Downhole Motors
About this chapter
The positive-displacement mud motor (PDM) is the most indispensable tool at the DD’s
disposal. It is vital that the DD understand how to utilize the PDM to best advantage. The
basics of PDM design are covered in this chapter.
With the PowerPak mud motor, Anadrill has added a reliable and high-quality tool to its
range.
It is recommended that, in the short term, the DD be aware of the exact specifications of
"third party" PDMs which he may have to use.
PDM design, specifications, operating procedures, etc., are covered in this chapter. The
basics of steerable PDMs and steerable BHA design are also covered in this chapter.
Objectives of this Chapter
On completing this chapter the directional driller should be able to do the following
exercises:
1. Draw a diagram of a PDM, showing the major components. Describe the function
and purpose of each component.
2. Explain the main differences in construction between 1:2 lobe and multilobe PDMs.
3. Explain the uses of a rotor nozzle.
4. Describe what is meant by hydraulic thrust when using a PDM.
5. Explain the procedure involved in making up a PDM with a bent sub in a kickoff
BHA.
6. Describe the basic service which is done to a PDM after POOH, prior to laying it
down. Assume water-base mud.
7. Describe what precautions are necessary when drilling with a PDM.
8. Explain what surface indication(s) the driller has of PDM operation (and possible
problems) downhole.
9. Explain the main difference(s) in design between a straight PDM and a steerable
PDM.
10. Explain how an estimate is made of the buildup rate achievable with a bent-housing
steerable BHA.
11. Give examples of typical steerable BHAs designed to build inclination from vertical
to maximum angle and to hold this inclination until the next casing point.
12. Explain the effect the upper (string) stabilizer has on steerable BHA performance.