Solid control is an important mechanical process to keep the drilling fluids in their optimum parameters to perform operations safely and effectively.
The solid control can be classified according to the applied method as follows:
- Screen separation using for example shale shakers
- Settling separation in sand traps and settling mud pits
- Gas separation inside the degassers and surface vacuum
- Forced separation by applying a centrifugal force in desanders, desilters and centrifuges.
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Fig 1 - Solid Control system |
Screen Separation
The shale shaker is a screen device and it contains one or more vibrating screens which mud passes through. During operations, mud comes out from the well through the flow line to the mud box, then the mud is distributed to the vibrating shale shakers. Normally, the mud pass through the screens and the drill cuttings are segregated out of the drilling fluids system. If the shakers work effectively and screens are the correct type and size, up to 80% of drill cuttings can be separated.
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Fig 2 - Shale Shaker |
According to the vibration motion, the shakers are categorized in two types: elliptical and linear motion. Related to the oil field experience, it seems that the elliptical shakers are suitable for water based mud and linear motion shakers are more effective for oil based mud.
There are some rig recommendations for optimized operations which can be summarized as follows:
Three shale shakers is recommended for normal operations to allow quick and simple screens replacements.
Screen sizes should be chosen to separate maximum solids without affecting mud components, or overloading the shakers which can lead to mud surface losses.
Drilling fluids, when passing over the screens, should not exceed two third (2/3) of the total screens lengths to avoid surface losses and also to permit the conveying of dry solids toward shakers exit.
If the screen blinding is faced, reducing the flow rate can be a solution without affecting the hole cleaning.
In order to keep the screen and shakers in good shape and to avoid the premature damage, never run the shale shakers dry.
Settling Separation
This type of separation is based on the settling process where the solids are allowed to settle down inside mud pits forced by the gravity force. These control method works on an over flow principle. The solids can settle first at the sand trap which is fed by the segregated mud from shale shakers. The large heavy solids normally settle down at the sand trap. Medium size cuttings require more time to settle in slow condition, however smaller solids needs longer time to be separated for instance silt particles can take days. There are some conditions which can improve the settling process such as drilling with low viscosity drilling mud, using mechanical means to improve the gravitational impact.
Gas Removal
Under some circumstances such as well control situation, gas can come into the wellbore and can affect the mud density. In order to avoid losing the applied hydrostatic pressure, the drilling fluids are allowed to flow through surface degasser to separate the gas from the drilling fluids system. The poor boy is a gas separator that is used when circulating through the choke. The separated gas is vented away from the rig using the vent line. A vacuum degasser is used when the mud logging unit detects a certain percentage of gas in the mud. The gas is separated when the drilling fluids flow over internal baffle plates inside the degasser.
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Fig 3 - Surface Vacuum |
Forced Settling
This process is performed by creating centrifugal forces which force the solids to separate from the drilling fluids. Both, desanders and desilters use the same principle. The mud is injected inside the hydrocyclones tangentially leading to the creation of centrifugal forces which drive the solids to the wall of the cones. Then, the solids with small amount of fluids are discharged from the bottom of the hydrocyclones and the processed drilling fluids flow from the top of hydrocyclones to drilling fluid active system.
Desanders are hydrocyclones with 6 inches diameters or larger. They are used mostly for the top hole drilling with water based muds in order to maintain low mud weights. The use of desanders helps to avoid overloading the desilters cones and improve their efficiency by reducing the solids content at the desilter inlet. Desanders should not be used with oil based mud. Desilters are hydrocyclones with diameters less than 6 inches and they are designed to remove the silt sized particles.
Mud cleaner is a combination of fine screened shale shakers and desilter which is installed above the screens. This combination helps to recover the barite and reused it. They are used when it becomes difficult to keep low mud weight. Processing with the mud cleaner should be minimized because surface mud losses are not uncommon.
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Fig 4 - Mud Cleaner |
Centrifuge is also another mechanical solid control equipment which applies centrifugal forces to remove solids from the liquids. It consist of double screw conveyor inside a conical steel bowl. Typical speed for the centrifuge is between 1800 to 4000 rev/min. when mud enters the centrifuge, the developed force pushes the solids against the wall, then conveyor blades transfer the solids towards narrow end of the bowl where they are damped with no free liquids.
References
Hussain Rabia. Well Engineering and Construction. 2001
M.E. Hossain & A.A. Al-Majed. Fundamentals of Sustainable Drilling Engineering. Wiley, 2015.
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