Artificial Lift Optimization: A Case Study of Ultra Long Stroke Pumping Systems Performance, Enhanced with Modified NEMA C Motors
Authors: Dollens D.L. , Becker R.L. , Adair R.L. 
Published: December 2007

It is widely accepted that a long, slow stroke pumping unit, utilizing the largest bore pump permitted under loading constraints of the selected equipment, is the preferred lift system of many artificial lift technicians. An evolutionary development in artificial lift equipment technology has occurred in the last decade, allowing operators access to this long sought-after pumping system. This paper will review the goals of the new technology, and examine actual field results utilizing an ultra long stroke pumping system (ULSPS). For the purpose of this discussion, an ultra long stroke unit is defined as being capable of delivering twenty-four feet of polish rod travel and having a configuration not requiring extra large gearboxes.

Data collected over a three-year period on individually monitored wells has provided strong evidence to support the rationalization regarding long, slow stroke units. The original pilot test has been expanded three times to include a total of thirteen wells. Electrical costs per barrel and failure frequency will be compared to electrical submersible pump systems.

Additionally, information obtained while attempting to further optimize the "system" will be reported. Another evolutionary product, in the form of a modified NEMA C motor, was installed on two of the test wells with the goal of further reducing KWh cost per barrel (one of the highest controllable expenses in artificial lift).

Introduction

The wells selected for testing the use of an ultra long stroke pumping system are located in the East Vacuum Grayburg San Andres Unit (EVGSAU), in the eastern side of the Vacuum Field, approximately fifteen miles northwest of Hobbs, New Mexico. The Vacuum Field was discovered in 1929 by Socony Vacuum Oil Company's Bridges State Well No. 1. Primary development of the field began ten years later and was completed by 1941 with the drilling of 330 producing wells. Waterflood development in the Vacuum area began in 1958 and gradually spread across the field. EVGSAU was one of the last areas in the Vacuum Field to be unitized (December, 1978). The initial waterflood was developed on an 80-acre inverted nine-spot pattern and commenced in early 1979 with oil production reaching its peak in 1983. Tertiary CO2 injection commenced at EVGSAU in September of 1985.

The EVGSAU produces from the Grayburg and San Andres intervals, at an average depth of 4400 feet, reservoir temperature of 101 degrees F, and average reservoir pressure of 2000 psig. The oil gravity is 38 degrees API and the original solution gas/oil ratio was 465 SCF/STB. The average net pay interval is 71 feet. Corrosion is considered moderate to heavy due to high concentrations of H2S. Of the 209 wells currently in operation, 140 are equipped with beam pumping units, 49 produce with downhole submersible pumps, 13 utilize a ULSPS and seven are naturally flowing.

Description of Lift Systems

Under primary recovery, beam pumping units were sufficient for keeping the wells pumped-off and efficiently producing. However, as the waterflood response began, many wells were switched to electrical submersible pumps (ESP) to take advantage of higher production potential and to lift increased water volumes.

A previous (1987) study of EVGSAU indicated that wells with producing capacities of less than 500 BFPD were more economically lifted with conventional beam pumping units.

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