Page 106 - Shale Shakers Drilling Fluid Systems
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INTRODUCTION 89
vertical and nearly vertical (up to 35°) wells. An wellbore. Because the mud weight and annular
empirical relationship has been published relating velocity will not usually be changed, the K-value
a carrying capacity index (CCI) to the product needed can be calculated. Generally, the plastic
of the mud weight (MW), annular velocity (AV), viscosity will not change significantly when the
and a characteristic viscosity (K). This can be ex- yield point is increased. Consequently, the curve
pressed as: in Figure 1-1 (or a similar curve) can be used to
determine the yield point needed to clean the hole.
DRILLING FLUID COSTS
The "400,000" constant was empirically deter-
Drilling fluid density can be increased using
mined by observing hole cleaning conditions on
drilled solids for the lowest initial weight-up cost.
many rigs over an 8- to 10-year period. A CCI If drilled solids had no other effect, using them for
value of "one" seems to indicate good hole clean- a weighting agent would be cost-effective. How-
ing in both water-based and oil-based drilling flu-
ever, using drilled solids could potentially increase
ids. The constant is probably not accurate to more
drilling costs because slow drilling, poor cement
than one significant figure. jobs, stuck pipe, and lost circulation will frequently
The "K" viscosity is the viscosity from the Power
Law Rheological Model, expressed as equivalent follow. Downtime on a rig or reduced penetration
rates are far more expensive than the cost of us-
centipoise. It can be related to the Plastic Viscos- ing adequate weighting material.
ity (PV) and Yield Point (YP) through the equations:
Removal of drilled solids as they reach the sur-
face usually costs less than any other method of
mitigating their effect. Conversely, controlling drilled
solids with dilution is usually the most expensive
and method. Although drilled solids concentration can
be reduced by half with dilution by doubling the
system volume with clean drilling fluid, this clean
fluid is expensive.
Polymer drilling fluids, with X-C, PHPA, and
At a constant yield point value, increasing the so on, require relatively low drilled-solids concen-
plastic viscosity decreases the value of the viscos- trations. The polymer attaches to all solids in the
ity. This is clear from the curves in Figure 1-1. system whether they are desirable or not. Failure
If the cuttings on the shaker screen indicate that to keep a low drilled-solids concentration will re-
they are not arriving at the surface without con- sult in using excessive quantities of chemical (poly-
siderable tumbling, the CCI equation can be used mer). This can make the use of polymer drilling
to calculate the yield point needed to clean the fluids cost-prohibitive. The use of polymer systems
requires the utmost in planning solids control
equipment for optimum performance.
A word of caution is appropriate here. Neo-
phytes in drilling have a tendency to try to mini-
mize the cost of each category of drilling expenses
with the misconception that this will minimize the
total cost of the well. It is important to realize that
additional expenses can be incurred because of
inadvisable decisions to cut costs in easily moni-
tored expenses while drilling wells. When line-
items are independent of each other, minimiza-
tion of each line-item will result in the lowest
possible cost. When line-items are interconnected,
minimization of each line-item may be very ex-
pensive. Drilled solids concentrations and trouble
costs (or the cost of unscheduled events) are very
closely related.
For example, a common mistake is to allow an
initial increase in mud weight to occur with drilled
FIGURE 1-1. K-viscosity. solids. Clearly, less money will be spent on the
