Page 315 - Drilling Technology in Nontechnical Language

306 Drilling Technology in Nontechnical Language Second Edition

no weighting materials such as barite are added. The density
could be further reduced by the following:
a. Mixing the mud with fresh water rather than sea water, if the
supply situation permits. Sea water has a higher density than
fresh water.

b. Aerating the mud using compressed nitrogen injected at the
standpipe. (Aerated muds are discussed in chapter 7.)

2. Reduce the density of the drilling fluid when it is contaminated
with cuttings, which it will be in the annulus while drilling.
Several methods may be easily employed:
a. Increase the viscosity of the mud at the shear rates present in
the annulus so that the mud lifts out cuttings more efficiently.

Polymers (such as starches) can be added to do this.
b. Drill more slowly by applying less weight on the bit, so that
cuttings contaminate the mud at a lower rate. This is called
controlled drilling because the ROP is controlled at a level
below which the bit could potentially drill if the drilling
parameters were optimized to drill as fast as possible. This
also tends to generate smaller cuttings, which are more easily
lifted up with the mud flow.

c. Increase the circulation rate. This lifts out cuttings more
quickly. It will also increase the annular pressure losses
slightly, but this effect is very small due to the large-diameter,
shallow hole typically drilled for surface casing. In most
cases, the reduction in contaminated mud density causes
a greater drop in bottomhole pressure than the increased
annular pressure loss.

d. From time to time, pump around a pill of highly viscous mud
to try to clean out any cuttings not moving out of the well.

3. Reduce the height of the mud column. On an offshore rig, surface
hole might be drilled with fluid returns to the seabed, rather

than trying to create a closed circulating system to bring fluid

to the rig. It is also possible to weld a remotely controlled large-
diameter valve on the riser close to the sea surface. If losses are
taken, the valve can be opened, and returns exit at sea level. If the
well flows, this valve is closed at the same time as the diverter.

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