A Rod Pumping System To Reduce Lifting Costs
Authors: Tait Howard C. , Hamilton Robert M. 
Published: December 2007

Rising costs of artificial lift operations are a continuing concern to oil producers in maintaining efficient and profitable performance. A new long-stroke sucker rod profitable performance. A new long-stroke sucker rod pumping system has been developed to minimize the impact pumping system has been developed to minimize the impact of these rising costs.

This new system results from a broad development project based on system design considerations having major project based on system design considerations having major influence on overall cost effectiveness of the system. The basis of these evaluations is discussed, resulting equipment design is described, and field operating results are presented. Performance results to date confirm presented. Performance results to date confirm achievement of reduced overall operating costs as a result of the performance characteristics of this system. performance characteristics of this system. Introduction

The cost effectiveness of the artificial lift system ultimately becomes a primary consideration in determining the economic life of a producing oil well. Higher crude oil prices have offered incentive to extend the pumping life, prices have offered incentive to extend the pumping life, but unfortunately, operating costs also have risen, diluting this incentive.

To improve cost effectiveness of the lift system, a development program was initiated more than 4 years ago to restudy the sucker rod pumping method of artificial lift from a system standpoint. The broad objective was to develop a rod lift system design capable of improved overall performance that would result in lower cost for pumping the average oil well. The measure of this pumping the average oil well. The measure of this performance would be in terms of lower cost per barrel of performance would be in terms of lower cost per barrel of fluid lifted.

The product of this development program is identified as the Natl. Supply Co.'s LIFTRONIC TM pumping system. Installations are now in operation on producing oil wells, and are proving to offer measurable reduction in overall lifting costs. A typical LIFTRONIC installation is shown in Fig. 1.

System Design Concepts

In planning this development project, three major areas for design consideration was identified: well system efficiency, mechanical equipment design, and operating control of the system. Each of these was considered from the standpoint of impact on cost effectiveness of the system.

Well System Efficiency. A convenient measure of well system efficiency for a rod lift system can be made by developing the calculated polished rod horsepower (PRHP) required for the well conditions and equipment selected. API RP11L presents a widely used calculation method for developing this and other operating data for the conventional beam pumping system. API Bull. 11L3 tabulates these calculated operating parameters for a wide variety of equipment combinations, depths, and producing rates, providing a helpful reference for evaluating producing rates, providing a helpful reference for evaluating comparative well system performance for various equipment combinations.

The complex dynamics of the rod pumping system require that specific analysis be made for the various equipment options to identify the best choice for the required lift conditions. However, these API publications may be used for general evaluation of the various design parameters. parameters. API 11L3 tabulations show for a given depth, producing rate, and rod string, as pump size is increased, producing rate, and rod string, as pump size is increased, the PRHP decreases. Further, for a given pump size within these listings, an increased stroke length and lower pumping speed (strokes/minute [SPM]) typically produces a pumping speed (strokes/minute [SPM]) typically produces a lower PRHP, lower peak polished rod load (PPRL), and a higher minimum polished rod load (MPRL). These results reflect the effect of two nondimensional factors as defined and used in API RPIIL as follows.

Pumping speed factor, N/No', decreases as SPM is reduced and rod stretch factor, Fo/Skr, decreases as stroke length increases.

These factors in turn influence additional nondimensional calculation factors as follows.

PPRL factor, F1/Skr, MPRL factor, F2/Skr, and PRHP factor, F3/Skr, all decrease as N/No' and Fo/Skr PRHP factor, F3/Skr, all decrease as N/No' and Fo/Skr decrease.

From the trends of these calculation factors, and their use in the API formulas, a general conclusion can be drawn that a longer stroke, operating at a lower pumping speed, tends to reduce PRHP, peak load, and load range. Graphic evidence of this can be seen in API Bull. 11L2, which catalogs analog computer dynamometer cards for varying N/No' and Fo/Skr values.

In addition to these beneficial effects on power requirements and rod stress, the lower pumping speed also contributes to improved service life of valves in the downhole pump by reducing the frequency of valve closures. The displacement efficiency of the pump may also be improved because of the higher compression ratio generated by the longer stroke.

The conclusions drawn from this and other analyses led to the selection of a long stroke pumping action, operating at a low pumping speed, as a means of gaining improved well system efficiency.

JPT

P. 1971