An introduction on the shale shakers
In the drilling industry, a vibrating screen called shale shaker is the first equipment that does the filtration process. The purpose here is minimizing cutting solids in the mud. A shale shaker is the first line of defense in minimizing the cuttings content because it separates the largest solids first. The screens consist of different layers of mesh and are vibrated in order to increase the filtration efficiency.
Shaker screen areas
A shale shaker should employ all screen areas to remove solids from drilling fluid and minimize the drilling fluid loss. The screen vibration pushes the particles uphill over the screen and mud is collected at the underside of the screen. There is a limitation in shale shakers’ operations in which filtration performance alters as the feed properties change. Typical vibrating screens vibrate with a constant speed and constant motors forces which results in acceleration on the screen. In handling the huge volume of drilling mud, the acceleration usually decreases as mudflows into the screen. Shakers operating in the oil industry have higher acceleration than the required magnitude to be able to have enough acceleration when heavily loaded.
In the new technologies developed for shale shakers, constant-g technology is becoming a popular technique. This technique measures the screen acceleration and sends the signals to a variable frequency drive to keep constant acceleration even under varying loads.
Drilling liquid is returned to the well surface and then flows on the shale shaker screens. After the drilling mud was processed by the shaker, it flows to the mud tanks where other solid-liquid separation equipments separate the finer particles from mud. The separated particles are sent to a holding tank where they further will be disposed of.
Types of the shale shakers
Two types of end-feed and center-feed shale shakers are used in the drilling industry which the end-feed shaker is the most common one. The screen of an end feed shaker is rectangular while center feed screens are circular. Because drilling fluid flow pattern is the difference for both screens, so the vibration pattern for end-feed and center feed shale shakers is not identical. The screen motion dominates particle velocity on the screen and drilling flow rate though cake and screen.
In an end feed shale shaker, the motion of the imaginary line created by the intersection of vertical plane parallel with the walls and screen cloth is elliptical. All points on the line perpendicular to the vertical plane parallel with the walls and passing through the screens have identical motion. In a direction perpendicular to the vertical plane parallel with the walls, motion is zero. This type of vibration results in the motion of particles across the screen in a straight path to the mud pits.
In the radial distance from the center of the circular feed shaker, the motion of all points on the screen is elliptical. All points vibrate in a vertical plane perpendicular to the radial plane. In this kind of shaker, particles also move in a circular shape in a horizontal plane perpendicular to the radial plane and the screen experiences a 3-D motion. In the elliptical motion screens, motion is identical in all angular location around the center.
Hoberock proposed that the linear vibration than the circular motion model results in higher efficiency in solids conveyance. He also showed that even elliptical vibration shows higher efficiency compared to the linear motion as a result of that screen life is increased.
For the multi-deck shale shakers, it is recommended that the coarsest mesh size is placed at the top, then the finer mesh size is used as the middle screen and finally, the finest mesh should be placed as the bottom screen. This configuration allows the shaker to collect the finer particles with the highest efficiency. The problem of multi-deck shakers is in maintaining the bottom screen.
Parameters affecting the performance of a shale shaker
It has been shown that the performance of a shale shaker depends on the large number of parameters. The most important variables affecting the capacity of a shale shaker are fluid rheological properties, concentration and size distribution of solids, screen mesh and area, vibration frequency, vibration pattern, acceleration, and deck angle.
Capacity of shaker
The maximizing capacity of a shale shaker is a trade-off between the content of separated cuttings off the screen and filtrated drilling mud passed through the screen. For example, if the shaker deck is inclined downward to enhance particles transfer more drilling mud flows off the shaker channel, and cuttings at the outlet have more moisture while tilting the screen up decreases solids velocity but more fluid is saved. There is an optimum angle for each shaker, depend on the manufacturer, which tilting the screen up. more than that causes solids accumulation on the screen and blocking the screen pores. The physical mechanisms justifying the effect of vibration on the fluid displacement in porous media are not yet known.
It is suggested that an increase in flow rate is caused by changes in the pore structure and particle rearrangement. A research was conducted on the effect of vibration on the flow rate of Hexadecane as a non-wetting phase in a column filled with water and sand, the Hexadecane flow rate increased by increasing amplitude. Another explanation for the effect of vibration on the flow rate is based on the capillary trapping. The capillary trapping mechanism is the most promising one. The idea for this mechanism is based on the interfacial tension which is considered as the most significant parameter on multi-phase flow in porous media.
