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.


Drilling with positive displacement motors using compressible fluids is not new to the industry. Many such jobs have been successfully carried out using both coiled tubing and drill pipe using both air and commingled mixtures of liquid and gas. However, there are significant additional difficulties to contend with when using compressible fluids and the success rate or motor life is typically much lower as compared to conventional drilling with single-phase fluids.

Prior research and testing has been undertaken to show how the down hole motors themselves respond when compressible fluids are pumped through them1,2. Naturally, the motor testing done is based on steady state conditions. Whereas the steady state behavior of the motor is very important, the transient behavior is critically important.

Controlling a PDM with compressible fluids is made difficult by the fact that the coil operator receives little help from his surface instruments. Changes in the differential pressure across the motor are not directly reflected in changes in the surface pump pressure. This is true for steady state conditions and particularly true for transient conditions.

Down hole, the load on the motor can change quite suddenly, responding to changes in WOB or changes in the target through which the motor is drilling. The compressible fluid inside the coiled tubing acts as a giant accumulator. The combined effect of the "accumulator" and the motor can be to impose destructive flow rates through the motor under transient conditions. These destructive transients can be totally transparent to the coil operator on surface.

Also, the tubing weight indicator gauge can be a poor or insufficient indicator of motor performance, depending on the motor/bit configuration used3.

The drilling system of coil, motor and bit can be unstable and practically impossible to control if not engineered taking transients into account. An unstable system can be avoided when the physics of the system are understood and the drilling system is configured correctly. An understanding of steady state behavior is required to design the target conditions for a coil drilling job. An understanding of the transient conditions is required to prevent unstable and self-destructive behavior of the motor down hole. The fluid transient behavior is mostly a function of the fluids, the coil, additional flow restrictions, and the operating pressure range of the motor.

Basic Concepts of Compressible Fluid Drilling

Figure 1 shows a much-simplified picture denoting the basic components of a coil unit drilling with a down hole motor.

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