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The Anatomy of an ESP System

In ESP systems, an electric motor and a multistage centrifugal pump run on a production string, connected back to a surface control mechanism and transformer via an electric power cable (see Figure 1 below). Careful consideration must be given to each downhole and surface component of the system in the design stage. An ESP can pump intermittently or continuously. Because an ESP can be easily adapted to automation and control systems, numerous surface control and communication devices are available. Additionally, the downhole components can vary depending on the specific application or conditions.

Downhole Components

Due to limited well casing diameters, the lift or head developed by an individual stage is relatively low. Stages must be stacked together to meet the lift requirements for various applications. Each stage of the multistage centrifugal pump adds energy to the fluid in the form of increased velocity and pressure. The impeller accelerates the fluid and increases the kinetic energy, which is then converted into potential energy (pressure) in the diffuser that redirects the flow to the next impeller. Diffusers also act as a bearing surface, providing additional stability to the pump shaft. The fluid flow is described by the fundamentals of classical physics—conservation of mass, momentum and energy.

Motor—The energy to turn the pump comes from a high voltage (3 to 5 kV) alternating current source to drive a special motor that can work at high temperatures up to 500-deg F and high pressures up to 5000-psi and from deep wells up to 15,000-ft deep with high energy requirements up to 1000-hp. Submersible two-pole, squirrel cage, induction electric motors are manufactured in a variety of horsepower ratings, operating voltages and currents to meet pressure extremes and temperature requirements. The motor size is designed to lift the estimated volume of production. Wellbore fluids passing over the motor housing act as cooling agents. The motor is powered from the surface via submersible electric cable.

Seal Section—The seal section is located between the motor and the intake and performs the following functions:

• Houses the thrust bearing that carries the axial thrust developed by the pump
• Isolates and protects the motor from well fluids
• Equalizes the pressure in the wellbore with the pressure inside the motor
• Compensates for the expansion and contraction of motor oil due to internal temperature changes  

Power Cable—Available in flat or round configurations, specially engineered and manufactured cable systems provide dependability in the harsh, hot, gassy and corrosive conditions found in most downhole ESP applications. A variety of materials, duty ranges and constructions allow selection of a particular cable for specific applications. The cable is connected to the top of the motor, runs up the side of the pump, is strapped to the outside of every joint of tubing from the motor to the surface of the well and is extended on the surface to the control junction box. In most cases, the cable is flat as it stretches from the motor up beside the pump to the tubing, at which point the flat cable is spliced to a round one.

Surface Components

Fixed or Variable Speed Controllers and Drives—Intelligent RTU programmable controllers (fixed speed or variable speed) maintain the proper flow of electricity to the pump motor. They allow the well to be operated continuously or intermittently, or be shut off. They also provide protection from power surges or other electricity changes.

A variable speed drive (VSD) offers ESP systems continuous duty variable flow and pressure control, which in turn increase productivity, process control flexibility and energy savings. Direct speed control over the pump motor provides maximum system efficiency and reduced maintenance when compared with across the line (full voltage) operation. The VSD provides the essential reduced voltage starting characteristics of a soft starter combined with continuous duty variable frequency operation. This directly results in increased life of the mechanical equipment and reduced incidence of downtime.

Electrical Supply System—Electricity is generally provided by a commercial power distribution system. The highest available voltage produces the most efficient performance. In offshore applications, the nature of the power supply is strictly dependant on a portable source—namely that of a diesel generator. In a situation where generator-fed power is the primary supply, strict design requirements must be recognized in order to prevent a costly, time-consuming failure and/or redesign and retrofit. Selection of the generator requires careful calculation of the system power requirements it is meant to supply.