Particles and drilling fluid
Changing in pore sizes of porous media trap the fluid which leads to variations in capillary pressures. This pressure imbalance changes the flow rate of liquid through the porous media. By applying vibration, we see that vibration of the screen will result in an inertial body force acting on the fluid which this movement pushes the trapped fluid to reflow. Vibration creates an internal circulation in the mud and it gives more time to the fluid to touch the screen and this might be one of the effects of vibration on the enhancement of the flow rate.
Particle size distribution and concentration both have an effect on the process of solids-liquid separation. Increasing the solids concentration in drilling mud reduces the performance of the drilling operations. Experimental work shows that muds containing more than 10% by mass solids caused the failure infiltration process. Microbit drilling results indicated that very fine particles in a drilling mud have more adverse effects on the flow rate than larger sizes.
It is claimed that particles smaller than 1μ are much more damaging to the filtration process than particles larger than 1μ. All solid-liquid separation tools in the drilling industry are designed to remove particles larger than 1μ. The shale shaker changes the formation of particle structure in the drilling mud due to vibration. Shear stress of the drilling fluid is decreased due to vibration while polymeric drilling fluid is not affected by imposing vibration.
|Material||Size range μ|
Drilling mud viscosity and shaker
Research on the effect of plastic viscosity and yield values shows that plastic viscosity of drilling mud flowing through the screen and cake has a significant effect on the capacity of a shale shaker while yield value has a slight effect on the performance. It has been also shown that increasing the plastic viscosity and yield value of a drilling fluid increases the required screen area used in a shaker. The capacity of a shale shaker can be increased by decreasing plastic viscosity and increasing screen area, shaker angle, and acceleration.
The install location of the vibrating motors on the shale shakers can be considered as one of the parameters involving in the design of shale shakers. Some manufacturers say that if a vibrator is precisely mounted on the shaker support there is no need to incline the shaker downward to get desired mass rate of solids on the screen but one should aware that inclining the screen downward decreases the drilling mud flow rate and increases the moisture content of the particles leaving out the channel of the shaker.
What factors can affect the shaker’s working?
In an experimental work done by Porter on a vibrating electromagnetic screen, the capacity improved by increasing frequency and decreased by amplitude. Their results showed that there is optimum operational conditions which after passing the optimal point, flow rate decreased. Angle 33° was found as the most effective angle.
It has been shown that frequency is one of the important parameters affecting screen performance while other researches showed the reverse results. The interaction between frequency and particle size shows that for a feed whose particle size is close to the opening, frequency is the most effective parameter. Two experimental works claimed that screening efficiency decreased as frequency increased.
Shaker’s deck angle
A study showed that an increase in deck angle increased the effective mesh area and number of contacts per unit screen length. An increase in the deck angle enhanced the passage of particles. It was found that angles more than 15° decreased the effectiveness.
A work by Hoberock on an experimental shaker working in acceleration 4g and two frequencies 20 and 60 Hz showed that frequency has an insignificant effect on the fluid capacity of the shaker. His work showed that flow rate at 60Hz is slightly less than that in 20Hz. Their results on a 100*100 mesh screen with three types of drilling fluids showed that the capacity of a shale shaker depend heavily upon the acceleration.
A screen whose conductance is higher than the similar screens shows higher performance. The proposed mechanism for this improvement is in considering permeability and screen thickness than solely the pore area percentage.
A work by Dorry shows that capacity of a shale shaker increases by increasing g-force. His work revealed that the rate of increase in capacity of the shale shaker reached a minimum plateau. It indicates that there is a threshold g-force which after passing that point increasing acceleration does not have any effect on the performance of the shaker.
Fundamental parts of a shale shaker
A typical industrial shale shaker is composed of the following major parts.
Usually, a shale shaker works with two motors that apply the vibratory motion on the shaker screen. There are two eccentric weights in the motors to generate a vibrating force when they rotate. The vibrators rotate in opposite directions and create a force on the screen. The force pushes the particles along with the screen and off the screen outlet. The motors can be installed on the vibrating deck or on the support frame.
Screen angling system
The shaker screen plane should be capable of tilting to handle fluctuations in mud flow rates and maximize the use of the screen area. Depend on the type of the shale and drilling process, different angling systems are used which mechanical, hydraulic, and pneumatic mechanisms are the most common. It is reported that mechanical and hydraulic systems are faster than pneumatic mechanism and need less energy to function.
This part works as a platform for the shaker and collector for the drilling mud passed through the shaker screen. Depend on flow rate of mud, different depths can be used.
This part is the most important part of the shale shaker which most of the efforts in improving the performance of a vibrating shale shaker concentrate on this part. The screen removes drilled cuttings and sends them to the base and make the filtration process more convenient.
