Page 165 - Shale Shakers Drilling Fluid Systems
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148 SHALE SHAKERS AND DRILLING FLUID SYSTEMS
the 30 to 50 micron range. They are primarily used Summary
to remove high solids volume associated with
fast drilling of large-diameter top holes. In water- Desanders should be used in unweighted mud
based drilling fluids, desanders make a median when shakers are unable to screen down to 140
separation cut of 2.6 specific gravity solids in the mesh (100 micron). The role of desanders is to
50 to 80 micron size range. The desander removes reduce loading downstream on desilters. Installing
sand-size and larger particles that pass through the a desander ahead of the desilter relieves a signifi-
shale shaker screens. cant amount of solids loading on the desilter and
Desanders are installed immediately downstream improves its efficiency. High rates of penetration,
of the shaker and degasser. Suction is taken from especially unconsolidated "surface hole" where the
the immediate upstream tank, usually the degasser largest diameter bits are used, results in generat-
discharge tank. Discharge from the desander is ing larger concentrations of drilled solids. This may
made into the tank immediately downstream. Suc- place desilters in rope discharge. For this reason,
tion and discharge tanks are equalized through desanders, which have greater volumetric capac-
valves located on the bottom of each tank. ity and can make separations of coarser drilled
Desanders are used continuously while drilling. solids, are placed upstream of desilters. Desanders
Plumbing can be arranged to process all total sur- remove a higher mass (coarser drilled solids) dur-
face pit volume after beginning a trip. ing periods of high solids loading. Desilters can then
Use of desanders is generally discontinued af- efficiently process the reduced solids content over-
ter barite and/or expensive polymers are added flow of the desanders.
to the drilling mud because a desander discards If the drill rate is slow, generating only a few
such a high proportion of these materials. Use of hundred pounds per hour of drilled solids, the
desanders is generally not cost effective with an desander may be turned off and the desilter used
"oil-based" drilling fluid because the larger cones to process the entire circulating system.
discharge a significant amount of the liquid phase Desilters should be used on all unweighted,
(see Mud Cleaners discussed later in this chapter). water-based mud. These units are not used on
weighted muds because they discard an appre-
ciable amount of barite. Most barite particles fall
Desilters within the silt-size range.
Desilter operation is important for all unweighted
Desilter cones are manufactured in a variety of fluids, however, in oil-based muds with high viscosi-
dimensions, ranging from 2 to 6 inches, and make ties (as found in deep water drilling) the apex dis-
drilled solids separations in the 12 to 40 micron charge may be centrifuged for "oil-phase" salvage.
range. They will also separate barite particles in Hydrocyclones should process all drilling fluid
the 8 to 25 micron range. Desilters are installed entering their suction compartments independent
downstream from the shale shaker, sand trap, of the drilling fluid circulation rate.
degasser, and desander.
Desilter cones differ from desander cones only
in dimensions and operate on exactly the same Median (D 50) Cut Points
principles. Common desilter cone sizes are be-
tween 2 and 5 inches. Desilter cones should be fed In spray discharge, for any set of cone diameter,
by a centrifugal pump dedicated only to providing feed slurry compositions, flow properties, volumet-
fluid to the desilter. ric flow rates, and pressure conditions, some par-
These units make the finest particle-size sepa- ticles size (mass) is 100% discarded from the apex.
rations of any full-flow solids control equipment— For every size and design of cone operating at
down to 12 microns of drilled solids. The desilter, a given pressure with feed slurry of a given vis-
therefore, is an important device for reducing av- cosity, density, and solids distribution, there is a
erage particle size and eliminating drilled solids. certain size (mass) of particle that shows no pref-
Desilter suction is also taken from the immedi- erence for either top or bottom discharge. As a
ate upstream tank, usually the desander discharge result, 50% of this particular size exits through the
tank. Desilter suction and discharge tanks are, vortex and 50% exits through the apex. This par-
again, equalized through a valve(s) located on the ticle size is termed the "median cut," "median size
bottom of each tank. Suction should not be taken particle," or more frequently in drilling operations,
from the tank into which chemicals and other the D 50 cut point.
materials (barite and bentonite) are added because The median cut, or D 50 cut point, does not mean
valuable treating materials may be lost. that all larger particles exit at the apex and smaller
