Evaluation and Prediction of the Performance of Positive Displacement Motor (PDM)
Authors: Li J. , Tudor R. , Ginzburg L. , Robello G. , Xu H. , Grigor C. 
Published: March 2010

Abstract
Positive displacement motors (PDM) are being used extensively due to the increased drilling of horizontal/extended reach wells as well as the deepening of vertical wells. Knowing motor performance is essential to ensure drilling success. To enhance the technology and make the system effective, a complete performance envelope of the motor in use is required. In the traditional way, the performance of a PDM can be described by pressure drop vs. torque and rotational speed. For a particular type of motor, the RPM curves are different at different flow rates. Although it is very difficult to fully describe the motor performance with limited testing, a good prediction is critical for motor selection and operation. In addition, the performance of a PDM operating with compressible fluid decreases drastically as compared to the operation with incompressible fluids. In this paper, analysis of the test results for various PDMs from different PDM suppliers are presented. Correlations have been developed, based on the test data, to describe PDM performance with incompressible fluids.

Introduction
Drilling with coiled tubing is a very complex process. In general, the coiled tubing unit consists of surface equipment and downhole equipment. Most downhole tools and equipment associated with coiled tubing drilling may be summarized in the following categories: drill bit, PDM, and bottom hole assembly (BHA) for non-directional and directional drilling. For a particular well, the proper combination of a bit and a downhole motor must be selected. To optimize the drilling process, a variety of parameters have to be fully understood, such as rate of penetration, which can bemodelled with a solid mechanism model(1), and the performanceof a PDM which can be predicted with a suitable correlation.
The PDM based on the Moineau principle is considered in this study. A PDM consists of a by-pass valve, power section, transmission assembly and bearing assembly. In the power section, a helical rotor with one or more lobes is placed eccentrically inside a stator having one cavity more than the rotor lobes. This difference between the rotor/stator lobe configurations creates the cavities. Under pressure, the drilling fluid will drive the rotor in an eccentric rotation, which is then translated into concentric rotation through the transmission assembly and further transferred to the drill bit.
In general, the performance of a PDM depends on the crosssection of the flow area between the rotor and stator, rotor-stator lobe ratio, stage length (or lobe spiral pitch), number of stages and flow rate. The performance parameters of a PDM include output torque, rotation speed, pressure drop and mechanical power. For a given type of PDM, the rotation speed is proportional to the flow rate pumped through the PDM, and the pressure drop across the motor is proportional to the torque output(2).
Usually, the manufacturer-supplied operation manuals present the data in the form of pressure drop vs. torque output, and rotational speed vs. pressure drop based on limited test data. These plots do not depict the actual performance, as the plotted curves are different for different flow rates.