Electrical Submersible Pump

component of ESP 

a. Subsurface components

- Pump - Motor

- Seal electric cable

- Gas separator

b. Surface components

- Motor controller (or variable speed controller)

- Transformer

- Surface electric cable

HOW ESP WORK :- 

The overall ESP system operates like any electric pump commonly used in other industrial applications. In ESP operations, electric energy is transported to the down-hole electric motor via the electric cables. These electric cables are run on the side of (and are attached to) the production tubing. The electric cable provides the electrical energy needed to actuate the down-hole electric motor. The electric motor drives the pump and the pump imparts energy to the fluid in the form of hydraulic power, which lifts the fluid to the surface.

Pump 

ESPs are pumps made of dynamic pump stages or centrifugal pump stages. Figure A gives the internal schematic

of a single-stage centrifugal pump. Figure B shows a cutaway of a multistage centrifugal pump.                

The electric motor connects directly to the centrifugal pump module in an ESP. This means that the electric motor shaft connects directly to the pump shaft. Thus, the pump rotates at the same speed as the electric motor.

Like most down-hole tools in the oil field, ESPs are classified by their outside diameter (from 3.5 to 10.0 in.). The number of stages to be used in a particular outside diameter sized pump is determined by the volumetric flow rate and the lift (height) required. Thus, the length of a pump module can be 40–344 in. in length. 

ESP motors :- 

Types of electric motors are

1. three-phase (AC)
2. squirrel cage
3. induction type

 They can vary from 10 to 750 hp at 60 Hz or 50 Hz (and range from33⁄4 to 71⁄4 in. in diameter). Their voltage requirements vary from 420–4,200 V.

The seal system (the protector) :-

separates the well fluids from the electric motor lubrication fluids and the electrical wiring.

The electric controller (surface) :-

The electric controller (surface) serves to energize the ESP, sensing such conditions as overload, well pump off, short in cable, and so on. It also shuts down or starts up in response to down-hole pressure switches, tank levels, or remote commands.

These controllers are available 

1. conventional electromechanical or solid-state devices

2. the variable speed controller 

• Conventional electromechanical controllers give a fixed speed, fixed flow rate pumping. To overcome this limitation. 

• the variable speed controller has been developed(solid state). These controllers allow the frequency of the electric current to vary. This results in a variation in speed (rpm) and, thus, flow rate. Such a device allows changes to be made (on the fly) whenever a well changes volume(static level), pressure, GLR, or WOR. It also allows flexibility for operations in wells where the PI is not well known.

The transformer

The transformer (at surface) changes the voltage of the distribution system to a voltage required by the ESP system.

ESP LIMITATION 

ESPs can operate over a wide range of parameters (depths and volumes), to depths over 12,000 ft and volumetric flow rates of up to 45,000 bbl/day. Certain operating variables can severely limit ESP applications, including the following:

. Free gas in oil :- Excessive free gas results in pump cavitation that leads to motor fluctuations that ultimately reduces run
life and reliability. 
. Temperature at depth :-  High temperature at depth will limit the life of the thrust bearing, the epoxy encapsulations (of electronics, etc.), insulation, and elastomers.
. Viscosity of oil :-  
Increased viscosity of the fluid to be pumped reduces the total head that the pump system can generate, which leads to an increased number of pump stages and increased horsepower requirements. 
. Sand content fluid
. Paraffin content of fluid :-  Sand and paraffin content in the fluid will lead to wear and choking conditions inside the pump.

ESP Applications

The following factors are important in designing ESP applications:
. PI of the well
. Casing and tubing sizes
. Static liquid level
ESPs are usually for high PI wells. More and more ESP applications are found in offshore wells. The outside diameter of the ESP down-hole equipment is determined by the inside diameter (ID) of the borehole. There must be clearance around the outside of the pump down-hole equipment to allow the free flow of oil/water to the pump intake. The desired flow rate and tubing size will determine the total dynamic head (TDH) requirements for the ESP system. The ‘‘TDH’’ is defined as the pressure head immediately above the pump (in the tubing). This is converted to feet of head (or meters of head). This TDH is usually given in water equivalent. Thus, TDH ¼ static column of fluid (net) head + friction loss head + backpressure head.

Advantages

ESPs provide a number of advantages.

• Easy to install and operate, they can lift extremely high volumes from highly productive oil reservoirs

• Can be use in highly deviated/crooked holes.
•  ESPs are applicable to offshore operations. Lifting costs for high volumes are generally very low. 
• Adaptable to required subsurface wellheads 6 ft apart for maximum surface-location density.
• Permit use of minimum space for subsurface controls and associated production facilities.
• Quiet, safe, and sanitary for acceptable operations in an offshore and environmentally conscious area.
• Generally considered a high-volume pump.
• Provides for increased volumes and water cuts brought on by pressure maintenance and secondary recovery operations.
• Permits placing wells on production even while drilling and working over wells in immediate vicinity.
• Applicable in a range of harsh environments.

Disadvantages

• ESPs have some disadvantages that must be considered.electricity availability, not applicable to multiple completions, not suitable to deep and high-temperature oil reservoirs, gas and solids production is troublesome, and costly to install and repair.
•  ESP systems have higher horsepower, operate in hotter applications, are used in dual installations and as spare down-hole units, and include down-hole oil/water separation. 
• Sand and gas problems have led to new products. Automation of the systems includes monitoring, analysis, and control.
• Will tolerate only minimal percentages of solids (sand) production, although special pumps with hardened surfaces and bearings exist to minimize wear and increase run life.
• Costly pulling operations and lost production occur when correcting downhole failures, especially in an offshore environment.
• Below approximately 400 B/D, power efficiency drops sharply; ESPs are not particularly adaptable to rates below 150 B/D.
• Need relatively large (greater than 4½-in. outside diameter) casing size for the moderate- to high-production-rate equipment.

• Long life of ESP equipment is required to keep production economical.