Pumps & Systems, April 2008
In the oil and gas industry, electric submersible pump (ESP) systems are probably best known as an effective artificial lift method of pumping production fluids to the surface. ESPs are especially effective in wells with low bottomhole pressure, low gas/oil ratio, low bubblepoint, high water cut or low API gravity fluids.
Over the last several years, ESP technology has developed a reputation as a low-maintenance, cost-effective alternative to vertical turbine, split case and positive displacement pumps in various fluid-movement surface applications in the petroleum industry.
Artificial Lift Technologies
Artificial lift uses some means to increase the flow of liquids (i.e., crude oil or water with some amount of gas included) to the surface of a production well. This is usually achieved by (1) a mechanical device inside the well, such as a pump; (2) decreasing the weight of the liquid/gas mixture via high pressure gas or (3) improving the lift efficiency of the well via velocity strings. An artificial lift system is needed in wells with insufficient pressure in the reservoir to boost the liquid to the surface. Also, these systems are sometimes used in flowing wells to increase the naturally occurring flow rate.
More than 60 percent of producing oil wells require some type of assisted lift technology to produce the recoverable oil. Several artificial lift (or pumping) technologies are employed, including plunger lift, beam/sucker rod pumps, gas lift, progressive cavity pumps (PCP) or electric submersible pumps.
This artificial lift method is used primarily in gas wells to remove relatively small volumes of liquid. Functionally, a plunger-lift system provides a mechanical interface between the produced liquids and gas. Using the well's own energy for lift, liquids are pushed to the surface by the movement of a free-traveling piston (plunger) traveling from the bottom of the well to the surface. This mechanical interface eliminates liquid fallback, which boosts the well's lifting efficiency. In turn, the reduction of average flowing bottomhole pressure increases inflow.
Plunger travel is normally provided by formation gas stored in the casing annulus during a well shut-in period. As the well is opened and the tubing pressure decreases, the stored casing gas moves around the end of the tubing and pushes the plunger to the surface. This intermittent operation is repeated several times per day.
Sucker Rod Pumps
Beam pumps, rod pumps or sucker rod pumps refer to an artificial lift system that uses a surface power source to drive a downhole pump assembly. A beam and crank assembly at the surface (often called a "pump jack") creates reciprocating motion, which is converted to a vertical motion in a sucker-rod string that connects to the downhole pump assembly. The pump contains a plunger and valve assembly to impart vertical fluid movement. Due to its long history, sucker rod pumping is a very popular means of artificial lift. Roughly two-thirds of the producing oil wells around the world use this type of lift. Limitations to this system technology arise with deeper and/or more deviated wells. Sucker rod pumps are not generally considered applicable to offshore installations.
Gas-lift systems that inject gas into the crude are sometimes used in conjunction with surface operating reciprocating pumps or horizontal centrifugal pumps. However, these systems become far less efficient in deeper, deviated wells. Gas-lift systems often increase the degree of component flow constriction caused by scaling and paraffin crystal accumulation. Additionally, these techniques require an abundant supply of gas to be stored at the surface. Gas that is separated and vented is not easily retained for re-injection, and gas that is re-injected rapidly becomes contaminated with oxygen, carbon monoxide and hydrogen sulphide that can corrode production string components. Older gas-lift systems (sometimes prominent in offshore applications) burdened with high water cut are being converted more frequently to ESP systems.
Progressive Cavity Pumps
The PCP is a closely related technology to the ESP. PCPs consist of a helical bore that rotates inside a similar helical cavity. The rotation of the bore creates cavities with negative pressure (vacuum) to open and close, forcing fluid up through the pump body. The PCP offers proven performance in extracting crude oil at high viscosity. However, PCPs are vulnerable to damage from abrasive materials and are limited to well depths of approximately 5000-ft. PCPs do not perform well in deviated wells.
About 15 to 20 percent of almost one million wells worldwide are pumped with some form of artificial lift employing electric submersible pumps. In addition, ESP systems are the fastest growing form of artificial lift pumping technology. They are often considered high volume and depth champions among oil field lift systems.
Found in operating environments all over the world, ESPs are very versatile. They can handle a wide range of flow rates from 70-bpd to 64,000-bpd or more and lift requirements from virtually zero to as much as 15,000-ft of lift. As a rule, ESPs have lower efficiencies with significant fractions of gas, typically greater than about 10 percent volume at the pump intake. Given their high rotational speed of up to 4000-rpm and tight clearances, they are also only moderately tolerant of solids like sand. If solid-laden production flows are expected, special running procedures and pump placement techniques are usually employed. When very large amounts of free gas are present, downhole gas separators and/or gas compressors may be required in lieu of a standard pump intake.