When mechanical separation equipment is used to remove suspended solids from liquids, capture determinations provide a simple means of evaluating its performance. Capture is defined as the fraction of incoming suspended solids that report to the discarded stream.
The Process of Centrifuge Deal With Drilling Fluids
Before hydrocyclones and centrifuges became available for drilling applications, shale shakers and dilution were the only means of controlling the solids content of drilling fluids. Consequently, solids too fine to be separated by the shaker screens in use could be controlled only by dilution. During drilling with weighted muds, once the solids content reached the maximum acceptable level, the continuing and unavoidable incorporation of drilled solids made it necessary to add a continuous stream of water to control viscosity, while adding barite to control the mud weight. This was, obviously, a costly procedure that generated large quantities of excess drilling fluid.
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Mud Cleaner Performance
Usually, if solids are being removed with shale shakers, a mud cleaner will probably be beneficial. Solids removed by mud cleaners will cover a wide range of quantities depending on formations drilled, borehole stability, dispersion of solids as they move up the borehole, type of drill bit, type of drilling fluid, and other variables.
SHAKER SCREEN CLOTHS
Shale shakers remove solids by processing solids-laden drilling fluid over
the surface of a vibrating shaker screen. Particles smaller than the shaker screen openings pass through the screen along with the liquid phase of the drilling fluid.
Larger particles are separated into the shaker overflow for discard.
The shaker screen acts as a ‘go no-go’ gauge. That is, particles larger than
the screen openings remain on the screen and are discarded. Particles
finer than the screen openings go through the screen with the drilling
fluid. The criterion for early shale shaker screens was a long screen
life. This demand for screen life was consistent with the shaker designs
and solids-removal philosophies of the time period. Early shale shakers
could remove only large solids from the drilling fluid. The sand trap,
reserve and settling pits, and downstream hydrocyclones (if utilized)
removed the bulk of drilled solids. Today’s shale shakers are capable
of utilizing finer screens that remove more solids. Desirable characteristics
for a shaker screen are:
1. Economical drilled-solids removal
2. Large liquid flow rate capacity
3. Plugging and blinding resistance
4. Acceptable service life
5. Easy identification
For any particular shale shaker, the size and shape of the shaker screen openings have a great effect on solids removal. This means that the performance of any shaker is largely controlled by the screen cloth used.
The first four items in the preceding list are largely controlled by
choice of screen cloth and by the screen panel technology. Large gains
in shale shaker performance are a direct result of improved screen
cloth and panel fabrication. shaker Screens used on shale shakers have evolved into complex opening patterns.