This paper presents a method of overcoming some of the problems associated with drilling with positive displacement motors (PDM’s) using compressible fluids. The specific problems addressed are:

  1. motor over-speeding resulting from sudden drops in motor load, often leading to stator "chunking".

  2. repeated stalls resulting from poor feedback of motor performance to the operator, resulting in poor effective rates of penetration (ROP’s) and reduced motor life.

Transient flow conditions dictate the phenomena listed above. This paper explains the theory behind the transient conditions that occur when the load on the motor suddenly changes due to inevitable changes in drilling conditions. A methodology is given that predicts the PDM’s rotational speed resulting from these sudden load changes. A solution is put forward that ensures that the total drilling "system" of the coil, the bottom hole assembly (BHA) and the motor is stable, efficient, controllable and not liable to self-destruct.

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