. •Desilters use 4-to 6-in. hydroclones, with 12 or more 4-in. hydroclones being common.
•Clay ejectors or microclones use 2-in. hydroclones, with 20, 2-in. hydroclones being common. •The hydroclone discharge, or underflow, must be evaluated to ensure that the hydroclone is operating efficiently.
•The discharge should be in the form of a fine spray, with a slight suction felt at its center. •When drilling a large diameter hole at high ROP, the feed may become overloaded with solids and result in a rope-type discharge. –At times, this may have to be tolerated, since shutting the unit down would be worse.

Solids Control –Shale Shakers Screens

•A shale shaker is only as good as the mesh size and quality of its screen.
•A number of screen types are available today and performance varies.
•For example, a 100-mesh “square”screen removes 100% of the particles greater than 140 microns, while a high flow-rate, 100-mesh “layered”screen removes 95% of the particles greater than 208 microns.
–This layered-screen performance is equal roughly to only a 70-mesh “square”screen.
•Screen mesh:
–The number of openings per linear inch.
–For example, a 30 x 30-mesh “square”screen has 30 openings along a 1-in. line in both directions.
–A 70 x 30-mesh “oblong”(rectangular opening) screen will have 70 openings along a 1-in. line one way and 30 openings on a 1-in. line perpendicular.
•Depending on the manufacturer,wiresize and weave, this 70 x 30-mesh screen may be described as:
–An “oblong”or “rectangular”70-mesh screen,
–An “oblong 80”in an attempt to rate the effective rectangular opening in terms of a square equivalent or possibly
–A 100-mesh screen.
•Avoid using mesh designations when comparing screen types.
•In addition to mesh count, various wire sizes and weave patterns are used that affect the opening size and flow-rate for a particular mesh size.
•The 100-mesh square, layered, oblong and bolted screens each removes different particle sizes.


•Shaker screen values for U.S. standard sieve equivalents, square mesh market screen.

Solids Control –Types of Shale Shakers

•The circular-motion shaker,
–Older design
–Produces the lowest G-force
–Fast conveyance of cuttings
–Often used as scalping shakers
–Works well with sticky, clay-type solids
–Has a low capacity for drying cuttings, so wet cuttings are often discharged
•The elliptical-motion shaker,
–Modification of the circular motion type in which the center of gravity is raised above the deck and counter-weights are used to produce an egg-shaped motion that varies in both intensity and throw as solids move down the deck.
–Slow transport
–Dryer cuttings
•The linear-motion shaker
–Uses two circular-motion motors mounted on the same deck.
–The motors are set for opposite rotation to produce a downward G-force and an upward G-force when the rotations are complementary, but no G-force when the rotations are opposed.
–The G-force on most linear-motion shakers is variable from about 3 to 6.
•The linear-motion shaker cont
–Most versatile design
–produces fairly high G-force
–fast transport depending on the rotational speed, deck angle and vibrator position.

Solids Control –Solids Separation–Shale Shakers

Solids Separation

•Settling
–Settling pits are seldom used in modern drilling operations; however, they can be found from time to time.
–The rate of solids settling in settling pits or sand traps depends on
•Size, shape and specific gravity
•density of the drilling fluid
•viscosity of the drilling fluid
•type of fluid-flow regime
•residence time in the pit.
•Settling continued
–On a drilling rig with inferior shale shakers, a sand trap or settling pit will remove some of these large drill solids.
–Most modern shale shakers will remove sand-size and larger solids without the need for sand traps and/or settling pits.
•None of the solids-control equipment used in drilling will remove 100% of the solids generated.
•To compare the efficiency of solids-control equipment, a cut point particle-size rating is used.
–The cut point refers to the combination of a micron size and the percentage of that particle size removed.
•Cut point designations should include the percentage of the stated size removed.
–Cut points should always be denoted with the letter “D”with a subscript indicating the percentage removed.
–Without this percentage, no two cut point sizes can be compared.
–A D50cut point of 40 microns means that 50% of the 40-micron size particles have been removed and 50% have been retained in the mud system.

Shale Shakers

•The most important solids-control devices are shale shakers, which are vibrating screen separators used to remove drill cuttings from the mud •As the first step in the mud-cleaning/solids-removal chain, they represent the first line of defense against solids accumulation. •Shale shakers differ from other solids-removal equipment in that they produce nearly a 100% cut (D100) at the screen opening size. •Many potential problems can be avoided by observing and adjusting the shale shakers to achieve maximum removal efficiency for the handling capacity. •Using screens of the finest mesh to remove as many drill solids as possible on the first circulation from the well is the most efficient method of solids control. –It prevents solids from being re-circulated and degraded in size until they cannot be removed. –As much as 90% of the generated solids can be removed by the shale shakers •Unless the shale shakers are operating properly, and have screens of the finest mesh possible, all other equipment is subject to overloading and inefficient operation

•The mud flow should be spread over as much of the screen surface as possible by using feed-control gates located between the possum belly (flow line-to shaker transitional reservoir and the screen surface.
–The mud should cover 75% of the screens (About 1 foot from the end of the screens)

Solids Control –Particle Size Classification

•It is important to understand how particle sizes in drilling mud are classified and the types of solids that fall into each category.
•Particles in drilling mud can range from very small clays, (less than 1/25,400th of an inch), to very large drill cuttings (larger than an inch).
•Due to the extremely small particles, sizes are expressed in micron units.
•A micron is one-millionth of a meter
–(1/1,000,000 or 1 x 10 -6m).
–1 in. equals 25,400 microns.

Solids Control –Particle Size Classification

•It is important to understand how particle sizes in drilling mud are classified and the types of solids that fall into each category. •Particles in drilling mud can range from very small clays, (less than 1/25,400th of an inch), to very large drill cuttings (larger than an inch). •Due to the extremely small particles, sizes are expressed in micron units. •A micron is one-millionth of a meter –(1/1,000,000 or 1 x 10 -6m). –1 in. equals 25,400 microns.


•In a drilling mud, viscosity increases proportionally with the surface area of solids. –The surface area of all solids must be wetted. –As the amount of liquid is reduced due to increased surface area, fluid viscosity increases and performance declines. –Colloidal solids produce most of the viscosity in drilling muds due to this surface area increase. –The volume of colloidal-size solids contained in drilling mud must be controlled for the sake of economy and efficiency. •The picture shows how the same volume of material takes up substantially more surface area

Solids Control

•The types and quantities of solids present in drilling mud systems determine the fluid’s density, viscosity, gel strengths, filter-cake quality and filtration control, and other chemical and mechanical properties. •Solids and their volumes also influence mud and well costs, including factors such as –Rate of Penetration (ROP), –hydraulics, –dilution rates, –Torque and drag, –surge and swab pressures, –Differential sticking, –lost circulation, –Hole stability, –balling of the bit and the bottom-hole assembly. •Since it is not possible to remove all drill solids either mechanically or by other means they must be considered a continual contaminant of a mud system. •Solids removal is one of the most important aspects of mud system control, since it has a direct bearing on drilling efficiency. •Money spent for solids control and for solving problems related to drill solids represents a significant portion of overall drilling cost