Downhole oil–water separation (DOWS) technologies are apparatuses and methods that separate production fluids into a

petroleum Petroleum, also known as crude oil or simply oil, is a naturally occurring, yellowish-black liquid chemical mixture found in geological formations, consisting mainly of hydrocarbons. The term ''petroleum'' refers both to naturally occurring un ...

-rich stream and

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

-rich stream within an

oil well An oil well is a drillhole boring in Earth that is designed to bring petroleum oil hydrocarbons to the surface. Usually some natural gas is released as associated petroleum gas along with the oil. A well that is designed to produce only gas m ...

. A DOWS system installed in a

borehole A borehole is a narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water ( drilled water well and tube well), other liquids (such as petr ...

will receive the fluids from an oil-producing zone in an

oil reservoir A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in Porosity, porous or fractured rock formations. Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by t ...

and separate the mixture into a stream that is mostly water and a stream that is primarily

crude oil Petroleum, also known as crude oil or simply oil, is a naturally occurring, yellowish-black liquid chemical mixture found in geological formations, consisting mainly of hydrocarbons. The term ''petroleum'' refers both to naturally occurring u ...

and

natural gas Natural gas (also fossil gas, methane gas, and gas) is a naturally occurring compound of gaseous hydrocarbons, primarily methane (95%), small amounts of higher alkanes, and traces of carbon dioxide and nitrogen, hydrogen sulfide and helium ...

and direct the streams to different destinations. After the separation in the borehole, DOWS systems pump the petroleum-rich stream to the surface and inject the water-rich stream into a different zone or formation accessible to the same wellbore.

Economics

An oil producing well is usually completed to pump all produced fluids to the surface where the fluids will be separated into their constituent components. In the early life of most oil producing wells, more petroleum will usually be produced than

produced water Produced water is a term used in the oil industry or geothermal industry to describe water that is produced as a byproduct during the extraction of oil and natural gas, or used as a medium for heat extraction. Water that is produced along with t ...

. The produced water is considered to be

wastewater Wastewater (or waste water) is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of do ...

that will need to be treated. Over time, as the petroleum products are being depleted from the reservoir, the water cut (the ratio of water produced compared to the volume of total liquids produced) may increase. Or, when the field is undergoing secondary recovery with a waterflood, the water cut may become very high. Alternatively, the well may develop other problems requiring a

workover The term workover is used to refer to any kind of oil well well intervention, intervention involving invasive techniques, such as wireline (cabling), wireline, coiled tubing or snubbing. More specifically, a workover refers to the expensive proc ...

. When the water cut becomes high, or when a workover is considered, the economics will be evaluated and may favor the installation of DOWS technology so that the waste water does not need to be treated at the surface. In those cases, the economics may favor

injection Injection or injected may refer to: Science and technology * Injective function, a mathematical function mapping distinct arguments to distinct values * Injection (medicine), insertion of liquid into the body with a syringe * Injection, in broadca ...

of the produced water into a different permeable zone accessible to the borehole. However, not all oil wells are good candidates for deploying DOWS.

Operation

The producing zone that a DOWS system is located in must be sealed from the other zones of the borehole by

packers The Green Bay Packers are a professional American football team based in Green Bay, Wisconsin. The Packers compete in the National Football League (NFL) as a member of the National Football Conference (NFC) North division. They are the third ...

. This allows the

intake An intake (also inlet) is an opening, structure or system through which a fluid is admitted to a space or machine as a consequence of a pressure differential between the outside and the inside. The pressure difference may be generated on the ins ...

of a DOWS system to receive the petroleum and water mixture. In some types of DOWS systems, a

pump A pump is a device that moves fluids (liquids or gases), or sometimes Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy. Mechanical pumps serve in a wide range of application ...

will force the received mixture through the oil/water separation system. In some DOWS systems, the pressure of the fluids in the borehole will be enough to force the water-rich stream into a permeable zone below the DOWS system. In other DOWS systems, a separate pump is needed to inject the water-rich stream into a permeable zone. An artificial lift pump is used to lift the petroleum-rich stream to the surface. DOWS systems do not entirely separate petroleum from water in the borehole. Instead, DOWS systems will decrease the amount of water brought to the surface. Some entrained droplets of water will be retained in the petroleum-rich stream, and some droplets of petroleum and dissolved gas will be entrained within the water-rich stream. Trials have shown that a majority of properly operating DOWS systems may reduce the amount of water brought to the surface by around 75%. Research into further reducing the amount of water brought to the surface continues.

Types

Two traditional types of DOWS systems have been developed. Initially, a

gravity separation Gravity separation is an industrial method of separating two components, either a suspension, or dry granular mixture where separating the components with gravity is sufficiently practical: i.e. the components of the mixture have different specific ...

mechanism was developed. However, gravity separation requires time downhole for the components to separate, which results in lower production rates. Hydrocyclone DOWS systems were developed to speed up the separation and production rate. Hydrocyclone type DOWS systems usually use electric submersible pumps to overcome any

pressure head In fluid mechanics, pressure head is the height of a liquid column that corresponds to a particular pressure exerted by the liquid column on the base of its container. It may also be called static pressure head or simply static head (but not ''sta ...

differential needed to inject water into another zone. Additionally, the supplemental draw of fluids into the wellbore by the pump has been known to increase the production of oil from some wells. A new line of research is being pursued in which permeable membranes are deployed in the wellbore to separate water from the oil and gas.

References

Petroleum technology {{petroleum-stub