This part collects the drilling mud before it flows into the shaker channel. Different types of feeders are used in the drilling industry which the most common one is called weir feeder. This feeder is capable of distributing the drilling mud along the entire shaker screen surface. The feeder has a bypass streamline that sends the mud directly to the collecting tank without being processed by the screen.
A tank is used when the shaker is being repaired or screens are being changed. In the situations that drilling mud is too thick to pass through the screen, the screen is blinded or plugged which in this situation tank is used. A feed tank has a bypass port which allows the drilling mud goes to the mud circulation system.
Shale Shaker Screen
Layer of shaker screen
Different types of shaker screens with different methods of characterizations of the screen cloth are used in the filtration industry. In the drilling industry, the plain square mesh is the most common one. The number of wires per inch is called mesh. A higher mesh number means finer particles can pass through it. For preventing problems such as plugging in the square screens, rectangular mesh screens are usually used. These screens enhance the ratio of the opening area. Layered screens are known as the best option for preventing plugging. Tilting the screen changes flow capacity, conveyance and cuttings moisture. Drilling fluid is lost due to the failure in the borehole and conveyance reduces due to the particle plugging close to the outlet of the shaker.
API and shaker screen
The layered shale shaker screens are non-plugging and easily changed. API set some instructions for the shale shaker’s screens mesh. APR recommends that numbering a mesh in both directions should be followed in parentheses by opening size in microns and the percentage of open area. For example, a screen with a specification of 85 *85 (642 *642, 49) means a square screen with 85 openings per inch in each direction which has an opening size of 642 and an open area of 49%. Screen with the specification of 14090 (211585, 56) means a rectangular mesh screen with 140 openings in one direction and 90 openings in another direction. Openings in 140 mesh direction are 211 micron and in 90 mesh direction has the size of 585 microns.
Some screen manufactures recommend that calculated length of required screen should be increased by one-third to consider for the drainage zone for wet filter cake. Screens with mesh number 40*80 are the most common screens in the drilling industry.
Shaker screens types
The most common types of screens used in the drilling industry can be classified as the following groups:
- Synthetic screen: The screen materials are usually steel, polyurethane, and rubber and used in the drilling operations in which wear life is the most important concern. Rubber is used as a highly resistant material and mounted as the top layer. In some drilling operations in which high frequencies are not desirable, rubber screen is used to reduce the maintenance cost.
- Different weaving techniques have been developed to increase the open area percentage. Woven wire cloth has been traditionally used in the drilling industry for more than 70 years. It is made of metal woven wires which is the most common screen due to being inexpensive and easily available.
Recently, a new technology called ‘pyramid screen’ has been introduced in the screen industry. In this technique, the maximum area of a shale shaker screen can be achieved. Pyramid screens have a flat bottom and corrugation shape on top. The shaker screens maximize the performance of the screen using building up without the requirement of having a larger screen which results in less expensive shale shakers.
Shale Shaker Screen Parts
The most important parts of a shale shaker screen used in the drilling industry are:
- Screen Type (mesh) – In the screens, metal wires are woven together to create a net of metal cloth. Mesh generating technology has been improved over many years to increase screen life and conductance. To maximize the screen conductance, we need to minimize the solids which can be fulfilled by either thinning the wire diameter or changing the geometrical shape of the mesh. Conductance is the criteria that show the amount of drilling mud that is capable of passing through the screen. It is said the rectangular mesh increases the performance of the screen while square mesh has a lower conductance. Screens made with several layers of mesh increase screen life and protect the cloth against solids accumulation and wire wear.
- Screen frame – Each screen should be supported by a frame to function well and depend on the manufacturer differs in material and shape. It is made of either steel or plastic composites. A-frame has a rectangular shape composed of some inner panels whose shape can be hexagonal, rectangular, or square.
- Binder – The binder attaches the frame to the mesh and designed to tolerate the difficult operational situations such as high heat and vibrations, abrasive particles, and corrosive mud.
Existing technologies using in the filtration of drilling mud
Industrial reports show that a constant-g control shale shaker is capable of filtration of finer solids than the typical shale shakers. The efficiency of solids removal is improved with a constant-g control shale shaker.
Vibration of shale shaker
Vibration acceleration of a shale shaker is calculated by Newton’s second law of motion. As the drilling fluid flows onto the screen, the system mass increases which results in decreasing the acceleration. A shale shaker vibrates at a constant frequency which generates a constant force. When the flow rate decreases, the acceleration increases and it causes higher surface area which results in screen failure.
