Recent advances in coiled tubing drilling have opened new applications for slim holes, in particular, high-angle, extended-reach horizontal wells drilled with downhole motors. This has created a need for high performance drilling engines. The present study covers torque and flow rate requirements based on the geometry of these wells. It shows how "slim" these motors can be and still satisfy the power requirements at the available flow rate. The analysis is supported with practical examples. Positive displacement, turbines, and electric motors are covered. Comparisons based on performance are also presented. The motors' compatibility with coiled tubing systems is also discussed.

The modeling reveals that the available downhole motors are capable of producing more torque than is actually needed for drilling horizontal holes. It is therefore possible to have slimmer motors whose lower torque is still sufficient.


The use of downhole motors to replace conventional rotary drilling has increased over the years, and its performance has been the focus of research in recent years. The use of coiled tubing to drill horizontal, slim holes has also increased the use of these motors. Currently there are three types of downhole motors: turbines, positive displacement (PDM), electric. Even though turbine downhole motors were introduced to the United States in 1960, their use has been limited to less than 1% of the total footage drilled. The positive displacement motor (PDM) has been the downhole motor of choice in the United States since 1968. Electrical downhole motors are less common than turbines and PDM's and have been mostly used in the Soviet Union. In 1976 General Electric (in a joint project with the Energy Research and Development Administration, ERDA) developed and tested an electric type downhole motor built by General Electric, the main contractor. The project was later abandoned due to non-technical reasons.

Among the most important characteristics to consider when selecting or designing any of the three motors mentioned above are: size (outside diameter), torque output at the bit, and the availability of power to drive them. The power available to turn the motor becomes more important when using a turbine or a PDM because it depends on the hole cleaning requirements and on erosional velocities. Insufficient flow rates could result in inappropriate cuttings transport, while excessively high flow rates (mainly when drilling with air) could cause erosion problems. In this study, torque, horsepower, and volumetric flow requirements are used as criteria for modeling the design and selection of slim downhole motors to be used in drilling horizontal (short radius), re-entry wells using coiled tubing. The configurations considered are schematically shown in Fig. 1.

In estimating torque requirements it is necessary to consider the characteristics of the formation being drilled as well as the mechanical behavior of the bottom hole assembly (BHA).

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