The first line of defense for a properly maintained drilling fluid has been, and will continue to be, the shale shaker. Without proper screening of the drilling fluid during this initial removal stop, reduced efficiency and effectiveness of all downstream solids control equipment on the rig is virtually assured.
The shale shaker, in various forms, has played prominent role in oilfield solids control schemes for several decades. Shakers have evolved from small, relatively simple devices capable of running only the coarsest screens to the models of today. Modern, high-performance shakers of today are able to use 10 mesh and finer screens at the flow-line in most applications.
This evolutionary process has taken us through three distinct eras of shale shaker technology and performance as shown in the diagram on the next page. These eras of oilfield screening development may be defined by the types of motion produced by the machines:
Elliptical, Unbalanced design
Circular, Balanced design
Linear, straight-line design
The unbalanced, elliptical motion machines have a downward slope and shown in the diagram above, A. This slope is required to properly transport cuttings across the screen and off the discharge end. However, the downward slope reduces fluid retention time and limits the capacity of this design. Optimum screening with these types of shakers is usually in the 30-40 mesh (400-600 micron) range.
The next generation of machine, introduced into the oilfield in the late 1960s and early 1970s, produces a balanced or circular motion. The consistent, circular vibration allows adequate solids transport with the basket in a float horizontal orientation, as shown in the diagram above, B. This design often incorporates multiple decks to split the solids load and to allow finer mesh screens, such as 80-100 square mesh (150-180 micron) screens.
The newest technology produces linear, or straight-line, motion, C in the diagram above. This motion is developed by a pair of eccentric shafts rotating in opposite directions; linear motion provides superior cuttings conveyance and is able to operate at an uphill slope to provide improved liquid retention. Better conveyance and longer fluid retention allow the use of 200 square mesh (74 micron) screens.