Page 181 - Shale Shakers Drilling Fluid Systems
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CHAPTE R EIGH T
Dilution
Dilution is the surface addition of base fluid to a drilling rig. These variables play an important role
the drilling fluid system. This addition has the in minimizing drilling fluid and drilling waste costs:
effect of reducing the concentration of all other
components or constituents of the drilling fluid. 1. The dryness of discarded solids varies sig-
Although the shale shaker frequently removes nificantly with various types of solids control
a larger fraction of drilled solids, it should be equipment. Shale shakers with coarse mesh
regarded as only one component of solids removal screens (such as 12 to 30 mesh) discard very
equipment in a drilling fluid processing system. In dry solids (60% to 80% volume solids) but
addition to solids removal equipment, the detri- also pass a large fraction of solids presented.
mental effect of drilled solids can be reduced with Shakers with fine mesh screens (such as 100
chemical treatment or dilution. Both of these to 200 mesh) can remove more solids but
methods are expensive but, unlike chemical treat- also discard more liquid drilling fluid. The
ments, dilution is always effective. This chapter fluid removed with these fine mesh screens
will focus on this process, provide some basic may have 30% to 45% volume solids and
guidelines for its use, and demonstrate how ex- contain more or less weighting material than
pensive dilution can be. the drilling fluid in the pits. Hydrocyclones
settle solids inside the cones in the whole
drilling fluid. Spray discharge is desirable but
INTRODUCTION seldom do all cones on a drilling rig dis-
charge solids in a spray discharge. The solids
One method of managing the drilled solids level concentration in hydrocyclone discharges
is to simply empty some of the drilling fluid con- can vary from 20% to 40% volume. A centri-
taining the drilled solids and replace it with clean fuge underflow usually has a solids concen-
drilling fluid. One half of the solids can be eliminated tration between 55% to 65% volume and the
if one half of the system is emptied and replaced overflow has a solids concentration between
with clean fluid. Generally, this is an expensive pro- 5% and 15% volume. The mud cleaner screen
cess, therefore, mechanical equipment is used. discharge in an unweighted drilling fluid has
With solids control equipment, both solids and approximately 30% to 45% volume solids. A
liquid are removed. The total volume removed weighted drilling fluid usually resembles the
decreases the volume of the circulating system. concentration of the centrifuge underflow.
Consequently, new drilling fluid must be built to Thus, a typical solids concentration of the
keep the pit levels constant. This additional fluid overall system discard is approximately 35%.
helps dilute the remaining drilled solids in the pit. 2. The concentration of drilled solids targeted
If the solids-removal equipment is ineffective at in the drilling fluid is a function of the type
removing a substantial quantity of the drilled solids, of drilling fluid. Polymeric additives that ad-
too much new drilling fluid must be built. In the here to active solids are attracted to most
case where no drilled solids or liquid are removed clay-type drilled solids. These drilling fluids
from the system, no volume addition is required. demand low drilled-solids concentrations.
Three primary variables—dryness of discarded The inhibitive fluids also need a low solids
solids, targeted concentration of drilled solids in concentration, but also aid the removal pro-
the drilling fluid, and removal efficiency—are impor- cesses by not allowing clay-type solids
tant in evaluating a solids management system on to disperse.
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