Predicting Rotor-Stator Fit in Positive Displacement Motors PDMs

Abstract One of the most challenging aspects of using positive displacement motors (PDMs) is understanding the influence of each variable on the motor functioning and failure modes. Rotor-stator fit is the most critical factor in achieving maximum service life and performance. The objective of this paper is to present a new method to predict the rotor-stator fit that accounts for all main downhole variables. This new methodology will enable the selection of a rotor-stator combination that delivers a balance between performance, efficiency, and service life. Fit selection is complex and should be made based on physics models, testing data, field data, and experience. However, downhole temperature is commonly the only variable used to calculate thermal expansion of the elastomer and the resulting rotor-stator fit. Effects of environmental conditions and operating parameters are normally neglected. This paper presents a calculation method for predicting rotor-stator fit in PDMs that includes environmental conditions and operating parameters. A mathematical model is proposed to account for additional expansion caused by internal heat buildup (hysteresis). Additionally, a time-sensitive equation is used to estimate elastomer volume changes resulting from the interaction between the fluid and the elastomer. In conjunction with the equations developed, collected rotor and stator wear data captures the effect of rotor coatings, drilling fluid, solids content, and elastomer mechanical properties. An additional equation is also utilized to estimate the change of elastomer volume caused by the system hydraulics (downhole pressure and flow rate). Final fit prediction is obtained by combining all these effects. Normalized fit ranges can then be correlated with expected performance and risk of failure. After testing and validation, results from the new method have proven to be accurate in applications with both low and high downhole circulating temperatures and for different elastomer types, drilling fluids, and varying operational conditions. A real case is presented. Unlike any current temperature-based prediction method, this new model integrates the effects of the most important downhole variables to accurately predict the rotor-stator fit. As a result, uncertainty in operations decrease; risk of failure is drastically reduced; and the reliability, performance, and service life of positive displacement motors are improved.