The shale shakers are the first mechanical treatment of the returning drilling fluid for solids control.
None of the other mechanical devices can cope with solids control without the pre-treatment of
the fluid in the shale shaker.
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A shale shaker or vibration screen is a spring-mounted screen that is vibrated by the rotation of
an eccentric shaft mounted on top of the screen frame (Figure 1). To provide sufficient mud
handling capacity, multiple shakers are normally used. Historically a land rig (check also Drilling Rig Types) would have perhaps two shakers and make heavy use of secondary solids control equipment.
The shale shaker’s performance can be easily observed, all aspects of its operation are visible. Shale shakers provide the advantage of not degrading soft or friable cuttings. When well operated and maintained, shale shakers can produce a relatively dry cuttings discharge.
In unweighted muds, the shale shaker’s main role is to remove as many solids as possible and reduce the solids loading to the downstream hydro-cyclones and centrifuges to improve their efficiency. In muds containing solid weighting agents (Check WBM Additives – OBM Additives) such as barite (barite in drilling), the shale shaker is the primary solids removal device. It is usually relied upon to remove all drilled cuttings coarser than the weighting material. Downstream equipment will often remove too much valuable weighting material.
Enough shakers should be installed to process the entire circulating rate with the goal of removing as many drilled cuttings ( You may be interested in Unrepresentative drilling cuttings) as economically feasible. Given the importance of the shale shaker, the most efficient shakers and shakers screens should be selected to achieve the optimum economic performance of the solids control system.
Shale Shaler Mechanism
Simply stated, a shale shaker works by channeling mud and solids onto vibrating screens. The mud and fine solids pass through the screens and return to the active system. Solids coarser than the screen openings are conveyed off the screen by the vibratory motion of the shaker. The shaker is the only solids removal device that makes a separation based on physical particle size. Hydrocyclones and centrifuges separate solids based on differences in their relative mass.
Shale Shaker Types
Three major types of shale shakers are used.
- Single deck shakers
- Differential single deck shakers
- Double and multiple screen shakers.
Singe Deck Shaker
A single deck shaker is shown in the section in Figure 2. In the past, the majority of shakers in use were of this type. They had fairly coarse screens. This meant that only the coarser formation particles (cuttings and cavings and coarse sand) could be removed, whereas the finer sand and silt remained in the drilling fluid. The other problem with this type of screen was its low efficiency.
Differential single deck shakers
The construction of the differential single-deck shaker is shown Figure 3. The screens are said to be in “parallel” and the angle of the screen slope varies.
Double-deck Shale shakers
Most modern drilling units now have double deck shakers fitted. These have a second, finer screen in “series” which removes the majority of the finer particles (see Figure 4). The size of the second screen can be up to 150 mesh (104 μm).
Multiple deck shakers
Multiple screen shakers have a single-deck construction with three or four screens placed at different levels in a “series” arrangement. This type of arrangement is illustrated in Figure 5 & 6.
In the types of shale shakers article, we have given more details about its types and applications
Shale Shakers Performance & Design Characteristics
Improved efficiency of the shaker has managed to reduce the load on secondary solids control equipment. Modern systems may now have four or five shakers, especially offshore or when handling large volumes (Figure 6).
The volume of fluid that can be processed by a shaker is determined by:
- The size of the openings in the wire screen(s).
- The percentage of open area.
- The speed and amplitude of the vibrations.
- The type of motion (vibrator position).
- Its fluid flow properties.
- The type, size and amount of solids.
- The rate of solids discharge depends on:
- The type of motion.
- The speed and amplitude of vibration.
- The mesh design.
- The screen strength.
Shale shakers are characterized in three ways.
- Amplitude and speed
- Motion types
The amplitude, or one-half of a stroke of a shaker, is determined by the vibrator’s eccentric weight. Normally, shakers use low amplitudes and high vibrator speeds. Fine screen shakers have high amplitudes at lower speeds to prevent plugging of the screens. Speed of vibration is important to ensure efficient removal of cuttings from the screen. Shale shakers are now available with variable speed control.
Most modem shakers use linear motion. Linear motion shakers have the vibrator mounted at the front of the basket through the center of gravity. Linear motion is achieved by using two counter-rotating vibrators/shafts which, because of their positioning and vibration dynamics, will naturally operate in phase. They are located so that a line drawn from the shakers center of gravity bisects at 90° a line drawn between the two axes of rotation. This gives a sawtooth-type motion allowing longer residence time on the screen and increased throughput compared to unbalanced and balanced motion-type shakers.
We have discussed the shale shaker design and selection in the previous article
estimating number of shakers required (Rule Of Thumb)
Base the number of shakers required on the economics and physical constraints of the specific application.
A “ballpark” estimate of shaker requirements, based on average drilling conditions can be made from the following table. This is a very rough estimate and should be used only as a guide.
|GPM / Max Viscosity||5 cP||10 cP||15 cP||20 cP||25 cP||30 cP||40 cP||50 cP||60 cP|
The guide however does not reflect the performance of the most modern market-leading shale shakers. The Derrick Flo-Line Cleaner 514 will outperform smaller units and allow fewer units to be used with finer screens to produce the same fluid handling ability.
position in the system
Positioned downstream of the gumbo trap and flow distribution system. May comprise a single set or a dual set or cascade system.
setting up Shale Shaker Tips
- Use enough shakers to provide a sufficient non-blanked screen area to run 100 mesh or finer screens. Shaker setup should be sufficient to process solids-laden fluids at maximum flow rates over any significant hole interval.
