Paper: Effects of Free Gas and Downhole Separation Efficiency on the Volumetric Efficiency of Sucker Rod Pumps and Progressing Cavity Pumps.

The prediction of the volumetric efficiency of the pump is one of the most important factors in the analysis of sucker rod and progressing cavity pumping systems. In most cases, the reduction of production due to gas interference in the pump exceeds by far the effect of leakage in the pump components. In this paper, the equations to determine the effect of free gas on the volumetric efficiency of a sucker rod pump and a progressing cavity pump, as a function of the downhole separation efficiency, the pump intake conditions and the fluid properties, are presented. Computer programs were written to simulate the hydrodynamic behavior of a sucker rod pump and of an idealized progressing cavity pump. The sucker rod pump was simulated for harmonic motion and for constant velocity motion. A comparison between the three cases is presented. Curves for sensitivity of the sucker rod pump volumetric efficiency to the compression ratio and clearance ratio are also presented. Finally, a much more simplified model based on constant flow rate is presented for the case of sucker rod pump. The results obtained with this model are in good agreement with those corresponding to the simulation based on harmonic motion.