Electro-Submersible Pumping (ESP): A Comprehensive Guide

Posted on Aug 28, 2024 in Technology

Electro-Submersible Pumping (ESP)

Basic Principle

The electro-submersible pumping (ESP) system utilizes an electro-centrifugal pump powered by a submersible electric motor to transmit energy to well fluids in the form of pressure. The entire unit is submerged in the well fluid and suspended from the tubing. A cable connects the unit to the surface, providing power to the motor. The motor and pump are coupled via grooved axes, ensuring the pump’s impeller rotates with the motor shaft.

Surface Equipment

Electrical Transformer Bank

This bank consists of transformers that modify the alternating current voltage. These units, typically immersed in oil for insulation and cooling, ensure the equipment receives the correct voltage for operation. Various types, including single-phase, bi-phasic, and tri-phasic, are available to match the primary voltage of the electric line to the motor’s requirements.

Control Panel

The control panel protects and manages the well’s operations. It includes components like start and stop switches, voltage selectors, fuses, ammeters, and signal lights. Some panels may incorporate specialized equipment for remote, intermittent pumping control.

Frequency Inverter (Variable Frequency Drive – VFD)

This device provides variable frequencies and voltages to the motor. Key benefits include:

  • Reduced stress on the pump and motor components during startup by allowing low-speed operation.
  • Minimized wear and abrasion by operating below the nominal frequency.
  • Protection against electrical variations.

Vent Box

This junction box connects the surface power cable to the motor’s cable. It also vents gases, preventing them from reaching the control panel.

Discharge Head

The wellhead features a probe or a closed hydrant-type packing to create a secure seal around the cables and pipes.

Miscellaneous Equipment

  • Ammeter Recorder: Continuously records the motor’s amperage, aiding in diagnosing anomalies and reflecting pumping conditions.
  • Centralizers: Center the pump and motor in deviated wells, preventing cable damage from friction.
  • Cable Clamps: Secure the power cable to the tubing.
  • Cable Protectors: Prevent cable abrasion against the casing.
  • Downhole Pressure Gauges: Provide pressure data for optimizing pump size, injection rates, and well reconditioning.

Downhole Equipment

Motor

Typically a three-phase, bipolar, squirrel-cage induction motor, operating at 3500 rpm (60 Hz) or 2915 rpm (50 Hz). It’s enclosed in a steel jacket filled with dielectric oil for lubrication and heat exchange. Proper placement above the perforations is crucial.

Protector

Located between the motor and pump, the protector performs four key functions:

  1. Connects the pump shaft to the motor shaft.
  2. Absorbs axial loads from the pump.
  3. Compensates for motor expansion and contraction.
  4. Allows well fluid entry into the motor.

Fluid Intake Section

This section, often slotted, serves as the fluid inlet and may include a filter or gas separator.

Gas Separator

Reduces free gas entering the pump. Common types include reverse flow (static) and centrifugal (dynamic) separators. The separation process occurs in a rotating chamber, utilizing centrifugal force to separate gas from the liquid.

Pump

The core component of the ESP system, the multi-stage centrifugal pump draws fluid and imparts energy. Each stage comprises a rotating impeller and a diffuser. The impeller, driven by the motor, ejects fluid at a higher pressure. The number of stages corresponds to the required hydraulic load. The diffuser converts the fluid’s velocity into pressure.

Affinity Laws (for low viscosity fluids):
  • Pumped volume is directly proportional to impeller diameter.
  • Hydraulic load is directly proportional to the square of the impeller diameter.
  • Power output is dependent on the impeller diameter.

Manufacturers provide performance curves for various pumps, typically based on water. For other liquids, power requirements should be adjusted based on specific gravity.

Valves:
  • Check Valve: Reduces hydrostatic pressure on pump components.
  • Drain Valve: Provides a safety factor for fluid transfer between the well casing and tubing.

Cable

The three-phase electric cable transmits power from the surface to the downhole motor. Available in flat and round configurations, these cables are designed to withstand the harsh downhole environment.

Applications, Advantages, and Limitations

Applications:

ESP systems are suitable for high-volume wells with:

  • High productivity
  • Low bottomhole pressure
  • High water-oil ratio
  • Low gas-liquid ratio

Advantages:

  • High production volumes
  • Simple operation
  • Suitable for deviated wells
  • Applicable offshore
  • Low operating costs for high volumes
  • Easy installation of downhole sensors
  • Minimal environmental impact
  • Easy implementation of corrosion and scale treatments

Limitations:

  • Requires a reliable power source
  • Requires high voltages
  • Not practical for low-productivity, shallow wells
  • Limitations based on casing size
  • Cable handling challenges
  • Cable degradation at high temperatures
  • Challenges with solids and gas production
  • Not recommended for depths greater than 10,000 feet
  • Complex analysis and software requirements

Equipment Failure

Motor:

  • Excessive voltage load
  • Seal leakage
  • Corrosion
  • Operational issues due to dirt or moisture

Pump:

  • Wear and tear
  • Sediment buildup
  • Shaft damage
  • Corrosion

Protector:

  • Seal breakage
  • Vibrations
  • Frequent stops and starts
  • Cycling changes

Cable:

  • Damage during installation
  • Excessive amperage load
  • Poor connection