Underbalanced drilling with coiled tubing requires the use of a positive displacement motor (PDM) to drive the drill bit. Motor performance characteristics are essential to ensure drilling success and efficiency, and the pressure drop across the motor must be specified to accurately simulate underbalanced conditions at bottom hole. In this study, five positive displacement motors were tested under a variety of mixed ratios of nitrogen and water and different back pressures. The experimental results in this study show that the performance of PDMs powered by commingled fluids is different from water baseline tests. With the commingled fluid at the same effective flow rate, the outputs of torque and mechanical power and the pressure drop across the motor are lower than those for a water baseline test. The torque output is more sensitive to the water rate, while the RPM is more sensitive to the nitrogen rate. It was also found that back pressure has a significant impact on the performance of a PDM for commingled fluids. A higher back pressure results in a lower output of torque, mechanical power and pressure drop across the motor for constant of N2 and water rates.

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