Design Tapered Electric Submersible Pumps For Gassy Wells

A tapered electric submersible pump (ESP) is mainly used to pump wells with high gas oil ratio. Free gas is separated and vented via a shroud or gas separator. Or, it is compressed using a tapered larger-than-normal pump or specially-designed gas handler below the “normal” pump. Although tapered ESP has been used for decades in petroleum production, few articles have discussed its design. After studying the pressures and flow rates stage by stage using a computer program, the paper presents basic criterion to design a tapered pump. Free gas in pumped fluid stream reduces pump performance, and may cause surging and gas lock. For tapered pumps the free gas effect becomes vital since generally tapered pumps handle considerable amount of free gas. The paper discusses traditional homogeneous model and multiphase pumping model. By comparing pump performances of the two models using examples, the paper presents that the traditional model designs fewer stages and will produce smaller rate than desired rate. Further, without considering free gas effect, the pump above bottom pump may work out of its operating range. For a tapered pump, pumping stability should be checked and pump degradation should be included in stage by stage calculation. Also, fluid flow pattern should be checked to avoid slug flow at the place of pump intake. Also presented are optimal design methods for both single and tapered pumps. Widely used design methods are using desired liquid rate at surface or the liquid rate at pump intake to select a pump with closest best efficiency point. The paper illustrates by examples that the two liquid rate methods fail to design high efficiency when pumping high gas/liquid fluid, and proposes two methods of using total rate at pump discharge and using average total rate. The two design methods will improve a well’s pumping efficiency and running life. Introduction Electric submersible pump (ESP) is a widely used artificial lift method especially for producing large volume fluid. The base to design an ESP is its standard pump curve (or called standard catalog graph) provided by ESP manufacturers. As shown in Fig. 1, the standard pump curve generally consists of three performance curves, pump head performance, brake horsepower performance and efficiency performance. Also, marked on the graph is the pump’s operating range. The pump should be operated in the recommended range to avoid low efficiency and mechanical damage. The performances are measured at laboratory condition with pure water at standard frequency (60 Hz or 50 Hz). Another important parameter for selecting an ESP is its best efficiency point (BEP) as marked in Fig. 1. It is sometimes called design point and, in most cases, it is the rate at which the efficiency is the highest. Therefore, a widely used method in an ESP design is to find an ESP with a BEP closest to desired production liquid rate at pump intake condition. The BEP divides the operating range into lower and upper ranges. The low end of the operating range is generally set by maximum downward thrust force with reasonable pump life, and the high end is set by a value which is a little bit less than neutral thrust point. When a pump works beyond its high end, the pump may generate an upward thrust force that is harmful to pump life. On the other hand, once pumping fluid goes below the pump’s low end, the thrust bearing of the pump may suffer excessive wearing. Although a pump’s standard catalog graph is a major reference for pump design, one may not use it directly in oil wells. Pump manufacturers generate the graph at standard temperature and frequency (60 Hz or 50 Hz) with pure water. At wellbore condition, fluid may not be pure water and its viscosity may affect the pump’s performance. When pumping viscous fluid, pump brake horsepower will increase, and its capacity, head and efficiency will decrease. Therefore, the standard catalog graph should be corrected for viscosity before used in well condition. One may also correct viscous fluid to equivalent pure water and use the standard catalog graph. The correcting correlations for viscosity are