- For double-deck shakers, run a coarser screen on top and a finer screen on the bottom. The coarser screen should be at least two meshes coarser. Watch for a torn bottom screen. Replace or patch torn screens at once. Cover 75% to 80% of the bottom screen with mud to maximize utilization of the available screen area. Flow back pans are recommended for improving coverage and throughput.
- For a single deck shaker with parallel screens, try to run all the same mesh screens. If coarser screens are necessary to prevent mud loss, no more than two meshes should be on the shaker at one time, with the finer mesh screen closest to the possum belly. The two meshes should have approximately the same size opening. For example, use a combination of 100 mesh (140u) and 80 mesh (178u), not 100 mesh (140u) and 50 mesh (279u). Cover 75% to 80% of the screen area with mud to properly utilize the screen surface area.
- Use spray bars (mist only) as needed for sticky clay, etc. Use spray bars only with unweighted water-based muds. Spray bars are not recommended for weighted or oil-based muds.
- Do not bypass or operate with torn screens; these are the main causes of plugged hydrocyclones. Use screens with mesh back-up so that coarser solids will be screened out when the finer mesh tears.
- For improved screen life with non-pre-tensioned screens, make sure the components of the screen tensioning system, including any rubber supports, nuts, bolts, springs, etc are in place and in good shape. Install screens according to the manufacturer’s recommended installation procedure.
- Check the bearing lubrication according to the manufacturer’s maintenance schedule. Lubricate and maintain the unit according to manufacturers’ instructions.
- Rig up with sufficient space and walkways with handrails around the shaker skid to permit easy service. The shaker skid should be level.
- Check for correct direction of motor rotation for shakers with one vibrator.
- The flow line should enter at the bottom of the possum belly to prevent solids from settling and build-up in the possum belly. If the flow line enters over the top of the possum belly, the flow line should be extended to within 8 -10 ins (20 – 25 cms) of the bottom of the possum belly.
- Rig up for equal fluid and solids distribution when more than one shaker is used. A cement bypass is desirable.
Shale Shaker Limitations
We consider the shaker limit when large amounts of drilling fluids begin to discharge over the shaker end. Such capacity is specified by interfering of two factors:
- The maximum fluid flow rate that can be handled by the shaker screen which is called fluid limit.
- The maximum amount of solids that can be seperated off the end of the shaker which is called the solids limit.
Once the amount of solids is increased, the amount of fluid that can be handled by the shaker decreases. Major factors that affect these limits are:
- Mud Rheology properties
- Wire Wettability: the screens wire shall be oil wet when drilling with oil base mud. Water adhering to a screen wire lowers the effective opening size for oil to pass through so shaker will not be able handle the flow of OBM “sheeting”.
- fluid surface tension: as this increased, it will be difficult for fluids to pass through shakers.
- Fluid Density: it is usually increased by adding a weighting agent (solids) to the drilling fluid that makes it more difficult to screen the drilling mud.
- Quantity of solids
- Solids: Particles that are only slightly larger than the opening size, can become wedged in the openings which lead to screen plugging. Also solids such as gumbo will also decrease the screen effective area.
- Hole Cleaning: If cuttings accumulated in the annulus, they will start disintegration into small solids and by time will increase some PV and solids content.
Fluid Only Capacity
IS the capacity of the shale shaker incase of no solids is separated by the screens assuming that their volume is small and can pass through screens. It depends on many factors such as:
- characteristics of the shaker (“G”-factor, vibrational frequency, type of motion, and angle of the screen deck)
- the screen (area and conductance)
- the fluid properties (density, rheology, additives, and fluid type)
Let’s consider that the screens are permeable media, so the fluid capacity will increase as the fluid viscosity decreases and as the density increase ( as the density will push the fluid to drop through the screens).
This limit is reached easily when drilling large diameter holes, sticky formation, and high penetration rates.
- The shale shaker is the only solids control device that makes a separation based on the physical size of the particle. The separation size is dictated by the opening sizes in the shaker screens. Hydrocyclones and centrifuges separate solids based on differences between their relative mass and the fluid.
- Shale shakers with linear vibratory motion are preferred for most applications because of their superior processing capacity and fine screening ability. Circular motion or unbalanced elliptical motion shakers are recommended as scalping shakers in cascading systems.
- Vibration of the shaker basket creates G-forces which help drive shear thinning fluids such as drilling mud through the screens. Vibration also conveys solids off the screens. Most linear motion shakers operate in the range of 3 to 4 G’s to balance throughput with screen life. G-force is a function of vibration frequency (rpm) and stroke length.
- “High-Speed” should not be equated with “high performance”. Laboratory tests indicate that, in the normal operating range for linear motion shale shakers, lower frequency vibration and longer stroke lengths improve throughput capacity. Most linear motion shakers operate at 1200 to 1800 rpm.
- Avoid deck inclinations above 3˚. High deck angles reduce solids conveyance and increase the risk of grinding soft or friable solids through the screens.
- Shakers are designed to accept either hookstrip or rigid frame screen panels. Hookstrip screen panels are the most common and are usually cheaper, although cuttings wetness can be a concern due to deck curvature. Flat, rigid frame panels promote even fluid coverage, but can cost more.
- Shakers may have single or tandem screening decks. Single deck shakers offer mechanical simplicity and full access to the screening surface. Single deck shakers may be arranged to process mud sequentially as a “cascading” system to improve performance under high solids loading conditions. Tandem deck shakers offer improved processing capacity under high solids loading conditions when space is limited.
- Manifolds should provide even distribution of mud and solids to each shaker. Avoid branch tee’s.
- Recommended manifold designs are illustrated.
- Operating guidelines are provided for optimising screen life and cuttings dryness, handling sticky solids, polymer muds, blinding and LCM problems.