Page 149 - Shale Shakers Drilling Fluid Systems

132 SHALE SHAKERS AND DRILLING FLUID SYSTEMS
contact time with the screen, and presence
of other solids all affect the ability of the
solids to pass through the holes in the screen.
These variables, therefore, affect the percent
separated curve.
• Surface tension of the fluid causes solids to
agglomerate together as they exit a pool of
fluid. If solids finer than the screen openings
leave the pool of fluid, then they are held by
the surface tension and have very little chance size distribution expressed as equivalent spherical
of traveling through the screen. Adding a
spray wash to the last screen panel, disperses
these solids and allows finer solids to be
washed through the screen.
• Blinding or plugging of the screen cloth dra-
matically affects both the amount of fluid
passing through the screen and the separa-
tion the screen makes. Many of the screen
openings effectively become smaller and
fewer solids will pass through. The screen
then makes a much finer separation than
originally intended, and the screen capacity
descreases significantly.
• Fine solids are also trapped in the moving
solids bed. Many of the solids rejected by
the screens are finer, or smaller, than the
screen openings.
• Reported values for percent separated curves
are also affected by how the measurement
was made in the laboratory. The greatest
error is often the measurement of particle-
size distribution. Particle sizing by sieve analy- A comparison of calculated Image Analysis (IA)
sis is the best way to characterize solids be-
ing screened since the sieving process is
similar to screening. Unfortunately, sieving
is a tedious and slow process. Forward laser
light scattering particle size analyzers, such
as the Malvern and Cilas Granulometer, tend
to report size distributions somewhat larger
than sieve analysis. These instruments report
particle sizes in terms of equivalent spheri-
cal diameters. Some drilled solids may be
more rectangular in shape, therefore, the
equivalent spherical diameter may not exactly
agree with the sieve analysis.
In summary, the percent separated curve rep-
resents the fraction of solids rejected by the screen
as a function of size. From the above discussion,
it may be noted that the percent separated curve
is dependent on the conditions that exist when the
data are taken. As a result, in actual drilling con-
ditions, the percent separated curve will most
likely vary as drilling fluid properties change,
as the shapes of the solids change, and as the
screen blinds.
Comparison of Separation Potential
and Percent Separated Curves
The separation potential curve is a represen-
tation of the screen opening ellipsoidal volume
diameters. The percent separated curve (cut
point curve) is a representation of the actual sepa-
ration of solids made by a screen, based on mea-
surements of a particular shale shaker.
Separation potential curves have the same shape
as percent separated curves for many types of
screens. The separation potential curve provides
a way to compare a range of complex screen types
in a uniform manner. Since the separation poten-
tial curve can be obtained for all screens that have
a straight-through flow path, it can be calculated
for all oil field screens and provide some informa-
tion on the confusion of openings in layered screens.
Screens with large length-to-width ratio open-
ings (3:1 or larger) are not well represented with
the Image Analysis method. Particles in the drill-
ing fluid normally have a maximum length-to-
width ratio of less than 3:1. Ellipsoids that barely
fit through the screen openings are not a fair rep-
resentation of the particle size that the screen's
openings will reject. Thus, the opening sizes may
not necessarily match the actual performance of
a shaker as measured by cut point curves.
d 50 values and actual measured cut points (D 50) for
a particular shaker is shown in Table 6-9.
Comparison of Separation Potential
Curves for Various Screens
The separation potential curves of several woven,
wire screen cloths are shown in Figure 6-8 for
market grade 80 x 80 mesh, rectangular 80 x 40
mesh, and layered 84 mesh screens. The follow-
ing observations may be made:
• The area or region to the left of these curves
represents solids returned to the active system.
The area or region to the right of these curves
represents solids rejected by the shale shaker.
• The market grade 80 x 80 cloth has a nar-
row distribution of openings, with the small-
est openings equaling 165 microns and the
largest openings, 193 microns. This type of