Mechanical Pumps: Concepts and Operation

Mechanical Pump: Concept and Operation

The mechanical pump is the most used artificial lift method worldwide. This method consists of an underground pump with reciprocating action, which is supplied with power produced by a string of rebar. Energy comes from an electric motor or internal combustion engine, which mobilizes a unit area through a gear system and straps. The mechanical pump has its largest application in the global production of heavy and/or extra-heavy crudes, but is also used in the production of medium and light crudes.

Many theories explain the behavior of production wells by mechanical pumping. Some operators prefer to produce a coated pit open to the atmosphere and closed with the casing; this mechanism really depends on the volume of gas production associated with the oil. These mechanisms are applied if the well completion has no anchor to anchor the pipe or gas production and gasket.

The displacement capacity required to pump a well depends on factors such as the production of oil, gas, and water; gas ventilation efficiency in the well; the shrinkage factor; and the properties of free gas in the production fluids. Free gas may affect the pumping capacity requirements, as its management for the subsurface pump results in a loss of pumping efficiency and, hence, reduced production. Therefore, for the design of a mechanical pump installation, factors such as bubbling pressure, formation volume factor, and gas-oil ratio solution should be considered.

Equipment

The mechanical pumping system comprises surface equipment and subsurface equipment, which are formed as follows:

• Surface equipment
  • Pumping unit
  • Motor drive
  • Wellhead
• Subsoil Team
  • Tubing
  • Cabillas
  • Gas pump
  • Anchor

Pumping Unit

Its main function is to provide reciprocating motion to trigger a string of rebar and, in turn, to the pump groundwater. By the action of belts and gears, the properties of rotation are reduced. The resulting rotary motion is transformed into a reciprocating motion through the crank, the connecting rods, and the actual swing. The surface unit can be rocking or hydraulic. In this section, only the conventional rocker unit is described.

Other Features of the Rocking Unit are:

• The change in speed to the rpm of the driving machine.
• The variation of the length of the race.
• The variation of the counterweight, which acts against the burdens of the pins and the well fluid.

The unit is in the rocking clutch to start or stop the movement; it also has a brake that can stop the unit in any desired position.

The design of the rocking unit has three essential aspects:

• Speed Reduction System
• Joint System
• Counterweight system

Engine

Provides energy to the pumping unit to lift well fluids. They can be internal combustion or electric, the latter being the largest. The electric motor speed can be constant or variable. The change in velocity of the beam is achieved by modifying the motor pulley.

Gearbox

A gear system aims to reduce the rotation speed between the primary motor and the rod-crank system. The reduction system can be single, double, or triple. The gearbox is one of the most expensive parts of the pump unit.

Crank

Transmits the movement of the gearbox or transmission of the beam connecting rods that are attached to them by pins. They are subject to the low-speed shaft of the gearbox, and each carries the same number of holes in which pins are placed for tie-down cranks. Each hole represents a particular race of the beam; changing a pinhole to another is commonly called a change of shot.

Weights or Counterweights

Usually located in the handle and, in some units, on the main beam at the opposite end, the headstock is used to balance the unequal forces that arise on the engine during the upstream and downstream running of the beam. In balanced air units, the balance is performed with compressed air in a cylinder.

Stuffing Box

Used to seal the space between the polished rod and tubing, preventing oil spills. It consists of a cylindrical chamber containing packing elements that conform to the polished rod to make the seal.