Abstract
The introduction of downhole motors to the drilling industry has helped to reduce costs and to develop new drilling technologies.
The idea of using a downhole motor directly on top of the bit instead of turning the whole drill string is not new. The first commercial downhole motor was a turbine, but it turned too fast for most existing bits and applications, especially directional drilling. So other ideas were tried.
An electric downhole motor was experimented with, but it failed because of power transmission problems. Finally, the positive displacement motor (PDM) was developed. Like the turbine, the PDM converts the hydraulic power of the mud flow into mechanical power at the bit. But it turns slower, and thus has become the preferred downhole motor for most operations.
