Page 214 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 190

[ ] Well Programming
2.4.4

allow freedom in alignment, the nozzles have to be threadlocked in
place. A set of these nozzles was run in a PDC bit in the Nile Delta.
While there were no directly comparable runs to draw technical con-
clusions against, the run was very impressive with the bit drilling at
controlled ROPs over 160 fph with 1 to 3 kips on the bit and 80-100
RPM. It was determined that the nozzles had contributed significantly
to this performance.
Whatever is decided about hydraulics at the bit, a rule of thumb is
to use an annular velocity over 100 ft/minute to ensure that cuttings
can be lifted up the annulus. However, this is only a general guide.
Higher angle wells may need turbulent flow to prevent cuttings beds
from building up (if the mud rheology does not allow turbulent flow,
then thin, turbulent slugs may be needed). Washouts are particularly
detrimental in lowering AV and allowing cuttings beds buildup. If the
flow rate is restricted by losses, the AV should never fall below what is
needed to lift cuttings up to the loss zone, even if total losses are occur-
ring and you are drilling blind. You should use 50 ft/min as the mini-
mum AV.
Mud rheology becomes very complex in non-Newtonian fluids.
There are many different theories and methodologies to relate mud
shear rates to shear stress. Polymer fluids can exhibit complex behav-
ior. In addition to shear thinning (exhibiting lower viscosity at higher
flow rates), they may be time-dependent and exhibit different veloci-
ties at a particular shear rate depending on whether the pump speed is
increasing or decreasing! Temperature, and to a lesser extent pressure,
will also make a significant difference so that modeling the actual effect
on bottom of particular flow rates can get tricky.
If these complex fluids are in use and it is important to calculate
accurate pressure drops (such as in deep and/or slimhole wells), the
fluid itself needs to be tested and a suitable model needs to be used for
the circumstances.

2.4.4. Using Log Data to Aid in Bit Selection

Sonic, lithology, and porosity logs can be interpreted to give vari-
ous rock mechanical properties, such as Bulk Compressive Strength,
Shear Modulus, Young’s Modulus of Elasticity, and Poisson’s Ratio.
Some of the major bit vendors have done work on relating bit selection
to these properties. (Note: The definitions of these mechanical proper-
ties are covered in Section 1.4, “Casing Design.”)

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