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  • Electric submersible pump
  • Introduction to electrical submersible pump system for oil fields
    At the surface, transformers change the voltage to a value equal to the motor nameplate voltage plus the cable voltage loss. The switchboard is used to turn the unit on or off; to monitor voltage, current, starts, and stops; and to protect against emergencies. The vent box breaks the cable to prevent well gas from traveling though the cable to the switchboard. The cable is connected at the wellhead to the downhole cable or is packed off against rubbers. The cable is banded to the tubing and usually has some protectors to prevent damage on run-in. A check and drain may be installed above the pump to allow the tubing fluid to flow out before pulling the tubing.
    Electric submersible pump is a stack of centrifugal impellers keyed to the shaft running inside a stack of diffusers. The intake can be a standard intake or a rotary gas separator intake. The seal section (or protector or equalizer) is located below the intake to protect the motor. It carries the thrust of the shaft extending from the pump and allows the fluid in the motor to expand without leaking wellbore fluids to the motor. The seal can be a tortuous-path or a bag-type protector that contains shaft seals to prevent fluids from traveling along the shaft and carries the motor torque to the pump above. The motor is on the bottom of the unit so that it can be cooled by fluids traveling to the intake. A three-phase current is supplied to a two-pole, squirrel-cage motor through a cable connected to the top of the motor. The motor can be a single, a tandem, or even a triple motor, although single motors are preferred if they have enough horsepower for the job. The motor is filled with dielectric oil which cools and insulates the motor. The magnet wire in the motor is insulated; some new insulations have been suggested to insulate the wire even if water gets into the motor. Instrumentation for pressure and temperature is sometimes housed on the bottom end of the motor.
    With the features of simple structure, high efficiency and large displacement, electric submersible pump is especially suitable for the water-injection forced production and easily reaches the automatic control, to become the oil production equipment with stable and high production and good economic benefits.
     
    Introduction to electrical submersible centrifugal pump for oil fields
    A typical submersible pumping unit consists of an electric motor, seal section, intake section, multistage centrifugal pump, electric cable, surface installed switchboard, junction box, and transformer. Additional miscellaneous components of installation will include means of securing the cable alongside the tubing and well head supports. Optional equipment may include a pressure sentry for sensing bottom-hole temperature and pressure, check and bleeder valves.
    In its operating position, the standard down hole equipment is suspended from discharge tubing and submerged in well fluid. The setting depth, or bottom hole pressure, creates no problems as the seal section equalizes internal pressure in the motor with the submergence pressure in the well. Such installations in directional wells are common. The submersible pump is also used in producing high viscosity fluids, gassy wells, and high temperature wells. With additional experience and improved technology, wells that were once considered non-feasible for submersible are now being pumped economically.
    1)  Advantages of electrical submersible centrifugal pump
    ① High volume lift capacity is the main advantage of this type of lift. However, electrical submersible are finding more application in low volume wells.
    ② Although more engineering supervision may be required initially it does not require a lot of knowledge to operate since it either does or does not run.
    ③ The pump can increase the volume in a dump flood from a water zone above to the waterflood zone below.
    2)  Disadvantages of electrical submersible centrifugal pump
    ① Due to horsepower rating of the electric power motor, depth is limited. Depth is also limited by size of tubulars and high temperature. Larger high HP equipment may not provide enough annular clearance to cool the motor resulting in failure. High temperature will also limit both motor and cable.
    ② Inition cost may be high, since multistage high volume and high HP Pumps are expensive. The cable is also a high cost item, especially if non corrosive or high temperature sheathing is required. Transformers must be provided to insure proper voltage.
    ③ Cable failures occur and require pulling the tubing to be repaired. High temperatures, corrosion, and poor handling lead to cable failure. Replacement cost may be excessive on high failure marginal operations.
    ④ Motor failures are also due to high temperature, corrosion, and abrasives. High GOR may result in low efficiency and failure is due to free gas locking the pump.
    Additional engineering supervision is required in design, installation, maintenance and trouble-shooting this type of lift.
     
    REDA456.540 series electric submersible pump
    In the 1990s, Tianjin Electric Motor General Plant has successfully developed the electric submersible pump with sand-prevention, high temperature resistance and high head for Xinjiang Tarim Desert Oilfield in China. With the oil production per day of 300m3,the submersible electric pump has represented the top level of the production technology of electric submersible pumps at that time in China. At present, the annual production capacity of the company reaches 2600 sets and the company can produce the complete set of electric submersible pumps applicable for the barrel diameter of 140mm and 178mm, the temperature from 50 to 150, and the pressure of 105MPa. With the maximum head of 3000m and the maximum displacement of about 500m3, the electric submersible pump meets the technical performance and quality requirements of U.S. DA Company.
    Working conditions of electric submersible pump—REDA456.540 series unit are as follows:
    1.  Adaptive oil-well casing: 140mm, 178mm;
    2.  Well temperature: 50, 90,120, 150;     
    3.  Working medium is oil-gas-water mixer, the PH value is 6.5, and the gas-oil ratio is 60m3/m3, 100m3/m3, 200m3/m3;   
    4.  The sand content is less than 0.05%.
     
