Shale shaker is the main solids removal equipment. Today, offshore and onshore drilling projects where more effective fluids/solids separation is required in addition to the reduction of costs related to shale shakers performance.
Why Choose Shale Shaker in Drilling
Since the screening of shale shaker is a relatively low-cost process that does not waste any appreciable amount of barite or chemical, it is the most desirable means of solids control. In essence, the only cost of screening mud is the rental cost of the equipment and replacement of screens.
Efforts should be made to maximize the amount of drilled solids removed by shale shaker. This means that 100 percent of the mud stream should be screened at all times through as fine a screen as possible without screening out an excessive amount of barite. API specifications on barite call for a maximum of 3 percent barite retained on a 200-mesh screen. This means that screens approaching 200 mesh can be used without wasting an excessive amount of barite.
Shaker screen performance
A woven wire cloth made from stainless steel is normally used for mud screening on shakers. This wire cloth is usually specified by mesh. The mesh is the number of openings per linear inch. Unfortunately, the mesh designation is not sufficient to describe the screen. The wire size which is used also influences the opening size and the percent open area of the screen. The finer the wire diameter the greater will be the screen capacity. However, screen life will be less and the opening size greater — both of which are undesirable.
There must be a compromise between opening size, capacity, and screen life.
Screen designations have also been complicated by screens that have rectangular rather than square openings. Such a screen may have 80 openings per inch in one direction and 40 in the other direction. This screen would be specified as an 80 x 40 mesh screen. Obviously, it would have a greater capacity than a square 80 mesh but would allow some particles to pass that would be retained on a square 80 mesh.
Much confusion has resulted from insufficient screen description. Through indiscriminate use of the term “equivalent”, screens have been represented as one size when they are actually something else. To properly describe a screen, the mesh and opening size in both directions and the percent open area are needed.
Particle sizes and screen openings are usually measured in microns. A micron is 1/1000 of a millimeter or 0.0000394 inches. Figure 1. shows the size ranges of various types of particles and the size ranges removed by different screens and solids control equipment.
Drilled solids may exist in the mud system in any size ranging from the finest dispersed bentonite, 0.05 microns, up to the largest cutting or sloughing. The purpose of shale shaker screening is to remove as much as possible of the drilled solids larger than the barite particles. In this manner bentonite, barite, and the liquid phase of the mud containing the chemicals are salvaged; the coarse, drilled solids are discarded. Since only a very small amount of mud that adheres to these larger particles is lost from the system, screening is a very economical way to remove drilled solids.
The drilled solids normally reduce in size with time and continued circulation in the mud system. When they enter the barite size range, it becomes impossible to separate them from the barite until they are further reduced in size and become smaller than the baste particles. At this time a centrifuge can be used to separate a portion of the liquid fraction of the mud containing these small particles along with the chemical and bentonite. This operation is expensive and removes only a small fraction of the total drilled solids. This makes it imperative that we remove as much of the drilled solids as possible the first time they reach the surface.
For many years, shakers did not exist that would handle the high volume requirements with fine screens. Conventional shakers were generally equipped with 12 to 20 mesh screens. This allowed all particles less than about 840 microns to become incorporated in the mud and caused dilution to be necessary to control the mud properties.
Recent improvements in shale shakers allow the use of much finer screens than is possible with the conventional shakers. These new shakers are normally somewhat larger and operate at a higher vibrational frequency than conventional shakers. Some are designed so that the speed and amplitude can be easily changed to fit the needs of various muds. Under normal conditions, they will handle about 400 gal/min of mud with an 80-mesh, 178-micron, screen. Table 1. shows the opening size, wire diameter, and percent open area for a number of different screens representative of those most commonly used on shakers. These specifications may vary somewhat, depending on the manufacturer, but should serve as a guide.
|Table 1 -Representative Shaker Screen Descriptions|
|/||Opening Size||Wire Dia.||Percent|
Increased mud viscosity will reduce shaker screen capacity. As a rule, the capacity will be reduced by about 2 percent for every 10 percent increase in viscosity. Often, it is necessary to start with coarse screens and change to finer screens as the flow rate is decreased or the drilled solids content is reduced. Where flow rates are low, it has sometimes been possible to screen high-weight muds with two shale shakers using screens as fine as 200 mesh.
Careful attention should be given to maintaining the shale shaker in good working condition and ensure that the finest screens that will handle the entire mud stream are used at all times. Every pound of solids that are missed by the shaker will cost several times as much to be treated or removed from the mud system by another means.
If the pounds of drilled solids removed by the shale shaker per day can be estimated, this number divided into the shaker rental and screen replacement costs will give the cost per pound of solids removed. A comparison of these two numbers will indicate the advantage derived from the shaker. In most situations, the shaker costs will be less than dilution costs from spud to total depth.
Muds that retard dispersion or lift the cuttings out of the hole in a minimum length of time also aid screening efficiency. This ensures that the particles remain as large as possible, thus allowing the maximum amount to be removed by screening. Oil muds are particularly good at inhibiting the dispersion of shale particles. Normally, a fine-screen shaker will control the buildup of drilled solids content in an oil mud with only a minimum amount of dilution being necessary.