There is solids-control equipment that is used to remove solid contaminants and gas entrapped in mud. A typical solids-removal system is shown in Figure 1, which depicts a layout for solids control equipment for a weighed mud system. Solids can be removed from mud in four stages:
- Screen separation – shale shakers, scalper screens and mud cleaner screens;
- Settling separation in non-stirred compartments – sand traps and settling pits;
- Removal of gaseous contaminants by vacuum degassers or similar equipment;
- Forced settling by the action of centrifugal devices including hydrocyclones (i.e. desanders, desilters and micro-cones) and centrifuges.
Screen separation
The shale shaker is the most common screen device as shown in Figure 3. It contains one or more vibrating screens through which mud passes. Mud loaded with solids passes over the vibrating shaker where the liquid part of mud and small solids pass through the shaker screens. The drill cuttings are then collected at the bottom of the shaker. If the correct type of shaker is used and runs in an effi cient manner, the shale shaker and scalper screens (Gumbo shakers) can eff ectively remove up to 80% of all solids from a drilling fluid. Shale shakers are classified into two types based on its motion: a) circular or elliptical motion, and b) linear motion. Circular or elliptical motion shakers are also known as rumba shakers. Elliptical rollers are used to generate a circular rocking motion to provide better solids removal through the screen. A linear-motion shaker uses a straight back-and-forth rocking motion to keep the fluid circulating through the screens. Field experience indicates that elliptical shakers work better with water-based muds and linear-motion shakers are more suitable for oil-based muds. An absolute minimum of three shale shakers is recommended to have an efficient separation of solids.
Settling separation
A low-cost solids control method is to allow time for the drilling fluid to settle down. In this case, the contaminated mud needs to circulate through a settling pit. These settling control pits work on an overflow principle. The sand trap is the first one (Figure 2), which is fed by the screened mud from the shale shakers. There should not be any agitation from suction discharge lines or paddles. Any large heavy solids will settle out here and will not be carried on into the other pits. Particles above colloidal size will eventually settle out in a slow condition. However, the smaller the particle, the longer it will take to settle. In some cases, for silt-sized particles, it may take days. Basically the solids will settle out more readily when a) the solid particles are large and heavy, b) the mud is light and has a low viscosity, and c) the gravitational force can be increased by mechanical means. Particle-settling velocities are given in Figure 1.
Gas removal
The trapped gas in mud must be removed in order to maintain the desired density to a level needed to control downhole formation pressures. Figure 4 shows a typical vacuum degasser, which is used to remove gases from mud. There are also some simple equipment such as a vacuum pump and a float assembly. Th e vacuum pump creates a low internal pressure that allows gas-cut mud to be drawn into the degasser vessel. It is then allowed to flow in a thin layer over an internal baffle plate. The combination of low internal pressure and thin liquid film causes gas bubbles to expand in size, which rise to the surface of the mud inside the vessel. As a result, the gas breaks down from the mud. As the gas moves toward the top of the degasser it is removed by the vacuum pump. The removed gas is routed away from the rig and is then either vented to atmosphere or flared.
Forced settling
Desanders and desilters are similar devices, which are called hydrocyclones. They work on the principle of separating solids from a liquid by creating centrifugal forces inside the hydrocyclone. Mud is injected tangentially into the hydrocyclone and the resulting centrifugal forces drive the solids to the walls of the hydrocyclone. Finally hydrocyclone discharges the solids from the apex with a small volume of mud (Figure 5). Th e fluid portion of mud leaves the top of the hydrocyclone as an overflow and is then sent to the active pit to be pumped downhole again. Hydrocyclones come in various sizes and shapes and usually specified by the particle sizes they are designed to remove. In general, there are four types of hydrocyclones: a) desanders, b) desilters, c) mud cleaners, and d) centrifuges.
Desanders
Desanders are hydrocyclones with 6-inch inner diameters or larger (Figure 6). The primary use of desanders is in the top-hole sections when drilling with water-based mud to help maintain low mud weights. Use of desanders prevents
overload of the desilter cones and increases their efficiency by reducing the mud weight and solids content of the feed inlet. Desanders should be used if the sand content of the mud rises above 0.5% to prevent abrasion of pump liners. Desanders should never be used with oil-based muds because of its very wet solids discharge. Th e desander makes a cut in the 40 to 45 micron size range. With a spray discharge, the underfl ow weight should be between 2.5 to 5.0 ppg heavier than the input mud.
Desilters
Desilters are the hydrocyclones made up of large number of small diameter cones (i.e. < 6 in inner diameter). Figure 7 shows a typical desilter arrangement. Desilters along with desanders should be used to process low mud weights that are used to drill top-hole sections (Figure 9). If the mud weight needs to rise, adding the barites must do this. Low gravity solids should not be allowed in such case. Desilters are designed to remove silt-sized particles.
Mud Cleaners
Mud cleaners are the combination of a fine-screened (roughly 320 mesh) shale shaker under a desilter and are placed above a high energy vibrating screen (Figure 8). They are used for weighed muds because barite tends to be removed with silt-sized particles. By using a mud cleaner, barite can be recovered and reused. Mud cleaners must be used when it becomes impossible to maintain low mud weights by use of the shale shakers alone. It is far more efficient to use desilters and process the underflow with a centrifuge than to use the screens of a mud cleaner. The use of mud cleaners with oil-based muds should be minimised because experience has shown that mud losses of 3 to 5 bbls/hr being discharged are not uncommon.
Decanter Centrifuges
Decanter centrifuges use centrifugal forces to remove heavy solids from the liquid and lighter components of the mud. Figure 3.16a shows a decanting centrifuge, which consists of a horizontal conical steel bowl rotating at a high speed. Th e bowl contains a double-screw type conveyor that rotates in the same direction as the steel bowl at a slightly lower speed (Figure 3.16a). Normally, the decanter centrifuge involves slender cylindrical- or conical-bowl sections with a relatively large aspect ratio (Figure 3.16b). Typical bowl speeds are 1,800 to 4,000 rev/min. When mud enters the centrifuge, the centrifugal force developed by the bowl holds the mud in a pond against the walls of the pond. In this pond the silt and sand particle settle against the walls and the conveyor blade scrapes and pushes the settled solids towards the narrow end of the bowl where they are collected as damp particles with no free liquid. Th e liquid and clay particles are collected as overfl ow from ports at the large end of the bowl. Centrifuge effi ciency is aff ected predominantly by the feed fl ow rate. However, it is also aff ected by the operating parameters such as bowl speed (rpm), bowl conveyer diff erential speed (rpm), and pool depth.
Table 1 API classifi cation of particle sizes.
| Particle size (μ) | Classifi cation | Sieve size (mesh) |
| > 2000 | Coarse | 10 |
| 2000 – 250 | Intermediate | 60 |
| 250 – 74 | Medium | 200 |
| 74 – 44 | fine | 325 |
| 44 – 2 | Ultra-fine | – |
| 2 – 0 | Colloidal |