    Inversion type pressure boosting and water injection electric submersible pump
    The Inversion type pressure boosting and water injection electric submersible pump is installed outdoors without building the pump house, with the features of no noise, no media loss, not polluting the environment, long running service, the pressure and flow performance reasonably matching up with water injection well. The pump has been applied in six wells of Shengli Oilfield, Liaohe Oilfield with prominent pressure boosting water injection result, so the Inversion type pressure boosting and water injection electric submersible pump is applicable to the pressure boosting and water injection of the remote wells.
     
    Electric submersible pump with guard shield
    The guard shield of electric submersible pump system with guard shield is a kind of device to protect the electric submersible motor for proper operation. 
    Electric submersible pump with guard shield is suitable for the following four services:
    1. In the low-yield heavy oil wells, the electric submersible pump is equipped with the guard shield. Viscosity reducer or water can be injected from the wellhead to reduce the viscosity of the crude oil and to increase the flow velocity through the submersible motor, which meets the hoisting requirement of the multi-stage centrifugal pump, and helps cool the submersible motor and prolong the working life of the full set of pump system.  
    2. In the oil wells with oil and gas ratio, the electric submersible pump system with guard shield is installed at the lower reservoir and the gas can not enter the centrifugal pump, which can prevent the airlock and ensure the continuous running of the electric submersible pump system.  
    3. In the low-yield large-diameter casing wells, the drilling fluid is forced to flow through the annular passage between the shell and the cap of the submersible motor, which can increase the heat dissipating capacity of the submersible motor, prevent the short trouble caused by that the magnetic winding unit on the motor makes the electric insulation be overaging owing to the high temperature, and prolong the working life of the submersible motor.
    4. In the pocket well with single casing and bottom sealing, the electric submersible pump system with guard shield is installed to carry out pressure boosting and water injection for the low-pressure water without leakage.
    Field applications show that the electric submersible pump stack with guide shield matching with the surface water blending can be applied in the low-yield oil wells with the production capacity per day less than 30m3 and the viscosity of the crude oil up to 1000mPa·s. When the electric submersible pump stack with guide shield is lowed down to a position under the production layer, it can operate continuously and normally in the wells with gas-fluid ratio of 400m3/t. 
     
    Variable frequency control system for the new type electric submersible pump
    The functions of each component in the electric-pump control system
    1.  Frequency changer (FC):
    The universal FC equipped with the vector current control card is adopted and has such functions as the speed regulation of the motor, the measurement and control of the torque current.
    2.  Protective and control units:
    They consist of control and protective units of the programmable controller and take charge of the protection and control of system on electric pump. The units have the following functions:
    1) Soft-start function: it can make the machine, at the place of 2000m below the surface, start smoothly, which decreases the wear of the start current on the motor insulation and the damage of the mechanical impulsive torque on the electric pump, and avoids the sand- sticking phenomenon caused by suddenly increasing the pumping force.
    2) Voltage-stabilized protection: the variable frequency control technology is adopted to automatically control the voltage at the motor end. It can do the voltage-stabilized treatment if the over voltage occurs.
    3) Underload protection: after the variable frequency control system is adopted, the output voltage can be reduced automatically to make submersible motor keep the proper voltage when the over excitation occurs.
    4) Soft-stall function: when the subsurface electric pump produces a mechanical failure, which makes the motor jammed. The soft-stall function can control the torque not to exceed the rated level, and can conveniently make the pump invert to spit out sands to ensure that the units run normally.
    5) Two control functions: one is the torque-current control, the other one is the intermittent control. They are mainly applied in occasions with less well fluids. The intermittent control method is adopted, which can make the motor slowly run, accumulate the fluid level and save energy in the conditions with less well fluids.
    3.  Waveform processing unit: this unit can carry out the wave-shaped compensation processing according to the coefficient of cable capacity, cable length, the inductance coefficient of transformer and motor. 
    4.  Special transformer: Special transformer has the following technical requirements:
    1) Special transformer has enough torque output in the frequency range of 5 to 50Hz;
    2) Special transformer can overcome the influence of high harmonic wave and the asymmetric components of the direct current;
    3) The howling sound can not occur at any frequency.
     
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