Dynamic Behavior of the Polished Rod for the Coalbed Methane Pumping Installations

Abstract

A modern methodology has been proposed for the system design of dynamic behavior in order to provide the accurate pertinent analysis of CoalBed Methane (CBM) pumping installations. Dynamic hanging loads on polished rod can fully reflect the kinematic and dynamic characteristics of CBM pumping installations due to their combined action of the inertial hanging loads with the loads generated by vibration and hydrodynamic friction phenomena. The practical dynamometer cards were determined by computing dynamic hanging loads with the increments of stroke length. The interpretations of results show that the maximum errors of within 2% between the calculated and measured values are obtained with the comparison of calculated hanging loads by American Petroleum Institute (API), Schafer, Gibbs and proposed method. The ratios of inertial and vibration to polished-rod load are relatively high in producing CBM wells and calculated to be 10% and 8%, respectively. And the inertial and vibration hanging load ratios decrease rapidly for two-phase CBM wells. Moreover, the effect of friction loadings on polished rod is more obvious than that of inertial and vibration hanging loads for single-phase gas CBM wells. The total deformations of rod string and tubing are within 15% of polished-rod stroke length in producing CBM wells. And the dynamic loadings enhance the imbalance of hanging loads and improve the power consumption of CBM pumping installations. The total hanging loads on polished rod are variable in a large scale for the pumping prophase and single-phase water CBM wells and the variation range of dynamic hanging loads is much lower for the two-phase and single-phase gas CBM wells.