Modern, linear motion shale shakers are the most significant advance in solids removal technology. They permit the use of much finer screens than had been used in the past. By reducing the quantity of material to be removed by the desanders desilters or mud cleaners, they permit them to function much more efficiently and effectively.
Unfortunately, there appears to be a trend toward the removal of hydroclones from the system when modern shakers are available. The justification cited is that inasmuch as the finer shaker screens remove much of the material which would have been separated by the desanders desilters or mud cleaners, they are no longer needed. While it is true that the additional material removed by the finer screens would have had to be separated by the hydroclones, the idea that the use of finer screens eliminates the need for their use is erroneous.
While unweighted muds are in use, desanders, desilters and mud cleaners are necessary for effective solids removal. (Mud System Equipment Requirements)The desander makes a 50% cut at 40 – 45 microns (μ), the desilter’s cut point is approximately 20 μ. Even with the use of a pair of traditional shale shakers for primary separation and three linear motion units for secondary separation, it is seldom possible to run screens with openings finer than 74 μ in hole larger than 8-3/4″. (See Table. 1). Total reliance upon shale shakers for solids control assures the incorporation into the mud system of most of the solids which reach the surface as particles finer than the screen openings. Many, perhaps most, of these solids are separable by hydroclones and should be separated. Failure to remove them causes the accelerated accumulation of colloids and ultrafine solids and the need for massive dilution.
TABLE 1. SIZE OF PARTICLES REMOVED, MICRONS
|HOLE SIZE||26" to 17-1/2"||12-1/4"||8-3/4" to 5-7/8"|
|SHALE SHAKERS (PRIMARY)||> 400||> 230||> 140|
|SHALE SHAKERS (SECONDARY)||> 180||> 120||> 74|
|MUD CLEANER||--||> 74||--|
Modern shale shakers as part of a comprehensive solids control system, permit the maintenance of superior drilling fluids at reduced cost. The replacement of traditional shakers with linear motion machines, coupled with the elimination of desanders and desilters, assures reduced mud quality and higher mud and drilling costs.