Page 171 - Shale Shakers Drilling Fluid Systems

154 SHALE SHAKERS AND DRILLING FLUID SYSTEMS

flow diameter, feed head, and plastic viscosity have
Benefits of hydrocyclones: the greatest impact on hydrocyclone performance.
Barite particles 3 microns and smaller have a
1. Replacement of pump fluid end parts is reduced deleterious effect on drilling fluid viscosity due to
a surface charge imbalance resulting from unsat-
and pumps operate more efficiently.
isfied broken bonds on the ultra-fine's surface.
2. Less drill string torque and drag equates to less Therefore, if a centrifuge is set so that its median
wear on string and less key-seating (a major poten- (D 50) cut in 14-pound per gallon mud is a 3-
tial for stuck pipe). Casing is run easier. micron barite particle, it's median cut for drilled
solids will be 5 (or 4.9) microns.
3. Bit life is extended due to less abrasion.
4. Penetration rates increase.
MUD CLEANERS
5. Water dilution to maintain low mud weights is re-
duced. This is reflected in smaller waste pits and Until the development of linear motion shale
shakers, an 80-mesh screen was about the finest
smaller drilling fluid volumes.
screen that could process drilling fluid at a rig.
6. Material additions are decreased. With this limitation, drilled solids smaller than 177
microns remained in the drilling fluid. The mud
7. Additions of weight material are made with little or
cleaner was originally designed to remove drilled
no difficulty.
solids from a weighted drilling fluid in sizes
ranging from 177 to 74 microns (200 to 80 mesh).
8. Downhole tools set and release with little or no
Because barite is generally ground so that the
interference from drilled cuttings.
majority is smaller than 74 microns, most of it
should pass through the screen on a mud cleaner.
The mud cleaner is a combination of hydro-
cyclones and a fine-mesh screen shaker. Weighted
Generally hydrocyclones are most efficient when drilling fluid is first processed through hydrocyclones.
solids have a diameter greater than 10 microns Enough cones should be used to ensure that all
and are spherical in shape. If the solids are flat fluid entering the desilter suction compartment is
(such as mica), movement tends to be random de- treated. The underflow of the hydrocyclones con-
pendent on whether the flat surface or edge is to- tains drilled solids, weighting material, and some
ward the gravitational force created in the vortex. whole mud. The shaker sieves the fluid through a
Since separation efficiency somewhat depends 150- to 200-mesh screen. This allows most of the
on the freedom and velocity of the solid moving weighting material to pass through the screen. The
through the liquid phase, it is logical to use the solids retained on the fine-mesh screen are dis-
lowest viscosity fluid possible. carded. If the fluid on the screen leaves the ma-
Because fine solids have more surface per unit terial too soon, the discarded material contains
volume ("specific area"), the amount of liquid dis- appreciable quantities of weighting material. A
charged per pound of solids is higher with fine light mud-spray on the screen helps the separa-
solids than with coarse solids. Therefore, the differ- tion process.
ence between the feed and underflow densities is One advantage of the mud cleaner is that the
not a reliable indicator of hydrocyclone performance. shaker screen only processes ab^~ i i to 2 gallons
Pressure drop is a measure of the energy being for every 50 gallons passing through 4-inch desilters.
expended in the cone. Thus, a higher pressure The D 50 cut point of a 4-inch hydrocyclone process-
drop results in a finer separation. If the D 50 is in- ing a weighted drilling fluid may be as high as 60
creased to 75 microns, 25% of the 100-micron to 70 microns. Therefore, most of the weighting
particles are retained and only 25% of the 55- material is not presented to the mud cleaner screen.
micron particles are discharged. Usually, the screen discharge from a 200-mesh
The purpose of a hydrocyclone is to discharge mud cleaner screen has less whole mud associ-
maximum abrasive solids with minimal fluid loss. ated with it than a primary shale shaker discharge.
Larger particles have a greater probability of dis- This relatively dry discharge can give the appear-
charging through the bottom underflow (apex), ance of being primarily weighting material. Mea-
while smaller and lighter particles have a greater surements show, however, that the discharge from
probability to discharge through the top, or over- a 200-mesh shale shaker screen and a properly
flow, opening. Cone diameter, cone angle, under- treated 200-mesh mud cleaner screen are similar.