Development of a New High-Reliability Downhole Pumping System for Large Horsepowers
Authors: Ryall M.L. , Grant A.A. 
Published: December 2007

Summary
This paper describes the design concepts incorporated in an entirely new patented hydraulically powered downhole pumping system that has been the basis for a major development program undertaken by Weir Pumps Ltd. with the support of two large oil companies and the U.K. Dept. of Energy. This system is shown to be readily adaptable to high-power artificial lift requirements for both oil and water, and typical installations for several different market applications, both onshore and offshore, are described in detail. The comprehensive laboratory and field trials of the prototype units are described, and it is shown that early expectations of a major improvement in reliability offered by the new system have been met fully by the highly successful field performance.

Background
Recent expansion of secondary oil recovery operations both offshore and onshore has led to the need for higher-horsepower artificial lift systems. A large proportion these systems have incorporated electric submersible pump (ESP) sets downhole, either for pumping oil or, particularly in the Middle East, for lifting water from subterranean aquifers for reinjection. Many of these applications have required considerably larger downhole pump sets than had been used previously, either because the boosted well flow rate is high or because high pumping heads are required. This large-horsepower requirement for downhole motors generally has had to be met within the diameter limitations of well casings-often 9 5/8-in. (24.5-cm) OD or less--and this has resulted in the evolution of very long submersible motors of tandem or triple tandem construction, operating in series, with high electric supply voltages. These motors, when coupled to correspondingly long seal and pump sections, have resulted in large-horsepower ESP's that are typically more than 98 ft. (30 m) long [in some cases more than 130 ft (40 m) long], very slender and flexible, and difficult to manufacture and install. Therefore, problems in operating these units could have problems in operating these units could have been expected, particularly since the environmental conditions for these high-horsepower units frequently are very adverse. In the North Sea, experience with large-horsepower ESP's so far has been fairly limited, but a typical experience of short operating life and unreliability has been reported. The report identifies the following offshore problem areas.1. The difficulty in installing the long units in deviated wells frequently results in damage to motors, potheads, and cables.2. Ingress of saline well fluid into cable connectors causes cable failure to earth.3. Ingress of well fluid into the motor through the sealing section causes motor insulation failure.4. Motor failure results from overheating because of high currents and poor cooling at startup.5. Sand erodes pump components.6. Wear rates on thrust bearings are high, particularly at flows away from the duty rating. particularly at flows away from the duty rating. 7. There are high costs associated with onsite assembly and transporting unwieldy proportions of the pump sets.
JPT
p. 1709