Page 151 - Shale Shakers Drilling Fluid Systems
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134 SHALE SHAKERS AND DRILLING FLUID SYSTEMS
When the layered 84 curve is compared with • The pool of fluid on most linear motion shale
the market grade 80, both screens have a d so shakers may significantly alter the separation
of approximately 183-micron particles. The made by the layered screen. Solids in the
layered screen will discard fewer coarser par- pool have a greater opportunity to find a hole
ticles between 183 and 245 microns. Both in the screen to pass through. If every par-
screens will discard all solids larger than ticle in the slurry had the same chance to
245 microns. Conversely, the layered screen seek out all of the screen openings, then the
discards more smaller solids between 100 actual percent separated curve would be the
and 183 microns. Both screens will allow dimension of the largest hole. In reality, even
most of the solids smaller than 100 microns in a pool of fluid, all particles do not have a
to pass through. chance to find the largest hole opening. Lay-
ered screens without a pool (circular and
The area to the left of the MG80 and to the unbalanced elliptical motion shale shakers)
right of the layered 84 screens indicate a may provide solids a better chance to ag-
large quantity of smaller solids would be glomerate in patties. Agglomeration severely
rejected by the layered 84 that would be limits the mobility of the individual solid par-
returned to drilling fluid by the MG80.
ticles so that they are unable to find open-
The areas to the right of the MG80 and to the ings to pass through.
left of the layered 84 indicate that a large
quantity of large "sand" size particles would Separation potential curves for a layered 140
be returned to the drilling fluid by the layered (130/100/30) screen compared with a MG120 screen
84 that would be rejected by the MG84. are shown in Figure 6-9. The cumulative distri-
bution of equivalent spherical sizes in a barite
The square opening cloth makes a size sepa- sample, which has been marked with API approval,
ration somewhat independent of particle shape. has been added to this curve. Eighty-three volume
Since the opening size is the same in both percent of this barite is finer than 44 microns,
directions, particles that pass through the which means that 17% of this barite is coarser
openings must be smaller than the opening than 44 microns. The layered 140 screen has open-
in both directions. Furthermore, long slivers ings in the 44 to 185 micron range. For this rea-
that might pass through the screen if turned son, some barite may be discarded by this screen.
on end, probably will not because they will The MG120 should eliminate very little of this bar-
tend to remain flat on the screen surface. ite since all openings are larger than the largest
barite particle. (API specifications allow three weight
The rectangular opening cloth will have a
percent separated curve, or an actual sepa- percent barite to be larger than 75 microns, or 200
ration, which is a function of particle shape. mesh.) The barite shown in Figure 6-9, contains
For instance, if all of the particles being sepa- more than allowed by API standards. Generally,
rated are round, the percent separation curve 3 Ib from every 100-lb sack would be discarded if
will fall on the narrow dimension of the hole presented to a 200-mesh screen. If the slurry pre-
opening. Since the separation potential curve sented to the screen is dehydrated, such as hydro-
represents the largest ellipsoid that will barely cyclone underflows, both screens may remove
fit in the screen openings, the separation po- substantial amounts of barite.
tential curve approximates the separation this
rectangular screen makes of oblate sphe- Review
roids. Particles with a long length can be
separated even though the equivalent spheri-
cal diameter is much less than the equivalent The separation potential curve is similar in shape
ellipsoid that will pass through the screen. to the actual separation made by a screen. Shale
shakers classify and separate solid particles based
Layered screens have a broad range of open- on size and shape. Particles coarser than the screen
ing sizes, which is typical of layered screens. opening are removed and particles finer than the
The separation potential curve indicates that screen opening are retained. Particles have many
many openings are both larger than the d so chances to find openings to pass through as they
(or median) cut point and smaller than the are conveyed from the feed end to the discharge.
d 50. The layered screen has the greatest po- This is particularly true while they are in a pool
tential to have an actual percent separated of fluid. When they exit the pool, they tend to ag-
curve that differs from the separation poten- glomerate with other solids. This agglomeration is
tial curve. caused by surface tension between the air and the
