Development of Prime Mover Loading on the Sucker Rod Lift During Coalbed Methane Production

Abstract

A modern methodology has been proposed for the system design of prime mover loading in order to reduce the motor size and improve the efficiency of pumping system. Polished rod loading, plunger stroke length, gearbox torque loading and motor power are chosen as the basic design variables to implement the process of design calculation in producing coalbed methane (CBM) wells. Based on the kinematic and dynamic analysis of sucker rod pumping mechanism, the kinematic relations of polished rod loading are developed with the limits of specific CBM well conditions along rod string. The gearbox torque is determined by combining the counterbalance effect with the calculated dynamometer cards and torque factors. Then the design of prime mover is computed by combining the pumping speed with motor and drive efficiencies. The application characteristics of this method are demonstrated by the example of producing CBM wells in Ordos Basin. The interpretations of results show that compared with oil/gas fields, the load ratios of dynamic and friction to the total loads are relatively high and the computation of polished rod loads should involve the static loads as well as the dynamic and friction loads. The net gearbox torque is cyclic loading for the balanced operation. And the extreme value of net gearbox torque occurring on the upstroke is approximately equal to that on the downstroke. The maximum value of net gearbox torque is less than 50% of the peak value of unbalanced gearbox loadings. Moreover, the variation of instantaneous motor power versus crank angle is consistent with that of gearbox torque versus crank angle. The negative motor power results from the combination of the various moving components which drive the motor, exceed its synchronous speed and make the prime mover operate in a “regenerative power” mode. The value range of negative motor power is much smaller than that of unbalanced gearbox loadings. The result of this work is a mathematical model and a complex computer program used for the design calculation of prime mover loading in producing CBM wells. This makes it possible to maintain the gearbox torque in balance, load the prime mover more uniformly and provide a reasonable basis for the selection of pumping units.