Artificial lift

Artificial lift involves the use of artificial means to increase the flow of liquids, such as crude oil or water, to the surface of a production well. Generally this is achieved by a mechanical device inside the well, such as a pump; decreasing the weight of the liquid/gas mixture via high pressure gas; or improving the lift efficiency of the well via velocity strings. Artificial lift is needed in wells when there is insufficient pressure in the reservoir to lift the liquid to the surface, but often used in naturally flowing wells (which don't technically need it) to increase the flow rate above what would flow naturally. The produced fluid can be oil and/or water, typically with some amount of gas included. The artificial lift provides additional energy to the system so that the fluids can be lifted to surface.

The most common method of artificial lift in oil wells is to use pumps. The most recognized type is the pumpjack or rod pump (also called a sucker-rod pump, beam pump, or "Nodding Donkey") seen in land-based oil fields worldwide. The rod pump works by creating a reciprocating motion in a sucker-rod string that connects to the downhole pump assembly. The pump contains a plunger and valve assembly to convert the reciprocating motion to vertical fluid movement. This type of pump is used in low-rate wells where 10's to 100's of barrels of liquid are produced per day. Other types of commonly used pumps are Electric Submersible Pump (ESP's), Progressing-Cavity pumps (PCP's), Jet Pumps, and Hydraulic Pumps. These pump systems must be installed in the well down hole. They also include a ground-level power-supply device that can be mechanical (rod pumps and PCP's), electrical (ESP's), or even hydraulic (jet and hydraulic pumps). The alternative to adding in extra pressure - such as with pumps - is to reduce the weight of the fluid. This is the objective of gas lift, where high-pressure gas is injected into the well at depth. By the injection of gas, average density of the produced liquid/gas mixture decreases, and therefore the reservoir pressure is high enough to maintain flow.

Why use Artificial Lift

Any liquid-producing reservoir will have a 'reservoir pressure': some level of energy or potential that will force fluid (liquid and/or gas) to areas of lower energy or potential. You can think of this much like the water pressure in your municipal water system. As soon as the pressure inside a production well is decreased below the reservoir pressure, the reservoir will act to fill the well back up, just like opening a valve on your water system. Depending on the depth of the reservoir (deeper results in higher pressure requirement) and density of the fluid (heavier mixture results in higher requirement), the reservoir may or may not have enough potential to push the fluid to the surface. Most oil production reservoirs have sufficient potential to produce oil and gas - which are light - naturally in the early phases of production. Eventually, as water - which is heavier than oil and much heavier than gas - encroaches into production and reservoir pressure decreases as the reservoir depletes, all wells will stop flowing naturally. At some point, most well operators will implement an artificial lift plan to continue and/or to increase production. Most water production wells, by contrast, will need artificial lift from the very beginning of production because they do not benefit from the lighter density of oil and gas.

Hydraulic Pumping Systems

Hydraulic pumping systems, such as Jet Pumps, transmit energy downhole by means of pressurized power fluid that flows in the wellbore tubular. This method of transmitting energy downhole is reasonably efficient. The two methods of converting the energy downhole is to have either a downhole hydraulic pump, which has a set of coupled reciprocating piston, one is powered by the injected fluid while the other pumps the wellbore fluid to surface. The jet pump works by taking the injected fluid and turning it into a high velocity jet that mixes with the wellbore fluid and helps lift it to surface.

These systems are very versatile and have been used in shallow depths (1000ft) to deeper wells (18,000ft), low rate wells with production in the 10’s of barrels per day to wells producing in excess of convert|10000|oilbbl/d|m3/d. In addition to this certain fluids can be mixed in with the injected fluid to help deal or control with corrosion, paraffin and emulsion problems. They are also suitable for wells where conventional pumps such as the rod pump are not possible due to crooked or deviated wells.

ESP

Electric Submersible Pumps consist of three parts, a downhole pump and motor, electric power cable from the pump to the surface and surface controls to operate the pump. The centrifugal pump (the part that introduces energy to the fluid) attaches to the bottom of the production tubing and couples to a submersible electric motor that turns the pump. The power cable connects the surface control system to the downhole motor. ESP’s are a very versatile artificial lift method and can be found in operating environments all over the world. They can handle a very wide range of flow rates (from 200 to 90,000 bpd) and lift requirements (from virtually zero to convert|10000|ft|m|abbr=on of lift). They can be modified to handle contaminants commonly found in oil, aggressive corrosive fluids such as H2S and CO2, abrasive contaminants such as sand, exceptionally high downhole temperatures, and high levels of gas production. Increasing water cut has been shown to have no significant detrimental effect on the ESP performance. It is possible to locate them in vertical, deviated, or horizontal wells, but it is recommended for optimum runlife performance they are located in straight section of casing.

Rod Pumps

Rod Pumps are large cylinders with both fixed and moveable elements inside. The most important components are: the barrel, valves (traveling and fixed) and the piston. It also has another 18 to 30 components which are called "fitting". The pump is designed to be inserted inside the tubing of a well and its main purpose is to gather fluids from beneath it and lift them to the surface.

Components

Every part of the pump is important for its correct operation. The most commonly used parts are described below:

- Barrel: The barrel is a large cylinder which can be from 10 to 36 feet long and a diameter from 1.25 to convert|3.75|in|mm. After using several materials for its construction, the API (American Petroleum Institute) standardized the use of 2 materials or compositions for this part which are carbon steel and brass, both with an inside coating of chrome. The advantage of brass against carbon steel, weather is a more soft material, is its 100% resistance to corrosion.

- Piston: This is a nickel-metal sprayed steel cylinder, that goes inside the barrel. Its main purpose is to create a sucking effect that lift the fluids beneath it and then, with the help of the valves, take that fluids above it and, progressively, out of the well. It achieves this with a reciprocal up and own movement.

- Valves: The valve has two components - the seat and the ball - which create a complete seal when closed. After trying several materials, the most commonly used seats are made of carbon nitride and the ball is often made of silicon nitride. In the past, balls of iron, ceramic and titanium were used. This last type of balls, made of titanium, are still being used but only where crude oil is extremely dense and/or the quantity of fluids is too much. The most common configuration of a rod pump, requires two valves, called the traveling valve and fixed or static valve.

- Piston Rod: It's a rod that connects the piston with the outside of the pump. Its main purpose is to transfer the engine produced by the "Nodding Donkey" above in an up/down reciprocal movement.

- Fitting: The rest of the parts of the pump is called fitting and is, basically, small pieces designed to keep everything hold together in the right place. Most of these parts, are designed to let the fluids pass uninterrupted.

- Filter: The job of the filter, as guessed, is to stop big parts of rock, rubber or any other garbage that might be loose in the well from going into the pump. There are several types of filters, being a common iron cylinder with enough holes in it to permit the entrance of the amount of fluid the pump needs the most commonly used.

References

* [http://www.slb.com/content/services/artificial/index.asp " Schlumberger Page on Artificial Lift"] Accesed Jan 24 2007

*Petroleum Engineering Handbook Bradley H, Society of Petroleum Engineers, Richardson, TX, U.S.A, 1987