Mass Implementation of Ultrahigh-Speed ESP Systems Increases Reliability, Savings
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This paper presents the results of a 3-year project aimed at mass field implementation of ultrahigh-speed (UHS) electric submersible pump (ESP) systems in western Siberia. The project had a successful outcome, with more than 200 installations performed. The project was aimed at increasing the efficiency and safety of oil and gas production and reduction of total cost of ownership (TCO).
The authors discuss the project as an endeavor of a joint venture developing the Salym group of oil fields. Since the beginning of asset development in 2003, ESP technology has been used as the primary artificial-lift method. Nearly 750 ESP systems are used at the time of writing, provided by three vendors.
Field Conditions. Salym reservoirs are found at 2100 to 2500 m true vertical depth (TVD). Downhole temperatures range between 80 and 100°C. Each of the Salym fields is typical for the region, experiencing gradual water-cut increase and flow-assurance challenges such as downhole inorganic scale deposition and various corrosion processes. Frequent debris from the unconsolidated near-wellbore zone and low inflows are recognized within the region historically as the greatest challenges facing ESP systems.
Technology Trials. Challenging production conditions and increasing operating expense led to the development of a technology study in 2014 focused on researching, testing, and evaluating the latest ESP technologies that could help reduce TCO. UHS ESP was selected among others for no-cure, no-pay performance evaluation in 2014 and 2015. Field-trial results were found to be much better than target-success criteria, specifically in terms of power consumption; UHS ESP systems consumed 40% less on average compared with standard (STD) ESP systems of an equivalent flow and head.
According to TCO analysis, UHS ESP technology lowered overall ESP-related costs by 30% (through power efficiency, reduced maintenance, and workover optimization and reduction of nonproductive time).
Project Work Flow, 2016–2018. Because of the compact design, wide operating range, and minimum on-site preparation requirements of UHS ESP systems, monthly delivery and return of equipment between the field base and the central repair facility was possible without a need to construct a field workshop. UHS ESP manufacturing and repairs were centralized in Kirov, 1750 km southwest of the Salym fields. Monthly delivery with a single 12-m-long truck can accommodate up to 30 downhole strings, fully assembled, full-string tested, and ready to run in hole. Dismantling, inspection, and failure analysis (DIFA) was scheduled on a monthly basis and corrective actions were finalized within 10 days of receipt of pulled equipment in the repair center.
UHS ESP Technology
UHS ESP systems can operate at speeds between 1,000 and 12,000 rev/min. Because of its high operating speed (10,000 rev/min), the total length of the UHS ESP string in this study is significantly shorter than lengths of conventional ESP systems. The compact design makes it possible to assemble and test the full downhole string at the manufacturing facility. The UHS ESP installation routine incorporates preparing the motor lead extension and attaching it to the motor. As a result, the system requires minimum installation time and labor costs.
Special high-speed-oriented stages are distributed between several pump modules. Pump-component materials were influenced by aerospace technology. Advanced metallurgy using hard alloys, bimetals, and trimetals has made the pump tolerant to highly abrasive fluids, with a minimal wear rate.
The UHS ESP uses a synchronous motor with permanent magnets imbedded into a six-pole rotor. The system comprises a pump impeller serving to allocate generated heat through motor-oil circulation between the rotor, a seal with an increased oil capacity, and a built-in heat exchanger providing additional area for heat transfer. As a result of these design advances, lower heat rise is observed during operation, and high efficiency is achieved over a wide range of motor loads.
Application Design Considerations
Although three types of UHS ESP systems are used at the Salym fields, a standardized LX600 UHS ESP system was designed to ensure coverage of flow ranges and minimum inventory needs of the most problematic wells. Fig. 1 shows the layout of the system. The power-efficient system was designed for a nameplate flow range of 45–115 m3/d with a best efficiency point of 80 m3/d.
A few trials were conducted at the end of 2014, with several more in 2015 (three phases, 10 systems per batch); subsequent monthly supplies in 2016 and 2017 resulted in a total number of installations exceeding 200 by the last quarter of 2018. UHS ESPs can be operated continuously rather than intermittently over a corresponding number of STD ESPs within the studied flow-rate segment of the Salym fields. A flow-rate-based ESP segment analysis is discussed in the complete paper.
Run-Life Statistics and Failure Analysis
Shortly after successful trials, three failures were discovered to have common rotor-bearing failure modes (wear). Upon the completion of an immediate DIFA program, all stocked equipment was recalled to the manufacturing facility for retrofitting. Because some systems already had been installed, operations personnel could observe similar failures in a range of 200 to 300 days from start. A second major discovery, bearing sleeve spin, was observed in units that ran for over 1 year. A third issue was related to pump and seal-fatigue effects on shaft and stage bearings. Designs were changed accordingly, providing good potential to further extend installation success, with 2 years’ target run life set as a minimum.
For the purposes of run-life comparison, 10 wells were analyzed in detail with regard to their historical operations cycle. Each of these wells had between two and nine installations of STD ESPs before conversion to UHS ESP. Run-life increment with UHS ESP systems ranges from 37 to 640 days, with an average of 396 days. This indicates significant benefits of conversion from STD to UHS ESP systems, especially considering that UHS ESP data are for still-running equipment, while STD ESP data reflect historical maximums.
UHS ESP systems offer improved reliability for reasons including
- Fewer parts at risk of failure because of compact equipment design
- Complete factory acceptance test for every string, with reduced risk of human error and environmental impact
The experience of UHS ESP implementation in the Salym fields reflects increased efficiency and reliability with reduced TCO.
Testing, Research, and Development
Solids. Development of an abrasion-resistant pump design at ultrahigh operational speeds is a result of a research and development project based on achievements of aerospace-materials science. Application of modern metrology, physical control of materials, and a number of other newly developed techniques has made the UHS pump practically tolerant to sandy production. In the case described in the complete paper, extreme solid content was observed during the ramping-up period. Maximum sand production exceeded the rated value, but reduced rampup time was achieved.
Without being affected by highly abrasive fluids, UHS ESP run life hit 1,270 days, operating in intermittent mode, with over 15,000 running cycles being performed for the whole period of operation.
Viscosity. Viscosity effects are considered to have greater effect on pump performance at high speeds. The UHS ESP performance was verified with a large number of experimental data in a wide range of viscosities using non-Newtonian emulsions prepared by various types of oil and water. Results so far indicated normal degradation rate of UHS and STD ESP systems.
Gas. Simulation of a variety of well conditions along with a deep study of pump performance has been achieved through the development of a unique experimental testing bench, allowing for fine simulation of well fluids using surfactant-based mixtures with target free-gas content and gas-bubble size. With high speed contributing to better dispersion, research observations have proved similar or even better performance of UHS centrifugal pump stages when pumping gassy fluids vs. equivalent standard-speed stages.
Mass Implementation of Ultrahigh-Speed ESP Systems Increases Reliability, Savings
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27 March 2020