Performance of positive displacement downhole motors under two-phase flow

Positive displacement motors have been used extensively in underbalanced dr illing operations. Motor performance characteristics are essential to ensur e drilling success and efficiency. Pressure drop across the motor must be u nderstood to accurately simulate underbalanced conditions bottom-hole. In t his study, five positive displacement motors were tested under a variety of mixing ratios of nitrogen and water with different back pressures. Compari ng with the baseline test at maximum liquid flow rate, the replacement of w ater with nitrogen decreased the motor performance by as much as 95% in ter ms of maximum torque output and maximum mechanical power output. The pressu re drop across the motor was lower with commingled fluid, and the greater t he nitrogen ratio, the lower the pressure drop across the motor. It was als o found that back pressure decreased the motor performance. This paper also discusses the testing procedures, the results, and how commingled fluid af fects underbalanced drilling operations.