Performance of Positive Displacement Downhole Motors Under Two-phase Flow
Authors: Li J. , Tudor R. , Sonego G. , Varcoe B. 
Published: March 2010

Abstract
Positive displacement motors have been used extensively in underbalanced drilling operations. Motor performance characteristics are essential to ensure drilling success and efficiency. Pressure drop across the motor must be understood to accurately simulate underbalanced conditions bottom-hole. In this study, five positive displacement motors were tested under a variety of mixing ratios of nitrogen and water with different back pressures. Comparing with the baseline test at maximum liquid flow rate, the replacement of water with nitrogen decreased the motor performance by as much as 95% in terms of maximum torque output and maximum mechanical power output. The pressure drop across the motor was lower with commingled fluid, and the greater the nitrogen ratio, the lower the pressure drop across the motor. It was also found that back pressure decreased the motor performance. This paper also discusses the testing procedures, the results, and how commingled fluid affects underbalanced drilling operations.

Introduction
Underbalanced drilling is a process where the aerated or nitrified fluid is pumped through the drilling string to the bottom-hole. The pressure at the bottom-hole should be less than the formation pressure and allow wellbore fluids to flow during the drilling period. The circulated fluids power the downhole motor to rotate the drill bit to cut the formations. The cuttings are then picked up by the jetting action of the nozzles on the bit and transported by the drilling fluid to the surface through the annulus between the drilling string and casing. Currently, there are two main types of downhole motors: positive displacement (PDM) and turbines. In general, turbine motors have a high rotational speed, whereas PDMs have a low speed and high torque output. The application of turbine motors has been limited to less than 1% of the total footage drilled in the United States. PDMs are the most widely used in the world at the present time(l).
The PDM is a hydraulically-driven downhole motor that is based on the Moineau principle. It consists of a bypass valve, power section, transmission assembly, and bearing assembly (Figure 1). In the power section of a PDM, a helicoidal rotor with one or more lobes is placed eccentrically inside a stator having one more cavity than the rotor. This difference between the rotor/stator lobe configuration creates 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 transferred to the drill bit.
Motor performance characteristics are essential to ensure drilling success and efficiency. Pressure drop across the motor must be understood to accurately simulate underbalanced conditions at the bottom-hole. In general, motor suppliers provide motor performance data at the baseline condition. A baseline test consists of testing a motor at designated flow rates with water and motor outlet open to atmosphere. When conventional drilling fluids are used, output torque and bit speed are determined based on differential pressure and flow rate. Due to the compressible nature of gas, it is difficult to predict the gas flow and RPM/torque relationship based on baseline tests.