The performance of a shale shaker depends on the vibration intensity and shaker structure. The vibration has effect on the agglomeration of particle. Different techniques such as high temperature, solvent extraction, and soap washing have been proposed to separate oil from cuttings. These techniques have limitations such as safety issues and high energy consuming.
shale shaker screen cloth
Very few experimental studies have been done on the filtration of drilling mud using shale shakers. Cagle et.al compared two shale shakers experimentally to investigate the screen cloth effect on the filtration process. Hoberock developed a model on a full-scale shale shaker to predict the fluid handling capacity of the vibrating screens. It is shown that a shale shaker efficiency in treating mud is a function of vibration frequency and acceleration, shaker angle, fluid rheological properties, type and
amount of drilled solids, mud height, and type of screen and mesh size.
In a new design in the drilling industry, a vacuum conveyor separator (VCS) system was innovated to improve the efficiency of removal of solids from drilling mud. In this system, blinding does not happen and there is no need to install respiratory systems. VCS systems are able to monitor fluid and solids volume simultaneously and record and transmit fluid data. In these kinds of solids separation equipment, there is no need to install degasser, pressure washers, and solids dryer so operation cost becomes minimum.
One of the concerns with using fine mesh screens in viscous mud systems is that screen life and flow capacity decrease and the plugging screen is observed repeatedly. The typical layered screens are composed of two fine mesh layers supported by a coarse screen.
A field report shows that mud viscosity has a significant effect on the performance of the screen. Increasing viscosity decreases capacity of a screen exponentially. The results show that the capacity of a screen in handling drilling mud is not linear function of the covered surface of the screen.
Principles of screening process
One of the most common physical separation techniques in drilling industry is the mechanical screening. Separating tools are classified into moving and static screen equipment in which the machine can be inclined or horizontal.
The process of screening is controlled by physical variables such as particle shapes, acceleration, vibration type and bed density. The vibration motion and screen mesh size have their pros and cons in the process of screening. The most effective pattern of vibration is sinusoidal vibration which is applied on the angled screen relative to the horizontal.
The actual flow rate of a shale shaker infiltration of drilling mud is less than the capacity of a shaker which processes only fluid. Because of the presence of the solids in drilling mud, the capacity of a shale shaker may be reduced due to one of the following effects:
- The solids larger than the screen openings tend to cover the pores which result in decreasing the screen area for passing drilling fluid and finer solids. For overcoming this problem, frequency should be increased to increase the solid’s velocity. Another way is that the downward angle should be increased but it causes more mud to run off the screen.
- When screen openings are slightly smaller than particles, the blinding effect reduces the performance of a shaker.
- The particles which are slightly smaller than openings pass through the pores barely due to weak orientation and low velocity.
Vibrational motion patterns
The five types of mechanical vibration are used in solids separation industry: Circular motion shakers in which motion at the low angels gives the best performance. This vibrator works by an eccentric drive or mass offsets that cause the shaker to vibrate in the orbital pattern. The solids move across the screen and leave out the screen due to gravity and directional shifts. The shaker is inclined between 2 to 5 degrees and is used for clean cuttings.
Circle-throw machine is another type of vibrator in which an eccentric shaft shakes the screen at a given angle. As the vibrator returns to the steady-state, the cuttings drop down by gravity to the collector. This equipment is usually used in the mining industry for solids size which varies from 5 to 20 in. This shaker is employed for large solids and a high volume rate at the outlet.
The solids capable of passing through the screen cloth return back to a crusher and then is mixed with crushed solids. The most common application of this shaker is in the washing process. The results of vibrating screens used in the mining industry cannot be generalized for the shale shakers in the drilling field because the main material in which shale shaker process is fluid rather than solids.
High-frequency vibrators vibrate only the screen and are usually used for particle sizes smaller than 20 mm. These vibrators fulfill a secondary filtration for more separation process and their angles vary from 3º to 12º.
Tumbler screen is another separator in which elliptical motion does the filtration process. In these screens, the fine material blinds the screen center, and larger particles move to the collector. Particles on the screen are broke down and leave the screen cloth. For improving the separation efficiency of a tumbler screen, adding more decks is recommended.
In the G-Control technique, the screen acceleration is measured and then a signal is sent to a variable frequency drive to keep constant g-force. As drilling fluid enters the shaker screen, vibration frequency decreases. The current shale shakers in the industry have high acceleration than required one to handle the situations the screen is heavily loaded. In recent developments in the shaker industry, a new technology called G-Control came to the market to overcome the problem of decreasing acceleration of the shaker screens due to being loaded. By applying Newton’s second law of motion, we can easily see that acceleration is inversely proportional to the drilling mud mass. The shakers in the fieldwork at a constant frequency which results in decreasing acceleration as the mass of mud increases Industrial reports claim that these types of shakers can remove finer solids than the typical shakers used in the fields.
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