The ESP submersible pumping system consists of both downhole and surface components. The surface components are transformers, motor controllers, junction box and wellhead.
The wellhead accommodates the passage of the power cable from the surface to the wellbore.
The main down-hole components are the motor, seal, pump, and cable. Additional accessory equipment may include the gas separators, check and drain valves, cable bands and protectors, and downhole sensors.
Technologies, types, recommended practices and selection criteria of each compound of the ESP pumping system are discussed in the following list of 22 posts.
The Motor Lead Extension is a “special power cable extending from the pothead on the motor to above the end of the pump where it connects with the power cable (API RP 11S4).
A low-profile cable (flat configuration) is usually needed in this area due to limited clearance between the pump housing and the well casing”.
It is recommended to select a length at least 6 ft. (1.8 m) longer than the upper end of the pump. The length of MLE has to be select in a way to avoid a splice over a tubing collar. Doing so could allow the cable to catch on the wellbore casing and damage the equipment.
The motor lead extension operates under extremely adverse conditions. This is due to the restricted size, the high mechanical stresses, and the high temperatures involved. Because of these effects, motor lead extensions are usually replaced every time a cable is reused.
ESP compounds have different sizes and can be assembled in a variety of combinations. These combinations must be carefully determined to operate the ESP with production requirement, downhole conditions, material strength and temperature limits, etc. to select the optimum size of compounds.
To determine the required number of stages of the pump to produce the anticipated capacity; just divide the Total Dynamic Head (TDH) by the Head developed by Stage.
The Head developed per stage is deducted from the published performance curve which shows the discharge head developed by the pump. It is an experimental curve given by the manufacturer and obtained with fresh water at 60 F under controlled conditions detailed in API R11 S2. Refer to the articles “Pump Performance Curves – part 01” and “Pump Performance Curves – part 02” for more details.
Once calculated, divide the TDH by the Head developed per stage to get the Total Number of Stages required to produce the anticipated capacity.
The electric submersible motor is simple in construction, rugged and reliable. In this article, the ESP motor compounds are detailed as well as their main functions.
Introduction to ESP motor:
ESP motor is installed below the motor seal and above the downhole sensor. In cases where a downhole sensor is not installed, the motor is installed at the very bottom of ESP string, generally attached to a motor guide.
ESP motor is an induction motor, two poles, three phases, squirrel cage type stator winding filled with specific motor oil, high dielectric strength (> 28 KV). The motor is rated for a specific horsepower, voltage, & current. Its role is to drive the downhole pump and seal section.
The ESP motor rotates at approximately 3500 RPM at 60 Hertz. The difference between actual and the synchronous speed (3,600 RPM) is called “motor slippage” and it is due to losses inside the motor. The actual RPM is usually noted on the motor nameplate (example: 3500 RPM / 60 Hz – 2917 RPM / 50 Hz).
The ESP motor is constructed of rotors and bearings stacked on the shaft and loaded in a wound stator, the motor compounds will be detailed in the next section.
The motor contains synthetic dielectric mineral oil for lubrication, insulation, and for the homogeneous distribution of the heat generated inside the motor (cooling). Heat is then drawn off by the produced fluid past the housing OD on the way to the intake.
NB: ESP motor is close to the same design type as motors used on beam pumping units. Of course, it must be small in diameter in order to fit inside oil well casing sizes.