If we were to list the primary solids removal devices in their best order of series operation, it would be: shale shaker, sand trap (or settling tank, or shale tank, etc.), mud cleaner, hydrocyclone desander, hydrocyclone desilter, and decanter centrifuge. Regardless of drilling and mud economics improvement with the other mechanical devices, none at present can operate properly and continuously without the protection of the shale shakers. The sand trap
should serve to protect the downstream equipment temporarily if something happens to a shale shaker screen.
The term “shale shaker” is used in drilling mud work to cover all the devices that in another industry might be differentiated as “shaking” screen, “vibrating screens,” and “oscillating screens.” 7 All three of these types are in use, although most would probably fall in the “vibrating screen” classification . The old cylindrical rotating, or “squirrel cage” screen is rarely seen.
The trend in the last decade to “fine screen shakers” (using screen cloth finer than 30 mesh), whether replacing the conventional mesh screens or operating in series down stream on the liquid-undersize solids discharge of the coarser mesh conventional shale shaker, has magnified all the problems of operatin g shale shakers. Many disappointments were suffered when solids problems often were more severe with the new fine screen shakers than with the older shaker and coarser screens. Out of the confusion and disappointments some better and long-overdue information on shale shaker operation has become available.
The particle size a shale shaker can remove depends almost completely upon the size and the shape of the mesh openings in the screen cloth. See Figure 1. An oblong mesh allows a heavier gauge wire, for the same minimum dimension opening as a square mesh, without excessive sacrifices in open area. However, it is obvious that under some conditions the square mesh will make a separation finer than the “equivalent” oblong mesh.
Obviously, this finer separation due to coating of the wires is a step in the direction of coating of the apertures and complete loss of mud, and there is more tendency for this problem in the square mesh.
If a shale shaker unit (one vibrating mechanism including screen cloth and all appurtenances) has multiple screens in a series arrangement, as Figures 2A and 2B, the particle size separation will be determined by the finest mesh screen, which should be the bottom screen. If a shale shaker unit had multiple screens in a “parallel” arrangement, as Figures 3A and 3B , the size separation will be determined by the coarsest screen in the unit through which liquid passes.
The capacity of a shale shaker to handle oversize solids is determined Primarily by the screen amplitude and motion. The amplitude, or one-half the stroke, of a shaker is determined by the vibrator eccentric weight, and is of primary importance in moving oversize solids. Whether the motion is eccentric or circular at various parts of the cloth also affects the transport of solids.
Less effective motion can be overcome by sloping the deck towards the solids discharge end (see Figure 4A), but this may be at the expense of Wetter oversize solids carrying over more drilling fluid. The balanced motion, with the vibrator in the center of gravity of the vibrating mass imparts a uniform circular motion capable of moving the oversize solids even slightly uphill, the transport being relatively independent of the angle. See Figure 4B.
Liquid throughput capacity, if mud properties and formation solids in the feed remain equal, depends first and foremost on the transport of oversize solids from the screen cloth. If the mesh openings plug with near-size particles, or if the mesh openings coat over with sticky, undersize particles in a viscous mud, there is negligible liquid throughput capacity, and the screen cloth is “blinded.” For this reason selection of screen cloth mesh can be critical to the entire solids removal installation in mud system.
If the screen installed on a shale shaker creates a blinding problem for the shale shaker unit , there are three alternatives :
- Change the screen mesh to a coarser mesh cloth if the problem is undersize solids coatings, or possibly to a finer mesh if the problem is near-size plugging;
- continue to blind and lose all mud over the shale shaker , endangering the job and wasting a sizeable amount of money;
- bypass the shale shaker, permitting all formation solids to pass to the downstream removal equipment, immediately plugging the desanders and /or desilters and endangering the hole and drill string. The only sensible course of action is number one .
Unfortunately, the resultant chain of events not being visibly obvious , the most common choice in the field is number three.
Even a shale shaker of best solids transport design and fitted with a screen or proper mesh selection for the particle size distribution can be overloaded with solids. The solids to be removed always varies with the hole (volume) drilling rate, plus the sloughing rate if that is occurring. If a solids overload occurs, it is also reflected in a reduction in liquid throughput capacity. Figure 5 illustrates this effect both for series and for single screen arrangements. (Review Fig . 2). Figure 5 must be considered general and indicative, as screen size and other shale shaker details, viscosity and other mud specifications , hole size, etc. are not specified.
For example, if the cuttings were so fine , or if the first stages of screens in a series were of such a coarse mesh , that no cuttings were removed before the slurry reached the last (finest mesh) screen cloth, the series screen curve in this figure would be the same as for a single screen of the same mesh. This does sometimes occur in the field, and then no benefit is realized from the series arrangement.
Figure 6, representing the effect of slurry weight on the liquid throughput ca pacity of a screen, is generar for the finest mesh cloth of a series screen, or of a single screen, since the effect of cuttings load is omitted. The effect portrayed here is a combination of that of viscosity and of fine particles as well as the reduction in effective screening area for some typical meshes
The frequency and severity of screen overload problems (or bypassing ) is explained in great measure by the fact that the effects shown in Figures 5 and 6 are additive to each other. Screen liquid capacity decreases as drilling rate increases , as mesh size decreases, and as mud weight increases. Also as cuttings size decreases toward mesh size, and as viscosity increases, the liquid capacity decreases, both being additive to those conditions already mentioned. If the specification of shale shakers for an operation, or for a rig , is left to someone familiar only with the usual “catalogspecs” without consideration for the most adverse combination of shale shaker conditions probable, the shale shakers will be inadequate.
From casual observation of conventional shale shakers operating with screen cloths of mesh coarser than 30 x 30 , it has been assumed for years that shale shakers remove solids with relatively little loss of mud. Recent Preliminary investigations strongly indicate that mud loss ranges from one gallon of mud per gallon of net dry cuttings removed under the best conditions and coarser screens , to four gallons of mud loss per gallon of net dry solids removed with severe conditions and very fine screens. This range of loss was found with primary removal screens operating “efficiently,” not blinded by coating or plugging.
Some shale shaker designs no doubt must be superior to others in removing excess liquid from oversize solids. For example, a screen cloth over which the solids move uphill should logically salvage more liquid mud (from the solids surface area) through the screen than one moving the oversize solids downhill; however, no investigation of the difference is available, and with a heavy solids loads any difference would tend to be lessened. Also, as particles smaller than “medium” size lose their free liquid film, they become very cohesive.
Economic limits exist for all methods of separation. As conditions become more severe, the shale shaker design principles and construction details become more critical, and maintenance costs climb rapidly. Anyone responsible for shaker specifications must familiarize himself with the field operating characteristics and limits of the various machines available.