Page 437 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 413

Drill Bits [ ]
3.6.3

3.6.3. Mud Motors, Steerable Systems, and Turbines

Other considerations apply when planning to run a downhole
motor. They include:

1. Will the maximum bit RPM be exceeded if the string is rotated
while drilling with a motor?
2. Will the flow through the motor clean the hole at the envisaged
flow rates?
3. Are there any limitations on the bit pressure drop imposed by the
motor?
4. Is the proposed bit suitable to use on the type of motor to be used?
5. Can LCM be pumped through the motor and if not, should a cir-
culating sub be run above? Might LCM be needed while drilling
with the motor?
6. What size liners are needed in the pump for the necessary flow
rates and pressures?
7. Are there any problems with the mud properties? Chemical com-
patibility with seals, sand content, etc. Check with the motor sup-
plier.
8. What is the plan for before and after the motor run? For example,
if running in for straight hole turbodrilling with a PDC bit, the pre-
vious assembly should be a fairly stiff one to avoid reaming in with
the turbine. Any junk in the hole would require a junk run first.
Otherwise, if a steerable system run is to terminate at casing point,
then a wiper trip with a rotary assembly should be made to ream to
bottom and reduce the chance of mechanically stuck casing by
reducing doglegs and ledges.

3.6.4. Monitoring Bit Progress While Drilling

Cost per foot calculations should be done while drilling. Once the
cost per foot starts to increase, the bit will be nearing the end of its eco-
nomic life. However, several other factors should be considered when
making a decision to pull the bit.
Pull the bit earlier if there are indications of bearing failure (high
and/or fluctuating torque on bottom compared to steady, reasonable
torque just off bottom).

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